Patent Publication Number: US-8973577-B2

Title: Leak-compensated pressure regulated volume control ventilation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 12/408,414, entitled “LEAK-COMPENSATED PRESSURE REGULATED VOLUME CONTROL VENTILATION,” filed on Mar. 20, 2009, the entire disclosure of which is hereby incorporated herein by reference. 
    
    
     INTRODUCTION 
     In mechanical ventilation, pressure regulated volume control (PRVC) ventilation is a type of pressure regulated treatment that provides the capability to control the volume of gas delivered to the patient&#39;s lungs by adjusting the inspiratory target pressure. In PRVC, a selected volume is delivered to a patient by changing the pressure of the respiratory gas. During PRVC ventilation, the ventilator will evaluate the volume delivered to the patient and compare it against the desired volume set by the therapist. If delivered volume is less than the desired, therapist-selected volume, then the target pressure for the next breath will be increased proportionally. On the other hand, if the delivered volume exceeded the desired volume, the target pressure will be lowered. 
     In one implementation of PRVC, the ventilator estimates the compliance of the patient&#39;s lungs and uses the estimated lung compliance to calculate the target pressure that will result in the delivery of the pre-selected volume of gas. The magnitude and shape of the inspiratory flow delivered by the ventilator will be a function of the patient lung characteristics, breathing pattern, and other ventilator settings. 
     The lung compliance and lung resistance of a patient may be collectively referred to as the respiratory mechanics of the lung or, simply, the patient&#39;s respiratory mechanics. Because PRVC relies on the patient&#39;s respiratory mechanics when determining what pressure to provide for each breath, more accurate determination of respiratory mechanics is essential to performance of the ventilator when providing PRVC ventilation. 
     Leak-Compensated Pressure Regulated Volume Control Ventilation 
     This disclosure describes systems and methods for compensating for leakage when during delivery of gas to a patient from a medical ventilator in a pressure regulated volume control (PRVC) ventilation mode. The technology described herein includes systems and methods that compensate the delivery of PRVC ventilation for leakage in the patient circuit by using leak-compensated lung flows as well as respiratory mechanics (lung compliance and lung resistance) estimated in a manner that compensates for elastic and inelastic leaks from the ventilation system. 
     In part, this disclosure describes a method of compensating for leakage in a ventilation system during delivery of pressure regulated volume control ventilation to a patient. The method starts with monitoring an instantaneous flow of respiratory gas in the ventilation system based on one or more measurements of pressure and flow in ventilation system. Leakage of gas from the system is modeled as a first leakage component through a first orifice of a fixed size and a second leakage component through a second orifice of a varying size, in which the first and second leakage components are different functions of instantaneous pressure in the ventilation system. A leak-compensated delivered lung volume is then estimated for at least one breath based on the one or more measurements, the first leakage component and the second leakage component. The leak-compensated delivered lung volume and a predetermined respiratory mechanics model are then used to estimate a leak-compensated lung compliance. A target pressure to be delivered to the patient for a subsequent pressure-based breath is then calculated based on a desired lung volume, the leak-compensated delivered lung volume and the leak-compensated lung compliance. The target pressure is then delivered to the patient during the inspiratory phase of the next breath. The leak-compensated lung compliance may be estimated based on the leak-compensated delivered lung volume and a pressure difference, such as the difference between an end inspiratory pressure of a first breath and an end expiratory pressure of the first breath. 
     The disclosure also describes a method of compensating for leakage in a ventilation tubing system during delivery of gas from a medical ventilator to a patient. The method includes measuring leakage from the ventilation tubing system during a first breath and calculating a leak-compensated delivered lung volume for the first breath based on the leakage. The method then estimates a lung compliance of the patient based on the leak-compensated delivered lung volume and pressure measurements taken during the first breath. Ventilation is then delivered to the patient in a second breath at a pressure determined based on a desired delivered lung volume, the leak-compensated delivered lung volume and the leak-compensated lung compliance. As part of measuring the leakage, the method may include identifying an inelastic leakage from the ventilation tubing system as a first function of at least one of a pressure measurement and a flow measurement in the ventilation system and identifying an elastic leakage from the ventilation tubing system as a second function of at least one of the pressure measurement and the flow measurement in the ventilation system. 
