Respiratory pressure therapy device

Abstract

A respiratory pressure therapy (RPT) device comprises a pressure generator for producing a flow of air for respiratory pressure therapy, and a controller configured to control at least one operational parameter of the pressure generator in order to compensate for local air density.

Description

[0001] RESPIRATORY PRESSURE THERAPY DEVICE 1 BACKGROUND OF THE TECHNOLOGY 1.1 FIELD OF THE TECHNOLOGY [1] The present technology relates to one or more of the screening, diagnosis, monitoring, treatment, prevention and amelioration of respiratory-related disorders. The present technology also relates to medical devices or apparatus, and their use. 1.2 DESCRIPTION OF THE RELATED ART 1.2.1 Human Respiratory System and its Disorders [2] The respiratory system of the body facilitates gas exchange. The nose and mouth form the entrance to the airways of a patient. [3] The airways include a series of branching tubes, which become narrower, shorter and more numerous as they penetrate deeper into the lung. The prime function of the lung is gas exchange, allowing oxygen to move from the inhaled air into the venous blood and carbon dioxide to move in the opposite direction. The trachea divides into right and left main bronchi, which further divide eventually into terminal bronchioles. The bronchi make up the conducting airways, and do not take part in gas exchange. Further divisions of the airways lead to the respiratory bronchioles, and eventually to the alveoli. The alveolated region of the lung is where the gas exchange takes place, and is referred to as the respiratory zone. See “Respiratory Physiology”, by John B. West, Lippincott Williams & Wilkins, 9th edition published 2012. [4] A range of respiratory disorders exist. Certain disorders may be characterised by particular events, e.g. apneas, hypopneas, and hyperpneas. [5] Examples of respiratory disorders include Obstructive Sleep Apnea (OSA), Cheyne-Stokes Respiration (CSR), respiratory insufficiency, Obesity Hypoventilation Syndrome (OHS), Chronic Obstructive Pulmonary Disease (COPD), Neuromuscular Disease (NMD) and Chest wall disorders.Therapies [6] Various respiratory therapies, such as Continuous Positive Airway Pressure (CPAP) therapy, Non-invasive ventilation (NIV), Invasive ventilation (IV), and High Flow Therapy (HFT) have been used to treat one or more of the above respiratory disorders. 1.2.1.1 Respiratory pressure therapies [7] Respiratory pressure therapy is the application of a supply of air to an entrance to the airways at a controlled target pressure that is nominally positive with respect to atmosphere throughout the patient’s breathing cycle (in contrast to negative pressure therapies such as the tank ventilator or cuirass). [8] Continuous Positive Airway Pressure (CPAP) therapy has been used to treat Obstructive Sleep Apnea (OSA). The mechanism of action is that continuous positive airway pressure acts as a pneumatic splint and may prevent upper airway occlusion, such as by pushing the soft palate and tongue forward and away from the posterior oropharyngeal wall. Treatment of OSA by CPAP therapy may be voluntary, and hence patients may elect not to comply with therapy if they find devices used to provide such therapy one or more of: uncomfortable, difficult to use, expensive and aesthetically unappealing. 1.2.2 Respiratory Therapy Systems [9] These respiratory therapies may be provided by a respiratory therapy system or device. Such systems and devices may also be used to screen, diagnose, or monitor a condition without treating it.

[0010] A respiratory therapy system may comprise a Respiratory Pressure Therapy Device (RPT device), an air circuit, a humidifier, a patient interface, an oxygen source, and data management. 1.2.2.1 Patient Interface

[0011] A patient interface may be used to interface respiratory equipment to its wearer, for example by providing a flow of air to an entrance to the airways. The flow of air may be provided via a mask to the nose and / or mouth, a tube to the mouth or a tracheostomy tube to the trachea of a patient. Depending upon the therapy to be applied, the patient interface may form a seal, e.g., with a region of the patient's face, to facilitate the delivery of gas at a pressure at sufficient variance with ambient pressure to effect therapy, e.g., at a positive pressure of about 10 cmH2O relative to ambient pressure. For other forms of therapy, such as the delivery of oxygen, the patient interface may not include a seal sufficient to facilitate delivery to the airways of a supply of gas at a positive pressure of about 10 cmH2O. For flow therapies such as nasal HFT, the patient interface is configured to insufflate the nares but specifically to avoid a complete seal. One example of such a patient interface is a nasal cannula. 1.2.2.2 Respiratory Pressure Therapy (RPT) Device

[0012] A respiratory pressure therapy (RPT) device may be used individually or as part of a system to deliver one or more of a number of therapies described above, such as by operating the device to generate a flow of air for delivery to an interface to the airways. The flow of air may be pressure-controlled (for respiratory pressure therapies) or flow- controlled (for flow therapies such as HFT). Thus RPT devices may also act as flow therapy devices. Examples of RPT devices include a CPAP device and a ventilator.

[0013] Air pressure generators are known in a range of applications, e.g. industrial- scale ventilation systems. However, air pressure generators for medical applications have particular requirements not fulfilled by more generalised air pressure generators, such as the reliability, size and weight requirements of medical devices. In addition, even devices designed for medical treatment may suffer from shortcomings, pertaining to one or more of: comfort, noise, ease of use, efficacy, size, weight, manufacturability, cost, and reliability.

[0014] An example of the special requirements of certain RPT devices is acoustic noise.

[0015] Table of noise output levels of prior RPT devices (one specimen only, measured using test method specified in ISO 3744 in CPAP mode at 10 cmH2O).

[0016] One known RPT device used for treating sleep disordered breathing is the S9 Sleep Therapy System, manufactured by ResMed Inc. Another example of an RPT device is a ventilator. Ventilators such as the ResMed Stellar™ Series of Adult and Paediatric Ventilators may provide support for invasive and non-invasive non-dependent ventilation for a range of patients for treating a number of conditions such as but not limited to NMD, OHS and COPD.

[0017] The ResMed Astral™ 100 / 150 ventilators ventilator may provide support for invasive and non-invasive dependent ventilation suitable for adult or paediatric patients for treating a number of conditions. These ventilators provide volumetric and barometric ventilation modes with a single or double limb circuit. RPT devices typically comprise a pressure generator, such as a motor-driven blower or a compressed gas reservoir, and are configured to supply a flow of air to the airway of a patient. In some cases, the flow of air may be supplied to the airway of the patient at positive pressure. The outlet of the RPT device is connected via an air circuit to a patient interface such as those described above.

[0018] The designer of a device may be presented with an infinite number of choices to make. The designer may also have to design for a range of environments. The designer may also have to design for interoperability with other elements of a respiratory therapy system. Design criteria often conflict, meaning that certain design choices are far from routine or inevitable. Furthermore, the comfort and efficacy of certain aspects may be highly sensitive to small, subtle changes in one or more parameters. For example, the ability to deliver a controlled target pressure with an acceptable level of accuracy may be impacted by some design criteria. 1.2.2.3 Air circuit

[0019] An air circuit is a conduit or a tube constructed and arranged to allow, in use, a flow of air to travel between two components of a respiratory therapy system such as the RPT device and the patient interface. In some cases, there may be separate limbs of the air circuit for inhalation and exhalation. In other cases, a single limb air circuit is used for both inhalation and exhalation. 1.2.2.4 Humidifier

[0020] Delivery of a flow of air without humidification may cause drying of airways. The use of a humidifier with an RPT device and the patient interface produces humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort. In addition, in cooler climates, warm air applied generally to the face area in and about the patient interface is more comfortable than cold air. 1.2.2.5 Vent technologies

[0021] Some forms of treatment systems may include a vent to allow the washout of exhaled carbon dioxide. The vent may allow a flow of gas from an interior space of a patient interface, e.g., the plenum chamber, to an exterior of the patient interface, e.g., to ambient. 2 BRIEF SUMMARY OF THE TECHNOLOGY

[0022] The present technology is directed towards providing medical devices used in the screening, diagnosis, monitoring, amelioration, treatment, or prevention of respiratory disorders having one or more of improved comfort, cost, efficacy, ease of use and manufacturability.

[0023] A first aspect of the present technology relates to apparatus used in the screening, diagnosis, monitoring, amelioration, treatment or prevention of a respiratory disorder.

[0024] Another aspect of the present technology relates to methods used in the screening, diagnosis, monitoring, amelioration, treatment or prevention of a respiratory disorder.

[0025] An aspect of certain forms of the present technology is to provide methods and / or apparatus that mitigate against mask pressure inaccuracy in respiratory therapy.

[0026] An aspect of certain forms of the present technology is to provide methods and / or apparatus that mitigate against the effects of impedance variation in respiratory therapy systems.

[0027] One form of the present technology comprises an RPT device that corrects for impedance variation in respiratory therapy systems.

[0028] One form of the present technology comprises an RPT device that controls at least one operational parameter of a pressure generator based on in air density.

[0029] One form of the present technology comprises a respiratory pressure therapy (RPT) device comprising: a pressure generator for producing a flow of air for respiratory pressure therapy; and a controller configured to control at least one operational parameter of the pressure generator in order to compensate for local air density.

[0030] In one form of the technology, the controller is configured to estimate local air density and to control the at least one operational parameter on the basis of the estimated local air density.

[0031] In one form of the technology, the RPT device has transducers for producing signals indicative of blower pressure, motor speed, and blower flow, and the controller is configured to estimate local air density from the signals indicative of blower pressure, motor speed and blower flow.

[0032] In one form of the technology, the controller estimates local air density based on a polynomial equation that relates blower pressure to motor speed, blower pressure and local air density.

[0033] In one form of the technology, the polynomial equation comprises at least one polynomial coefficient that is a function of motor speed and local air density.

[0034] In one form of the technology, local air density (ρlocal) is estimated based on the equation:⍴local =−(^^1 ∗ ^^^^^^^^ − ^^^^^^^^^^^^^^^^ ) + √(^^1 ∗ ^^^^^^^^ − ^^^^^^^^^^^^^^^^ )2 ∗ ^^1 ∗ ⍴ref ∗ Flow^2 where Flow is the blower flow rate, Pressure is the blower pressure, and A1, B1, and C1 are coefficients characterising operation of a fan of the pressure generator at a reference air density (ρref).

[0035] In one form of the technology, the RPT device is associated with an air circuit and the controller is configured to estimate pressure drop across the air circuit based on the estimated local air density and control the at least one operational parameter based on the estimated pressure drop.

[0036] In one form of the technology, the pressure drop is estimated as a function of blower flow rate and at least one function of air density.

[0037] In one form of the technology, the pressure drop (^P) is estimated according to the equation: ∆^^ = ^^1. ^^2 + ^^2. ^^where Q is blower flow rate and Z1 and Z2 are functions of air density.

[0038] In one form of the technology, the controller is configured to modify at least one operational parameter of the pressure generator in response to a change in local air density.

[0039] Another aspect of one form of the present technology is an RPT system comprising an RPT device as described above and an air circuit.

[0040] Another aspect of one form of the present technology is a non-transitory computer-readable medium comprising instructions, which when executed by a processor, cause the processor to control at least one operational parameter of a pressure generator of an RPT device to produce a flow of air for respiratory pressure therapy compensated for local air density.

[0041] In one form of the technology, when the instructions are executed, they cause the processor to estimate local air density and to control the at least one operational parameter on the basis of the estimated local air density.

