Patent Publication Number: US-2020289772-A1

Title: Adaptive patient circuit compensation with pressure sensor at mask apparatus

Description:
RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application Ser. No. 14/434,626, filed Apr. 9, 2015, which is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/1B2013/059276, filed on Oct. 10, 2013, which claims the benefit of U.S. Provisional Patent Application No. 61/711,791, filed on Oct. 10, 2012. These applications are hereby incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure pertains to a system and method for providing respiratory therapy through a pressure support device, and, in particular, to modeling, estimating, and compensating for one or more effects of transport delay through tubing within the pressure support device, including but not limited to a pressure drop between the output of a pressure generator and the point of delivery of a pressurized flow of breathable gas. 
     BACKGROUND 
     It is well known that some types of respiratory therapy involve the delivery of a flow of breathable gas to the airway of a subject. It is known that a flow of breathable gas may be pressurized at varying levels of pressure, even during a single therapy session. It is known that one or more algorithms may operate to control and/or adjust the pressure level or flow used in respiratory therapy. It is known that measurements or estimations of various gas parameters can be used in a feedback or feedforward manner to control and/or adjust the pressure level used in respiratory therapy. It is known that there are practical limitations to the responsiveness and/or stability of a respiratory therapy device, due, in part, to the transport delay of a pressure wave propagating through the respiratory therapy device on its way to the point of delivery, such as the tubing and mask of a patient. 
     SUMMARY 
     Accordingly, it is an object of one or more embodiments of the present invention to provide a system for providing respiratory therapy during a therapy session to a subject. The system comprises a pressure generator configured to generate a pressurized flow of breathable gas for delivery to the airway of the subject, the pressure generator having an output configured to expel the pressurized flow of breathable gas; a subject interface configured to guide the pressurized flow of breathable gas from the output of the pressure generator to a point of delivery at or near the airway of the subject, wherein the subject interface causes a pressure drop between the output of the pressure generator and the point of delivery during delivery of the pressurized flow of breathable gas; one or more sensors configured to generate output signals conveying information related to one or more gas parameters of the pressurized flow of breathable gas; one or more processors configured to execute processing modules, the processing modules comprising: a model module configured to adjust one or more model parameters of a parameter-based model that models the subject interface, the one or more model parameters being different than the one or more gas parameters, and wherein the parameter-based model includes one or more model parameters related to pneumatic impedance of the subject interface; an estimation module configured to estimate the pressure drop between the output of the pressure generator and the point of delivery during delivery of the pressurized flow of breathable gas based on the generated output signal and based on the one or more adjusted model parameters of the parameter-based models; and a control module configured to adjust levels of one or more gas parameters of the pressurized flow of breathable gas based on the estimated pressure drop. 
     It is yet another aspect of one or more embodiments of the present invention to provide a method for providing respiratory therapy during a therapy session to a subject implemented in a system including a pressure generator, a subject interface, and one or more sensors, the method comprising: generating a pressurized flow of breathable gas for delivery to the airway of the subject via an output of the pressure generator; guiding the pressurized flow of breathable gas from the output of the pressure generator to a point of delivery at or near the airway of the subject via the subject interface, wherein the subject interface causes a pressure drop between the output of the pressure generator and the point of delivery during delivery of the pressurized flow of breathable gas; generating output signals conveying information related to one or more gas parameters of the pressurized flow of breathable gas; adjusting one or more model parameters of a parameter-based model that models the subject interface, the one or more model parameters being different than the one or more gas parameters, and wherein the parameter-based model includes one or more model parameters related to pneumatic impedance of the subject interface; estimating the pressure drop between the output of the pressure generator and the point of delivery during delivery of the pressurized flow of breathable gas based on the generated output signal and based on the one or more adjusted model parameters of the parameter-based models; and adjusting levels of one or more gas parameters of the pressurized flow of breathable gas based on the estimated pressure drop. 
