Patent Publication Number: US-9895083-B2

Title: Non-invasive ventilation measurement

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/IB2012/053777, filed on Jul. 25, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/527,186, filed on Aug. 25, 2011. These applications are hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure pertains to systems and methods for measuring respiratory parameters of a subject during application of non-invasive respiratory therapy, and, in particular, to measuring lung compliance and lung resistance on a breath-by-breath basis during spontaneous breathing and/or non-spontaneous breathing of the subject undergoing respiratory therapy. 
     2. Description of the Related Art 
     It is well known that the proper administration of respiratory therapy hinges on having accurate and up-to-date information regarding the lung mechanics of a patient (a.k.a a subject). Lung mechanics may include lung compliance, lung resistance, and/or other parameters. Respiratory therapy involving invasive mechanical ventilation is well-known, and during application of such therapy a monitor device can measure, e.g., subject flow in order to determine, e.g., lung compliance and/or lung resistance. Respiratory therapy involving non-invasive mechanical ventilation is well-know. However, during application of this type of therapy accurate and up-to-date information regarding the lung mechanics of a subject cannot usually be determined due to leaks in the system. Leaks may be intentional or non-intentional, known or unknown, variable or non-variable. In particular, non-intentional, unknown, and/or variable leaks may preclude the determination of accurate and up-to-date information regarding the lung mechanics of a subject. 
     SUMMARY 
     Accordingly, it is an object of one or more embodiments of the present disclosure to provide a system configured to measure respiratory parameters during application of non-invasive respiratory therapy of a subject. The system includes a timing module, a passive compliance module, a passive resistance module, an active compliance module, and an active resistance module. The timing module is configured to determine transitions between inhalations and exhalations. The passive compliance module is configured to determine a passive exhalation lung compliance during an exhalation, wherein the determination by the passive compliance module is based on a passive lung model, and wherein the exhalation is demarcated based on the transitions. The passive resistance module is configured to determine a passive exhalation lung resistance during the exhalation, wherein the determination by the passive resistance module is based on a passive lung model. The active compliance module is configured to determine an active lung compliance that reflects an active lung model, wherein the active lung compliance is determined based on the passive exhalation lung compliance and a subject breathing mode. The active resistance module is configured to determine an active lung resistance that reflects an active lung model, wherein the active lung resistance is determined based on the passive exhalation lung resistance and the subject breathing mode. 
     It is yet another aspect of one or more embodiments of the present disclosure to provide a method for measuring respiratory parameters during non-invasive respiratory therapy of a subject. The method comprises determining transitions in breathing between inhalations and exhalations; determining a passive exhalation lung compliance, during an exhalation, based on a passive lung model, wherein the exhalation is demarcated based on the transitions; determining a passive exhalation lung resistance, during the exhalation, based on the passive lung model; obtaining a subject breathing mode; determining an active lung compliance that reflects an active lung model, wherein the determination is based on the passive exhalation lung compliance and the subject breathing mode; and determining an active lung resistance that reflects an active lung model, wherein the determination is based on the passive exhalation lung resistance and the subject breathing mode. 
     It is yet another aspect of one or more embodiments to provide a system configured for measuring respiratory parameters during application of non-invasive respiratory therapy of a subject. The system comprises means for determining transitions in breathing between inhalations and exhalations; means for determining a passive exhalation lung compliance, during an exhalation, based on a passive lung model, wherein the exhalation is demarcated based on the transitions; means for determining a passive exhalation lung resistance, during the exhalation, based on the passive lung model; means for obtaining a subject breathing mode; means for determining an active lung compliance, reflecting an active lung model, based on the passive exhalation lung compliance and the subject breathing mode; and means for determining an active lung resistance, reflecting an active lung model, based on the passive exhalation lung resistance and the subject breathing mode. 