     The disclosure further describes a pressure support system, such as a respiratory ventilator. The system includes: a pressure generating system adapted to generate a flow of breathing gas; a ventilation tubing system including a patient interface device for connecting the pressure generating system to a patient; one or more sensors operatively coupled to the pressure generating system or the ventilation tubing system, in which each sensor is capable of generating an output indicative of a pressure or flow of the breathing gas in the ventilation tubing system; a leak estimation module that identifies leakage in the ventilation tubing system; a delivered lung volume module that calculates a leak-compensated delivered lung volume for a first breath based on the leakage during the first breath and the flow of the breathing gas in the ventilation tubing system; a respiratory mechanics calculation module that generates a leak-compensated lung compliance based on the leak-compensated delivered lung volume and at least one output indicative of a pressure of the breathing gas during the first breath; and a pressure control module that causes the pressure generating system to deliver a second breath to the patient at a target pressure calculated based on the leak-compensated lung compliance and the leak-compensated delivered lung volume. 
     The disclosure also describes a controller for a medical ventilator that includes a microprocessor, a module (which may be a software program executed by the microprocessor, or a component comprising software, hardware and/or firmware that is separate from the microprocessor) that calculates leak-compensated delivered lung volume and leak-compensated lung compliance based on instantaneous elastic leakage and instantaneous inelastic leakage of breathing gas from a ventilation system, and a pressure control module that provides pressure regulated volume control ventilation at a pressure determined based on the leak-compensated delivered lung volume and the leak-compensated lung compliance. 
     These and various other features as well as advantages which characterize the systems and methods described herein will be apparent from a reading of the following detailed description and a review of the associated drawings. Additional features are set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the technology. The benefits and features of the technology will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawing figures, which form a part of this application, are illustrative of described technology and are not meant to limit the scope of the invention as claimed in any manner, which scope shall be based on the claims appended hereto. 
         FIG. 1  illustrates an embodiment of a ventilator connected to a human patient. 
         FIG. 2  schematically depicts example systems and methods of ventilator control. 
         FIG. 3  illustrates an embodiment of a method of compensating for leakage in a ventilator providing pressure-regulated volume control ventilation to a patient. 
         FIG. 4  illustrates an embodiment of a method for calculated leak-compensated lung flow and delivered lung volume while providing pressure-regulated volume control ventilation to a patient. 
         FIG. 5  illustrates a functional block diagram of modules and other components that may be used in an embodiment of ventilator that compensates for elastic and rigid orifice sources of leaks when determining the target pressure during pressure-regulated volume control ventilation. 
     
    
    
     DETAILED DESCRIPTION 
     Although the techniques introduced above and discussed in detail below may be implemented for a variety of medical devices, the present disclosure will discuss the implementation of these techniques in the context of a medical ventilator providing pressure regulated volume control (PRVC) ventilation to a human patient. The reader will understand that the technology described in the context of a medical ventilator for human patients could be adapted for use with other systems such as ventilators for non-human patients and general gas transport systems in which leaks may cause a degradation of performance. 
     In PRVC ventilation, a ventilator may evaluate the volume delivered to the patient over a breath or a specified time period and compare it against the volume set by the therapist. If the delivered volume is less than the setting, then the pressure target is increased proportionally. 
       FIG. 1  illustrates an embodiment of a ventilator  20  connected to a human patient  24  that is adapted to provide PRVC ventilation. Ventilator  20  includes a pneumatic system  22  (also referred to as a pressure generating system  22 ) for circulating breathing gases to and from patient  24  via the ventilation tubing system  26 , which couples the patient to the pneumatic system via physical patient interface  28  and ventilator circuit  30 . Ventilator circuit  30  could be a dual-limb or single-limb circuit for carrying gas to and from the patient. In a dual-limb embodiment as shown, a wye fitting  36  may be provided as shown to couple the patient interface  28  to the inspiratory limb  32  and the expiratory limb  34  of the circuit  30 . 
     The present systems and methods have proved particularly advantageous in noninvasive settings, such as with facial breathing masks, as those settings typically are more susceptible to leaks. However, leaks do occur in a variety of settings, and the present description contemplates that the patient interface may be invasive or non-invasive, and of any configuration suitable for communicating a flow of breathing gas from the patient circuit to an airway of the patient. Examples of suitable patient interface devices include a nasal mask, nasal/oral mask (which is shown in  FIG. 1 ), nasal prong, full-face mask, tracheal tube, endotracheal tube, nasal pillow, etc. 
     Pneumatic system  22  may be configured in a variety of ways. In the present example, system  22  includes an expiratory module  40  coupled with an expiratory limb  34  and an inspiratory module  42  coupled with an inspiratory limb  32 . Compressor  44  or another source(s) of pressurized gas (e.g., air and oxygen) is coupled with inspiratory module  42  to provide a gas source for ventilatory support via inspiratory limb  32 . 