     It is yet another aspect of one or more embodiments to provide a system configured to providing respiratory therapy during a therapy session to a subject. The system comprises pressure means for generating a pressurized flow of breathable gas for delivery to the airway of the subject; guiding means for guiding the pressurized flow of breathable gas from an output of the pressure means to a point of delivery at or near the airway of the subject, wherein the guiding means causes a pressure drop between the output of the pressure means and the point of delivery during delivery of the pressurized flow of breathable gas; means for generating output signals conveying information related to one or more gas parameters of the pressurized flow of breathable gas, wherein the output signals are generated in an ongoing manner during the therapy session; means for adjusting one or more model parameters of a parameter-based model that models the subject interface, the one or more model parameters being different than the one or more gas parameters, and wherein the parameter-based model includes one or more model parameters related to pneumatic impedance of the subject interface; means for estimating the pressure drop between the output of the pressure generator and the point of delivery during delivery of the pressurized flow of breathable gas based on the generated output signal and based on the one or more adjusted model parameters of the parameter-based models; and means for adjusting levels of one or more gas parameters of the pressurized flow of breathable gas based on the estimated pressure drop. 
     These and other objects, features, and characteristics of the present invention, 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 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 the limits of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a system for providing respiratory therapy to a subject in accordance with one or more embodiments; 
         FIG. 2  schematically illustrates an exemplary system for providing respiratory therapy to a subject; 
         FIG. 3  illustrates a method for providing respiratory therapy to a subject; and 
         FIG. 4  schematically illustrates an electrical circuit representation of a system for providing respiratory therapy to a subject in accordance with one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE 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  for providing respiratory therapy to a subject  106 . System  100  may be implemented as, integrated with, and/or operating in conjunction with a respiratory therapy device. System  100  dynamically models, measures, determines, and/or estimates one or more effects of transport delay through tubing and/or other pneumatic system parameters within a respiratory therapy device, including but not limited to a pressure drop dynamically during a therapy session and compensates for one or more effects in order to improve the quality, responsiveness, and/or stability of the respiratory therapy device and/or the provided respiratory therapy. 
     Quality of a respiratory therapy device and/or the provided respiratory therapy may pertain to the precision of the level and/or timing of one or more gas parameters of a delivered pressurized flow of breathable gas, in particular in response to load-side disturbances such as flow changes from the subject and/or components of system  100 . Alternatively, and/or simultaneously, quality may pertain to the bandwidth of the respiratory therapy device. Alternatively, and/or simultaneously, quality may pertain to the amount of noise or the signal-to-noise ratio within a respiratory therapy device. Responsiveness of a respiratory therapy device may pertain to how well and/or how rapidly the device handles load disturbances (and/or other flow changes) and/or set-point changes within the system. Such changes may include, without limitation, breathing, sneezing, coughing and/or other actions by subject  106 , as well as changes due to hardware components, such as a tube moving, bending, etc. In some cases, responsiveness may be characterized by a response rate. Stability of a respiratory therapy device may pertain to the likelihood of introducing oscillations within the device during a therapy session. Alternatively, and/or simultaneously, stability may be characterized by a gain margin and a phase margin. 