     These and other objects, features, and characteristics of the present disclosure, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals may designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of any limits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a system configured to measure respiratory parameters during application of non-invasive respiratory therapy of a subject, according to certain embodiments; and 
         FIG. 2  illustrates a method for measuring respiratory parameters during non-invasive respiratory therapy of a subject, according to certain embodiments. 
         FIG. 3A-3B  illustrate standard lung models. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     As used herein, the singular form of “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. As used herein, the statement that two or more parts or components are “coupled” shall mean that the parts are joined or operate together either directly or indirectly, i.e., through one or more intermediate parts or components, so long as a link occurs. As used herein, “directly coupled” means that two elements are directly in contact with each other. As used herein, “fixedly coupled” or “fixed” means that two components are coupled so as to move as one while maintaining a constant orientation relative to each other. 
     As used herein, the word “unitary” means a component is created as a single piece or unit. That is, a component that includes pieces that are created separately and then coupled together as a unit is not a “unitary” component or body. As employed herein, the statement that two or more parts or components “engage” one another shall mean that the parts exert a force against one another either directly or through one or more intermediate parts or components. As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality). 
     Directional phrases used herein, such as, for example and without limitation, top, bottom, left, right, upper, lower, front, back, and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein. 
       FIG. 1  schematically illustrates a system  100  configured to measure respiratory parameters during application of non-invasive respiratory therapy of a subject  106 , according to certain embodiments. System  100  may be implemented as, integrated with, and/or operating in conjunction with a respiratory therapy device. 
     System  100  may include one or more of a pressure generator  140 , a subject interface  180 , one or more sensors  142 , an electronic storage  130 , a user interface  120 , a processor  110 , a timing module  111 , a passive compliance module  112 , a passive resistance module  113 , an active compliance module  114 , an active resistance module  115 , a flow module  116 , and/or other components. 
     Pressure generator  140  may be integrated, combined, or connected with a ventilator and/or (positive) airway pressure device (PAP/CPAP/BiPAP®/etc.) and configured to provide a pressurized flow of breathable gas for delivery to the airway of subject  106 , e.g. via subject interface  180 . Subject  106  may or may not initiate one or more phases of respiration. Pressure support may be implemented as a higher and lower positive pressure of a (multi-level) PAP device. For example, to support inspiration, the pressure of the pressurized flow of breathable gas may be adjusted to an inspiratory pressure. Alternatively, and/or simultaneously, to support expiration, the pressure of the pressurized flow of breathable gas may be adjusted to an expiratory pressure. Other schemes for providing respiratory support through the delivery of the pressurized flow of breathable gas are contemplated. Pressure generator  140  may be configured to adjust pressure levels, flow, humidity, velocity, acceleration, and/or other parameters of the pressurized flow of breathable gas in substantial synchronization with the breathing cycle of the subject. In certain embodiments, pressure generator  140  is part of an airway pressure device configured to provide types of therapy other than positive airway support therapy. 
     A pressurized flow of breathable gas may be delivered from pressure generator  140  to the airway of subject  106  by a subject interface  180 . Subject interface  180  may include a conduit  182  and/or a subject interface appliance  184 . Conduit  182  may be a flexible length of hose, or other conduit, that places subject interface appliance  184  in fluid communication with pressure generator  140 . Conduit  182  forms a flow path through which the pressurized flow of breathable gas is communicated between subject interface appliance  184  and pressure generator  140 . 
