Method of delivering variable ventilation with AVAPS

Abstract

A method for controlling a ventilator to provide variable volume (VV) with average volume guaranteed pressure support (AVAPS) includes generating a VV target volume using a VV distribution function; generating a volume error Verror that is a difference between the VV target volume and a measured volume of a previous breath; scaling the volume error Verror; generating a VV target difference that is a difference between the VV target volume and a VV target volume of the previous breath; generating a modified volume error by adding the VV target difference to the scaled volume error Verror; generating a delta pressure support ΔPS based on the modified volume error and a dynamic compliance; and generating a current pressure support value based on the delta pressure support ΔPS and a pressure support value of the previous breath.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This patent application claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 63 / 037,620, filed on June 11, 2020, the contents of which are incorporated herein by reference. Technical Field

[0003] Various exemplary embodiments disclosed herein generally relate to systems and methods for delivering variable ventilation with average volume assured pressure support (AVAPS). Background Art

[0004] Variable ventilation has been clinically demonstrated to improve cellular health in mechanically ventilated patients. This ventilation mode intentionally varies the size of individual breaths while maintaining a constant average breath size. AVAPS is a mode that allows volume-targeted ventilation to be delivered to chronic patients using a non-invasive ventilation (NIV) interface. AVAPS is beneficial for spontaneously breathing patients because it limits pressure and does not overreact to incidental events such as swallowing, coughing, glottis closure, speaking, and movement. Summary of the Invention

[0005] The following presents an overview of various exemplary embodiments. Some simplifications and omissions may be made in the following overview, which is intended to highlight and introduce some aspects of the various exemplary embodiments, rather than to limit the scope of the present invention. A detailed description of the exemplary embodiments sufficient to allow one of ordinary skill in the art to make and use the inventive concept will be provided in the following sections.

[0006] Various embodiments relate to a method for controlling a ventilator to provide a variable volume (VV) with average volume assured pressure support (AVAPS), the method comprising: generating a VV target volume using a VV distribution function; generating a volume error, Verror, which is the difference between the VV target volume and a measured volume of a previous breath; scaling the volume error, Verror; generating a VV target difference that is the difference between the VV target volume and the VV target volume of the previous breath; generating a modified volume error by adding the VV target difference to the scaled volume error, Verror; generating a delta pressure support, ΔPS, based on the modified volume error and a dynamic compliance; and generating a current pressure support value based on the delta pressure support, ΔPS, and a pressure support value of the previous breath.

[0007] Various embodiments are described where generating delta pressure support ΔPS further comprises dividing the modified volume error by the dynamic compliance.

[0008] Various embodiments are described in which the volume error Verror is scaled by a factor of 2.

[0009] Various embodiments are described that also include limiting the current pressure support by a minimum pressure support value and a maximum pressure support value.

[0010] Various embodiments are described that also include generating a breath profile based on current pressure support and positive end expiratory pressure (PEEP) values.

[0011] Various embodiments are described that also include rise time filtering of the respiration profile.

[0012] Various embodiments are described that also include applying a breathing profile to the user's lungs and measuring the volume of the resulting breath.

[0013] Various embodiments are described in which the VV distribution function has a mean value based on a received target volume value.

[0014] Various embodiments are described in which the VV distribution function can be selected from a plurality of different VV distribution functions.

[0015] Various additional embodiments relate to a controller configured to control a ventilator to provide a variable volume (VV) with an average volume assured pressure support (AVAPS), the controller comprising: a memory; a processor coupled to the memory, wherein the processor is further configured to: generate a VV target volume using a VV distribution function; generate a volume error, Verror, which is a difference between the VV target volume and a measured volume of a previous breath; scale the volume error, Verror; generate a VV target difference that is the difference between the VV target volume and the VV target volume of the previous breath; generate a modified volume error by adding the VV target difference to the scaled volume error, Verror; generate a delta pressure support, ΔPS, based on the modified volume error and a dynamic compliance; and generate a current pressure support value based on the delta pressure support, ΔPS, and a pressure support value of the previous breath.

[0016] Various embodiments are described where generating delta pressure support ΔPS further comprises dividing the modified volume error by the dynamic compliance.

[0017] Various embodiments are described in which the volume error Verror is scaled by a factor of 2.

[0018] Various embodiments are described wherein the processor is further configured to limit the current pressure support by a minimum pressure support value and a maximum pressure support value.