     The pneumatic system may include a variety of other components, including sources for pressurized air and/or oxygen, mixing modules, valves, sensors, tubing, accumulators, filters, etc. Controller  50  is operatively coupled with pneumatic system  22 , signal measurement and acquisition systems, and an operator interface  52  may be provided to enable an operator to interact with the ventilator (e.g., change ventilator settings, select operational modes, view monitored parameters, etc.). Controller  50  may include memory  54 , one or more processors  56 , storage  58 , and/or other components of the type commonly found in command and control computing devices. 
     The memory  54  is computer-readable storage media that stores software that is executed by the processor  56  and which controls the operation of the ventilator  20 . In an embodiment, the memory  54  comprises one or more solid-state storage devices such as flash memory chips. In an alternative embodiment, the memory  54  may be mass storage connected to the processor  56  through a mass storage controller (not shown) and a communications bus (not shown). Although the description of computer-readable media contained herein refers to a solid-state storage, it should be appreciated by those skilled in the art that computer-readable storage media can be any available media that can be accessed by the processor  56 . Computer-readable storage media includes volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules or other data. Computer-readable storage media includes, but is not limited to, RAM, ROM, EPROM, EEPROM, flash memory or other solid state memory technology, CD-ROM, DVD, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computer. 
     As described in more detail below, controller  50  issues commands to pneumatic system  22  in order to control the breathing assistance provided to the patient by the ventilator. The specific commands may be based on inputs received from an operator, the patient  24 , the pneumatic system  22  and sensors, the operator interface  52  and/or other components of the ventilator. In the depicted example, operator interface includes a display  59  that is touch-sensitive, enabling the display to serve both as an input and output device. 
       FIG. 2  schematically depicts exemplary systems and methods of ventilator control. As shown, controller  50  issues control commands  60  to drive pneumatic system  22  and thereby circulate breathing gas to and from patient  24 . The depicted schematic interaction between pneumatic system  22  and patient  24  may be viewed in terms of pressure and/or flow “signals.” For example, signal  62  may be an increased pressure which is applied to the patient via inspiratory limb  32 . Control commands  60  are based upon inputs received at controller  50  which may include, among other things, inputs from operator interface  52 , and feedback from pneumatic system  22  (e.g., from pressure/flow sensors) and/or sensed from patient  24 . 
     In an embodiment, before the respiratory mechanics of a patient can be determined, the mechanics of the ventilation tubing system may be determined. For example, when modeling the delivery of gas to and from a patient  24  via a closed-circuit ventilator, one simple assumption is that compliance of the ventilator circuit  30  (the “circuit compliance”) is fixed and that all gas injected into the ventilator circuit  30  that does not exit the circuit  30  via the expiratory limb  34  (in a dual-limb embodiment) fills the circuit as well as the patient&#39;s lungs and causes an increase in pressure. As gas is injected (L 1 ), the lung responds to the increased gas pressure in the circuit  30  by expanding. The amount the lung expands is proportional to the lung compliance and is defined as a function of gas pressure differential (e.g., lung compliance=volume delivered/pressure difference). As discussed in greater detail below, this assumption is not valid when leaks are present. 
     The term circuit compliance is used to refer to the relationship between the pressure in the ventilator circuit  30  (or ventilator circuit  30  and attached patient interface  28 , depending on how the compliance is determined) changes based on changes in volume delivered into the circuit. In an embodiment, the circuit compliance may be estimated by pressurizing the ventilator circuit  30  (or circuit  30  and interface  28  combination) when flow to the patient is blocked and measuring the volume of additional gas introduced to cause the pressure change (compliance=volume delivered/pressure difference). 
     The term circuit resistance is used to refer to the amount the pressure changes between two sites upstream and downstream the ventilator circuit as a function of volumetric flow rate through that circuit. Circuit resistance may be modeled as a two-parameter function of flow and several methods for modeling and calculating circuit resistance are known in the art. For example, in an embodiment, the circuit resistance may be estimated by passing several fixed flow rates through the circuit and measuring the pressure difference between certain upstream and downstream sites and finding the best curve fit to the collected data. 
     Methods of determining circuit compliance and circuit resistance (such as those described above) may be executed by the operator prior to attaching the patient to the ventilator as part of the set up of the ventilator  20  to provide therapy. Other methods of determining circuit compliance and/or resistance during therapy are also known and could be adapted for use with the disclosed leak-compensation systems and methods described herein. 
     In many cases, it may be desirable to establish a baseline pressure and/or flow trajectory for a given respiratory therapy session. The volume of breathing gas delivered to the patient&#39;s lung (L 1 ) and the volume of the gas exhaled by the patient (L 2 ) are measured or determined, and the measured or predicted/estimated leaks are accounted for to ensure accurate delivery and data reporting and monitoring. Accordingly, the more accurate the leak estimation, the better the baseline calculation of delivered and exhaled flow rates and volumes. 