     Subject interface appliance  184  may be configured to deliver the pressurized flow of breathable gas to or near the airway of subject  106 . As such, subject interface appliance  184  may include any appliance suitable for this function. In one embodiment, pressure generator  140  is a dedicated ventilation device and subject interface appliance  184  is configured to be removably coupled with another interface appliance being used to deliver respiratory therapy to subject  106 . For example, subject interface appliance  184  may be configured to engage with and/or be inserted into an endotracheal tube, a tracheotomy portal, and/or other interface appliances. In one embodiment, subject interface appliance  184  is configured to engage the airway of subject  106  without an intervening appliance. In this embodiment, subject interface appliance  184  may include one or more of an endotracheal tube, a nasal cannula, a tracheotomy tube, a nasal mask, a nasal/oral mask, a full face mask, a total face mask, a partial rebreathing mask, or other interface appliances that communicate a flow of gas with an airway of a subject. The present disclosure is not limited to these examples, and contemplates delivery of the pressurized flow of breathable gas to subject  106  using any subject interface. 
     Electronic storage  130  comprises electronic storage media that electronically stores information. The electronic storage media of electronic storage  130  may include one or both of system storage that is provided integrally (i.e., substantially non-removable) with system  100  and/or removable storage that is removably connectable to system  100  via, for example, a port (e.g., a USB port, a FireWire port, etc.) or a drive (e.g., a disk drive, etc.). Electronic storage  130  may 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., EPROM, EEPROM, RAM, etc.), solid-state storage media (e.g., flash drive, etc.), and/or other electronically readable storage media. Electronic storage  130  may store software algorithms, information determined by processor  110 , information received via user interface  120 , and/or other information that enables system  100  to function properly. For example, electronic storage  130  may record or store one or more respiratory parameters (as discussed elsewhere herein), information indicating whether the subject adequately complied with a therapy regimen, information indicating whether and/or when a respiratory event occurred, and/or other information. Electronic storage  130  may be a separate component within system  100 , or electronic storage  130  may be provided integrally with one or more other components of system  100  (e.g., processor  110 ). 
     User interface  120  is configured to provide an interface between system  100  and a user (e.g., user  108 , subject  106 , a caregiver, a therapy decision-maker, etc.) through which the user can provide information to and receive information from system  100 . This enables data, results, and/or instructions and any other communicable items, collectively referred to as “information,” to be communicated between the user and system  100 . An example of information that may be conveyed to subject  106  is a report detailing the changes in determined lung mechanics throughout a period during which the subject is receiving therapy. An example of information that may be conveyed by subject  106  and/or user  108  is the breathing mode of subject  106 , i.e. spontaneous or non-spontaneous. Examples of interface devices suitable for inclusion in user interface  120  include a keypad, buttons, switches, a keyboard, knobs, levers, a display screen, a touch screen, speakers, a microphone, an indicator light, an audible alarm, and a printer. Information may be provided to subject  106  by user interface  120  in the form of auditory signals, visual signals, tactile signals, and/or other sensory signals. 
     By way of non-limiting example, user interface  120  may include a radiation source capable of emitting light. The radiation source may include, for example, one or more of at least one LED, at least one light bulb, a display screen, and/or other sources. User interface  120  may control the radiation source to emit light in a manner that conveys to subject  106  information related to breathing and/or the pressurized flow of breathable gas. Note that the subject and the user of system  100  may be one and the same person. 
     It is to be understood that other communication techniques, either hard-wired or wireless, are also contemplated herein as user interface  120 . For example, in one embodiment, user interface  120  may be integrated with a removable storage interface provided by electronic storage  130 . In this example, information is loaded into system  100  from removable storage (e.g., a smart card, a flash drive, a removable disk, etc.) that enables the user(s) to customize the implementation of system  100 . Other exemplary input devices and techniques adapted for use with system  100  as user interface  120  include, but are not limited to, an RS-232 port, RF link, an IR link, modem (telephone, cable, Ethernet, internet or other). In short, any technique for communicating information with system  100  is contemplated as user interface  120 . 
