System and method for quantifying lung compliance in a self-ventilating subject

The lung compliance of a subject that is at least partially self-ventilating is determined. The quantification of lung compliance may be an estimation, a measurement, and/or an approximate measurement. The quantification of lung compliance may be enhanced over conventional techniques and/or systems for quantifying lung compliance of self-ventilating subjects in the lung compliance may be quantified relatively accurately without an effort belt or other external sensing device that directly measures diaphragmatic muscle pressure, and without requiring the subject to manually control diaphragmatic muscle pressure. Quantification of lung compliance may be a useful tool in evaluating the health of the subject, including detection of fluid retention associated with developing acute congestive heart failure.

The invention relates to the quantification of lung compliance in a self-ventilating subject.

Systems for quantifying (e.g., measuring, estimating, etc.) lung compliance in subjects are known. Such systems include ventilator systems configured to mechanically ventilate subjects completely. These systems may be implemented, for example, with subjects that are incapable of self-ventilation.

The quantification of lung compliance in a self-ventilating subject is dependent in part on diaphragmatic muscle pressure during respiration. As such, some systems configured to quantify lung compliance in subjects that are self-ventilating require the implementation of an effort belt, or some other sensor that provides a direct measurement of diaphragmatic muscle pressure. Other systems configured to quantify lung compliance in self-ventilating subjects require that the subject be directed and/or taught to control diaphragmatic muscle pressure manually. However, this typically requires the subject and/or doctor to perform a special maneuver which, if not performed with precision, may negatively impact the precision and/or accuracy of the estimation of lung compliance.

One aspect of the invention is related to a system configured to quantify lung compliance of a subject that is at least partially self-ventilating. In one embodiment, the system comprises a pressure support device, one or more sensors, and one or more processors. The pressure support device configured to generate a pressurized flow of breathable gas to be delivered to the airway of a subject that is at least partially self-ventilating. The one or more sensors are configured to generate one or more output signals conveying information about one or more parameters of the pressurized flow of breathable gas. The one or more processors is operatively linked with the pressure support device and the one or more sensors, and is configured to execute one or more computer program modules. The one or more computer program modules comprise a control module, a pressure module, a transition module, and a compliance module. The control module is configured to control the pressure support device to adjust pressure of the pressurized flow of breathable gas during a series of consecutive breaths of the subject. The pressure module is configured to determine the pressure to which the pressurized flow of breathable gas should be adjusted by the control module during the series of consecutive breaths such that for a first inhalation the pressure is adjusted to a first pressure and for a second inhalation proximate in time to the first inhalation the pressure is adjusted to a second pressure that is different from the first pressure. The transition module is configured to identify a first transition point of the first inhalation and a second transition point of the second inhalation based on the one or more output signals generated by the one or more sensors, wherein the first transition point is identified at or near a point in time at which peak flow of the pressurized flow of breathable gas occurs during the first inhalation and the second transition point is identified at or near a point in time at which the peak flow of the pressurized flow of breathable gas occurs during the second inhalation. The compliance module is configured to quantify lung compliance of the subject based on the difference between the first pressure and the second pressure and the one or more output signals generated by the one or more sensors during the first inhalation and the second inhalation, wherein for the purposes of quantifying lung compliance the compliance module considers the first inhalation to have begun at the first transition point and considers the second inhalation to have begun at the second transition point

Another aspect of the invention relates to a method of quantifying lung compliance of a subject that is at least partially self-ventilating. In one embodiment, the method comprises delivering a pressurized flow of breathable gas to the airway of a subject that is at least partially self-ventilating; generating one or more output signals conveying information about one or more parameters of the pressurized flow of breathable gas; determining pressures to which the pressurized flow of breathable gas should be adjusted during a series of consecutive breaths of the subject, including determining a first pressure for a first inhalation and determining a second pressure that is different from the first pressure for a second inhalation proximate in time to the first inhalation; adjusting the pressure of the pressurized flow of breathable gas to the determined pressures during the series of consecutive breaths; identifying, based on the one or more output signals, a first transition point at or near a point in time at which peak flow of the pressurized flow of breathable gas occurs during the first inhalation; identifying, based on the one or more output signals, a second transition point at or near a point in time at which peak flow of the pressurized flow of breathable gas occurs during the second inhalation; and quantifying lung compliance of the subject based on the difference between the first pressure and the second pressure and the one or more output signals generated during the first inhalation and the second inhalation, wherein for the purposes of quantifying lung compliance the first inhalation is considered to have begun at the first transition point and the second inhalation is considered to have begun at the second transition point.

