System and method for measuring mechanical impedance of respiratory system

Abstract

A system for measuring the mechanical impedance of the respiratory system of a patient during spontaneous respiratory activity, characterized in that the system comprises a fan, a motor driving the fan; the motor and the fan are placed in the cavity; the cavity is divided into two sub-cavities separated from each other so as to prevent the passage of air, one sub-cavity containing the motor and the other sub-cavity containing the fan and including a start end and a tail end, both having a passage leading to the outside.

Description

Technical Field

[0001] The present invention relates to systems and methods for measuring the mechanical impedance of the respiratory system during the spontaneous breathing activity of an individual (e.g., a patient) using forced oscillation techniques (FOT or oscillometry).

[0002] State of the Art

[0003] Oscillometry is a method for measuring the mechanical properties of the airways and lungs, which is based on the analysis of the airflow generated by a small-amplitude pressure stimulus oscillating at a frequency higher than the spontaneous breathing frequency. This method was first proposed by the American physiologist Arthur Dubois in 1956 (Journal of Applied Physiology - May 1956, Vol. 8, No. 6, pp. 587-594) and has recently attracted increasing attention in the clinical community as a potentially new non-invasive and easy-to-use means for diagnosing and monitoring respiratory system dysfunctions.

[0004] This, combined with the emergence of new devices using this method on the market, has recently prompted the European Respiratory Society to issue a technical standard document to support the dissemination of such instruments in the clinical environment (Eur Respir J 2020; 55:1900753).

[0005] During oscillometry measurements, the respiratory system is subjected to a small-amplitude oscillatory mechanical stimulus generated by an external actuator. The difficulty of the subsequent movement of air through the respiratory system caused by the stimulus is quantified by calculating the mechanical impedance (Z), which is obtained from the complex ratio between the pressure (Pm) measured at the airway inlet and the flow and is related to any or all frequency components (f) of the stimulus:

[0006]

[0007] Mechanical impedance is a complex number, the real part of which is called resistance (R(f)) and is an indicator of the airway caliber and / or its patency, while the imaginary part is called reactance (X(f)) and summarizes the ability of the system to store energy and is thus determined by both the elastic and inertial properties of the respiratory system.

[0008] Various oscillometry measurement systems characterized by the use of different stimulus generation systems have been reported in the scientific and technical literature.

[0009] Upon initial use, the stimulator consisted of a cylinder connected to a piston and a set of sensors for measuring flow and pressure, with the outlet of the cylinder directly connected to the airway opening (nose or mouth) (Journal of Applied Physiology - May 1956, Vol. 8, No. 6, pp. 587 - 594, U.S. Patent 3,713,436 filed on October 23, 1970). These large and complex systems could not be used for measurements during spontaneous breathing, but only for short periods of apnea.

[0010] Subsequently, starting from the late 1960s, oscillometric measurement systems consisting of an oscillating loudspeaker connected to a set of pressure and flow sensors that directly contact the individual's airway opening and a parallel high - inertia path began to be more widely used to allow measurements during spontaneous breathing. The high - inertia path consists of a tube with sufficient diameter, length, and / or resistance (The Journal of Clinical Investigation - November 1975, Vol. 56, pp. 1210 - 1230, US 4,333,476, EP 1 551 293). However, the presence of the high - inertia tube significantly increases the dead space in the respiratory system and requires the use of an additional air - flow generator for ventilation, thereby increasing the size and complexity of the entire system.

[0011] As described in patents US 4,220,161 and US 6,066,101, smaller setups have been constructed that use an actuator to partially or fully occlude the airway during spontaneous breathing to cause a pressure disturbance within the circuit, where the stimulating energy is generated by the respiratory muscles.

[0012] While devices belonging to this latter category are cheaper and smaller in size, they are not suitable for low expiratory and inspiratory flows (e.g., at the end of inspiration and expiration) and are not suitable for measuring reactivity.

[0013] The owner of this patent application has described a small and potentially portable respiratory impedance measurement system in patent application ITBG20100042. The system includes a cavity through which the individual being measured breathes, and the cavity houses a motor connected to a fan, and the appropriately controlled movement of the fan can generate the pressure oscillations required to stimulate the respiratory system and measure respiratory impedance.

[0014] The aim of the present invention is to provide a system and method for mechanical measurement of the respiratory system that at least partially overcomes the drawbacks of existing systems or improves their performance.