     A therapy “session” of using system  100  may be defined as a period of substantially uninterrupted therapeutic usage of system  100 , not to exceed some upper threshold of (consecutive) hours. The upper threshold may be, for example, about 6 hours, about 8 hours, about 10 hours, about 12 hours, about 16 hours, about 24 hours and/or other time periods. If the respiratory therapy is used to treat sleeping disorders the related session length may correspond to the sleeping pattern of a subject. A typical session length may thus be about eight hours. Alternatively, and/or simultaneously, a therapy session may be defined as a period of substantially uninterrupted therapeutic usage of system  100 , not to span less than some lower threshold of (consecutive) units of time, and/or at least a minimum period of time apart from a previous session. The lower threshold may be, for example, about 15 minutes, about 30 minutes, about 1 hour, about 2 hours, about 3 hours, about 4 hours, about 5 hours and/or other time periods. For example, a minute of usage may be too short to be regarded as a session. For example, two 3-hour periods of usage separated by a 10-minute gap may be regarded as one session rather than two sessions. Individual therapy sessions may have a beginning and an end. 
     In some embodiments, one or more operative levels (e.g. pressure, volume, etc.) are adjusted on a relatively ongoing manner (e.g., between individual breaths, every few breaths, every few seconds, every minute, etc.) during an individual therapy session to titrate the therapy and/or to compensate for other changes in the patient circuit. 
     System  100  includes one or more of a pressure generator  140 , a delivery circuit  180 , one or more sensors  142 , an electronic storage  130 , a user interface  120 , a processor  110 , an estimation module  111 , a target module  112 , a control module  113 , a patient pressure module  114 , a device pressure module  115 , an error module  116 , a parameter determination module  117 , and/or other components. 
     Pressure generator  140  of system  100  in  FIG. 1  may be integrated, combined, coupled, and/or connected with a (positive) airway pressure device (PAP/CPAP/BiPAP®etc.). Pressure generator  140  may be configured to provide a pressurized flow of breathable gas for delivery to the airway of subject  106 , e.g. via an output  141  of pressure generator  140 , and/or via a delivery circuit  180 . Delivery circuit  180  may sometimes be referred to as subject interface  180 . Subject  106  may initiate one or more phases of respiration. Respiratory therapy may be implemented as pressure control, pressure support, volume control, and/or other types of support and/or control. 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 and/or flow of the pressurized flow of breathable gas may be adjusted to an expiratory pressure. Adjustments may be made numerous times in implementations using auto-titrating for providing respiratory support through the delivery of the pressurized flow of breathable gas. In addition to alternating between multiple levels, the inhalation pressure level may ramp up or down according to a predetermined slope (absolute and/or relative, e.g. dependent on breathing rate) for any specified section of a phase. Similar features may be available for exhalation phases. The pressure levels may be either predetermined and fixed, follow a predetermined dynamic characteristic, or they may dynamically change breath-to-breath or night-to-night depending on sensed breathing, breathing disorder, or other physiological characteristics. Pressure generator  140  is configured to adjust one or more of pressure levels, flow, humidity, velocity, acceleration, and/or other parameters of the pressurized flow of breathable gas, e.g. in substantial synchronization with the breathing cycle of the subject. 
     An airway pressure device may be configured such that one or more gas parameters of the pressurized flow of breathable gas are controlled in accordance with a therapeutic respiratory regimen for subject  106 . The one or more gas parameters include one or more of flow, volume, retrograde volume, pressure, humidity, velocity, acceleration, (intentional) gas leak, and/or other parameters. System  100  may be configured to provide types of therapy including types of therapy where a subject performs inspiration and/or expiration of his own accord or where the device provides negative airway pressure. 
     The functional relation between the pressure level at output  141  of pressure generator  140  and the pressure level at the point of delivery to subject  106  may be referred to as a transfer function. A parameter-based model that models one or more of subject interface  180 , interaction between subject interface  180  and subject  106 , and/or other components within system  100  may be used to analyze the transfer function in the context of a closed-loop feedback/feedforward system. The parameter-based model may contain a patient model as well as a patient interface circuit pneumatic model. The parameter-based model may be dynamic, e.g. the parameters may change in value dynamically, or model elements may be added, removed, and/or reconfigured dynamically to better estimate the patient model or patient interface circuit pneumatic model. Usage and/or analysis of the parameter-based model, e.g. pertaining to the transfer function, may pertain to the effects of a time delay, such as the transport delay of a pressure wave propagating through subject interface  180 , on, e.g., system responsiveness and stability. An example of a transfer function for an input X set  versus an output X Actual  is given by the equation below: 
     