[0042] In one form of the technology, when the instructions are executed, they cause the processor to process signals indicative of blower pressure, motor speed, and blower flow to estimate local air density.

[0043] In one form of the technology, when the instructions are executed, they cause the processor to estimate local air density based on a polynomial equation that relates blower pressure to motor speed, blower pressure and local air density.

[0044] In one form of the technology, the polynomial equation comprises at least one polynomial coefficient that is a function of motor speed and local air density.

[0045] In one form of the technology, when the instructions are executed, they cause the processor to estimate local air density (ρlocal) based on the equation:⍴local = where Flow is the blower flow rate, Pressure is the blower pressure, and A1, B1, and C1 are coefficients characterising operation of a fan of the pressure generator at a reference air density (ρref).

[0046] In one form of the technology, the RPT device is associated with an air circuit and when the instructions are executed, they cause the processor to estimate pressure drop across the air circuit based on the estimated local air density and control the at least one operational parameters based on the estimated pressure drop.

[0047] In one form of the technology, when the instructions are executed, they cause the processor to estimate pressure drop as a function of blower flow rate and at least one function of air density.

[0048] In one form of the technology, when the instructions are executed, they cause the processor to estimate pressure drop (^P) according to the equation:∆^^ = ^^1. ^^2 + ^^2. ^^where Q is blower flow rate and Z1 and Z2 are functions of air density.

[0049] In one form of the technology, when the instructions are executed, they cause the processor to modify at least one operational parameter of the pressure generator in response to a change in local air density.

[0050] Another aspect of one form of the present technology is a computer- implemented method comprising controlling at least one operational parameter of a pressure generator of an RPT device to produce a flow of air for respiratory pressure therapy compensated for local air density.

[0051] In one form of the technology, the computer-implemented method comprises estimating local air density and controlling the at least one operational parameter on the basis of the estimated local air density.

[0052] In one form of the technology, the computer-implemented method comprises causing the processor to process signals indicative of blower pressure, motor speed, and blower flow to estimate local air density.

[0053] In one form of the technology, the computer-implemented method comprises the processor to estimate local air density based on a polynomial equation that relates blower pressure to motor speed, blower pressure and local air density.

[0054] In one form of the technology, the computer-implemented method comprises the polynomial equation comprises at least one polynomial coefficient that is a function of motor speed and local air density.

[0055] In one form of the technology, the computer-implemented method comprises causing the processor to estimate local air density (ρlocal) based on the equation:⍴local = −(^^1 ∗ ^^^^^^^^ − ^^^^^^^^^^^^^^^^ ) + √(^^1 ∗ ^^^^^^^^ − ^^^^^^^^^^^^^^^^ )2 − 4 ∗ 1⍴ref∗ ^^1 ∗ ⍴ref ∗ Flow^22 ∗^^1⍴ref where Flow is the blower flow rate, Pressure is the blower pressure, and A1, B1, and C1are coefficients characterising operation of a fan of the pressure generator at a reference air density (ρref).

[0056] In one form of the technology, the RPT device is associated with an air circuit and the method comprises causing the processor to estimate pressure drop across the air circuit based on the estimated local air density and control the at least one operational parameters based on the estimated pressure drop.

[0057] In one form of the technology, the computer-implemented method comprises causing the processor to estimate pressure drop as a function of blower flow rate and at least one function of air density.

[0058] In one form of the technology, the computer-implemented method comprises causing the processor to estimate pressure drop (^P) according to the equation: ∆^^ = ^^1. ^^2 + ^^2. ^^where Q is blower flow rate and Z1 and Z2 are functions of air density.

[0059] In one form of the technology, the computer-implemented method comprises modifying at least one operational parameter of the pressure generator in response to a change in local air density.

[0060] The methods, systems, devices and apparatus described may be implemented so as to improve the functionality of a processor, such as a processor of a specific purpose computer, respiratory monitor and / or a respiratory therapy apparatus. Moreover, the described methods, systems, devices and apparatus can provide improvements in the technological field of automated management, monitoring and / or treatment of respiratory conditions, including, for example, sleep disordered breathing.

[0061] Of course, portions of the aspects may form sub-aspects of the present technology. Also, various ones of the sub-aspects and / or aspects may be combined in various manners and also constitute additional aspects or sub-aspects of the present technology.

[0062] Other features of the technology will be apparent from consideration of the information contained in the following detailed description, abstract, drawings and claims. 3 BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The present technology is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements including: 3.1 RESPIRATORY THERAPY SYSTEMS

[0064] Fig.1A shows a system including a patient 1000 wearing a patient interface 3000, in the form of nasal pillows, receiving a supply of air at positive pressure from an RPT device 4000. Air from the RPT device 4000 is humidified in a humidifier 5000, and passes along an air circuit 4170 to the patient 1000. A bed partner 1100 is also shown. The patient is sleeping in a supine sleeping position.

[0065] Fig.1B shows a system including a patient 1000 wearing a patient interface 3000, in the form of a nasal mask, receiving a supply of air at positive pressure from an RPT device 4000. Air from the RPT device is humidified in a humidifier 5000, and passes along an air circuit 4170 to the patient 1000.

[0066] Fig.1C shows a system including a patient 1000 wearing a patient interface 3000, in the form of a full-face mask, receiving a supply of air at positive pressure from an RPT device 4000. Air from the RPT device is humidified in a humidifier 5000, and passes along an air circuit 4170 to the patient 1000. The patient is sleeping in a side sleeping position. 3.2 RESPIRATORY SYSTEM AND FACIAL ANATOMY

[0067] Fig.2A shows an overview of a human respiratory system including the nasal and oral cavities, the larynx, vocal folds, oesophagus, trachea, bronchus, lung, alveolar sacs, heart and diaphragm. 3.3 PATIENT INTERFACE

[0068] Fig.3A shows a patient interface in the form of a nasal mask in accordance with one form of the present technology. 3.4 RPT DEVICE

[0069] Fig.4A shows an RPT device in accordance with one form of the present technology.

[0070] Fig.4B is a schematic diagram of the pneumatic path of an RPT device in accordance with one form of the present technology. The directions of upstream and downstream are indicated with reference to the blower and the patient interface. The blower is defined to be upstream of the patient interface and the patient interface is defined to be downstream of the blower, regardless of the actual flow direction at any particular moment. Items which are located within the pneumatic path between the blower and the patient interface are downstream of the blower and upstream of the patient interface.

[0071] Fig.4C is a schematic diagram of the electrical components of an RPT device in accordance with one form of the present technology.

[0072] Fig.4D is a schematic diagram of the algorithms implemented in an RPT device in accordance with one form of the present technology.

[0073] FIG.4E is a schematic diagram showing detail of FIG.4C.

[0074] FIG.4F is a plot of measurements of pressure drop for an air circuit having a 15 mm diameter air tube at different elevations and for different blower flows.

[0075] FIG.4F is a plot of measurements of pressure drop for an air circuit having a 15 mm diameter air tube at different elevations and for different blower flows.

[0076] FIG.4G is a plot of measurements of pressure drop for an air circuit having a 12 mm diameter air tube at different elevations and for different blower flows.

[0077] FIG.4H is a plot of measured and intended mask pressure waveforms at sea level.

[0078] FIG.4I is a plot of measured and intended mask pressure waveforms at 2600m.

[0079] FIG.4J shows a plurality of fan curves obtained at sea level.

[0080] FIGs.4K to 4M show second order polynomial coefficients obtained from the fan curves plotted as a function of RPM shown in order to obtain second order polynomial equation parameters.

[0081] FIG.4N compares estimated fan curves with measured fan curves at sea level using a sea level model.

[0082] FIG.4O to 4S are graphs showing the relationship between polynomial coefficients and air density.

[0083] FIGs.4T and 4U show a comparison of fan curves plotted from measured blower pressure and estimated blower pressure at a two different air densities using a pressure estimation model that compensates for air density.

[0084] FIG.4V is a schematic diagram of an algorithm of determining an air density compensated pressure drop.

[0085] FIG.4W is a plot of measured and intended mask pressure waveforms at 2600m with air density compensation.

[0086] FIG.4X is a plot of measured and intended mask pressure waveforms at sea level with air density compensation.

[0087] FIG. 4Y is a plot of coefficients of a compensated pressure drop algorithm.

[0088] FIG.4Z is a plot of pressure drop estimated using a compensated pressure drop algorithm at an altitude of 2600m superimposed on the measured values of FIG.4 G. 3.5 HUMIDIFIER

[0089] Fig.5A shows an isometric view of a humidifier in accordance with one form of the present technology.

[0090] Fig.5B shows an isometric view of a humidifier in accordance with one form of the present technology, showing a humidifier reservoir 5110 removed from the humidifier reservoir dock 5130. 3.6 BREATHING WAVEFORMS

[0091] Fig.6A shows a model typical breath waveform of a person while sleeping. 4 DETAILED DESCRIPTION OF EXAMPLES OF THE TECHNOLOGY

[0092] Before the present technology is described in further detail, it is to be understood that the technology is not limited to the particular examples described herein, which may vary. It is also to be understood that the terminology used in this disclosure is for the purpose of describing only the particular examples discussed herein, and is not intended to be limiting.

[0093] The following description is provided in relation to various examples which may share one or more common characteristics and / or features. It is to be understood that one or more features of any one example may be combinable with one or more features of another example or other examples. In addition, any single feature or combination of features in any of the examples may constitute a further example. 4.1 THERAPY

[0094] In one form, the present technology comprises a method for treating a respiratory disorder comprising applying positive pressure to the entrance of the airways of a patient 1000.

[0095] In certain examples of the present technology, a supply of air at positive pressure is provided to the nasal passages of the patient via one or both nares.

[0096] In certain examples of the present technology, mouth breathing is limited, restricted or prevented. 4.2 RESPIRATORY THERAPY SYSTEMS

[0097] In one form, the present technology comprises a respiratory therapy system for treating a respiratory disorder. The respiratory therapy system may comprise an RPT device 4000 for supplying a flow of air to the patient 1000 via an air circuit 4170 and a patient interface 3000 or 3800. 4.3 PATIENT INTERFACE

[0098] A non-invasive patient interface 3000, such as that shown in Fig.3A, in accordance with one aspect of the present technology comprises the following functional aspects: a seal-forming structure 3100, a plenum chamber 3200, a positioning and stabilising structure 3300, a vent 3400, one form of connection port 3600 for connection to air circuit 4170, and a forehead support 3700. In some forms a functional aspect may be provided by one or more physical components. In some forms, one physical component may provide one or more functional aspects. In use the seal-forming structure 3100 is arranged to surround an entrance to the airways of the patient so as to maintain positive pressure at the entrance(s) to the airways of the patient 1000. The sealed patient interface 3000 is therefore suitable for delivery of positive pressure therapy. 4.4 RPT DEVICE

[0099] An RPT device 4000 in accordance with one aspect of the present technology comprises mechanical, pneumatic, and / or electrical components and is configured to execute one or more algorithms 4300, such as any of the methods, in whole or in part, described herein. The RPT device 4000 may be configured to generate a flow of air for delivery to a patient’s airways, such as to treat one or more of the respiratory conditions described elsewhere in the present document.