[0019] Various embodiments are described in which the processor is further configured to generate a breathing profile based on current pressure support and positive end expiratory pressure (PEEP) values.

[0020] Various embodiments are described wherein the processor is further configured to perform rise time filtering on the respiration profile.

[0021] Various embodiments are described in which the processor is further configured to apply a breathing profile to the user's lungs and measure the volume of the resulting breath.

[0022] Various embodiments are described in which the VV distribution function has a mean value based on a received target volume value.

[0023] Various embodiments are described in which the VV distribution function can be selected from a plurality of different VV distribution functions. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] For a better understanding of the various exemplary embodiments, reference is made to the accompanying drawings, in which:

[0025] Figure 1 Illustrated is a plot of lung volume versus pressure showing the variable circuits made during VV with AVAPS and variable pressure support.

[0026] Figure 2 FIGURE 1 illustrates a block diagram of the AVAPS control method;

[0027] Figure 3 An embodiment of an AVAPS system supporting variable ventilation is illustrated;

[0028] Figure 4A 、 Figure 4B and Figure 4C Histograms of the number of breaths per 1000 breaths at various volumes Vt for narrow, medium, and wide distributions, respectively;

[0029] Figure 5A 、 Figure 5B and Figure 5C Illustration of the use of passive lung Figure 4A The simulation results of the narrow distribution AVAPS system;

[0030] Figure 6A 、 Figure 6B and Figure 6C Illustration of the use of assisted breathing with fixed rate and fixed muscle effort, using Figure 4A The simulation results of the narrow distribution AVAPS system;

[0031] Figure 7A 、 Figure 7B and Figure 7C Illustration of the use of assisted breathing with fixed rate and fixed muscle effort, using Figure 4C Simulation results of the wide-distribution AVAPS system 300; and

[0032] Figure 8 The diagram shows Figure 3 An exemplary hardware diagram of the AVAPS system.

[0033] To facilitate understanding, the same reference numerals have been used to designate elements having substantially the same or similar structure and / or substantially the same or similar function. DETAILED DESCRIPTION

[0034] Description and accompanying drawings have illustrated the principle of the present invention.Therefore, it will be appreciated that those skilled in the art will be able to design various arrangements that embody the principle of the present invention and are included within the scope thereof, although not explicitly described or shown herein.In addition, all examples recorded herein are mainly clearly intended to be used for teaching purposes, to help readers understand the principle of the present invention and the conception contributed by the inventor to promote this area, and all examples recorded herein are to be interpreted as not being limited to such specific recorded examples and conditions. In addition, unless otherwise indicated (for example, "otherwise" or "or in the alternative"), the term "or" as used herein refers to non-exclusive or (that is, and / or). Moreover, the various embodiments described herein are not necessarily mutually exclusive, because some embodiments can be combined with one or more other embodiments to form new embodiments.

[0035] Variable ventilation delivers a breath-to-breath varying tidal volume (TV) around a specified mean TV according to a predefined statistical distribution. Figure 1A plot 100 illustrating 1000 variable volume breaths by varying pressure support is shown. Plot 100 includes line 120, which illustrates the general relationship between pressure and volume as the lungs go from fully collapsed to fully expanded. Plot 100 shows various PV loops 105 resulting from this ventilation method, each loop illustrating pressure and volume values ​​during a breath. Inset 110 shows a histogram of how TV is distributed over a sequence of 1000 breaths. For conventional mechanical ventilation (MV), a dashed loop 130 is shown, where TV is a constant value CV. The pressure support required to produce this constant value is shown as dashed line 125 in plot 115. For VV, the vertical bars in plot 115 show how pressure support is varied for each breath to produce the desired change in TV according to distribution function 110 and the relationship between pressure and volume 120. More specifically, VV delivers a variable TV for each breath, where the average value is equal to the TV that would be specified in conventional MV. The intuition behind this ventilation strategy comes from the observation that biological systems exhibit inherent fluctuations. Subsystems such as the cardiovascular and respiratory systems exhibit variability even during steady-state conditions. Breath-to-breath variations in tidal volume and respiratory rate contribute to maintaining rapid state transitions while minimizing the ratio between tissue stress and strain. The coefficient of variation of tidal volume in healthy spontaneous breathing at rest is approximately 33%. Lefevre et al. first hypothesized that the use of physiological variability in breathing patterns, as observed in a healthy resting state, could benefit patients under controlled mechanical ventilation to improve lung function and reduce damage in diseased lungs. (See Lefevre, GR et al., Improved arterial oxygenation after oleic acid lung injury in the pig using a computer-controlled mechanical ventilator. Am J Respir Crit Care Med, 1996. 154(5): p. 1567-72).