Another aspect of the invention relates to a system configured to quantify lung compliance of a subject that is at least partially self-ventilating. In one embodiment, the system comprises means for delivering a pressurized flow of breathable gas to the airway of a subject that is at least partially self-ventilating; means for generating one or more output signals conveying information about one or more parameters of the pressurized flow of breathable gas; means for determining pressures to which the pressurized flow of breathable gas should be adjusted during a series of consecutive breaths of the subject, including determining a first pressure for a first inhalation and determining a second pressure that is different from the first pressure for a second inhalation proximate in time to the first inhalation; means for adjusting the pressure of the pressurized flow of breathable gas to the determined pressures during the series of consecutive breaths; means for identifying, based on the one or more output signals, a first transition point at or near a point in time at which peak flow of the pressurized flow of breathable gas occurs during the first inhalation; means for identifying, based on the one or more output signals, a second transition point at or near a point in time at which peak flow of the pressurized flow of breathable gas occurs during the second inhalation; and means for quantifying lung compliance of the subject based on the difference between the first pressure and the second pressure and the one or more output signals generated during the first inhalation and the second inhalation, wherein for the purposes of quantifying lung compliance the first inhalation is considered to have begun at the first transition point and the second inhalation is considered to have begun at the second transition point.

FIG. 1illustrates a system10configured to quantify lung compliance of a subject12that is at least partially self-ventilating. The quantification of lung compliance may be an estimation, a measurement, and/or an approximate measurement. The quantification of lung compliance by system10may be enhanced over conventional systems for quantifying lung compliance of self-ventilating subjects in that system10may quantify lung compliance relatively accurately without an effort belt or other external sensing device that directly measures diaphragmatic muscle pressure. Quantification of lung compliance may be a useful tool in evaluating the health of subject12, including detection of fluid retention associated with developing acute congestive heart failure. In one embodiment, system10includes one or more of a pressure support device14, electronic storage16, a user interface18, one or more sensors20, a processor22, and/or other components.

In one embodiment, pressure support device14is configured to generate a pressurized flow of breathable gas for delivery to the airway of subject12. The pressure support device14may control one or more parameters of the pressurized flow of breathable gas (e.g., flow rate, pressure, volume, humidity, temperature, composition, etc.) for therapeutic purposes, or for other purposes. By way of non-limiting example, pressure support device14may be configured to control the pressure of the pressurized flow of breathable gas to provide pressure support to the airway of subject12. The pressure support device14may include a positive pressure support device such as, for example, the device described in U.S. Pat. No. 6,105,575, hereby incorporated by reference in its entirety.

The pressure support device14may be configured to generate the pressurized flow of breathable gas according to one or more modes. A non-limiting example of one such mode is Continuous Positive Airway Pressure (CPAP). CPAP has been used for many years and has proven to be helpful in promoting regular breathing. Another mode for generating the pressurized flow of breathable gas is Inspiratory Positive Air Pressure (IPAP). One example of the IPAP mode is bi-level positive air pressure mode (BIPAP®). In bi-level positive air pressure mode, two levels of positive air pressure (HI and LO) are supplied to a patient. Other modes of generating the pressurized flow of breathable gas are contemplated. Generally, the timing of the HI and LO levels of pressure are controlled such that the HI level of positive air pressure is delivered to subject12during inhalation and the LO level of pressure is delivered to subject12during exhalation.