[0100] In one form, the RPT device 4000 is constructed and arranged to be capable of delivering a flow of air in a range of -20 L / min to +150 L / min while maintaining a positive pressure of at least 4 cmH2O, or at least 10cmH2O, or at least 20 cmH2O.

[0101] The RPT device may have an external housing 4010, formed in two parts, an upper portion 4012 and a lower portion 4014. Furthermore, the external housing 4010 may include one or more panel(s) 4015. The RPT device 4000 comprises a chassis 4016 that supports one or more internal components of the RPT device 4000. The RPT device 4000 may include a handle 4018.

[0102] The pneumatic path of the RPT device 4000 may comprise one or more air path items, e.g., an inlet air filter 4112, an inlet muffler 4122, a pressure generator 4140 capable of supplying air at positive pressure (e.g., a blower 4142), an outlet muffler 4124 and one or more transducers 4270, such as pressure sensors 4272 and flow rate sensors 4274.

[0103] One or more of the air path items may be located within a removable unitary structure which will be referred to as a pneumatic block 4020. The pneumatic block 4020 may be located within the external housing 4010. In one form a pneumatic block 4020 is supported by, or formed as part of the chassis 4016.

[0104] As shown in Fig.4C, the RPT device 4000 may have an electrical power supply 4210, one or more input devices 4220, a Global Navigation Satellite System (GNSS) sensor 4225, a central controller 4230, a therapy device controller 4240, a pressure generator 4140, one or more protection circuits 4250, memory 4260, transducers 4270, data communication interface 4280 and one or more output devices 4290. Electrical components 4200 may be mounted on a single Printed Circuit Board Assembly (PCBA) 4202. In an alternative form, the RPT device 4000 may include more than one PCBA 4202. 4.4.1 RPT device mechanical & pneumatic components

[0105] An RPT device may comprise one or more of the following components in an integral unit. In an alternative form, one or more of the following components may be located as respective separate units. 4.4.1.1 Air filter(s)

[0106] An RPT device in accordance with one form of the present technology may include an air filter 4110, or a plurality of air filters 4110.

[0107] In one form illustrated in Fig.4B, an inlet air filter 4112 is located at the beginning of the pneumatic path upstream of a pressure generator 4140.

[0108] In one form illustrated in Fig.4B, an outlet air filter 4114, for example an antibacterial filter, is located between an outlet of the pneumatic block 4020 and a patient interface 3000 or 3800. 4.4.1.2 Muffler(s)

[0109] An RPT device in accordance with one form of the present technology may include a muffler 4120, or a plurality of mufflers 4120.

[0110] In one form of the present technology (see e.g., Fig.4B), an inlet muffler 4122 is located in the pneumatic path upstream of a pressure generator 4140.

[0111] In one form of the present technology, an outlet muffler 4124 is located in the pneumatic path between the pressure generator 4140 and a patient interface 3000 or 3800. 4.4.1.3 Pressure generator

[0112] In one form of the present technology, a pressure generator 4140 for producing a flow, or a supply, of air at positive pressure is a controllable blower 4142. For example, the blower 4142 may include a brushless DC motor 4144 with one or more impellers, e.g. a fan. The impellers may be located in a volute. The blower may be capable of delivering a supply of air, for example at a rate of up to about 120 litres / minute, at a positive pressure in a range from about 4 cmH2O to about 20 cmH2O, or in other forms up to about 30 cmH2O when delivering respiratory pressure therapy. The blower may be as described in any one of the following patents or patent applications the contents of which are incorporated herein by reference in their entirety: U.S. Patent No.7,866,944; U.S. Patent No.8,638,014; U.S. Patent No.8,636,479; and PCT Patent Application Publication No. WO 2013 / 020167.

[0113] The pressure generator 4140 may be under the control of the therapy device controller 4240.

[0114] In other forms, a pressure generator 4140 may be a piston-driven pump, a pressure regulator connected to a high pressure source (e.g. compressed air reservoir), or a bellows. 4.4.1.4 Transducer(s)

[0115] Transducers may be internal of the RPT device, or external of the RPT device. External transducers may be located for example on or form part of the air circuit, e.g., the patient interface. External transducers may be in the form of non-contact sensors such as a Doppler radar movement sensor that transmit or transfer data to the RPT device.

[0116] In one form of the present technology (see e.g., Fig.4B), one or more transducers 4270 are located upstream and / or downstream of the pressure generator 4140. The one or more transducers 4270 may be constructed and arranged to generate signals representing properties of the flow of air such as a flow rate, a pressure or a temperature at that point in the pneumatic path.

[0117] In one form of the present technology, one or more transducers 4270 may be located proximate to the patient interface 3000 or 3800.

[0118] In one form, a signal from a transducer 4270 may be filtered, such as by low- pass, high-pass or band-pass filtering. 4.4.1.4.1 Flow rate sensor

[0119] A flow rate sensor 4274 in accordance with the present technology may be based on a differential pressure transducer, for example, an SDP600 Series differential pressure transducer from SENSIRION.

[0120] In one form, a signal generated by the flow rate sensor 4274 and representing a flow rate is received by the central controller 4230. 4.4.1.4.2 Pressure sensor(s)

[0121] Pressure sensor(s) 4272 comprise a flow pressure sensor 4272A that, in accordance with the present technology, is located in fluid communication with the pneumatic path. An example of a suitable pressure sensor is a transducer from the HONEYWELL ASDX series. An alternative suitable pressure sensor is a transducer from the NPA Series from GENERAL ELECTRIC.

[0122] In one form, a signal generated by the flow pressure sensor 4272A and representing a flow pressure is received by the central controller 4230.

[0123] In one form of the technology, pressure sensor(s) 4272 comprise an atmospheric pressure sensor 4272B. An example of a suitable atmospheric pressure sensor is a SM1131-EEN-S-165-000 transducer from Silicon Microstructures, Inc,

[0124] In one form, a signal derived from the atmospheric pressure sensor 4272B and representing an air density is received by the central controller 4230 and air density is determined from the measured atmospheric pressure.

[0125] In another form of the technology, the RPT device 4000 does not comprise an atmospheric pressure sensor. In one form air density is estimated from signals of other transducers using an air density algorithm described in further detail below. 4.4.1.4.3 Motor speed transducer

[0126] In one form of the present technology a motor speed transducer 4276 is used to determine a rotational velocity of the motor 4144 and / or the blower 4142. A motor speed signal from the motor speed transducer 4276 may be provided to the therapy device controller 4240. The motor speed transducer 4276 may, for example, be a speed sensor, such as a Hall effect sensor. 4.4.1.5 Anti-spill back valve

[0127] As shown in Fig.4B, one form of the present technology, an anti-spill back valve 4160 is located between the humidifier 5000 and the pneumatic block 4020. The anti-spill back valve is constructed and arranged to reduce the risk that water will flow upstream from the humidifier 5000, for example to the motor 4144. 4.4.2 RPT device electrical components 4.4.2.1 Power supply

[0128] A power supply 4210 may be located internal or external of the external housing 4010 of the RPT device 4000.

[0129] In one form of the present technology, power supply 4210 provides electrical power to the RPT device 4000 only. In another form of the present technology, power supply 4210 provides electrical power to both RPT device 4000 and humidifier 5000. 4.4.2.2 Input devices

[0130] In one form of the present technology, an RPT device 4000 includes one or more input devices 4220 in the form of buttons, switches or dials to allow a person to interact with the device. The buttons, switches or dials may be physical devices, or software devices accessible via a touch screen. The buttons, switches or dials may, in one form, be physically connected to the external housing 4010, or may, in another form, be in wireless communication with a receiver that is in electrical connection to the central controller 4230.

[0131] In one form, the input device 4220 may be constructed and arranged to allow a person to select a value and / or a menu option. 4.4.2.3 Central controller

[0132] In one form of the present technology, the central controller 4230 is one or a plurality of processors suitable to control an RPT device 4000. The central controller 4230 is show in Fig.4C.

[0133] Suitable processors may include an x86 INTEL processor, a processor based on ARM® Cortex®-M processor from ARM Holdings such as an STM32 series microcontroller from ST MICROELECTRONIC. In certain alternative forms of the present technology, a 32-bit RISC CPU, such as an STR9 series microcontroller from ST MICROELECTRONICS or a 16-bit RISC CPU such as a processor from the MSP430 family of microcontrollers, manufactured by TEXAS INSTRUMENTS may also be suitable.

[0134] In one form of the present technology, the central controller 4230 is a dedicated electronic circuit.

[0135] In one form, the central controller 4230 is an application-specific integrated circuit. In another form, the central controller 4230 comprises discrete electronic components.

[0136] The central controller 4230 may be configured to receive input signal(s) from one or more transducers 4270, one or more input devices 4220, and / or the humidifier 5000.

[0137] The central controller 4230 may be configured to provide output signal(s) to one or more of an output device 4290, a pressure generator 4140, a therapy device controller 4240, a data communication interface 4280, and / or the humidifier 5000.

[0138] In some forms of the present technology, the central controller 4230 is configured to implement the one or more methodologies described herein, such as the one or more algorithms 4300 which may be implemented with processor-control instructions, expressed as computer programs stored in a non-transitory computer readable storage medium, such as memory 4260. In some forms of the present technology, the central controller 4230 may be integrated with an RPT device 4000. However, in some forms of the present technology, some methodologies may be performed by a remotely located device. For example, the remotely located device may determine control settings for a ventilator or detect respiratory related events by analysis of stored data such as from any of the sensors described herein. 4.4.2.4 Clock

[0139] The RPT device 4000 may include a clock 4232 that is connected to the central controller 4230. 4.4.2.5 Therapy device controller

[0140] In one form of the present technology, therapy device controller 4240 is a therapy control module 4330 that forms part of the algorithms 4300 executed by the central controller 4230.

[0141] In one form of the present technology, therapy device controller 4240 is a dedicated motor control integrated circuit. For example, in one form a MC33035 brushless DC motor controller, manufactured by ONSEMI is used. 4.4.2.6 Protection circuits

[0142] The one or more protection circuits 4250 in accordance with the present technology may comprise an electrical protection circuit, a temperature and / or pressure safety circuit. 4.4.2.7 Memory

[0143] In accordance with one form of the present technology the RPT device 4000 includes memory 4260, e.g., non-volatile memory. In some forms, memory 4260 may include battery powered static RAM. In some forms, memory 4260 may include volatile RAM.

[0144] Memory 4260 may be located on the PCBA 4202. Memory 4260 may be in the form of EEPROM, or NAND flash.

[0145] Additionally, or alternatively, RPT device 4000 includes a removable form of memory 4260, for example a memory card made in accordance with the Secure Digital (SD) standard.

[0146] In one form of the present technology, the memory 4260 acts as a non- transitory computer readable storage medium on which is stored computer program instructions expressing the one or more methodologies described herein, such as the one or more algorithms 4300. 4.4.2.8 Data communication systems

[0147] In one form of the present technology, a data communication interface 4280 is provided, and is connected to the central controller 4230 (see e.g., Fig.4C). Data communication interface 4280 may be connectable to a remote external communication network 4282 and / or a local external communication network 4284. The remote external communication network 4282 may be connectable to a remote external device 4286. The local external communication network 4284 may be connectable to a local external device 4288.