[0036] Further studies have demonstrated that VV is superior to conventional MV currently in clinical use, with improved outcomes and a reduced likelihood of ventilator-induced lung injury (VILI). VV delivers a specific set of TVs and respiratory rates (f) such that the TVs are pulled from a specific probability distribution that is optimized to best restore the collapsed area of ​​the lung, and f is matched so that the delivered minute ventilation is constant. This research has shown that VV improves gas exchange and reduces lung injury compared to conventional MV. VV may use the following mechanisms to improve performance compared to conventional MV: VV maintains an open lung; VV promotes surfactant production and release; and VV downregulates inflammation in the lung.

[0037] In previous studies, VV has only been implemented in acute settings with volume-controlled ventilation (VCV), where the volume control set point was varied breath-by-breath according to the profile of Suki et al. (see Suki, B. et al., Life-support system benefits from noise. Nature, 1998. 393(6681): p. 127-8).

[0038] Pressure controlled ventilation (PCV) has become the preferred method for chronically ventilated patients because it prevents barotrauma and prevents asynchrony during periods of bulbar activity, coughing, glottal movement, and activity. Volume assured pressure support (VAPS) mode is a hybrid of VCV and PCV in which the ventilator will target the volume set point through continuous small changes in pressure support, but will maintain the control pressure within limits set by the clinician to optimize patient comfort and safety. VAPS has also been shown to be effective in non-invasive ventilation because it has leak compensation and algorithms that tolerate variable leaks. However, the slow response of VAPS to changing respiratory physiology makes it inherently difficult to target programmed changes in target volume Vt (such as in VV) unless the VAPS mode is modified at its core to support VV.

[0039] An embodiment of an Average Volume Assured Pressure Support (AVAPS) system is described that combines the algorithms of VV and AVAPS to produce a VV algorithm that performs as closely as possible to the VV algorithm demonstrated in previous studies on modified controlled ventilation (CV) for VV. The AVAPS system strives to maintain the comfort and protection of VAPS mode and all the other benefits in chronic patients who benefit from the open lung and cellular health of VV.

[0040] AVAPS attempts to target tidal volume by adjusting pressure support in a systematic approach. Figure 2 A block diagram of the AVAPS control method is illustrated. First, the AVAPS method 200 calculates the volume error Verr as the difference between the tidal volume Vt measured at each breath and the target volume Vtarget 205 (using adder 210). The divider 215 divides the error Verror in half. Note that the error Verror can also be divided by other values ​​to provide stability and robustness in the presence of noise or interference. This factor determines the gain of the adjustment controller. The output of the divider 215 is then attenuated by a best estimate of the dynamic compliance dyn C of the ventilator using attenuator 220 to determine the change in pressure support ΔPS to be applied to achieve the change in volume. The dynamic compliance dyn C is defined as the change in lung volume per unit change in pressure support. The rate limiter 225 limits the pressure support ΔPS so that ΔPS can only change so quickly, resulting in ΔPSlim The adder 230 converts the limited pressure support ΔPS lim Added to the previous pressure support value PS n-1 . The updated pressure support value is then limited 235 based on the minimum and maximum pressure support values ​​allowed by the user. This limited pressure support value is then used to generate a breath profile to deliver the specified pressure support and positive end expiratory pressure (PEEP) 245. For example, if a PEEP value of 5 cm H20 pressure with a PS of 12 cm H20 is specified, a breath profile will be generated that starts at 5 cm H20 pressure and rises to 17 cm H20 pressure. A rise time filter 250 can be used to further smooth the waveform used to generate the breath, and the rise time filter 250 can be, for example, a low pass filter. The lungs are monitored and the tidal volume Vt is measured 255, and this is the value that is fed back to the adder 210.

[0041] We will first explain the relationship between tidal volume delivery and pressure support when the AVAPS method is updated to accommodate variable ventilation. The expected closed-form relationship between flow to the passive lung and pressure support during the inspiratory phase is given by the following equation:

[0042]

[0043] Where PS is the delivered pressure support, C is the compliance of the lung, R is the resistance of the lung, τ is the time constant of the rise time filter, and Q(t) is the patient flow over time.