[0015] Object of the Invention

[0016] The main object of the present invention is to provide a system and method for measuring respiratory impedance, which reduces the drawbacks of the known techniques. Summary of the Invention

[0018] According to the present invention, this result has been achieved by constructing a portable system for measuring the mechanical impedance of a patient's respiratory system during spontaneous breathing, the portable system comprising: a system including a motor and a fan for generating a pressure stimulus capable of producing small variations in pressure and / or flow at the opening leading to the airway; a detection system capable of measuring the pressure and flow values generated by the stimulus and by the patient's spontaneous breathing activity; a microprocessor capable of: controlling the system for generating the pressure signal; receiving the data measured by the detection system; calculating a measured value of the mechanical impedance of the patient's respiratory system based on the values measured by the said detection system; characterized in that the portable system includes a first chamber containing the motor and a second chamber containing the fan, the first chamber and the second chamber being separated from each other such that there is no air passage between the first chamber and the second chamber, the second chamber having a rear opening capable of inhaling air from the external environment and a front opening capable of coming into contact with the patient and receiving the patient's inspiratory and expiratory flows.

[0019] The detection system preferably includes at least one pressure sensor. According to a preferred embodiment, the detection system further includes a flowmeter for measuring the flow generated by the patient's spontaneous breathing activity; alternatively, the flow generated by the patient's spontaneous breathing activity is estimated by the microprocessor based on one or more of the following data: fan speed, motor power consumption, pressure, air temperature and humidity.

[0020] In a preferred embodiment, the microprocessor is arranged to modulate the pressure stimulus such that the rotational speed is reduced during an initial measurement phase compared to a steady-state measurement phase. The fan speed in the initial phase can be controlled by the microprocessor to generate a pressure stimulus whose peak-to-peak amplitude increases linearly until a predetermined target value is reached. According to a possible alternative embodiment, the microprocessor can control the rotational speed of the fan in the initial phase to generate a pressure stimulus whose peak-to-peak amplitude increases exponentially until a predetermined target value is reached. According to a preferred embodiment, during the steady-state measurement phase (i.e., after the adaptation phase), the rotational speed of the fan is controlled by a closed-loop control system to generate a pressure stimulus whose peak-to-peak amplitude remains constant and is equal to a value determined based on one or more of the patient's pressure, flow, impedance, resistance and reactivity values measured during the initial measurement phase.

[0021] According to another aspect of the present invention, there is provided a method for operating a portable system as described above, the method comprising the steps of: generating a pressure stimulus to produce a small pressure change; measuring the pressure and flow values passing through the stimulus and generated by the patient's spontaneous breathing activity; receiving data measured by the sensing system from the microprocessor; calculating a measurement of the mechanical impedance of the patient's respiratory system based on the values measured by the sensing system.

[0022] There is also provided a computer program which, when executed by a microprocessor, implements the above method.

[0023] According to another aspect of the present invention, there is provided a kit for measuring the mechanical impedance of a patient's respiratory system during spontaneous breathing, comprising: the above system; test and calibration equipment including a hollow catheter having two relatively frustoconical ends, the smaller cross-sections of the two relatively frustoconical ends converging inwardly, and the two frustoconical ends being joined together by a generally cylindrical central portion; the test equipment having a known and predetermined impedance value.

[0024] The present invention allows for the development of a compact portable device which can be used for different individuals, is easy to clean and maximizes patient comfort during measurement.

[0025] Among the advantages obtained from the device manufactured according to an embodiment of the present invention, we will point out:

[0026] The separation between the chamber containing the motor and the chamber containing the fan allows the motor to be isolated from the air inhaled and exhaled by the patient. This can prevent the accidental inhalation of dust generated during the operation of the motor and prevent moisture and any saliva generated during breathing from damaging the motor's circuitry and electrical connections;

[0027] If contamination occurs, the separation also makes it possible to replace only the breathing chamber and the fan therein and allows the device to be used for different patients without having to replace the entire device;

[0028] As provided in an optional embodiment of the present invention, generating a gradually increasing stimulus during the adaptation phase of the measurement helps the individual to become accustomed to the presence of the stimulus oscillation, thereby increasing the acceptability of the test;

[0029] Generating a stimulus whose amplitude is adjusted based on the amplitude of the resulting flow oscillation provided by a possible embodiment of the present invention makes it possible to optimize the signal-to-noise ratio and utilize a pressure stimulus with a reduced amplitude in individuals with a low respiratory impedance, increasing comfort during measurement. Brief Description of the Drawings

[0031] Any person skilled in the art will better understand these and other advantages, objects, and features of the present invention from the following description and drawings, which relate to examples of illustrative embodiments but should not be understood in a restrictive sense, where:

[0032] Figure 1 is a schematic diagram of a system for measuring the mechanical impedance of the respiratory system according to a preferred embodiment of the present invention;

[0033] Figure 2 schematically depicts the pressure pattern at the patient's mouth during the adaptation and measurement phases according to a preferred embodiment of the present invention;

[0034] Figure 3 shows a mechanical device for calibrating the system according to the present invention and for automatically checking the correct system function.