     Sensor(s)  142  may be configured to generate output signals conveying measurements related to parameters of respiratory airflow or airway mechanics. These parameters may include one or more of flow, (airway) pressure, humidity, velocity, acceleration, and/or other parameters. Sensor  142  may be in fluid communication with conduit  182  and/or subject interface appliance  184 . 
     The illustration of sensor  142  including a single member in  FIG. 1  is not intended to be limiting. In one embodiment sensor  142  includes a plurality of sensors operating as described above by generating output signals conveying information related to parameters associated with the state and/or condition of an airway of subject  106 , the breathing of subject  106 , the gas breathed by subject  106 , the delivery of the gas to the airway of subject  106 , lung mechanics of subject  106 , and/or a respiratory effort by the subject. For example, a parameter may be related to a mechanical unit of measurement of a component of pressure generator  140  (or of a device that pressure generator  140  is integrated, combined, or connected with) such as rotor speed, motor speed, blower speed, fan speed, or a related measurement that may serve as a proxy for any of the previously listed parameters through a previously known and/or calibrated mathematical relationship. Resulting signals or information from sensor  142  may be transmitted to processor  110 , user interface  120 , electronic storage  130 , and/or other components of system  100 . This transmission can be wired and/or wireless. 
     Processor  110  is configured to provide information processing capabilities in system  100 . As such, processor  110  includes 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 processor  110  is shown in  FIG. 1  as a single entity, this is for illustrative purposes only. In some implementations, processor  110  includes a plurality of processing units. 
     As is shown in  FIG. 1 , processor  110  is configured to execute one or more computer program modules. The one or more computer program modules include one or more of timing module  111 , passive compliance module  112 , passive resistance module  113 , active compliance module  114 , active resistance module  115 , flow module  116 , and/or other modules. Processor  110  may be configured to execute modules  111 ,  112 ,  113 ,  114 ,  115 , and/or  116  by software; hardware; firmware; some combination of software, hardware, and/or firmware; and/or other mechanisms for configuring processing capabilities on processor  110 . 
     It should be appreciated that although modules  111 ,  112 ,  113 ,  114 ,  115 , and  116  are illustrated in  FIG. 1  as being co-located within a single processing unit, in implementations in which processor  110  includes multiple processing units, one or more of modules  111 ,  112 ,  113 ,  114 ,  115 , and/or  116  may be located remotely from the other modules. The description of the functionality provided by the different modules  111 ,  112 ,  113 ,  114 ,  115 , and/or  116  described below is for illustrative purposes, and is not intended to be limiting, as any of modules  111 ,  112 ,  113 ,  114 ,  115 , and/or  116  may provide more or less functionality than is described. For example, one or more of modules  111 ,  112 ,  113 ,  114 ,  115 , and/or  116  may be eliminated, and some or all of its functionality may be provided by other ones of modules  111 ,  112 ,  113 ,  114 ,  115 , and/or  116 . Note that processor  110  may be configured to execute one or more additional modules that may perform some or all of the functionality attributed below to one of modules  111 ,  112 ,  113 ,  114 ,  115 , and/or  116 . 
     One or more modules may be configured to determine one or more gas parameters, breathing parameters, and/or other parameters from output signals generated by sensor(s)  142 . One or more gas parameters may be related to and/or derived from measurements of one or more of (peak) flow, flow rate, (tidal) volume, pressure, temperature, humidity, velocity, acceleration, gas composition (e.g. concentration(s) of one or more constituents), thermal energy dissipated, (intentional) gas leak, and/or other measurements related to the (pressurized) flow of breathable gas. One or more breathing parameters may be derived from gas parameters and/or other output signals conveying measurements of the pressurized flow of breathable gas. The one or more breathing parameters may include one or more of respiratory rate, breathing period, inhalation time or period, exhalation time or period, respiration flow curve shape, transition time from inhalation to exhalation and/or vice versa, transition time from peak inhalation flow rate to peak exhalation flow rate and/or vice versa, respiration pressure curve shape, maximum proximal pressure drop (per breathing cycle and/or phase), and/or other breathing parameters. Some or all of this functionality may be incorporated or integrated into other computer program modules of processor  110 . 