[0044] The volume at the end of inspiration is approximated by the following equation:

[0045]

[0046] The above equation indicates that the volume delivered is linearly proportional to the pressure support. Note that when the patient is active, there is no closed form equation, but it is assumed that for neuromuscular patients, most of the volume comes from ventilator assist.

[0047] This suggests that it can be modified Figure 2 The AVAPS method 200 is used to support variable ventilation. Figure 3An embodiment of an AVAPS system that supports variable ventilation is illustrated. First, the AVAPS method 300 receives a target volume Vtarget 305. The VV distribution function 360 generates a variable target volume with Vtarget as the average. The AVAPS method 300 calculates the volume error Verr as the difference between the tidal volume Vt measured at each breath and the variable target volume (using adder 310). A divider 315 divides the error Verror in half. Note that the error Verror can also be divided by other values ​​to provide stability and robustness in the presence of noise or interference. This factor determines the gain of the adjustment controller. Adder 365 also adjusts the gain of the controller by subtracting the previous target volume value Vtarget from the current variable target value. n-1 The change in target volume is determined by adding the difference output by adder 365 to the output of divider 315. Adder 370 then uses attenuator 320 to attenuate the output of adder 370 by a best estimate of the ventilator's dynamic compliance, dyn C, to produce a change in pressure support, ΔPS, to be applied to achieve the change in volume. Adder 330 adds the pressure support change, ΔPS, to the previous pressure support value, PS. n-1 . The updated pressure support value is then limited 335 based on the minimum and maximum pressure support values ​​allowed by the user. This limited pressure support value is then used to generate a breath profile to deliver the specified pressure support and positive end expiratory pressure (PEEP) 345. For example, if a PEEP value of 5 cm H20 pressure with a PS of 12 cm H20 pressure is specified, a breath profile will be generated that starts at 5 cm H20 pressure and rises to 17 cm H20 pressure. A rise time filter 350 can be used to further smooth the waveform used to generate the breath, and the rise time filter 350 can be, for example, a low pass filter. The lungs are monitored and the tidal volume Vt is measured 255, and this is the value that is fed back to the adder 310.

[0048] Notice, Figure 2 The rate limiter of AVAPS system 200 in FIG2 is removed in AVAPS system 300 to allow the volume to vary with breath, but otherwise the main components, including the pressure limiter, are retained. The target is no longer a constant volume, but a volume that varies according to a specified distribution function.

[0049] Professor Bela Suki of Boston University has generated three distribution functions to be considered for variable ventilation. Figure 4A 、 Figure 4B and Figure 4C The histograms of the number of breaths per 1000 breaths at various volumes Vt for narrow distribution, medium distribution and wide distribution are shown respectively. Figure 4A 、 Figure 4B and Figure 4CAs shown, the distribution function changes over a 1000 breath sequence and a histogram of normalized volume in cc / kg ideal body weight (IBW). These various distributions are used to simulate Figure 3 Operation of the AVAPS system 300 is shown. Each of the three distributions has a mean value of 8 cc / kg. The target volume Vtarget 305 should be normalized to this ratio. Note that in the medium and wide distributions, there is a larger selected volume than in the narrow distribution.

[0050] Figure 5A 、 Figure 5B and Figure 5C Illustration of the use of passive lung Figure 4A Simulation results of the narrow distribution AVAPS system 300. The following parameters were used in the simulation: R = 20; C = 20; and Vtarget = 300 ml. Figure 5A is a plot of normalized volume versus breath number for both target VV (VVtarg) and actual volume. VVtarg is represented by a circle on the plot, and actual volume is shown by an x ​​on the plot. Figure 5B is the pressure support provided by the AVAPS system 300 with each breath. The pressure support varies from approximately 15 cm H20 to approximately 25 cm H20. Note also that these values ​​vary greatly from breath to breath, and this is possible because there is no rate limiting function in the AVAPS system 300. As can be seen in this case, the measured volume matches VVtarget very well because the circle and x on the plot are very closely aligned. Figure 5C is a histogram of the number of breaths at each normalized measured lung volume. Analysis of this data shows that an average tidal volume of 300.1 ml was achieved, compared to a Vtarget of 300 ml. The AVAPS system 300 achieved the desired average Vtarget value. Because a passive test lung was used, it was an easy situation to provide the required pressure support to achieve the desired volume with each breath. The use of a passive lung does accurately model neuromuscular patients, as they do not have a large respiratory drive.