[0148] In one form, data communication interface 4280 is part of the central controller 4230. In another form, data communication interface 4280 is separate from the central controller 4230, and may comprise an integrated circuit or a processor.

[0149] In one form, remote external communication network 4282 is the Internet. The data communication interface 4280 may use wired communication (e.g. via Ethernet, or optical fibre) or a wireless protocol (e.g. CDMA, GSM, LTE) to connect to the Internet.

[0150] In one form, local external communication network 4284 utilises one or more communication standards, such as Bluetooth, or a consumer infrared protocol.

[0151] In one form, remote external device 4286 is one or more computers, for example a cluster of networked computers. In one form, remote external device 4286 may be virtual computers, rather than physical computers. In either case, such a remote external device 4286 may be accessible to an appropriately authorised person such as a clinician.

[0152] The local external device 4288 may be a personal computer, mobile phone, tablet or remote control. 4.4.2.9 Output devices including optional display, alarms

[0153] An output device 4290 in accordance with the present technology may take the form of one or more of a visual, audio and haptic unit. A visual display may be a Liquid Crystal Display (LCD) or Light Emitting Diode (LED) display. 4.4.2.9.1 Display driver

[0154] A display driver 4292 receives as an input the characters, symbols, or images intended for display on the display 4294, and converts them to commands that cause the display 4294 to display those characters, symbols, or images. 4.4.2.9.2 Display

[0155] A display 4294 is configured to visually display characters, symbols, or images in response to commands received from the display driver 4292. For example, the display 4294 may be an eight-segment display, in which case the display driver 4292 converts each character or symbol, such as the figure “0”, to eight logical signals indicating whether the eight respective segments are to be activated to display a particular character or symbol. 4.4.3 RPT device algorithms

[0156] As mentioned above, in some forms of the present technology, the central controller 4230 may be configured to implement one or more algorithms 4300 expressed as computer programs stored in a non-transitory computer readable storage medium, such as memory 4260. In some forms, one or more algorithms may be added to the algorithms stored in memory 4260 of an RPT device 4000 by a software update process.

[0157] The algorithms 4300 are generally grouped into groups referred to as modules.

[0158] In other forms of the present technology, some portion or all of the algorithms 4300 may be implemented by a controller of an external device such as the local external device 4288 or the remote external device 4286. In such forms, data representing the input signals and / or intermediate algorithm outputs necessary for the portion of the algorithms 4300 to be executed at the external device may be communicated to the external device via the local external communication network 4284 or the remote external communication network 4282. In such forms, the portion of the algorithms 4300 to be executed at the external device may be expressed as computer programs, such as with processor control instructions to be executed by one or more processor(s), stored in a non-transitory computer readable storage medium accessible to the controller of the external device. Such programs configure the controller of the external device to execute the portion of the algorithms 4300.

[0159] In such forms, the therapy parameters generated by the external device via the therapy engine module 4320 (if such forms part of the portion of the algorithms 4300 executed by the external device) may be communicated to the central controller 4230 to be passed to the therapy control module 4330. 4.4.3.1 Pre-processing module

[0160] A pre-processing module 4310 in accordance with one form of the present technology receives as an input a signal from a transducer 4270, for example a flow rate sensor 4274 or flow pressure sensor 4272, and performs one or more process steps to calculate one or more output values that will be used as an input to another module, for example a therapy engine module 4320.

[0161] In one form of the present technology, the output values include the interface pressure Pm, the vent flow rate Qv, the respiratory flow rate Qr, and the leak flow rate Ql.

[0162] In various forms of the present technology, the pre-processing module 4310 comprises one or more of the following algorithms: air density estimation 4311, interface pressure estimation 4312, vent flow rate estimation 4314, leak flow rate estimation 4316, pressure drop estimation 4317 and respiratory flow rate estimation 4318. 4.4.3.1.1 Air density estimation

[0163] In one form of the present technology, an air density estimation algorithm 4311 receives as inputs a signal from the flow pressure sensor 4272 indicative of the pressure in the pneumatic path proximal to an outlet of the pneumatic block (the device pressure Pd), a signal from the flow rate sensor 4274 representative of the flow rate of the airflow leaving the RPT device 4000 (the device flow rate Qd) and the motor speed RPM from motor speed transducer 4276.

[0164] In one form of the present technology, an air density estimation algorithm 4311 may improve the accuracy of pressure delivery by the RPT device 4000. The technology may enable intended mask pressure to be delivered more accurately. The technology may enable the absolute pressure and / or pressure waveform to be more accurate.

[0165] In one form of the present technology, an air density estimation algorithm 4311 may enable an RPT device 4000 to be used with an air circuit that might otherwise introduce errors in pressure delivery. For example, as described below an air circuit may incorporate an air tube. An air tube having a relatively smaller diameter will have a higher variation in impedance as air density varies than an air tube of the same length (e.g.1.8m) with a relatively larger diameter. In some examples, an air tube having a relatively smaller diameter may be advantageous. For example, it may improve portability of a respiratory therapy system by allowing it to be packed into a smaller volume or it may enable a respiratory therapy system to take up less space or look more aesthetically pleasing. Such factors can lead to improved compliance. For example, a patient may be more inclined to take a respiratory therapy system on holiday or a work trip if it packs more compactly or more likely to leave it at a convenient position for use if it takes up less space or is more aesthetically pleasing.

[0166] FIGs.4F and 4G illustrate the impact of air density. In FIGs.4F and 4G, the x- axis 4410 is air flow rate in L / min STPD (Standard Temperature and Pressure, Dry) and the y-axis 4420 is pressure drop.

[0167] FIG.4F is a plot of measurements for a RPT device operating with an air tube having a 15 mm diameter. In this example, measurements were obtained for a plurality of different flow rates at sea level equivalent air density at both 25 degrees Celsius and 35 degrees Celsius and when plotted 4431 fall along a common trend line showing that any effect of temperature is relatively minor.

[0168] FIG.4F also shows measurements plotted 4432 for a plurality of different flow rates at an air density corresponding to an elevation of 8500 ft (2591 m) at 25 degrees Celsius and measurements plotted 4433 for a plurality of different flow rates at an air density corresponding to an elevation of 9870 ft (3008 m) at 25 degrees Celsius. It will be observed that these measurements show an increase in pressure drop at lower air densities (higher elevations) and that these differences are more marked at higher flow rates.

[0169] FIG.4G is a plot of measurements for a RPT device with an air tube having a 12 mm diameter. In this example, measurements were obtained for a plurality of different flow rates at sea level equivalent air density at 25 degrees Celsius, and 35 degrees Celsius and when plotted 4435 fall along a common trend line showing that any effect of temperature is relatively minor.

[0170] FIG.4G shows measurements plotted 4436 for a plurality of different flow rates at an air density corresponding to an elevation of 8500 ft (2591m) at 25 degrees Celsius and measurements plotted 4437 for a plurality of different flow rates at an air density corresponding to an elevation of 9870 ft (3008 m) at 25 degrees Celsius. It will be observed that these measurements show an increase in pressure drop at lower air densities (higher elevations) and that these differences are more marked at higher flow rates.

[0171] It will also be observed from a comparison of FIGs.4F and 4G that the effect of impedance variation due to air density / altitude is more significant for the smaller diameter 12 mm tube and hence will be a more significant contributor to mask pressure inaccuracy. For example, for a 12 mm diameter tube, the error is approximately 1.5 cmH2O at a flow rate of 60L / min. Such an error may lead to a degradation in performance.

[0172] In FIGs.4H and 4I, the x-axis is time 4501 and the y-axis 4502 is mask pressure. Each of FIGs.4H and 4I show a comparison between a computed mask pressure waveform (an intended pressure waveform) and a measured mask pressure waveform. FIG. 4H shows a comparison between the pressure waveforms at sea level, from which it will be apparent that the measured mask pressure waveform 4511 corresponds closely to the intended mask pressure waveform 4512.

[0173] FIG.4I shows a comparison between the measured and intended pressure waveforms at an elevation 2600 m, from which it will be apparent that the measured mask pressure waveform 4521 diverges significantly from intended mask pressure waveform 4522. Not only is there an observable shift in the base line pressure, there is also a change in shape of the measured waveform 4521 which may have an effect on breathing comfort.

[0174] The air density estimation algorithm of a form of the technology may be based on modelling the behaviour of the system in order produce an air density estimation algorithm, which, in turn may be used to produce an air density compensation pressure drop algorithm.

[0175] In one form of the technology, to develop a model, the fan curves 4611-4617 shown in FIG.4J were obtained for an example blower at a number of different constant motor speeds (RPM) at sea level and reference ambient conditions. Other types of blower (e.g. with different fans) may be characterized in the same way. Reference ambient conditions may be a temperature of 22 degrees C, an atmospheric pressure of 1010 hPa and a relative humidity of 40%. In FIG.4J, the x-axis is blower flow (L / min) and the y-axis is blower pressure (cmH20).

[0176] In one form of the technology, the fan curves may be modelled using the second order polynomial equation: In other forms of the technology, other equations may be used – e.g. first order, or higher order polynomial equations.

[0177] Coefficients of Equation 1 derived from the fan curves of FIG.4J are set out in Table 1 below. Table 1

[0178] These coefficients were plotted in FIGs.4K to 4M as a function of RPM 4701 and second order polynomial equation parameters (A1, B1and C1, obtained at reference ambient conditions) were then fitted to the plotted coefficients. In this respect, FIG.4K shows a plot 4711 of coefficient A14702 and the fitted second order polynomial 4712; and FIG.4M shows a plot 4731 of coefficient C14706 and the fitted second order polynomial 4732

[0179] This process produced the following form of Equation 1 with polynomial parameters:Equation 1: ^^^^^^^^^^^^^^^^ = ^^(^^^^^^^^, ^^^^^^) = ^^1 ∗ ^^^^^^^^2 + ^^1 ∗ ^^^^^^^^ + ^^1with ^^1 = ^^(^^^^^^) = −0.0000000035 ∗ ^^^^^^ − 0.0016979048^^1 = ^^(^^^^^^) = 0.00000271 ∗ ^^^^^^ − 0.04012255^^1 = ^^(^^^^^^) = 0.0000000297 ∗ ^^^^^^2 + 0.0000003698 ∗ ^^^^^^ + 0.0794352571

[0180] FIG.4N shows a comparison of the measured fan curves 4611-4617 of FIG.4J with fan curves 4621-4627 generated using Equation 1 for corresponding fan speeds. It will be apparent that the generated fan curves 4621-4627 closely follow the measured fan curves 4611-4617 which validates the model.

[0181] Having obtained an equation linking fan speed to pressure and flow rate, corrections for air density were determined. Density (⍴) is a function of Atmospheric Pressure, Temperature and Relative Humidity (RH): with: ^^0 = 1013 ℎ^^^^^^0 = 15 ^^^^^^^^ ^^^^ 288.15 ^^^^^^^^⍴ = 1.2256^^^^ 0^^3

[0182] RH is defined as the ratio of local vapour pressure (Pv) to local vapour pressure when saturated (ie saturation vapour pressure, or Psv), expressed as a percentage.

[0183] The local vapour pressure is what contributes to effects on density and viscosity, rather than RH directly. Both Pv and Psv are functions of temperature.