The pressurized flow of breathable gas is delivered to the airway of subject12via a subject interface24. Subject interface24is configured to communicate the pressurized flow of breathable gas generated by pressure support device14to the airway of subject12. As such, subject interface24includes a conduit26and an interface appliance28. Conduit conveys the pressurized flow of breathable gas to interface appliance28, and interface appliance28delivers the pressurized flow of breathable gas to the airway of subject12. Some examples of interface appliance28may include, for example, an endotracheal tube, a nasal cannula, a tracheotomy tube, a nasal mask, a nasal/oral mask, a full face mask, a total face mask, or other interface appliances that communication a flow of gas with an airway of a subject. The present invention is not limited to these examples, and contemplates delivery of the pressurized flow of breathable gas to subject12using any subject interface.

In one embodiment, electronic storage16comprises electronic storage media that electronically stores information. The electronically storage media of electronic storage16may include one or both of system storage that is provided integrally (i.e., substantially non-removable) with system10and/or removable storage that is removably connectable to system10via, for example, a port (e.g., a USB port, a firewire port, etc.) or a drive (e.g., a disk drive, etc.). Electronic storage16may include one or more of optically readable storage media (e.g., optical disks, etc.), magnetically readable storage media (e.g., magnetic tape, magnetic hard drive, floppy drive, etc.), electrical charge-based storage media (e.g., EEPROM, RAM, etc.), solid-state storage media (e.g., flash drive, etc.), and/or other electronically readable storage media. Electronic storage16may store software algorithms, information determined by processor22, information received via user interface18, and/or other information that enables system10to function properly. Electronic storage16may be (in whole or in part) a separate component within system10, or electronic storage16may be provided (in whole or in part) integrally with one or more other components of system10(e.g., device14, user interface18, processor22, etc.).

User interface18is configured to provide an interface between system10and subject12through which subject12may provide information to and receive information from system10. This enables data, results, and/or instructions and any other communicable items, collectively referred to as “information,” to be communicated between the subject12and one or more of device14, electronic storage16, and/or processor22. Examples of interface devices suitable for inclusion in user interface18include a keypad, buttons, switches, a keyboard, knobs, levers, a display screen, a touch screen, speakers, a microphone, an indicator light, an audible alarm, a printer, and/or other interface devices. In one embodiment, user interface18includes a plurality of separate interfaces. In one embodiment, user interface18includes at least one interface that is provided integrally with device14.

It is to be understood that other communication techniques, either hard-wired or wireless, are also contemplated by the present invention as user interface18. For example, the present invention contemplates that user interface18may be integrated with a removable storage interface provided by electronic storage16. In this example, information may be loaded into system10from removable storage (e.g., a smart card, a flash drive, a removable disk, etc.) that enables the user(s) to customize the implementation of system10. Other exemplary input devices and techniques adapted for use with system10as user interface18include, but are not limited to, an RS-232 port, RF link, an IR link, modem (telephone, cable or other). In short, any technique for communicating information with system10is contemplated by the present invention as user interface18.

One or more sensors20are configured to generate one or more output signals conveying information related to one or more parameters of the pressurized flow of breathable gas. The one or more parameters may include, for example, one or more of a flow rate, a volume, a pressure, a composition (e.g., concentration(s) of one or more constituents), humidity, temperature, acceleration, velocity, acoustics, changes in a parameter indicative of respiration, and/or other gas parameters. The sensors20may include one or more sensors that measure such parameters directly (e.g., through fluid communication with the pressurized flow of breathable gas at pressure support device14or in subject interface24). The sensors20may include one or more sensors that generate output signals related to one or more parameters of the pressurized flow of breathable gas indirectly. For example, one or more of sensors20may generate an output based on an operating parameter of pressure support device14(e.g., a motor current, voltage, rotational velocity, and/or other operating parameters), and/or other sensors. Although sensors20are illustrated at a single location at or adjacent to pressure support device14, this is not intended to be limiting. The sensors20may include sensors disposed in a plurality of locations, such as for example, within pressure support device14, within (or in communication with) conduit26, within (or in communication with) interface appliance28, and/or other locations.

Processor22is configured to provide information processing capabilities in system10. As such, processor22may include one or more of a digital processor, an analog processor, a digital circuit designed to process information, an analog circuit designed to process information, a state machine, and/or other mechanisms for electronically processing information. Although processor22is shown inFIG. 1as a single entity, this is for illustrative purposes only. In some implementations, processor22may include a plurality of processing units. These processing units may be physically located within the same device (e.g., pressure support device14), or processor22may represent processing functionality of a plurality of devices operating in coordination.