       
         
           
             
               
                 
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     Note that a time delay in may contribute a linearly increasing phase lag in which the degree of negative phase contribution is proportional to frequency. Adding a small time delay may affect only the phase, thereby decreasing the stability margins. If the time delay is large enough, a reduction in gain may be needed to maintain stability and/or limit other undesirable effects, effectively limiting the response speed. 
     A pressurized flow of breathable gas is delivered from pressure generator  140  to the airway of subject  106  via delivery circuit  180 . Delivery circuit  180  may include a conduit  182  and/or a subject interface appliance  184 . Conduit  182  may include a flexible length of hose, or other conduit, either in single-limb or dual-limb configuration 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 . Conduit  182  may comprise a standard 22 mm diameter hose (other common diameters range between ¾″ and 1″) or, in certain embodiments, a much smaller diameter hose that is in the range of ⅓ of a standard size hose. Such a hose, which may be referred to as a restricted flow hose or limited flow hose, (for example, having a diameter ranging between ¼″ and ⅓″, or alternatively between 6 mm and 9 mm) may have a greater resistance to gas flow and/or may be smaller and/or less obtrusive. 
     Subject interface appliance  184  of system  100  in  FIG. 1  is configured to deliver the pressurized flow of breathable gas to the airway of subject  106 . As such, subject interface appliance  184  may include any appliance suitable for this function. In some embodiments, 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 facemask, and/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  of system  100  in  FIG. 1  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, FRAM, 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 timing information (including duration of inhalation phases and exhalation phases as well as transitional moments), one or more (breathing) parameters and/or other parameters (as discussed elsewhere herein), pressure levels, pressure drop estimated at various moments, information indicating whether the subject adequately complied with a prescribed respiratory therapy regimen, information indicating whether a respiratory event (including Cheyne-Stokes respiration, central sleep apnea, obstructive sleep apnea, hypopnea, snoring, hyperventilation, and/or other respiratory events) occurred, information indicating adequacy of treatment, 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  of system  100  in  FIG. 1  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 user  108  is a report detailing occurrences of respiratory events throughout a period during which the subject is receiving therapy. 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 user  108  or subject  106  by user interface  120  in the form of auditory signals, visual signals, tactile signals, and/or other sensory signals. 
     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 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 . 
     One or more sensors  142  of system  100  in  FIG. 1  are configured to generate output signals conveying measurements related to gas parameters of respiratory airflow, parameters related to airway mechanics, and/or other parameters. Gas parameters may include flow, (airway) pressure, humidity, velocity, acceleration, and/or other gas parameters. Output signals may convey measurements related to respiratory parameters. Sensor  142  may be in fluid communication with conduit  182  and/or subject interface appliance  184 . Sensor  142  may generate output signals related to physiological parameters pertaining to subject  106 . Parameters may be 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 composition of the gas breathed by subject  106 , the delivery of the gas to the airway 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 valve drive current, rotor speed, motor speed, blower speed, fan speed, or a related measurement and/or unit that may serve as a proxy for any of the parameters listed herein through a previously known and/or calibrated mathematical relationship. Sensed signals may include any information obtained by or extracted from fundamental relationships involving control parameters or surrogates. 
     The illustration of sensor  142  including two members in  FIG. 1  is not intended to be limiting. In some hardware configurations, system  100  may use only one sensor  142 . The individual sensor  142  may be located at or near subject interface appliance  184 , or at other locations. In some hardware configurations, system may include a sensor  142  at or near output  141  of pressure generator  140 . The illustration of a sensor  142  at or near subject interface appliance  184  and a sensor  142  at or near output  141  of pressure generator  140  is not intended to be limiting. Resulting signals or information from one or more sensors  142  may be transmitted to processor  110 , user interface  120 , electronic storage  130 , and/or other components of system  100 . This transmission may be wired and/or wireless. 
     The one or more sensors  142  may be configured to generate output signals in an ongoing manner during a therapy session. This may include generating signals intermittently, periodically (e.g. at a sampling rate), continuously, continually, at varying intervals, and/or in other ways that are ongoing during at least a portion of the therapy session. For example, in some embodiments, the generated output signals may be considered as a vector of output signals, such that a vector includes multiple samples of information conveyed related to one or more gas parameters and/or other parameters. Different parameters may be related to different vectors. A particular parameter determined in an ongoing manner from a vector of output signals may be considered as a vector of that particular parameter. 