[0184] Saturated vapour pressure (Psv) may be defined in terms of temperature: ^^^^^^(^^^^^^^^ ) = 6.1078

[0185] Vapour pressure may be defined in terms of RH and temperature:

[0186] To determine the density correction factors that need to be applied to A1, B1and C1(i.e. the second order polynomial parameters in Equation 1), additional fan curves may be collected at different air densities (equivalent to 1500m and 2600m altitude) using a barometric chamber and the process described above may be repeated for each air density.

[0187] Then each coefficient (A, B and C) and each motor speed, may be plotted as a function of air density 4701.

[0188] FIG.4O shows plots of coefficient A 4702 at each motor speed. As shown by the close grouping 4710 of plots of coefficient A in FIG.4O, coefficient A varies as a linear function of air density. Expressing the ratio 4722 of the coefficients A (A2 / A14722 with A2 being the coefficient at local conditions and A1the coefficient at reference conditions) as a function of the inverse ratio of their respective densities (ρref / ρlocal 4721) demonstrates that they are equal – see FIG.4P where the grouping of plots 4728 fits to a line 4729 have a 1:1 ratio. Therefore: ^^2 = ^^(^^^^^^, ⍴local)

[0189] Similarly, FIG.4Q has plots 4731-4734 of coefficient C 4703 as a function of air density 4701. As shown by FIG.4R, the ratio of the coefficients C (C2 / C14724 with C2being the coefficient at local conditions and C1the coefficient at reference conditions) as a function of the ratio of their respective densities (ρlocal / ρref 4741) which shows that they are equal. Therefore: ^^ = ^^(^^^^^^, ⍴local)

[0190] However, as shown by the plots 4751-4754 of coefficient B 4706 in FIG.4S, for coefficient B (linear term) there is no obvious relationship with air density as there is for A and C. Accordingly, in this example of the technology no correction is applied for coefficient B at different air densities. In another example of the technology, a look up table could be used to apply a correction for coefficient B.

[0191] Accordingly, based on the above the relationship between blower pressure, blower flow, motor speed and local air density may be expressed as:Equation 2: ^^^^^^^^^^^^^^^^ = ^^(^^^^^^^^, ^^^^^^, ⍴local) = ^^2 ∗ ^^^^^^^^2 + ^^2 ∗ ^^^^^^^^ + ^^2With:⍴^^ = ^^ ^^^^^^, local = ^^ ∗^^^^^^ ⍴ (⍴ ) = (−0.0000000035^^^^^^ 21⍴∗ ^^^^^^ − 0.0016979048) ∗^^^^^^^^^^ ⍴ ^^^^^^^^^^ ^^2 = ^^1 = ^^(^^^^^^) = 0.00000271 ∗ ^^^^^^ − 0.04012255^^2 = ^^(^^^^^^, ⍴local) =(0.0000000297 ∗ ^^^^^^2 + 0.0000003698 ∗ ^^^^^^ + 0.0794352571)

[0192] FIGs.4T and 4U shows a comparison of measured fan curves 4631-4637, 4651-4657 estimated fan curves 4641-4647, 4661-4667 (dotted red lines), validating the blower pressure estimation model of Equation 2 when running at two different local ambient conditions (air density equivalent to 1500m (FIG.4T) and 2600m (FIG.4U)).

[0193] It will be apparent that the technology can be implemented by characterising the fan curves of a specific blower and / or by applying a gain to re-size the fan curve characteristic. In some examples, an RPT device may be configured (e.g. by instructions in memory) to conduct a self-calibration process in which it determined its own fan curves when at reference conditions.

[0194] Equation 2 may be rearranged as a second order polynomial equation with local air density being the unknown in order to estimate local air density as set out in Equation 3:⍴local =−(^^ ∗ ^^^^^^^^ − ^^^^^^^^^^^^^^^^ ) + √(^^1 ∗ ^^^^^^^^ − ^^^^^^^^^^^^^^^^ ∗ ⍴ref ∗ 2∗^^1⍴ref 4.4.3.1.2 Alternative local pressure estimation algorithm

[0195] In another form of the technology, a local pressure estimation algorithm receives as inputs a signal from GNSS sensor 4225 indicative of the location of the RPT device 4000 and a source of map data that includes altitude data. In this form of the technology, the algorithm uses the location and the map data to estimate the altitude of the RPT device 4000 and hence the air density. In an example, RPT device 4000 sends the signal from the GNSS sensor 4225 to a remote external device 4266 using remote external communication network 4282 and remote external device 4266 estimates the air density using the map data and returns the estimated air density to the RPT device 4000. 4.4.3.1.3 Interface pressure estimation

[0196] In one form of the present technology, an interface pressure estimation algorithm 4312 receives as inputs a signal from the pressure sensor 4272A indicative of the pressure in the pneumatic path proximal to an outlet of the pneumatic block (the device pressure Pd) and a signal from the flow rate sensor 4274 representative of the flow rate of the airflow leaving the RPT device 4000 (the device flow rate Qd). The device flow rate Qd, absent any supplementary gas 4180, may be used as the total flow rate Qt. The interface pressure algorithm 4312 uses an estimate of pressure drop ^P through the air circuit 4170 produced by a pressure drop algorithm 4317 using the estimated air density as described in further detail below. The interface pressure estimation algorithm, 4312 then provides as an output an estimated pressure, Pm, in the patient interface 3000 or 3800. The pressure, Pm, in the patient interface 3000 or 3800 may be estimated as the device pressure Pd minus the air circuit pressure drop ^P.

[0197] As show in FIG.4V, in one form of the technology, the above air density estimation algorithm 4311 is run at 100Hz and generates an estimated air density 4912, this may then be clipped 4914 (in this example to a range between 0.8 and 1.4 before being passed through a low pass filter 4916. In one form of the technology, a time constant of the dynamic low pass filter may be dynamically adjusted from 1.592sec to 159.2sec over the first 30 seconds of therapy. In a form of the technology, the dynamic low pass filter enables the algorithm to rapidly converge to correct air density compensation. In other forms of the technology different low pass filters may be used or different dynamic adjustments may be applied.

[0198] Pressure drop algorithm 4317 then estimates the pressure drop ^P 4918 from the low pass filtered air density and flow rate Qt.

[0199] In this respect, pressure drop ^P may be characterised using the following equation:Equation 4: ∆^^ = ^^1. ^^2 + ^^2. ^^ ; ^^^^ = ^^^^^^^^^^^^^^^^^^Where Z1 and Z2 are functions of air density. Z1 and Z2 were determined at a plurality of air densities and plotted as shown in FIG.4Y from which it was determined that Z1 = - .0315ρlocal + 0.0339; and Z2 = - 0.0006ρlocal + 0.0018.

[0200] FIG.4Z validates the above equation by showing a plot 4980 of pressure drop estimated using this algorithm at an altitude of 2600m superimposed on the measured values of FIG.4 G.

[0201] FIG.4W shows that with an air density compensated pressure drop module, delivered mask pressure 4531 closely follows intended mask pressure 4532 at an altitude of 2600m and provides significant improvement relative to FIG.4I. That is, central controller 4230 can adjust one or more operational parameters (e.g. motor speed, flow rate, treatment waveform) based on the compensated estimated pressure drop.

[0202] FIG.4X shows that with an air density compensated pressure module, delivered mask pressure 4541 still closely follows intended mask pressure 4542 sea level.

[0203] In other forms of the technology, a pressure drop model may estimate pressure drop from unfiltered estimated air density and flow rate. 4.4.3.1.4 Vent flow rate estimation

[0204] In one form of the present technology, a vent flow rate estimation algorithm 4314 receives as an input an estimated pressure, Pm, in the patient interface 3000 or 3800 from the interface pressure estimation algorithm 4312 and estimates a vent flow rate of air, Qv, from a vent 3400 in a patient interface 3000 or 3800. The dependence of the vent flow rate Qv on the interface pressure Pm for the particular vent 3400 in use may be modelled by a vent characteristic Qv(Pm). 4.4.3.1.5 Leak flow rate estimation

[0205] In one form of the present technology, a leak flow rate estimation algorithm 4316 receives as an input a total flow rate, Qt, and a vent flow rate Qv, and provides as an output an estimate of the leak flow rate Ql. In one form, the leak flow rate estimation algorithm estimates the leak flow rate Ql by calculating an average of the difference between total flow rate Qt and vent flow rate Qv over a period sufficiently long to include several breathing cycles, e.g. about 10 seconds.

[0206] In one form, the leak flow rate estimation algorithm 4316 receives as an input a total flow rate Qt, a vent flow rate Qv, and an estimated pressure, Pm, in the patient interface 3000 or 3800, and provides as an output a leak flow rate Ql, by calculating a leak conductance, and determining a leak flow rate Ql to be a function of leak conductance and pressure, Pm. Leak conductance is calculated as the quotient of low pass filtered non-vent flow rate equal to the difference between total flow rate Qt and vent flow rate Qv, and low pass filtered square root of pressure Pm, where the low pass filter time constant has a value sufficiently long to include several breathing cycles, e.g. about 10 seconds. The leak flow rate Ql may be estimated as the product of leak conductance and a function of pressure, Pm. 4.4.3.1.6 Respiratory flow rate estimation

[0207] In one form of the present technology, a respiratory flow rate estimation algorithm 4318 receives as an input a total flow rate, Qt, a vent flow rate, Qv, and a leak flow rate, Ql, and estimates a respiratory flow rate of air, Qr, to the patient, by subtracting the vent flow rate Qv and the leak flow rate Ql from the total flow rate Qt. 4.4.3.2 Therapy Engine Module

[0208] In one form of the present technology, a therapy engine module 4320 receives as inputs one or more of a pressure, Pm, in a patient interface 3000 or 3800, and a respiratory flow rate of air to a patient, Qr, and provides as an output one or more therapy parameters.

[0209] In one form of the present technology, a therapy parameter is a treatment pressure Pt.

[0210] In one form of the present technology, therapy parameters are one or more of an amplitude of a pressure variation, a base pressure, and a target ventilation.

[0211] In various forms, the therapy engine module 4320 comprises one or more of the following algorithms: phase determination 4321, waveform determination 4322, ventilation determination 4323, inspiratory flow limitation determination 4324, apnea / hypopnea determination 4325, snore determination 4326, airway patency determination 4327, target ventilation determination 4328, and therapy parameter determination 4329. 4.4.3.2.1 Phase determination

[0212] In one form of the present technology, the RPT device 4000 does not determine phase.

[0213] In one form of the present technology, a phase determination algorithm 4321 receives as an input a signal indicative of respiratory flow rate, Qr, and provides as an output a phase ^ of a current breathing cycle of a patient 1000.

[0214] In some forms, known as discrete phase determination, the phase output ^ is a discrete variable. One implementation of discrete phase determination provides a bi- valued phase output ^ with values of either inhalation or exhalation, for example represented as values of 0 and 0.5 revolutions respectively, upon detecting the start of spontaneous inhalation and exhalation respectively. RPT devices 4000 that “trigger” and “cycle” effectively perform discrete phase determination, since the trigger and cycle points are the instants at which the phase changes from exhalation to inhalation and from inhalation to exhalation, respectively. In one implementation of bi-valued phase determination, the phase output ^ is determined to have a discrete value of 0 (thereby “triggering” the RPT device 4000) when the respiratory flow rate Qr has a value that exceeds a positive threshold, and a discrete value of 0.5 revolutions (thereby “cycling” the RPT device 4000) when a respiratory flow rate Qr has a value that is more negative than a negative threshold. The inhalation time Ti and the exhalation time Te may be estimated as typical values over many respiratory cycles of the time spent with phase ^ equal to 0 (indicating inspiration) and 0.5 (indicating expiration) respectively.