     Timing module  111  is configured to determine respiratory timing parameters and/or other timing parameters related to the operation of system  100 , such as transitions in breathing between inhalations and exhalations. Respiratory timing parameters may include transitional moments that separate inhalation phases from exhalation phases and vice versa, breathing period, respiratory rate, inhalation time or period, exhalation time or period, start and/or end of inhalation phases, start and/or end of exhalation phases, and/or other respiratory timing parameters. Timing parameters related to the operation of system  100  may include therapy session length, average and/or cumulative daily and/or nightly usage, amount of usage since the most recent pressure adjustment, and/or other timing parameters related to the operation of system  100 . 
     Passive compliance module  112  is configured to determine a passive exhalation lung compliance during an exhalation. Passive compliance module  112  may be configured to determine a passive inhalation lung compliance during an inhalation. The inhalation and exhalation may be demarcated based on the transitions as determined by timing module  111 . Determination may be dynamic, i.e. per breathing cycle and/or phase. Determination by the passive compliance module is based on a passive lung model. A passive lung model may comprise a serial (fluid) coupling of a flow source (e.g., a ventilator), a lung resistance, and a lung compliance. By way of illustration,  FIG. 3A  illustrates a standard passive lung model, including a flow source labeled Q P , a proximal pressure labeled P, a lung resistance labeled R, a lung compliance labeled C, and an end expiratory pressure labeled P 0 . Proximal pressure and subject flow may be related as follows: 
     P=R·Q P /1000+V/C+P 0 , wherein the unit of pressure may be cmH 2 O, V may be subject volume in ml, which may be obtained independently, e.g., for each breath. By obtaining, estimating, and/or determining the proximal (or airway) pressure of the subject (e.g., at the start of an inhalation), the subject flow, and the pressure P 0  at the end of an exhalation directly following the inhalation, lung compliance and lung resistance may be determined, calculated and/or estimated, using the formula above. Note that leaks in the respiratory circuit are not explicitly accounted for in the formula above. 
     Flow module  116  is configured to determine the subject flow, while taking the presence of leaks into account. Subsequently, the passive lung model, as used by multiple modules in system  100 , may be based on the subject flow determined by flow module  116 . Determination of the subject flow may be based on circuit flow, which may in turn be determined based on output signals generated by sensors conveying measurements related to parameters of respiratory airflow or airway mechanics. 
     Leaks may include intentional or non-intentional leaks, known or unknown leaks, variable or non-variable leaks, and/or other leaks. For example, subject interface  184  may be a mask having an unintentional mask leak, intentional and/or controllable exhalation port leak, and/or other leaks. Other components of system  100  may include known and/or predictable leaks, such as the connections of pressure generator  140  to other components of system  100 . The instantaneous estimated estimate subject flow Q P (t) and the instantaneous total circuit leak Q Leak (t) may be summed according to the following two formulae: 
     Q P (t)=Q C (t)−Q Leak (t) and Q Leak (t)=Q L     known   (t)+Q L     unknown   (t), wherein Q C (t) represents the circuit flow in the respiratory circuit, Q L     known   (t) represents the instantaneous known circuit leak, which may include intentional leaks and a systemic leak flow that predictably varies with a given proximal pressure, and Q L     unknown   (t) represents the instantaneous unknown circuit leak, which may include an unintentional leak at or near a leak orifice of the subject. The systemic leak flow may be characterized and pre-stored in, e.g., electronic storage  130 . The unknown leak may be modeled as: 
                   Q     L   unknown       ⁡     (   t   )       =         P   ⁡     (   t   )               R     L   unknown         _         ,         
wherein P(t) is the proximal pressure and
 