     Processor  110  of system  100  in  FIG. 1  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 embodiments, 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 estimation module  111 , target module  112 , control module  113 , patient pressure module  114 , device pressure module  115 , error module  116 , parameter determination module  117 , model module  118 , and/or other modules. Processor  110  may be configured to execute modules  111 - 118  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 - 118  are illustrated in  FIG. 1  as being co-located within a single processing unit, in embodiments in which processor  110  includes multiple processing units, one or more of modules  111 - 118  may be located remotely from the other modules. The description of the functionality provided by the different modules  111 - 118  described herein is for illustrative purposes, and is not intended to be limiting, as any of modules  111 - 118  may provide more or less functionality than is described. For example, one or more of modules  111 - 118  may be eliminated, and some or all of its functionality may be incorporated, shared, integrated into, and/or otherwise provided by other ones of modules  111 - 118 . 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 - 118 . 
     Parameter determination module  117  of system  100  in  FIG. 1  is configured to determine one or more gas parameters, breathing parameters, and/or other parameters based on one or more of output signals generated by sensor(s)  142  and/or other information sources. Determinations may be based on measurements, calculations, estimations, approximations, previously known and/or calibrated mathematical relationships, and/or other ways to determine a parameter. The other information sources may include motor currents, motor voltage, motor parameters, valve parameters, and/or other sources. The determined parameters may include system parameters and/or controlled parameters, i.e. not just sensed signals. 
     Operation of parameter determination module  117  may be performed in an ongoing manner. 
     The one or more gas parameter may include and/or be related to one or more of (peak) flow rate, flow rate, (tidal) volume, pressure, temperature, humidity, velocity, acceleration, gas composition (e.g. concentration(s) of one or more constituents such as, e.g., CO 2 ), thermal energy dissipated, (intentional) gas leak, and/or other measurements related to the (pressurized) flow of breathable gas. One or more gas parameters may be determined at different locations and/or positions within system  100 , including within pressure generator  140 , at or near output  141  of pressure generator  140 , within subject interface  180 , at or near the point of engagement between pressure generator  140  and subject interface  180 , within conduit  182 , at or near an input of conduit  182 , at or near an output of conduit  182 , within subject interface appliance  184 , at or near an input of subject interface appliance  184 , at or near an output of subject interface appliance  184 , and/or at other locations and/or positions within system  100 . 
     Parameter determination module  117  may derive one or more breathing parameters from one or more determined gas parameters and/or generated output signals. 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. 
     Parameter determination module  117  may derive vectors of parameters in an ongoing manner during a therapy session from vectors of generated output signals and/or other (vectors of) determined parameters. 
     Model module  118  is configured to dynamically manage a parameter-based system model that models one or more of subject interface  180 , interaction between subject interface  180  and subject  106 , subject  106 , and/or other components within system  100 . The parameter-based model may be derived using electrical circuit representation of system  100 . The parameter-based model includes one or more model parameters related to one or more of pneumatic impedance, resistance, inertance/inductance, capacitance, and/or other characteristics. The parameter-based model may separately represent resistive, compliance, inertance, and/or other (pneumatic) components of the patient pneumatic model. Model module  118  may be configured to adjust one or more model parameters described herein in an ongoing manner during a therapy session. As used herein, “adjusting” a model parameter may include correcting a model parameter. Adjustments by model module  118  may be based on one or more of information from parameter determination module  117  and/or the output signals generated by one or more sensors  142 . 
     By way of illustration,  FIG. 4  schematically illustrates model  100   b , an electrical circuit representation of system  100  as shown in  FIG. 1 . Note that in  FIG. 4  “hose” may refer to conduit  182  as shown in  FIG. 1 , or subject interface  180  as shown in  FIG. 1 , without subject interface appliance  184  as shown in  FIG. 1 . Note that in  FIG. 4  “mask” refers to subject interface appliance  184  as shown in  FIG. 1 . As depicted in  FIG. 4 , P device  is the pressure level at the output of the pressure generator, R hose  is the “hose” resistance, L hose , is the “hose” inertance, R mask  is the resistance of the subject interface appliance, P patient  is the patient pressure or subject pressure, R leak  is the leak resistance, R patient  is the resistance of the patient airways and lungs, L patient  is the inertance of the patient airways, C patient  is the compliance of the patient airway and lungs, P mus  is the pressure generated by the patient diaphragm, Q total  is the total flow measured by the device, Q leak  is the leak flow, and Q patient  is the patient flow. Variations of model  100   b  in which inertance and/or compliance of subject interface appliance  184  are included are contemplated within the scope of this disclosure. 
     Through circuit analysis of model  100   b  in  FIG. 4 , the following equations representing relations within the system may be derived: 
     