[0215] Another implementation of discrete phase determination provides a tri-valued phase output ^ with a value of one of inhalation, mid-inspiratory pause, and exhalation.

[0216] In other forms, known as continuous phase determination, the phase output ^ is a continuous variable, for example varying from 0 to 1 revolutions, or 0 to 2^ radians. RPT devices 4000 that perform continuous phase determination may trigger and cycle when the continuous phase reaches 0 and 0.5 revolutions, respectively. In one implementation of continuous phase determination, a continuous value of phase ^ is determined using a fuzzy logic analysis of the respiratory flow rate Qr. A continuous value of phase determined in this implementation is often referred to as “fuzzy phase”. In one implementation of a fuzzy phase determination algorithm 4321, the following rules are applied to the respiratory flow rate Qr: 1. If Qr is zero and increasing fast then ^ is 0 revolutions. 2. If Qr is large positive and steady then ^ is 0.25 revolutions. 3. If Qr is zero and falling fast, then ^ is 0.5 revolutions. 4. If Qr is large negative and steady then ^ is 0.75 revolutions. 5. If Qr is zero and steady and the 5-second low-pass filtered absolute value of Qr is large then ^ is 0.9 revolutions. 6. If Qr is positive and the phase is expiratory, then ^ is 0 revolutions. 7. If Qr is negative and the phase is inspiratory, then ^ is 0.5 revolutions. 8. If the 5-second low-pass filtered absolute value of Qr is large, ^ is increasing at a steady rate equal to the patient’s breathing rate, low-pass filtered with a time constant of 20 seconds.

[0217] The output of each rule may be represented as a vector whose phase is the result of the rule and whose magnitude is the fuzzy extent to which the rule is true. The fuzzy extent to which the respiratory flow rate is “large”, “steady”, etc. is determined with suitable membership functions. The results of the rules, represented as vectors, are then combined by some function such as taking the centroid. In such a combination, the rules may be equally weighted, or differently weighted.

[0218] In another implementation of continuous phase determination, the phase ^ is first discretely estimated from the respiratory flow rate Qr as described above, as are the inhalation time Ti and the exhalation time Te. The continuous phase ^ at any instant may be determined as the half the proportion of the inhalation time Ti that has elapsed since the previous trigger instant, or 0.5 revolutions plus half the proportion of the exhalation time Te that has elapsed since the previous cycle instant (whichever instant was more recent). 4.4.3.2.2 Waveform determination

[0219] In one form of the present technology, the therapy parameter determination algorithm 4329 provides an approximately constant treatment pressure throughout a respiratory cycle of a patient.

[0220] In other forms of the present technology, the therapy control module 4330 controls the pressure generator 4140 to provide a treatment pressure Pt that varies as a function of phase ^ of a respiratory cycle of a patient according to a waveform template ^(^).

[0221] In one form of the present technology, a waveform determination algorithm 4322 provides a waveform template ^(^) with values in the range [0, 1] on the domain of phase values ^ provided by the phase determination algorithm 4321 to be used by the therapy parameter determination algorithm 4329.

[0222] In one form, suitable for either discrete or continuously-valued phase, the waveform template ^(^) is a square-wave template, having a value of 1 for values of phase up to and including 0.5 revolutions, and a value of 0 for values of phase above 0.5 revolutions. In one form, suitable for continuously-valued phase, the waveform template ^(^) comprises two smoothly curved portions, namely a smoothly curved (e.g. raised cosine) rise from 0 to 1 for values of phase up to 0.5 revolutions, and a smoothly curved (e.g. exponential) decay from 1 to 0 for values of phase above 0.5 revolutions. In one form, suitable for continuously-valued phase, the waveform template ^(^) is based on a square wave, but with a smooth rise from 0 to 1 for values of phase up to a “rise time” that is less than 0.5 revolutions, and a smooth fall from 1 to 0 for values of phase within a “fall time” after 0.5 revolutions, with a “fall time” that is less than 0.5 revolutions.

[0223] In some forms of the present technology, the waveform determination algorithm 4322 selects a waveform template ^(^) from a library of waveform templates, dependent on a setting of the RPT device. Each waveform template ^(^) in the library may be provided as a lookup table of values ^ against phase values ^. In other forms, the waveform determination algorithm 4322 computes a waveform template ^(^) “on the fly” using a predetermined functional form, possibly parametrised by one or more parameters (e.g. time constant of an exponentially curved portion). The parameters of the functional form may be predetermined or dependent on a current state of the patient 1000.

[0224] In some forms of the present technology, suitable for discrete bi-valued phase of either inhalation (^ = 0 revolutions) or exhalation (^ = 0.5 revolutions), the waveform determination algorithm 4322 computes a waveform template ^ “on the fly” as a function of both discrete phase ^ and time t measured since the most recent trigger instant. In one such form, the waveform determination algorithm 4322 computes the waveform template ^(^, t) in two portions (inspiratory and expiratory) as follows: ^= 0 ^ = 0.5

[0225] where ^i(t) and ^e(t) are inspiratory and expiratory portions of the waveform template ^(^, t). In one such form, the inspiratory portion ^i(t) of the waveform template is a smooth rise from 0 to 1 parametrised by a rise time, and the expiratory portion ^e(t) of the waveform template is a smooth fall from 1 to 0 parametrised by a fall time. 4.4.3.2.3 Ventilation determination

[0226] In one form of the present technology, a ventilation determination algorithm 4323 receives an input a respiratory flow rate Qr, and determines a measure indicative of current patient ventilation, Vent.

[0227] In some implementations, the ventilation determination algorithm 4323 determines a measure of ventilation Vent that is an estimate of actual patient ventilation. One such implementation is to take half the absolute value of respiratory flow rate, Qr, optionally filtered by low-pass filter such as a second order Bessel low-pass filter with a corner frequency of 0.11 Hz.

[0228] In other implementations, the ventilation determination algorithm 4323 determines a measure of ventilation Vent that is broadly proportional to actual patient ventilation. One such implementation estimates peak respiratory flow rate Qpeak over the inspiratory portion of the cycle. This and many other procedures involving sampling the respiratory flow rate Qr produce measures which are broadly proportional to ventilation, provided the flow rate waveform shape does not vary very much (here, the shape of two breaths is taken to be similar when the flow rate waveforms of the breaths normalised in time and amplitude are similar). Some simple examples include the median positive respiratory flow rate, the median of the absolute value of respiratory flow rate, and the standard deviation of flow rate. Arbitrary linear combinations of arbitrary order statistics of the absolute value of respiratory flow rate using positive coefficients, and even some using both positive and negative coefficients, are approximately proportional to ventilation. Another example is the mean of the respiratory flow rate in the middle K proportion (by time) of the inspiratory portion, where 0 < K < 1. There is an arbitrarily large number of measures that are exactly proportional to ventilation if the flow rate shape is constant. 4.4.3.2.4 Determination of Inspiratory Flow Limitation

[0229] In one form of the present technology, the central controller 4230 executes an inspiratory flow limitation determination algorithm 4324 for the determination of the extent of inspiratory flow limitation.

[0230] In one form, the inspiratory flow limitation determination algorithm 4324 receives as an input a respiratory flow rate signal Qr and provides as an output a metric of the extent to which the inspiratory portion of the breath exhibits inspiratory flow limitation.

[0231] In one form of the present technology, the inspiratory portion of each breath is identified by a zero-crossing detector. A number of evenly spaced points (for example, sixty-five), representing points in time, are interpolated by an interpolator along the inspiratory flow rate-time curve for each breath. The curve described by the points is then scaled by a scalar to have unity length (duration / period) and unity area to remove the effects of changing breathing rate and depth. The scaled breaths are then compared in a comparator with a pre-stored template representing a normal unobstructed breath, similar to the inspiratory portion of the breath shown in Fig.6A. Breaths deviating by more than a specified threshold (typically 1 scaled unit) at any time during the inspiration from this template, such as those due to coughs, sighs, swallows and hiccups, as determined by a test element, are rejected. For non-rejected data, a moving average of the first such scaled point is calculated by the central controller 4230 for the preceding several inspiratory events. This is repeated over the same inspiratory events for the second such point, and so on. Thus, for example, sixty-five scaled data points are generated by the central controller 4230, and represent a moving average of the preceding several inspiratory events, e.g., three events. The moving average of continuously updated values of the (e.g., sixty-five) points are hereinafter called the "scaled flow rate ", designated as Qs(t). Alternatively, a single inspiratory event can be utilised rather than a moving average.

[0232] From the scaled flow rate, two shape factors relating to the determination of partial obstruction may be calculated.

[0233] Shape factor 1 is the ratio of the mean of the middle (e.g. thirty-two) scaled flow rate points to the mean overall (e.g. sixty-five) scaled flow rate points. Where this ratio is in excess of unity, the breath will be taken to be normal. Where the ratio is unity or less, the breath will be taken to be obstructed. A ratio of about 1.17 is taken as a threshold between partially obstructed and unobstructed breathing, and equates to a degree of obstruction that would permit maintenance of adequate oxygenation in a typical patient.

[0234] Shape factor 2 is calculated as the RMS deviation from unit scaled flow rate, taken over the middle (e.g. thirty-two) points. An RMS deviation of about 0.2 units is taken to be normal. An RMS deviation of zero is taken to be a totally flow–limited breath. The closer the RMS deviation to zero, the breath will be taken to be more flow limited.

[0235] Shape factors 1 and 2 may be used as alternatives, or in combination. In other forms of the present technology, the number of sampled points, breaths and middle points may differ from those described above. Furthermore, the threshold values can be other than those described. 4.4.3.2.5 Determination of apneas and hypopneas

[0236] In one form of the present technology, the central controller 4230 executes an apnea / hypopnea determination algorithm 4325 for the determination of the presence of apneas and / or hypopneas.

[0237] In one form, the apnea / hypopnea determination algorithm 4325 receives as an input a respiratory flow rate signal Qr and provides as an output a flag that indicates that an apnea or a hypopnea has been detected.

[0238] In one form, an apnea will be said to have been detected when a function of respiratory flow rate Qr falls below a flow rate threshold for a predetermined period of time. The function may determine a peak flow rate, a relatively short-term mean flow rate, or a flow rate intermediate of relatively short-term mean and peak flow rate, for example an RMS flow rate. The flow rate threshold may be a relatively long-term measure of flow rate.

[0239] In one form, a hypopnea will be said to have been detected when a function of respiratory flow rate Qr falls below a second flow rate threshold for a predetermined period of time. The function may determine a peak flow, a relatively short-term mean flow rate, or a flow rate intermediate of relatively short-term mean and peak flow rate, for example an RMS flow rate. The second flow rate threshold may be a relatively long-term measure of flow rate. The second flow rate threshold is greater than the flow rate threshold used to detect apneas. 4.4.3.2.6 Determination of snore

[0240] In one form of the present technology, the central controller 4230 executes one or more snore determination algorithms 4326 for the determination of the extent of snore.