[0035] Detailed description

[0036] Referring to the accompanying drawings, and in particular in Figure 1 a system for measuring the mechanical impedance of the respiratory system according to a preferred embodiment of the present invention includes a motor 3 connected to a centrifugal fan 4.

[0037] The motor 3 is located in a first chamber 1 that is not in communication with the air inhaled and exhaled by the patient. The fan 4 is located in a second chamber 2 ("breathing chamber") that has a beginning end 7 and an end 6, both of which have openings to the outside. The first chamber and the second chamber are placed in communication with each other so as to transmit the mechanical movement from the motor to the fan, but do not allow air to pass between the two chambers.

[0038] It is also possible to include the chamber 1 containing the motor and the breathing chamber 2 in the same chamber (or cavity), for example by means of a partition wall that allows mechanical communication (e.g., the motor shaft rotates the fan) but prevents air from passing through, thereby obtaining two separate sub-chambers (or sub-cavity cavities) such that the air inhaled by the user (patient) does not contain any harmful dust generated by the motor. This is one of the advantages provided by the system according to the present invention compared to, for example, the system described in patent application ITBG20100042.

[0039] The beginning end 7 is designed to be connected to a mouthpiece or otherwise interfaced with the patient, and preferably has a diameter of about 2 - 4 cm.

[0040] In an alternative embodiment, the chamber 2 can accommodate an axial-flow type fan 4.

[0041] The distance that the air travels in the chamber is less than 25 cm, preferably about 15 cm, so that the device is easy to transport.

[0042] In the case where the volume of the breathing chamber 2 is greater than 50 ml, the breathing chamber 2 preferably includes one or more ventilation holes located approximately halfway between the starting end 7 and the sampling port 11 for pressure and flow signals, and these ventilation holes are required to ensure the outward diffusion of exhaled air.

[0043] According to a possible embodiment, the breathing chamber includes a flow meter 8, and the flow meter 8 includes a resistance element capable of generating a pressure drop, and this pressure drop can generate a known change in the pressure and function of the air flow passing through the flow meter. In an alternative embodiment, the flow measurement at the airway opening does not have a resistance element, and an ultrasonic or hot wire type sensor can be used to perform the measurement.

[0044] In another embodiment, the flow rate can be estimated based on the rotational speed and / or power consumption and / or pressure value measured inside the chamber 2 and in the environment.

[0045] According to a possible embodiment, the chamber 1 further includes a pressure sensor 9 (Pm) and a flow sensor which is arranged in a manner that communicates with the breathing chamber 2 via a pneumatic connection 11.

[0046] Associated with the chamber 1 is a microprocessor-based processing and control system 5 powered by mains electricity or a battery. The processing and control system 5 receives signals from the sensors 9 and 10, stores them in its memory, and performs the necessary processing to calculate the mechanical impedance of the respiratory system. The processing system also includes a motor drive circuit 3 and a module for communicating with the outside world to perform measurements and send / receive commands to / from the microprocessor.

[0047] According to one embodiment, the processing and control device 5 that manages the measurements performed includes a memory and an electronic interface for data retrieval. In another embodiment, in addition to the memory, the processing and control device 5 further includes a data processing system, thus directly providing the previously processed data.

[0048] In another embodiment, the device includes a system for sending wireless data.

[0049] In another embodiment, the device includes a system for sending data to an external processing and storage system via the Internet.

[0050] In another embodiment, the device may include sensors for measuring blood saturation and / or heart rate.

[0051] In another embodiment, the device may include a display for displaying the measured values and system information.

[0052] In another embodiment, the device may include an input system, such as buttons or a touch screen for inputting patient information and changing system settings.