As is shown inFIG. 1, processor22may be configured to execute one or more computer program modules. The one or more computer program modules may include one or more of a breathing parameter module30, a control module32, a pressure module34, a transition module36, a compliance module38, and/or other modules. Processor22may be configured to execute modules30,32,34,36, and/or38by software; hardware; firmware; some combination of software, hardware, and/or firmware; and/or other mechanisms for configuring processing capabilities on processor22.

It should be appreciated that although modules30,32,34,36, and38are illustrated inFIG. 1as being co-located within a single processing unit, in implementations in which processor22includes multiple processing units, one or more of modules30,32,34,36, and/or38may be located remotely from the other modules. The description of the functionality provided by the different modules30,32,34,36, and/or38described below is for illustrative purposes, and is not intended to be limiting, as any of modules30,32,34,36, and/or38may provide more or less functionality than is described. For example, one or more of modules30,32,34,36, and/or38may be eliminated, and some or all of its functionality may be provided by other ones of modules30,32,34,36, and/or38. As another example, processor22may be configured to execute one or more additional modules that may perform some or all of the functionality attributed below to one of modules30,32,34,36, and/or38.

The breathing parameter module30is configured to determine one or more breathing parameters of the subject. The one or more breathing parameters are determined based on the one or more output signals generated by sensors20. The one or more breathing parameters may include, for example, a tidal volume, a peak flow, a flow rate, a pressure, a composition, a timing (e.g., beginning and/or end of inhalation, beginning and/or end of exhalation, etc.), a duration (e.g., of inhalation, of exhalation, of a single breathing cycle, etc.), a breath rate, a respiration frequency, and/or other parameters. In one embodiment, breathing parameter module30determines the one or more breathing parameter on a per inhale and/or exhale basis. By way of non-limiting example, breathing parameter module30may determine at least one given breathing parameters for each exhalation in a series of consecutive exhalations. The at least one given breathing parameter may include, for instance, a tidal volume, a peak flow, and/or other breathing parameters.

The control module32is configured to control pressure support device14to adjust one or more parameters of the pressurized flow of breathable gas. For example, control module32may control pressure support device14to adjust a flow rate, pressure, volume, humidity, temperature, composition, and/or other parameters of the pressurized flow of breathable gas. In one embodiment, control module32controls pressure support device14to operate in a bi-level positive air pressure mode where pressure is elevated to a HI level during inhalation and reduced to a LO level during exhalation by subject12. The control module32may determine when to trigger changes from HI to LO and vice versa based on detection of breathing transitions by breathing parameter module30.

The pressure module34is configured to determine the pressure(s) to which the pressurized flow of breathable gas should be adjusted by control module32. The pressure of the pressurized flow of breathable gas may be determined by pressure module34based on a therapy regime (e.g., for positive airway pressure support), to enable a quantification of lung compliance, and/or for other purposes. Determining the pressure(s) to which the pressurized flow of breathable gas should be adjusted includes determining the HI and LO pressure levels for a bi-level positive air pressure mode.

As is discussed further below, in order to enable a quantification of lung compliance, the pressure of pressurized flow of breathable gas should be changed between a pair of inhalations that are proximate to each other in time. As used herein, the pair of inhalations that are proximate in time to each other may include a pair of inhalations that are directly adjacent (i.e., consecutive without intervening inhalations), or a pair of inhalations that are reasonably close to each other in time (e.g., within about 2 minutes, within about 1 minute, within about 30 second, within about 15 seconds, etc.). To facilitate such a determination, pressure module34is configured to determine a first pressure to which the pressurized flow of breathable gas should be adjusted during a first inhalation, and a second pressure (different from the first pressure) to which the pressurized flow of breathable gas should be adjusted during a second inhalation that is proximate in time to the first inhalation.