[0241] In one form, the snore determination algorithm 4326 receives as an input a respiratory flow rate signal Qr and provides as an output a metric of the extent to which snoring is present.

[0242] The snore determination algorithm 4326 may comprise the step of determining the intensity of the flow rate signal in the range of 30-300 Hz. Further, the snore determination algorithm 4326 may comprise a step of filtering the respiratory flow rate signal Qr to reduce background noise, e.g., the sound of airflow in the system from the blower. 4.4.3.2.7 Determination of airway patency

[0243] In one form of the present technology, the central controller 4230 executes one or more airway patency determination algorithms 4327 for the determination of the extent of airway patency.

[0244] In one form, the airway patency determination algorithm 4327 receives as an input a respiratory flow rate signal Qr, and determines the power of the signal in the frequency range of about 0.75 Hz and about 3 Hz. The presence of a peak in this frequency range is taken to indicate an open airway. The absence of a peak is taken to be an indication of a closed airway.

[0245] In one form, the frequency range within which the peak is sought is the frequency of a small forced oscillation in the treatment pressure Pt. In one implementation, the forced oscillation is of frequency 2 Hz with amplitude about 1 cmH2O.

[0246] In one form, airway patency determination algorithm 4327 receives as an input a respiratory flow rate signal Qr, and determines the presence or absence of a cardiogenic signal. The absence of a cardiogenic signal is taken to be an indication of a closed airway. 4.4.3.2.8 Determination of target ventilation

[0247] In one form of the present technology, the central controller 4230 takes as input the measure of current ventilation, Vent, and executes one or more target ventilation determination algorithms 4328 for the determination of a target value Vtgt for the measure of ventilation.

[0248] In some forms of the present technology, there is no target ventilation determination algorithm 4328, and the target value Vtgt is predetermined, for example by hard-coding during configuration of the RPT device 4000 or by manual entry through the input device 4220.

[0249] In other forms of the present technology, such as adaptive servo-ventilation (ASV), the target ventilation determination algorithm 4328 computes a target value Vtgt from a value Vtyp indicative of the typical recent ventilation of the patient.

[0250] In some forms of adaptive servo-ventilation, the target ventilation Vtgt is computed as a high proportion of, but less than, the typical recent ventilation Vtyp. The high proportion in such forms may be in the range (80%, 100%), or (85%, 95%), or (87%, 92%).

[0251] In other forms of adaptive servo-ventilation, the target ventilation Vtgt is computed as a slightly greater than unity multiple of the typical recent ventilation Vtyp.

[0252] The typical recent ventilation Vtyp is the value around which the distribution of the measure of current ventilation Vent over multiple time instants over some predetermined timescale tends to cluster, that is, a measure of the central tendency of the measure of current ventilation over recent history. In one implementation of the target ventilation determination algorithm 4328, the recent history is of the order of several minutes, but in any case should be longer than the timescale of Cheyne-Stokes waxing and waning cycles. The target ventilation determination algorithm 4328 may use any of the variety of well-known measures of central tendency to determine the typical recent ventilation Vtyp from the measure of current ventilation, Vent. One such measure is the output of a low-pass filter on the measure of current ventilation Vent, with time constant equal to one hundred seconds. 4.4.3.2.9 Determination of therapy parameters

[0253] In some forms of the present technology, the central controller 4230 executes one or more therapy parameter determination algorithms 4329 for the determination of one or more therapy parameters using the values returned by one or more of the other algorithms in the therapy engine module 4320.

[0254] In one form of the present technology, the therapy parameter is an instantaneous treatment pressure Pt. In one implementation of this form, the therapy parameter determination algorithm 4329 determines the treatment pressure Pt using the equation

[0255] where: • A is the amplitude, • ^(^^ t) is the waveform template value (in the range 0 to 1) at the current value of phase and t of time, and • P0 is a base pressure.

[0256] If the waveform determination algorithm 4322 provides the waveform template ^(^^ t) as a lookup table of values ^ indexed by phase ^^ the therapy parameter determination algorithm 4329 applies equation (1) by locating the nearest lookup table entry to the current value ^ of phase returned by the phase determination algorithm 4321, or by interpolation between the two entries straddling the current value ^ of phase.

[0257] The values of the amplitude A and the base pressure P0 may be set by the therapy parameter determination algorithm 4329 depending on the chosen respiratory pressure therapy mode in the manner described below. 4.4.3.3 Therapy Control module

[0258] The therapy control module 4330 in accordance with one aspect of the present technology receives as inputs the therapy parameters from the therapy parameter determination algorithm 4329 of the therapy engine module 4320, and controls the pressure generator 4140 to deliver a flow of air in accordance with the therapy parameters.

[0259] In one form of the present technology, the therapy parameter is a treatment pressure Pt, and the therapy control module 4330 controls the pressure generator 4140 to deliver a flow of air whose interface pressure Pm at the patient interface 3000 or 3800 is equal to the treatment pressure Pt. 4.4.3.4 Detection of fault conditions

[0260] In one form of the present technology, the central controller 4230 executes one or more methods 4340 for the detection of fault conditions. The fault conditions detected by the one or more methods 4340 may include at least one of the following: • Power failure (no power, or insufficient power) • Transducer fault detection • Failure to detect the presence of a component • Operating parameters outside recommended ranges (e.g. pressure, flow rate, temperature, PaO2) • Failure of a test alarm to generate a detectable alarm signal.

[0261] Upon detection of the fault condition, the corresponding algorithm 4340 signals the presence of the fault by one or more of the following: • Initiation of an audible, visual & / or kinetic (e.g. vibrating) alarm • Sending a message to an external device • Logging of the incident 4.5 AIR CIRCUIT

[0262] An air circuit 4170 in accordance with an aspect of the present technology is a conduit or a tube constructed and arranged to allow, in use, a flow of air to travel between two components such as RPT device 4000 and the patient interface 3000 or 3800.

[0263] In particular, the air circuit 4170 may be in fluid connection with the outlet of the pneumatic block 4020 and the patient interface. The air circuit may be referred to as an air delivery tube. In some cases there may be separate limbs of the circuit for inhalation and exhalation. In other cases a single limb is used.

[0264] In some forms, the air circuit 4170 may comprise one or more heating elements configured to heat air in the air circuit, for example to maintain or raise the temperature of the air. The heating element may be in a form of a heated wire circuit, and may comprise one or more transducers, such as temperature sensors. In one form, the heated wire circuit may be helically wound around the axis of the air circuit 4170. The heating element may be in communication with a controller such as a central controller 4230. One example of an air circuit 4170 comprising a heated wire circuit is described in United States Patent 8,733,349, which is incorporated herewithin in its entirety by reference. 4.6 HUMIDIFIER 4.6.1 Humidifier overview

[0265] In one form of the present technology there is provided a humidifier 5000 (e.g. as shown in Fig.5A) to change the absolute humidity of air or gas for delivery to a patient relative to ambient air. Typically, the humidifier 5000 is used to increase the absolute humidity and increase the temperature of the flow of air (relative to ambient air) before delivery to the patient’s airways.

[0266] The humidifier 5000 may comprise a humidifier reservoir 5110, a humidifier inlet 5002 to receive a flow of air, and a humidifier outlet 5004 to deliver a humidified flow of air. In some forms, as shown in Fig.5A and Fig.5B, an inlet and an outlet of the humidifier reservoir 5110 may be the humidifier inlet 5002 and the humidifier outlet 5004 respectively. The humidifier 5000 may further comprise a humidifier base 5006, which may be adapted to receive the humidifier reservoir 5110 and comprise a heating element 5240. 4.7 GLOSSARY

[0267] For the purposes of the present technology disclosure, in certain forms of the present technology, one or more of the following definitions may apply. In other forms of the present technology, alternative definitions may apply. 4.7.1 General

[0268] Air: In certain forms of the present technology, air may be taken to mean atmospheric air, and in other forms of the present technology air may be taken to mean some other combination of breathable gases, e.g. oxygen enriched air.

[0269] Ambient: In certain forms of the present technology, the term ambient will be taken to mean (i) external of the treatment system or patient, and (ii) immediately surrounding the treatment system or patient.

[0270] For example, ambient humidity with respect to a humidifier may be the humidity of air immediately surrounding the humidifier, e.g. the humidity in the room where a patient is sleeping. Such ambient humidity may be different to the humidity outside the room where a patient is sleeping.

[0271] In another example, ambient pressure may be the pressure immediately surrounding or external to the body.

[0272] Automatic Positive Airway Pressure (APAP) therapy: CPAP therapy in which the treatment pressure is automatically adjustable, e.g. from breath to breath, between minimum and maximum limits, depending on the presence or absence of indications of SDB events.

[0273] Continuous Positive Airway Pressure (CPAP) therapy: Respiratory pressure therapy in which the treatment pressure is approximately constant through a respiratory cycle of a patient. In some forms, the pressure at the entrance to the airways will be slightly higher during exhalation, and slightly lower during inhalation. In some forms, the pressure will vary between different respiratory cycles of the patient, for example, being increased in response to detection of indications of partial upper airway obstruction, and decreased in the absence of indications of partial upper airway obstruction.

[0274] Flow rate: The volume (or mass) of air delivered per unit time. Flow rate may refer to an instantaneous quantity. In some cases, a reference to flow rate will be a reference to a scalar quantity, namely a quantity having magnitude only. In other cases, a reference to flow rate will be a reference to a vector quantity, namely a quantity having both magnitude and direction. Flow rate may be given the symbol Q. ‘Flow rate’ is sometimes shortened to simply ‘flow’ or ‘airflow’.

[0275] In the example of patient respiration, a flow rate may be nominally positive for the inspiratory portion of a breathing cycle of a patient, and hence negative for the expiratory portion of the breathing cycle of a patient. Device flow rate, Qd, is the flow rate of air leaving the RPT device. Total flow rate, Qt, is the flow rate of air and any supplementary gas reaching the patient interface via the air circuit. Vent flow rate, Qv, is the flow rate of air leaving a vent to allow washout of exhaled gases. Leak flow rate, Ql, is the flow rate of leak from a patient interface system or elsewhere. Respiratory flow rate, Qr, is the flow rate of air that is received into the patient's respiratory system.

[0276] Leak: The word leak will be taken to be an unintended flow of air. In one example, leak may occur as the result of an incomplete seal between a mask and a patient's face. In another example leak may occur in a swivel elbow to the ambient.

[0277] Patient: A person, whether or not they are suffering from a respiratory condition.

[0278] Pressure: Force per unit area. Pressure may be expressed in a range of units, including cmH2O, g-f / cm2and hectopascal.1 cmH2O is equal to 1 g-f / cm2and is approximately 0.98 hectopascal (1 hectopascal = 100 Pa = 100 N / m2= 1 millibar ~ 0.001 atm). In this specification, unless otherwise stated, pressure is given in units of cmH2O.

[0279] The pressure in the patient interface is given the symbol Pm, while the treatment pressure, which represents a target value to be achieved by the interface pressure Pm at the current instant of time, is given the symbol Pt.