       
         
           
             
               
                 P 
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                 P 
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             = 
             
               
                 
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               + 
               
                 
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                 · 
                 
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     Additional specificity using a more detailed or more complex parameter-based model is contemplated and would be implemented in these equations by additional terms. In some embodiments, these equations may be solved using a least-squared error solution: 
     
       
         
           
             
                 
             
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     In some embodiments, other solutions (and/or other data fitting techniques) may be implemented and/or contemplated that may be used to determine and/or estimate one or more model parameters of the parameter-based model, such as, e.g., R circuit  and L hose , based on one or more output signals generated by one or more sensors  142  as depicted in  FIG. 1 . 
     Estimation module  111  is configured to estimate a pressure drop over at least part of subject interface  180 . For example, the estimated pressure drop may be between output  141  of pressure generator  140  (and/or a point near output  141 ) and the point of delivery of the pressurized flow of breathable gas (and/or a point near the point of delivery) during delivery of the pressurized flow of breathable gas. Pressure drop may be related to pneumatic impedance of subject interface  180  and/or other components of system  100 . In some embodiments, estimations by estimation module  111  may be based on one or more model parameters of the parameter-based model of model module  111 , such that adjustments of the one or more model parameters of the parameter-based model are dynamically reflected in corresponding adjustments by estimation module  111 . 
     Pressure drop may vary with differences in hose length (e.g. the length of conduit  182 ), conduit diameter, bends in a hose or tube, and/or other factors, including dynamic factors that change during a therapy session. Estimation by estimation module  111  may be based on the generated output signals. Estimation by estimation module  111  may be performed in an ongoing manner during a therapy session. Alternatively, and/or simultaneously, estimations by estimation module  111  may be triggered when a particular error within system  100  breaches a predetermined threshold. For example, when the difference between a particular measured parameter is greater than an estimation of the same parameter, this occurrence may trigger operations from one or more modules within system  100 . 
     In some embodiments, the estimated pressure drop {circumflex over (P)} drop  may be based on a function (e.g. a differential function) of the flow Q measured within subject interface  180  and/or elsewhere in the patient circuit, the pressure P patient  measured at or near the point of delivery of the pressurized flow of breathable gas, the pressure measured or estimated at or near output  141  of pressure generator  140 , and/or other information. The function may use current and past samples of the listed (vectors of) parameters. 
     In some embodiments, the functions used may be based on a particular model used to represent system  100  during use. Examples include electrical circuit representations of the pneumatic characteristics of system  100 . Through circuit analysis, the relations between, e.g., patient pressure and device pressure may be represented as differential equations that may be solved in various ways, including by way of a least-squared error solution. Other patient circuit models are contemplated, as well as other data fitting techniques to solve such models. 
     If one of the one or more sensors  142  is located at or near output  141 , the estimated pressure drop may be based on a measured pressure at or near output  141  referred to as P device  or device pressure. Alternatively, and/or simultaneously, the estimated pressure drop may be based on an estimated pressure at or near output  141  referred to as {circumflex over (P)} device . Such an estimated pressure may e.g. be based on flow Q and a priori information including, but limited to, blower speed, valve drive current, and/or any other 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, and/or a proxy of such a measurement. 
     In other words, when using {circumflex over (P)} device : 
         {circumflex over (P)}   drop   [k]=f ( {right arrow over (P)}   patient   , {right arrow over (Q)}, {circumflex over ({right arrow over (P)})}   device ), for the  k   th  sample in a vector 
     Target module  112  is configured to determine a target pressure P target  for the pressurized flow of breathable gas that compensates for the estimated pressure drop. The target pressure may interchangeably be referred to as P set . The target pressure may be in accordance with a therapy regimen, and may dynamically change and/or titrate during one or more therapy sessions. For example, the therapy regimen may prescribe a particular pressure referred to as P prescription . Determination by target module  112  may be performed in an ongoing manner during the therapy session. The target pressure may be adjusted as either the prescribed pressure and/or the estimated pressure drop change. 
     In other words (and by way of non-limiting example): 