             1         R     L   unknown         _           
represents the average resistance of the leak orifice of the subject, which may be determined using the following formula, e.g., for each breath:
 
               1         R     L   unknown         _       =         ∫     t   0       t   2       ⁢       [         Q   C     ⁡     (   t   )       -       Q     L   unknown       ⁡     (     P   ⁡     (   t   )       )         ]     ⁢   dt           ∫     t   0       t   2       ⁢         P   ⁡     (   t   )         ⁢   dt               
wherein t 0  is the start of an inhalation and t 2  is the end of the subsequent exhalation. The average resistance of the leak orifice of the subject may change over time, e.g. between breaths. Accordingly, the resistance of the leak orifice could be determined regularly, e.g., for every breath, every ten seconds, every minute, and/or using any other regular and/or recurring determination. Through substitution, the instantaneous estimated estimate subject flow Q P (t) may be determined using the following formula:
 
     
       
         
           
             
               
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     Passive compliance module  112  is configured to use the passive lung model, in particular the estimated estimate subject flow Q P (t), to determine the passive exhalation lung compliance as follows: Suppose the estimated proximal pressure is given by P est =R·Q P /1000+V/C+P 0 , and P represents the measured proximal pressure, P diff  represents the difference between P and P 0 . Using the least square method (and/or another suitable method to calculate C) minimizes the sum of the squared difference between measured and estimated pressure: 
     
       
         
           
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     Solving for C results in the following: 
     
       
         
           
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     Note that parameters m1 through m5 are set to zero at the start of the exhalation. The formula above can be used to determine the passive exhalation lung compliance. To determine the passive inhalation lung compliance, volume V and parameters m1 through m5 are set to zero at the start of the inhalation. The circuit flow Q C  and proximal pressure P are obtained, e.g., through sensors in system  100 , and the formula above can be used to determine the passive inhalation lung compliance. To determine both the passive exhalation lung compliance and the passive inhalation lung compliance, passive compliance module  112  may be configured to determine the passive inhalation lung compliance first, before determining the passive exhalation lung compliance, e.g. such that the exhalation used for determination immediately follows the inhalation used for determination. 
     Passive resistance module  113  is configured to determine a passive exhalation lung resistance during the exhalation, wherein the determination by the passive resistance module is based on a passive lung model. Passive resistance module  113  may be configured to determine a passive inhalation lung resistance. The resistance, for either breathing phase, may be the maximum pressure drop over the maximum flow rate that passes the airway of the subject. In other words: R=ΔP max /Q P     max   . Using the passive lung compliance as determined by passive compliance module  112 , the subject lung pressure P Lung  may be calculated using P Lung =V/C+P 0 , such that the pressure drop ΔP is ΔP=P−P Lung =P diff −V/C. Assuming that the pressure drop increases or decreases monotonically with Q P , ΔP max  occurs at the moment when subject flow reaches its maximum value. For that reason the passive resistance may be rewritten as follows: 
     
       
         