[0051] Note that when the patient is ambulating, the same control of tidal volume will not produce the same results as with a passive lung. However, due to the additional control loop in the AVAPS system 300, the AVAPS system 300 can maintain the average tidal volume.

[0052] Figure 6A 、 Figure 6B and Figure 6C Illustration of the use of assisted breathing with fixed rate and fixed muscle effort, using Figure 4ASimulation results for the AVAPS system 300 with a narrow distribution of . Thus, in this scenario, the patient performs some of their own breathing and is assisted in other breathing. The following parameters were used in the simulation: R = 20; C = 20; and Vtarget = 300 ml. Figure 6A is a plot of normalized volume versus breath number for both target VV (VVtarg) and actual volume. As before, VVtarg is represented by a circle on the plot, and actual volume is shown by an x ​​on the plot. Figure 6B is the pressure support provided by the AVAPS system 300 during each breath. The pressure support varies from approximately 13 cm H20 to approximately 23 cm H20. As can be seen in this case, the measured volume is close to the VVtarget value, but there is some variation as can be seen where the circle and x on the plot do not perfectly align. Figure 6C is a histogram of the number of breaths at each normalized measured lung volume. Analysis of this data shows that an average tidal volume of 300.3 ml was achieved, compared to a Vtarget of 300 ml. Again, the AVAPS system 300 achieved the desired average Vtarget value.

[0053] Figure 7A 、 Figure 7B and Figure 7C Illustration of the use of assisted breathing with fixed rate and fixed muscle effort, using Figure 4C Simulation results for the AVAPS system 300 with a wide distribution of . Thus, in this scenario, the patient performs some breaths on their own and is assisted in other breaths. The following parameters were used in the simulation: R = 20; C = 20; and Vtarget = 300 ml. Figure 7A is a plot of normalized volume versus breath number for both target VV (VVtarg) and actual volume. As before, VVtarg is represented by a circle on the plot, and actual volume is shown by an x ​​on the plot. Figure 7B is the pressure support provided by the AVAPS system 300 during each breath. The pressure support varies from approximately 11 cm H20 to approximately 32 cm H20. As can be seen in this case, the measured volume is close to the VVtarget value, but there is some variation as can be seen where the circle and x on the plot do not perfectly align. Figure 7C is a histogram of the number of breaths at each normalized measured lung volume. Analysis of this data shows that an average tidal volume of 299.9 ml was achieved, compared to a Vtarget of 300 ml. Again, the AVAPS system 300 achieved the desired average Vtarget value.

[0054] Various other simulations using different patient parameters and distributions showed that the mean Vtarget value was within 1 ml of a Vtarget value of 300. This demonstrates that the AVAPS system 300 achieves the desired mean tidal volume.

[0055] The AVAPS system 300 can be implemented using hardware to perform Figure 3 The hardware may be an integrated circuit that receives various input data and provides control outputs to the ventilator.

[0056] The AVAPS system 300 may also be implemented using a processor and software instructions on the processor to perform Figure 3 The functions shown. Figure 8 The diagram shows Figure 3 FIG8 is an exemplary hardware diagram 800 of an AVAPS system. As shown, the device 800 includes a processor 820, a memory 830, a user interface 840, a network interface 850, and a storage device 860 interconnected via one or more system buses 810. It will be understood that Figure 8 Some aspects constitute an abstraction, and the actual organization of components of device 800 may be more complex than illustrated.

[0057] The processor 820 may be any hardware device capable of executing instructions stored in the memory 830 or the storage device 860 or otherwise processing data. Thus, the processor may include a microprocessor, a graphics processing unit (GPU), a field programmable gate array (FPGA), an application-specific integrated circuit (ASIC), any processor capable of parallel computing, or other similar devices. The processor may also be a specialized processor that implements a machine learning model.

[0058] The memory 830 may include various memories, such as, for example, L1, L2, or L3 cache or system memory. Thus, the memory 830 may include static random access memory (SRAM), dynamic RAM (DRAM), flash memory, read-only memory (ROM), or other similar memory devices.

[0059] The user interface 840 may include one or more devices for enabling communication with a user and may present information to the user. For example, the user interface 840 may include a display, a touch interface, a mouse, and / or a keyboard for receiving user commands. In some embodiments, the user interface 840 may include a command line interface or a graphical user interface that may be presented to a remote terminal via the network interface 850.