[0053] The rotational speed of the motor 3 is controlled by a microprocessor to force external air into the breathing chamber 2, thereby generating a pressure change with a maximum peak-to-peak amplitude of 5 cm H2O (centimeters of water column), which pressure change has a predetermined shape, typically a sine curve with a frequency > 2 Hz or a sum of sine curves with a frequency > 2 Hz, and has an average pressure value less than or equal to 2.5 cm H2O near the starting end 7.

[0054] In a preferred embodiment, the average pressure value is between 0.75 and 1 cm H2O, so the peak-to-peak pressure value is between 1.5 and 2 cm H2O.

[0055] In one embodiment, the movement of the fan 4 is activated only when the individual exhibits respiratory activity.

[0056] The vibration caused by the rotation of the fan can cause discomfort to the patient. To overcome this problem, in a preferred embodiment of the present invention, during the initial stage of the fan operation, which is called the adaptation stage, the rotational speed of the fan 4 is controlled to generate a pressure stimulus, and the peak-to-peak amplitude of this pressure stimulus linearly increases to a predetermined target value.

[0057] In another embodiment, during the adaptation stage, the rotational speed of the fan 4 is controlled to generate a pressure stimulus, and the peak-to-peak amplitude of this pressure stimulus exponentially increases until it reaches a predetermined target value, such as 2 cm H2O.

[0058] In another embodiment, during the adaptation stage, the rotational speed of the fan 4 is controlled to generate a pressure stimulus, and the peak-to-peak amplitude of this pressure stimulus increases according to a curve with a predetermined shape until it reaches a predetermined target value, such as 2 cm H2O.

[0059] According to yet another embodiment, during the measurement stage, the rotational speed of the fan 4 is modulated by a closed-loop control system to generate a pressure stimulus, and the ideal peak-to-peak amplitude of this pressure stimulus remains constant and is equal to a preset value, or is a value determined based on one or more of the patient's pressure, flow rate, impedance, resistance value, and reactivity value measured during the adaptation stage.

[0060] In another embodiment, during the measurement stage, the rotational speed of the fan 4 is modulated according to a series of values to generate a pressure stimulus, and the peak-to-peak amplitude of this pressure stimulus is equal to a preset value, or is determined based on the patient's impedance, resistance value, and reactivity value measured during the adaptation stage.

[0061] In another embodiment, during the measurement stage, the rotational speed of the fan 4 is controlled to generate a pressure stimulus, and this pressure stimulus generates a flow oscillation not less than a predetermined value, or is determined based on the patient's impedance, resistance value, and reactivity value measured during the adaptation stage.

[0062] One problem that may arise when using the system according to the present invention is maintaining the correct calibration of the pressure and flow sensors used for impedance measurement. Figure 3 A test device is shown, which is shaped like two cones connected by one or more cylindrical pipes. By determining the dimensions of the area and length of the cylindrical pipes, a test object can be produced that is characterized by a mechanically stable impedance value over time. This test object can be used to automatically check the calibration status of the sensors in the system according to the present invention and, if possible, calibrate them. The test device can be provided in a kit together with the system according to the present invention to allow the end user to perform a calibration check on the system and, if necessary, calibrate it.

[0063] In another embodiment, the breathing chamber 2 and the fan 4 can be removed and replaced.

[0064] To allow the patient to breathe spontaneously through the circuit with minimal effort, the dimensions of the breathing chamber 2 and the air inlet and outlet areas near the start end 7 and the end 6 are preferably designed to have a maximum impedance of 1 cm H2O / L / s (measured at normal breathing frequencies and thus in the range of 0 - 1 Hz).

[0065] A possible procedure for measuring the mechanical impedance of the respiratory system using the system described herein is described below. When switched on, the system can ask the user to check the correct functioning using the provided test device. The patient is then invited to breathe through a connection interface (filter, mouthpiece) connected to the start end 7. After the presence of a breathing activity is recognized by the sensors, the system will start the motor 3 and the fan 4 and will initiate an adaptation phase during which the impedance, resistance value, and reactance value are determined and continuously updated by the system based on the pressure and flow values read by the sensors 9 and 10. At the end of the adaptation phase, the system will automatically switch to a measurement phase of a predetermined duration, at the end of which the measured values will be stored.