           
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     This formula applies to the passive exhalation lung resistance by using the passive exhalation lung compliance (e.g., from passive compliance module  112 ) and the maximum pressure drop ΔP max  during the exhalation. This formula applies to the passive inhalation lung resistance by using the passive inhalation lung compliance (e.g., from passive compliance module  112 ) and the maximum pressure drop ΔP max  during the inhalation. To determine both the passive exhalation lung resistance and the passive inhalation lung resistance, passive resistance module  113  may be configured to determine the passive inhalation lung resistance first, before determining the passive exhalation lung resistance, e.g. such that the exhalation used for determination immediately follows the inhalation used for determination. 
     Active compliance module  114  is configured to determine an active lung compliance that reflects an active lung model, wherein the active lung compliance is determined based on the passive exhalation lung compliance and a subject breathing mode. Determination of the active lung compliance may further be based on the passive inhalation lung compliance. The subject breathing mode may be a spontaneous breathing mode or a non-spontaneous breathing mode. System  100  may obtain the subject breathing mode, e.g. as a system setting and/or user input, and/or determine the subject breathing mode, e.g. through analysis of output signals generated by one or more sensors of system  100 . 
     By way of illustration,  FIG. 3B  illustrates a standard active lung model, which differs from a standard passive lung model (e.g., see  FIG. 3A ) at least due to “interference” by spontaneous breathing of the subject and/or inspiratory muscle pressure of the subject, represented by P mus . Notably, though, during exhalation, the interference of P mus  may be substantially negligible, as the lung of the subject may be passively relaxed during exhalation. For this reason, (dynamic) determination of the active lung compliance and active lung resistance spanning an entire breathing cycle may be accomplished by (dynamically) determining compliance and resistance separately during inhalation and exhalation, and subsequently combining the results depending on the breathing mode of the subject. 
     Active compliance module  114  may determine the active compliance C of the subject by combining the passive exhalation lung compliance C exh  and the passive inhalation lung compliance C inh  according to: C=weight*C inh +(1−weight)*C exh , wherein weight is, e.g., 0 for spontaneous breathing mode, and, e.g., 0.5 for non-spontaneous breathing mode. The value for weight should be between 0 and 1. 
     Active resistance module  115  is configured to determine an active lung resistance that reflects an active lung model (e.g., the standard active lung model of  FIG. 3B ), wherein the active lung resistance is determined based on the passive exhalation lung resistance and the subject breathing mode. Determination of the active lung resistance may further be based on the passive inhalation lung resistance. Active resistance module  115  may determine the active resistance R of the subject by combining the passive exhalation lung resistance R exh  and the passive inhalation lung resistance R inh  according to: R=weight*R inh +(1−weight)* R   exh , wherein weight is, e.g., 0 for spontaneous breathing mode, and, e.g., 0.5 for non-spontaneous breathing mode. The value for weight should be between 0 and 1. 
       FIG. 2  illustrates a method  200  for measuring respiratory parameters during non-invasive respiratory therapy of a subject. The operations of method  200  presented below are intended to be illustrative. In some embodiments, method  200  may be accomplished with one or more additional operations not described, and/or without one or more of the operations discussed. Additionally, the order in which the operations of method  200  are illustrated in  FIG. 2  and described below is not intended to be limiting. 
     In some embodiments, method  200  may be implemented in one or more processing devices (e.g., 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). The one or more processing devices may include one or more devices executing some or all of the operations of method  200  in response to instructions stored electronically on an electronic storage medium. The one or more processing devices may include one or more devices configured through hardware, firmware, and/or software to be specifically designed for execution of one or more of the operations of method  200 . 
     At an operation  202 , transitions in breathing between inhalations and exhalations are determined. In one embodiment, operation  202  is performed by a timing module similar to or substantially the same as timing module  111  (shown in  FIG. 1  and described above). 
     At an operation  204 , a passive inhalation lung compliance is determined, during an inhalation, based on a passive lung model, wherein the inhalation is demarcated based on the transitions. In one embodiment, operation  204  is performed by a passive compliance module similar to or substantially the same as passive compliance module  112  (shown in  FIG. 1  and described above). 
     At an operation  206 , a passive exhalation lung compliance is determined, during an exhalation, based on a passive lung model, wherein the exhalation is demarcated based on the transitions. In one embodiment, operation  206  is performed by a passive compliance module similar to or substantially the same as passive compliance module  112  (shown in  FIG. 1  and described above). 
     At an operation  208 , a passive inhalation lung resistance is determined, during an inhalation, based on a passive lung model. In one embodiment, operation  208  is performed by a passive resistance module similar to or substantially the same as passive resistance module  113  (shown in  FIG. 1  and described above). 
     At an operation  210 , a passive exhalation lung resistance is determined, during an exhalation, based on a passive lung model. In one embodiment, operation  210  is performed by a passive resistance module similar to or substantially the same as passive resistance module  113  (shown in  FIG. 1  and described above). 
     At an operation  212 , a subject breathing mode is obtained. In one embodiment, operation  212  is performed through a user interface to or substantially the same as user interface  120  (shown in  FIG. 1  and described above). 
     At an operation  214 , an active lung compliance that reflects an active lung model is determined. In one embodiment, operation  214  is performed by an active compliance module similar to or substantially the same as active compliance module  114  (shown in  FIG. 1  and described above). 
     At an operation  216 , an active lung resistance that reflects an active lung model is determined. In one embodiment, operation  216  is performed by an active resistance module similar to or substantially the same as active resistance module  116  (shown in  FIG. 1  and described above). 
     In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” or “including” does not exclude the presence of elements or steps other than those listed in a claim. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In any device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain elements are recited in mutually different dependent claims does not indicate that these elements cannot be used in combination. 
     Although the invention has been described in detail for the purpose of illustration based on what is currently considered to be the most practical and preferred embodiments, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.