[0060] The network interface 850 may include one or more devices for enabling communication with other hardware devices. For example, the network interface 850 may include a network interface card (NIC) configured to communicate according to the Ethernet protocol or other communication protocols including wireless protocols. In addition, the network interface 850 may implement a TCP / IP stack for communicating according to the TCP / IP protocol. Various alternative or additional hardware or configurations for the network interface 850 will be apparent.

[0061] The storage device 860 may include one or more machine-readable storage media, such as read-only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, or similar storage media. In various embodiments, the storage device 860 may store instructions for execution by the processor 820 or data that the processor 820 can operate on. For example, the storage device 860 may store a basic operating system 861 for controlling various basic operations of the hardware 800. The storage device 862 may store instructions for implementing the AVAPS system.

[0062] It will be apparent that various information described as being stored in storage device 860 may additionally or alternatively be stored in memory 830. In this regard, memory 830 may also be considered to constitute a "storage device," and storage device 860 may be considered to be a "memory." Various other arrangements will be apparent. Furthermore, both memory 830 and storage device 860 may be considered to be "non-transitory machine-readable media." As used herein, the term "non-transitory" will be understood to exclude transient signals, but to include all forms of storage devices, including both volatile and non-volatile memory.

[0063] Although the system 800 is shown as including one component of each described component, various components may be replicated in various embodiments. For example, the processor 820 may include multiple microprocessors that are configured to independently execute the methods described herein or are configured to execute the steps or subroutines of the methods described herein, such that the multiple processors collaborate to implement the functions described herein. Such multiple processors may be of the same or different types. In addition, in the case where the device 800 is implemented in a cloud computing system, the various hardware components may belong to separate physical systems. For example, the processor 820 may include a first processor in a first server and a second processor in a second server.

[0064] The AVAPS system described herein provides a technical improvement over existing AVAPS systems by allowing variable volume ventilation to be used in conjunction with AVAPS. As described above, the use of variable volume ventilation has various benefits for patients, and these benefits are combined with the benefits of AVAPS. Thus, better ventilation therapy can be provided to patients.

[0065] Any combination of specific software running on a processor to implement embodiments of the invention constitutes a specific special purpose machine.

[0066] As used herein, the term "non-transitory machine-readable storage medium" will be understood to exclude transitory propagating signals, but to include all forms of volatile and non-volatile memory.

[0067] Although various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it will be understood that the present invention is capable of other embodiments and that its details are capable of modification in various obvious respects. As will be readily apparent to those skilled in the art, variations and modifications may be made while remaining within the spirit and scope of the present invention. Thus, the foregoing disclosure, description, and drawings are for illustrative purposes only and do not in any way limit the present invention, which is defined solely by the claims.

Claims

1. A controller configured to control a ventilator to provide a variable volume (VV) with mean volume assured pressure support (AVAPS), the controller comprising: Memory (830); a processor (820) coupled to the memory, wherein the processor is further configured to: Use the VV distribution function to generate the VV target volume; generating a volume error, Verror, which is the difference between the VV target volume and a measured volume of a previous breath; Scaling the volume error Verror; generating a VV target difference as a difference between a VV target volume and the VV target volume of the previous breath; generating a modified volumetric error by adding the VV target difference to the scaled volumetric error Verror; generating a delta pressure support ΔPS based on the modified volume error and dynamic compliance; as well as A current pressure support value is generated based on the delta pressure support ΔPS and the pressure support value of the previous breath.

2. The controller of claim 1 , wherein generating delta pressure support ΔPS further comprises: The modified volume error is divided by the dynamic compliance.

3. The controller of claim 2, wherein the volume error Verror is scaled by a factor of 2.

4. The controller of claim 1, wherein the processor (820) is further configured to limit the current pressure support by a minimum pressure support value and a maximum pressure support value.

5. The controller of claim 1, wherein the processor (820) is further configured to generate a breathing profile based on the current pressure support and positive end-expiratory pressure (PEEP) values.

6. The controller of claim 5, wherein the processor (820) is further configured to perform rise time filtering on the respiration profile.

7. The controller of claim 6, wherein the processor (820) is further configured to apply the breathing profile to the user's lungs and measure the volume of the resulting breath.

8. The controller of claim 1, wherein the VV distribution function has a mean value based on the received target volume value.

9. The controller of claim 1, wherein the VV distribution function is selectable from a plurality of different VV distribution functions.

10. A ventilator for delivering variable ventilation with mean volume assured pressure support (AVAPS), the ventilator comprising a controller according to any one of claims 1 to 9.

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