         P   set   [k]=P   prescription   [k]+{circumflex over (P)}   drop   [k ], for the  k   th  sample in a vector 
     Control module  113  is configured to control operation of system  100  during a therapy session. Control module  113  may be configured to control the pressure generator to adjust one or more levels of gas parameters of the pressurized flow of breathable gas in accordance with one or more of a (respiratory) therapy regimen, based on target pressures determined by target module  112 , based on one or more algorithms that control adjustments and/or changes in the pressurized flow of breathable gas, and/or based on other factors. Control module  113  may be configured to control pressure generator  140  to provide the pressurized flow of breathable gas. Control module  113  may be configured to control pressure generator  140  such that one or more gas parameters of the pressurized flow of breathable gas are varied over time in accordance with a respiratory therapy regimen. 
     Parameters determined by parameter determination module  117 , and/or received through sensors  142  may be used by control module  113  and/or other modules, e.g. in a feedback manner, to adjust one or more therapy modes/settings/operations of system  100 . Alternatively, and/or simultaneously, signals and/or information received through user interface  120  may be used by control module  113  and/or other modules, e.g. in a feedback manner, to adjust one or more therapy modes/settings/operations of system  100 . Control module  113  may be configured to time its operations relative to transitional moments in the breathing cycle of a subject, over multiple breath cycles, and/or in any other timing relation. For example, estimation module  111  may be configured to estimate the pressure drop based, at least in part, on a flow rate within subject interface  180  as determined by parameter determination module  117 . 
     Some respiratory therapy devices that measure P patient  may determine a pressure error P error  (commonly to be used in a feedback manner to adjust the pressure level of a pressurized flow of breathable gas) based on the difference between either P prescription  or P target  and P patient . Some respiratory therapy devices that measure P device  may determine a pressure error P error  based on the difference between either P prescription  or P target  and P device . In either of these cases the quality, stability, and/or responsiveness of the respiratory therapy device may be negatively and/or non-negligibly affected by the transport delay of a pressure wave propagating through the subject interface (and/or any other tube or component of these respiratory therapy devices). By way of non-limiting example, transport delay may contribute to (linearly increasing) phase lag, and/or a reduced gain margin. 
     System  100  accounts and/or compensates for all or most of the negative effects of such transport delay by virtue of, in part, basing adjustments by control module  113  on the estimated and/or measured pressure at or near output  141  of pressure generator  140 . 
     Error module  116  is configured to determine a pressure error P error  based on a difference between (and/or other mathematical operations involving) the target pressure P set  (as e.g. determined by target module  112 ) and the measured pressure (P device ) or estimated pressure ({circumflex over (P)} device ) at or near output  141  of pressure generator  140  (as e.g. determined by parameter determination module  117  and/or device pressure module  115 ). The pressure error may be determined in an ongoing manner during (at least part of) a therapy session. For example, a vector of the pressure error may be updated intermittently using samples of generate output signals, estimated pressure drop, and/or other (vectors of) parameters or information such that subsequent determinations of the pressure error are less than about 1 second apart, less than about 10 seconds apart, less than about 30 seconds apart, less than about 1 minute apart, less than about 10 minutes apart, and/or less than other time periods apart. The pressure error may subsequently be used elsewhere in system  100 , for example by control module  113 , e.g. in a feedback manner, to adjust levels of one or more gas parameters of the pressurized flow of breathable gas. 
     In other words (and by way of non-limiting example using the estimated device pressure): 
         P   error   [k]=P   set   [k− 1 ]−{circumflex over (P)}   device   [k− 1], for the  k   th  sample in a vector 
     Rapid, ongoing, adaptive, and/or dynamic determination of pressure drop during a therapy session facilitates both improved quality, stability, and/or responsiveness of system  100 , as well as the ability for a patient to test more interface equipment in a shorter amount of time. 
     Patient pressure module  114  is configured to determine patient pressure P patient  at or near the point of delivery of the pressurized flow of breathable gas to the airway of subject  106 . Determination by patient pressure module  114  may be based on the generated output signals from one or more sensors  142 , in particular a sensor  142  located at or near the point of delivery, e.g. in subject interface appliance  184 . 
     Device pressure module  115  is configured to determine a device pressure at or near output  141  of pressure generator  140 . Determination may be based on measurements and/or estimations. If one of the one or more sensors  142  is located at or near output  141 , determination by device pressure module  115  may be based on the measured pressure P device . Alternatively, and/or simultaneously, determinations by device pressure module  115  may be based on an estimated pressure {circumflex over (P)} device . Such an estimated pressure may e.g. be based on flow Q and a priori information including, but limited to, blower speed, valve drive current, and/or any other 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, and/or a proxy of such a measurement. 