It will be appreciated that in some embodiments, the quantification of lung compliance may be based on measurements taken in two breaths that are not proximate in time, and for which the pressure of the pressurized flow of breathable gas is different. Although this may degrade the accuracy and/or precision of the quantification (due to assumptions made about patient physiology and/or respiratory conditions during the two breaths), such degradation may not be fatal to the usefulness of the quantification.

In embodiments in which system10is operating in a bi-level positive air pressure mode, control module32implements the first pressure as the HI pressure for the first inhalation, a LO pressure (determined by pressure module34) for the exhalation(s) between the first inhalation and the second inhalation, and the second pressure as the HI pressure for the second inhalation. By way of illustration,FIG. 2illustrates a plot of pressure as determined by a pressure module similar to or the same as pressure module34vs. time over a series of consecutive breaths. During the series of consecutive breaths, pressure module34determines pressure of the pressurized flow of breathable gas in accordance with a bi-level positive air pressure mode in which pressure is reduced to a LO level40during exhalations. In the plot shown inFIG. 2, there a number of pairs of directly adjacent pairs of inhalations that could be viewed as the first and second inhalations described above. These pairs are labeled inFIG. 2with reference numeral42.

Returning toFIG. 1, transition module36is configured to identify a first transition point of the first inhalation and a second transition point of the second inhalation. The first transition point is a point in time at or near the peak flow of the pressurized flow of breathable gas during the first inhalation. The second transition point is a point in time at or near the peak flow of the pressurized flow of breathable gas during the second inhalation. Transition module36is configured to identify the first and second transition points based on at least one of the breathing parameters determined by breathing parameter module30(which are determined based on the output signals generated by sensors20).

It will be appreciated that in a bi-level positive air pressure mode, as control module32controls pressure support device14to transition from a LO pressure to a HI pressure (e.g., at the commencement of each of the first inhalation and the second inhalation), the pressure is not increased in an ideal step. In other words, for practical purposes, the pressure cannot be controlled to instantaneously change from the LO pressure to the HI pressure. Although this transition may be completed in a relatively short amount of time, there is still some period of transition during which pressure is approaching the HI pressure from below.

In the quantification of lung compliance set forth below, the transition between LO pressure and HI pressure at the commencement of each of the first inhalation and the second inhalation is assumed to be ideal (e.g., instantaneous). This assumption may lead to inaccuracy and/or imprecision in the quantification of lung compliance. However, if the first and second transition points are taken by compliance module38to be the beginning of the first and second inhalations, respectively, at least some of the inaccuracy and/or imprecision due to the non-ideal pressure step is eliminated.

By way of illustration,FIGS. 3A-3Cshow plots of volume difference or the instantaneous difference in volume between the two inhalations, flow difference or the instantaneous difference in flow between the two inhalations, and pressure difference or the instantaneous difference in pressure between the two inhalations as a function of time for the two inhalations used to quantify compliance. As can be seen inFIGS. 3A-3C, the measured values of volume difference, flow difference, and pressure difference lag the ideal values by an amount of time between the beginning of the inhalation and a transition point44at which peak value for flow difference between the two inhalations occurs.FIGS. 3A-3Calso illustrate how if volume difference, flow difference, and pressure difference are shifted by the amount of time between the beginning of the inhalation and transition point44(e.g., by considering transition point44to be the beginning of the inhalation), the measured values correspond much more closely to the ideal values.

Returning toFIG. 1, compliance module38is configured to quantify lung compliance of subject12based on the difference between the first pressure and the second pressure, and the one or more output signals generated by sensors20during the first and second inhalations. In one embodiment, compliance module38determines the lung compliance of subject12by removing diaphragmatic muscle pressure from input-output equations modeling the respiratory system of subject12during the first inhalation and the second inhalation.

In one embodiment, the quantification of lung compliance by compliance module38implements a single-compartment lung and ventilator circuit shown inFIG. 4. InFIG. 4, Pdrepresents device pressure (e.g., the pressure of the pressurized flow of breathable gas generated by pressure support device14), R represents the resistance of the respiratory system of a subject, Palvrepresents alveolar pressure, C represents compliance, Pmusrepresents diaphragmatic muscle pressure, and Qprepresents the subject flow. In this model, it is assumed that the resistance of an exhalation port (e.g., exhalation port at interface appliance28inFIG. 1) is much greater than a resistance of a hose (e.g., conduit26inFIG. 1). Therefore, the pressure within the subject is approximately the same as the device pressure. Thus, subject pressure is simply represented as the device pressure in the circuit shown inFIG. 4. Further, it is assumed that the patient flow and patient volume can be estimated by using the difference between the measure total flow of the system and an estimated (or measured) leak flow.