[0280] Respiratory Pressure Therapy: The application of a supply of air to an entrance to the airways at a treatment pressure that is typically positive with respect to atmosphere.

[0281] Ventilator: A mechanical device that provides pressure support to a patient to perform some or all of the work of breathing. 4.7.2 Ventilation

[0282] Adaptive Servo-Ventilator (ASV): A servo-ventilator that has a changeable, rather than fixed target ventilation. The changeable target ventilation may be learned from some characteristic of the patient, for example, a respiratory characteristic of the patient.

[0283] Backup rate: A parameter of a ventilator that establishes the minimum breathing rate (typically in number of breaths per minute) that the ventilator will deliver to the patient, if not triggered by spontaneous respiratory effort.

[0284] Cycled: The termination of a ventilator's inspiratory phase. When a ventilator delivers a breath to a spontaneously breathing patient, at the end of the inspiratory portion of the breathing cycle, the ventilator is said to be cycled to stop delivering the breath.

[0285] Expiratory positive airway pressure (EPAP): a base pressure, to which a pressure varying within the breath is added to produce the desired interface pressure which the ventilator will attempt to achieve at a given time.

[0286] End expiratory pressure (EEP): Desired interface pressure which the ventilator will attempt to achieve at the end of the expiratory portion of the breath. If the pressure waveform template ^(^) is zero-valued at the end of expiration, i.e. ^(^) = 0 when ^ = 1, the EEP is equal to the EPAP.

[0287] Inspiratory positive airway pressure (IPAP): Maximum desired interface pressure which the ventilator will attempt to achieve during the inspiratory portion of the breath.

[0288] Pressure support: A number that is indicative of the increase in pressure during ventilator inspiration over that during ventilator expiration, and generally means the difference in pressure between the maximum value during inspiration and the base pressure (e.g., PS = IPAP – EPAP). In some contexts, pressure support means the difference which the ventilator aims to achieve, rather than what it actually achieves.

[0289] Servo-ventilator: A ventilator that measures patient ventilation, has a target ventilation, and which adjusts the level of pressure support to bring the patient ventilation towards the target ventilation.

[0290] Spontaneous / Timed (S / T): A mode of a ventilator or other device that attempts to detect the initiation of a breath of a spontaneously breathing patient. If however, the device is unable to detect a breath within a predetermined period of time, the device will automatically initiate delivery of the breath.

[0291] Swing: Equivalent term to pressure support.

[0292] Triggered: When a ventilator, or other respiratory therapy device such as an RPT device or portable oxygen concentrator, delivers a volume of breathable gas to a spontaneously breathing patient, it is said to be triggered to do so. Triggering usually takes place at or near the initiation of the respiratory portion of the breathing cycle by the patient's efforts. 4.8 OTHER REMARKS

[0293] A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in Patent Office patent files or records, but otherwise reserves all copyright rights whatsoever.

[0294] Unless the context clearly dictates otherwise and where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit, between the upper and lower limit of that range, and any other stated or intervening value in that stated range is encompassed within the technology. The upper and lower limits of these intervening ranges, which may be independently included in the intervening ranges, are also encompassed within the technology, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the technology.

[0295] Furthermore, where a value or values are stated herein as being implemented as part of the technology, it is understood that such values may be approximated, unless otherwise stated, and such values may be utilized to any suitable significant digit to the extent that a practical technical implementation may permit or require it.

[0296] Furthermore, “approximately”, “substantially”, “about”, or any similar term used herein means + / - 5-10% of the recited value.

[0297] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this technology belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present technology, a limited number of the exemplary methods and materials are described herein.

[0298] When a particular material is identified as being used to construct a component, obvious alternative materials with similar properties may be used as a substitute. Furthermore, unless specified to the contrary, any and all components herein described are understood to be capable of being manufactured and, as such, may be manufactured together or separately.

[0299] It must be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include their plural equivalents, unless the context clearly dictates otherwise.

[0300] All publications mentioned herein are incorporated herein by reference in their entirety to disclose and describe the methods and / or materials which are the subject of those publications. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present technology is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates, which may need to be independently confirmed.

[0301] The terms "comprises" and "comprising" should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.

[0302] The subject headings used in the detailed description are included only for the ease of reference of the reader and should not be used to limit the subject matter found throughout the disclosure or the claims. The subject headings should not be used in construing the scope of the claims or the claim limitations.

[0303] Although the technology herein has been described with reference to particular examples, it is to be understood that these examples are merely illustrative of the principles and applications of the technology. In some instances, the terminology and symbols may imply specific details that are not required to practice the technology. For example, although the terms "first" and "second" may be used, unless otherwise specified, they are not intended to indicate any order but may be utilised to distinguish between distinct elements. Furthermore, although process steps in the methodologies may be described or illustrated in an order, such an ordering is not required. Those skilled in the art will recognize that such ordering may be modified and / or aspects thereof may be conducted concurrently or even synchronously.

[0304] It is therefore to be understood that numerous modifications may be made to the illustrative examples and that other arrangements may be devised without departing from the spirit and scope of the technology.

Claims

CLAIMS 1. A respiratory pressure therapy (RPT) device comprising: a pressure generator for producing a flow of air for respiratory pressure therapy; and a controller configured to control at least one operational parameter of the pressure generator in order to compensate for local air density.

2. An RPT device as claimed in claim 1, wherein the controller is configured to estimate local air density and to control the at least one operational parameter on the basis of the estimated local air density.

3. An RPT device as claimed in claim 2, wherein the RPT device has transducers for producing signals indicative of blower pressure, motor speed, and blower flow, and the controller is configured to estimate local air density from the signals indicative of blower pressure, motor speed and blower flow.

4. An RPT device as claimed in claim 3, wherein the controller estimates local air density based on a polynomial equation that relates blower pressure to motor speed, blower pressure and local air density.

5. An RPT device as claimed in claim 4, wherein the polynomial equation comprises at least one polynomial coefficient that is a function of motor speed and local air density.

6. An RPT device as claimed in claim 5, wherein local air density (ρlocal) is estimated based on the equation:⍴local =where Flow is the blower flow rate, Pressure is the blower pressure, and A1, B1, and C1are coefficients characterising operation of a fan of the pressure generator at a reference air density (ρref).

7. An RPT device as claimed in any one of claims 1 to 6, wherein the RPT device is associated with an air circuit and the controller is configured to estimate pressure drop across the air circuit based on the estimated local air density and control the at least one operational parameter based on the estimated pressure drop.

8. An RPT device as claimed in claim 7, wherein the pressure drop is estimated as a function of blower flow rate and at least one function of air density.

9. An RPT device as claimed in claim 8, wherein the pressure drop (^P) is estimated according to the equation: ∆^^ = ^^1. ^^2 + ^^2. ^^where Q is blower flow rate and Z1 and Z2 are functions of air density.

10. An RPT device as claimed in any one of claims 1 to 9, wherein the controller is configured to modify at least one operational parameter of the pressure generator in response to a change in local air density.

11. An RPT system comprising an RPT device as claimed in any one of claims 1 to 10 and an air circuit.

12. An RPT system as claim in claim 11, wherein the air circuit has a geometry and impedance such that changes in air density significantly affect pressure drop of the air circuit.

13. A non-transitory computer-readable medium comprising instructions, which when executed by a processor, cause the processor to control at least one operational parameter of a pressure generator of an RPT device to produce a flow of air for respiratory pressure therapy compensated for local air density.

14. A non-transitory computer-readable medium as claimed in claim 13, wherein when the instructions are executed, they cause the processor to estimate localair density and to control the at least one operational parameter on the basis of the estimated local air density.

15. A non-transitory computer-readable medium as claimed in claim 13, wherein when the instructions are executed, they cause the processor to process signals indicative of blower pressure, motor speed, and blower flow to estimate local air density.

16. A non-transitory computer-readable medium as claimed in claim 15, wherein when the instructions are executed, they cause the processor to estimate local air density based on a polynomial equation that relates blower pressure to motor speed, blower pressure and local air density.

17. A non-transitory computer-readable medium as claimed in claim 16, wherein the polynomial equation comprises at least one polynomial coefficient that is a function of motor speed and local air density.

18. A non-transitory computer-readable medium as claimed in claim 17, wherein when the instructions are executed, they cause the processor to estimate local air density (ρlocal) based on the equation:⍴local =where Flow is the blower flow rate, Pressure is the blower pressure, and A1, B1, and C1 are coefficients characterising operation of a fan of the pressure generator at a reference air density (ρref).

19. A non-transitory computer-readable medium as claimed in any one of claims 13 to 18, wherein the RPT device is associated with an air circuit and wherein when the instructions are executed, they cause the processor to estimate pressure drop across the air circuit based on the estimated local air density and control the at least one operational parameters based on the estimated pressure drop.

20. A non-transitory computer-readable medium as claimed in claim 19, wherein when the instructions are executed, they cause the processor to estimate pressure drop as a function of blower flow rate and at least one function of air density.

21. A non-transitory computer-readable medium device as claimed in claim 19, wherein when the instructions are executed, they cause the processor to estimate pressure drop (^P) according to the equation: ∆^^ = ^^1. ^^2 + ^^2. ^^where Q is blower flow rate and Z1 and Z2 are functions of air density.

22. A non-transitory computer-readable medium device as claimed in any one of claims 13 to 21, wherein when the instructions are executed, they cause the processor to modify at least one operational parameter of the pressure generator in response to a change in local air density.

23. A computer-implemented method comprising controlling at least one operational parameter of a pressure generator of an RPT device to produce a flow of air for respiratory pressure therapy compensated for local air density.

24. A computer-implemented method as claimed in claim 23, comprising estimating local air density and controlling the at least one operational parameter on the basis of the estimated local air density.

25. A computer-implemented method as claimed in claim 24, comprising causing the processor to process signals indicative of blower pressure, motor speed, and blower flow to estimate local air density.

26. A computer-implemented method as claimed in claim 25, comprising causing the processor to estimate local air density based on a polynomial equation that relates blower pressure to motor speed, blower pressure and local air density.

27. A computer-implemented method as claimed in claim 26, wherein the polynomial equation comprises at least one polynomial coefficient that is a function of motor speed and local air density.

28. A computer-implemented method as claimed in claim 27, wherein comprising causing the processor to estimate local air density (ρlocal) based on the equation:⍴local =^^1 ∗ ⍴ref ∗ Flow^2where Flow is the blower flow rate, Pressure is the blower pressure, and A1, B1, and C1 are coefficients characterising operation of a fan of the pressure generator at a reference air density (ρref).

29. A computer-implemented method as claimed in any one of claims 23 to 28, wherein the RPT device is associated with an air circuit and the method comprises causing the processor to estimate pressure drop across the air circuit based on the estimated local air density and control the at least one operational parameters based on the estimated pressure drop.

30. A computer-implemented method as claimed in claim 29, comprising causing the processor to estimate pressure drop as a function of blower flow rate and at least one function of air density.

31. A computer-implemented method as claimed in claim 30, comprising causing the processor to estimate pressure drop (^P) according to the equation: ∆^^ = ^^1. ^^2 + ^^2. ^^where Q is blower flow rate and Z1 and Z2 are functions of air density.

32. A computer implemented method as claimed in any one of claims 23 to 31, comprising modifying at least one operational parameter of the pressure generator in response to a change in local air density.

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