     By way of illustration,  FIG. 2  schematically illustrates an exemplary system  100   a  for providing respiratory therapy to a subject in substantially the same or similar manner as system  100  of  FIG. 1 . Referring to  FIG. 2 , pressure generator  140  provides a pressurized flow of breathable gas, having a pressure of P device , to subject interface  180 . The output of subject interface  180  is in fluid communication with subject  106 , such that the provided pressure is P patient . The difference between P device  and P patient  is the pressure drop P drop . Through a sensor  142  depicted above subject  106 , a flow Q may be measured. Load disturbances within system  100   a  may be fed back, by way of non-limiting example to control module  113  as depicted, such that the operation of pressure generator  140  may be adjusted to compensate for load disturbances. Control of pressure generator may be performed by control module  113 . Through a sensor  142  depicted below subject  106 , patient pressure P patient  may be determined. The pressure drop {circumflex over (P)} drop  may be estimated by estimation module  111  based on the patient pressure, flow Q (as determined by a sensor  142  depicted above subject  106 ), and one or both of measured device pressure P device  and/or estimated device pressure {circumflex over (P)} device , depending on the hardware configuration used for system  100   a . The device pressure may be determined by device pressure module  115 . Target module  112  may determine pressure target P target  (also referred to as P set ) based on prescription pressure P prescription  and the estimated pressure drop. Error module  116  may determine pressure error P error  based on the target pressure from target module  112  and one or both of measured device pressure P device  and/or estimated device pressure {circumflex over (P)} device . Pressure error P error , target pressure P target , and/or the device pressure may be used by control module  113 , in addition to information about flow Q and/or the load disturbances, to control pressure generator  140 . Note that system  100   a  and its depicted components and interconnections in  FIG. 2  are merely exemplary, and not intended to be limiting in any way. 
       FIG. 3  illustrates a method  300  for providing respiratory therapy to a subject. The operations of method  300  presented below are intended to be illustrative. In certain embodiments, method  300  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  300  are illustrated in  FIG. 3  and described below is not intended to be limiting. 
     In certain embodiments, method  300  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  300  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  300 . 
     At an operation  302 , a pressurized flow of breathable gas is generated for delivery to the airway of the subject via an output of the pressure generator. In some embodiments, operation  302  is performed by a pressure generator similar to or substantially the same as pressure generator  140  (shown in  FIG. 1  and described herein). 
     At an operation  304 , the pressurized flow of breathable gas is guided from the output of the pressure generator to a point of delivery at or near the airway of the subject via a subject interface. The subject interface causes a pressure drop between the output of the pressure generator and the point of delivery during delivery of the pressurized flow of breathable gas. In some embodiments, operation  304  is performed by a subject interface the same as or similar to subject interface  180  (shown in  FIG. 1  and described herein). 
     At an operation  306 , output signals conveying information related to one or more gas parameters of the pressurized flow of breathable gas are generated. The output signals are generated in an ongoing manner during the therapy session. In some embodiments, operation  306  is performed by one or more sensors the same as or similar to sensors  142  (shown in  FIG. 1  and described herein). 
     At an operation  308 , the pressure drop between the output of the pressure generator and the point of delivery of the pressurized flow of breathable gas is estimated based on the generated output signals. The estimation is performed in an ongoing manner during the therapy session. In some embodiments, operation  308  is performed by an estimation module the same as or similar to estimation module  111  (shown in  FIG. 1  and described herein). 
     At an operation  310 , a target pressure for the pressurized flow of breathable gas is determined that compensates for the estimated pressure drop. The target pressure is in accordance with a therapy regimen. The determination is performed in an ongoing manner during the therapy session. In some embodiments, operation  310  is performed by a target module the same as or similar to target module  112  (shown in  FIG. 1  and described herein). 
     At an operation  312 , levels of one or more gas parameters of the pressurized flow of breathable gas are adjusted based on the determined target pressure. In some embodiments, operation  312  is performed by a control module the same as or similar to control module  113  (shown in  FIG. 1  and described herein). 
     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.