It will be appreciated that the implementation of a single-compartment lung model in the description of determining lung compliance is not intended to be limiting. The removal of diaphragmatic muscle pressure from equations modeling the function of the respiratory system of a subject is not dependent on this model, but is used herein because it is computationally less expensive than more complex models, and simplifies explanation.

The transfer function in the s-domain relating patient flow to pressure of the device and the diaphragm of the subject for the circuit inFIG. 3is given by:

Additionally, the patient volume is given by the equation:

Thus, the transfer function relating the pressure to patient volume is given by the equation:

V⁡(s)P⁡(s)=CRCs+1⇒V⁡(s)=CRCs+1⁢Pd⁡(s)+CRCs+1⁢Pmus⁡(s);(4)
where the response to Pmusis given by the equation:

Vint⁡(s)=CRcs+1⁢Pmus⁡(s);(5)
and where the external response is given by:

Now, if Pd(s) represents the pressure of the pressurized flow of breathable gas generated by a pressure support device, and the pressure during inhalation is varied between a first inhalation and a second inhalation that are proximate in time (e.g., directly adjacent), then equation (4) can be written for the first inhalation and the second inhalation in the following form:

V1⁡(s)=CRCs+1⁢Pd⁢⁢1⁡(s)+CRCs+1⁢Pmus⁢⁢1⁡(s);and(7)V2⁡(s)=CRCs+1⁢Pd⁢⁢2⁡(s)+CRCs+1⁢Pmus⁢⁢2⁡(s);(8)
where subscripts 1 and 2 correspond to the first inhalation and the second inhalation, respectively.

Since Pmusis unknown, the part of the total response associated with the internal response is also unknown. However, if the assumption is made that Pmusis relatively constant between the first inhalation and the second inhalation (since the first and second inhalations are proximate in time), then Pmus1(s) can be assumed to be equal to Pmus2(s).

Taking the difference between the volume responses in equations (5) and (6), and using the assumption that Pmus1(s) is equal to Pmus2(s), the unknown internal response can be eliminated to yield the following combination of equations (7) and (8):

Δ⁢⁢V⁡(s)=CRCs+1⁢Δ⁢⁢Pd⁡(s);(9)
where ΔV(s) is the difference between V1(s) and V2(s), and where ΔPd(s) is the difference between Pd1(s) and Pd2(s).

Since the first pressure and the second pressure are different, ΔPd(s) can be represented by a step excitation of the form:

If equation (10) is substituted into equation (9), the result can be simplified as follows:

Transformation of equation (11) into the time domain results in the following equation:
Δv(t)=C·ΔPd(1−e−t/RC)u(t);
where Δv(t) represents volume difference.

Since the pressures and volumes for the two inhalations (and/or the instantaneous differences therebetween) are known, any one of various known numerical estimation techniques can be used to determine resistance R and compliance C. By way of non-limiting example, the technique of least squared error could be implemented.

Returning toFIG. 1, compliance module38may quantify lung compliance based on breathing parameter(s) determined by breathing parameter module30(which are determined from output signals generated by sensors20), the known value(s) of the first pressure, the second pressure, and/or the difference between the first pressure and the second pressure in the manner described above. In order to reduce imprecision and/or inaccuracy caused by non-ideal pressure steps at the commencement of the first inhalation and the second inhalation, compliance module38may consider the first transition point and second transition point identified by transition module36to be the beginning of the first inhalation and the second inhalation, respectively. The quantification of lung compliance by compliance module38may then be implemented for one or more of a variety of different uses and/or in a variety of different contexts. For example, the quantification of lung compliance may be implemented to preemptively diagnose congestive heart failure, to prescribe treatment, and/or for other purposes.