[0066] The impedance of the respiratory system can be calculated using any impedance calculation algorithm, such as the least - squares optimization - based algorithm reported by Horowitz (Comput Biomed Res 1983 December; 16(6): 499 - 521.) and Kackza (Ann Biomed Eng 1999 May; 27(3): 340 - 55), and recently improved by Dellacà et al. (EP1551293). This algorithm is based on decomposing the pressure and flow signals into components of the pressure and flow signals generated by normal breathing activity and components of the pressure and flow signals generated by external stimuli. The latter are in turn decomposed into their constituent harmonics, and each of these constituent harmonics is applied to an iterative calculation process to identify the phase coefficients of each constituent harmonic within a time window W of a predetermined length.

[0067] The phase coefficients of the pressure and flow signals determined for each harmonic (f) give rise to an impedance Z(f) related to the data present within the time window Z w (f):

[0068]

[0069] The calculation is then repeated, shifting the time window forward by one or more samples in order to obtain a calculation of Z(f) for all the measurement data.

[0070] Subsequently, the values of Z(f), R(f), X(f), pressure, and flow are analyzed using threshold and statistical outlier identification methods to identify and exclude portions of the data measured at measurement artifacts (e.g., glottal closure, swallowing, coughing, phonation, or leakage at the mouthpiece / filter used for measurement). Finally, the measurement result is obtained by calculating one or more parameters that are derived from the values of Z(f), R(f), X(f), pressure, and flow related to the portion of the free data that does not contain artifacts.

Claims

1. A portable system for measuring the mechanical impedance of an individual's respiratory system during spontaneous breathing, comprising: A system for generating a pressure signal, which is capable of generating small changes in pressure and / or flow at the opening leading to the airway, and the system includes a motor and a fan; A detection system, which is capable of measuring the pressure and flow values generated by stimulating and by the individual's spontaneous breathing activity; A microprocessor, which is capable of: Controlling the system for generating a pressure signal; Receiving the data measured by the detection system; Calculating a measured value of the mechanical impedance of the individual's respiratory system based on the values measured by the detection system; Characterized in that the portable system includes a first chamber containing the motor and a second chamber containing the fan, and the first chamber and the second chamber are separated from each other such that there is no air passage between the first chamber and the second chamber, and the second chamber has a rear opening capable of exchanging air with the external environment and a front opening capable of contacting the individual and receiving the inspiratory and expiratory flows of the individual.

2. The portable system according to claim 1, wherein, The detection system includes one or more of the following: at least one pressure sensor; at least one flowmeter for measuring the flow generated by the individual's spontaneous breathing activity.

3. The portable system according to any one of the preceding claims, wherein, The flow generated by the individual's spontaneous breathing activity is estimated by the microprocessor based on one or more of the following data: fan speed, power consumption value of the motor, air pressure, air temperature, and air humidity.

4. The portable system according to any one of the preceding claims, further characterized in that, The microprocessor is arranged to modulate the pressure signal such that during the initial measurement phase, the rotational speed is reduced relative to the steady-state measurement phase.

5. The portable system according to claim 4, wherein, In the initial phase, the rotational speed of the fan is controlled by the microprocessor to generate a pressure signal, and the peak-to-peak amplitude of the pressure signal increases linearly until a predetermined target value is reached.

6. The portable system according to claim 4, wherein, In the initial phase, the rotational speed of the fan is controlled by the microprocessor to generate a pressure signal, and the peak-to-peak amplitude of the pressure signal increases exponentially until a predetermined target value is reached.

7. The portable system according to any one of claims 4, 5 or 6, wherein, During the steady-state measurement phase, the rotational speed of the fan is controlled by a closed-loop control system to generate a pressure signal, and the peak-to-peak amplitude of the pressure signal remains constant and is equal to a value determined based on one or more of the pressure, flow, impedance, resistance value, and reactivity value of the individual measured during the initial measurement phase.

8. A method for operating a portable system as defined in any one of the preceding claims, comprising the following steps: Generating a pressure stimulus to produce a pressure change; Measuring the pressure and flow values generated by stimulating and by the individual's spontaneous breathing activity; Receiving the data measured by the detection system from the microprocessor; Calculating a measured value of the mechanical impedance of the individual's respiratory system based on the values measured by the detection system.

9. A computer program, which when executed by a microprocessor, implements the method according to claim 8.

10. A kit for measuring the mechanical impedance of an individual's respiratory system during spontaneous breathing, comprising: The system according to any one of claims 1-7, Test and calibration equipment, including a hollow conduit having two relatively frustoconical ends, the smaller cross-sections of the two relatively frustoconical ends converging inwardly, the two frustoconical ends being joined together by a generally cylindrical central portion, the test equipment having a known and predetermined impedance value.

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