     Errors may be introduced due to leaks in the ventilation tubing system  26 . The term ventilation tubing system  26  is used herein to describe the ventilator circuit  30 , any equipment attached to or used in the ventilator circuit  30  such as water traps, monitors, drug delivery devices, etc. (not shown), and the patient interface  28 . Depending on the embodiment, this may include some equipment contained in the inspiration module  42  and/or the expiration module  40 . When referring to leaks in or from the ventilation tubing system  26 , such leaks include leaks within the tubing system  26  and leaks where the tubing system  26  connects to the pressure generator  22  or the patient  24 . Thus, leaks from the ventilation tubing system  26  include leaks from the ventilator circuit  30 , leaks from the patient interface  28  (e.g., masks are often provided with holes or other pressure relief devices through which some leakage may occur), leaks from the point of connection of the patient interface  28  to the patient  24  (e.g., leaks around the edges of a mask due to a poor fit or patient movement), and leaks from the point of connection of the patient interface  28  to the circuit  30  (e.g., due to a poor connection between the patient interface  28  and the circuit  30 ). 
     For the purpose of estimating how a leak flow rate changes based on changes in pressure in the ventilation tubing system  26 , the instantaneous leak may be modeled as a leak through a single rigid orifice or opening of a fixed size in which that size is determined based on comparing the total flow into the inspiratory limb  32  and out of the expiratory limb  34 . However, this leak model does not take into account any elastic component of leak source(s) in the system  26 , that is how much of the area of any of the holes or openings in the ventilation tubing system  26  through which leakage occurs may change due to an increase or decrease in pressure. 
     It has been determined that not accounting for elastic leakage from the ventilation tubing system  26  can cause many problems. First, if only the inelastic/fixed orifice model is used to estimate leak, the subsequent errors caused by ignoring the elastic effects of any actual leaks end up generating inaccurate estimates of flow rates into the lung. This can cause the ventilator  20  to estimate gas volume delivered into the lung inaccurately when, in fact, the elastic leaks in the system  26  have let more gas escape than estimated. Second, if the elasticity of the leak source is ignored, any other calculation, estimate, or action that the ventilator  20  may perform which is affected by the leak estimate will be less accurate. 
     In the systems and methods described herein, the provision of PRVC ventilation is made more accurate by compensating for leakage from the ventilation tubing system. In the embodiments described herein fixed (rigid) and elastic components of the system leakage are used when determining the lung flow, net lung volume, lung compliance and lung resistance of the patient. This results in a more accurate determination of lung compliance and lung resistance and, therefore, ventilation of patients based on respiratory mechanics. While the systems and methods are presented in the context of specific leakage models, the technology described herein could be used to compensate the respiratory mechanics determined by any model for leakage using any type of mechanical ventilator or other device that provides gas. 
       FIG. 3  illustrates an embodiment of a method of compensating PRVC ventilation for leakage during delivery of gas from a medical ventilator to a patient. In the method  300  shown, a medical ventilator such as that described above with reference to  FIGS. 1 and 2  is used to provide PRVC ventilation to a patient. 
     The method  300  illustrated starts with a setup operation  301  in which the operator directs the ventilator to provide PRVC ventilation. In the setup operation  301 , the operator selects a volume of gas to be delivered to the patient, that is to be delivered into the lung, over a specified time period such as a minute, a number of breaths, etc. This desired delivered volume is received by the ventilator and stored in memory for use during PRVC ventilation. 
     In the embodiment shown, the method  300  includes a circuit compliance and resistance operation  302 . In that operation  302 , the ventilator circuit compliance and resistance are estimated. In an embodiment, this may be performed prior to connecting the ventilator to the patient (as previously described). Alternatively, it may be dynamically determined periodically throughout the delivery of ventilatory support to the patient. The circuit compliance and resistance may be used in the remaining operations to correct for any losses in volume or effects on the volume delivered to the patient introduced due to the patient circuit. 
     After the circuit compliance and resistance have been determined, the ventilator is connected to the patient and an initialization operation  304  is performed. In the initialization operation  304  the ventilator operates for an initialization period in order to generate an initial estimate of lung compliance. If the ventilator already has some knowledge of the respiratory mechanics of the patient (e.g., the respiratory mechanics have been recently determined during provision of a different type of ventilation or an operator has provided initial settings for lung compliance and resistance), this operation  304  may be automatically or manually omitted in favor of the previously determined values. 
     A description of an embodiment of the initialization operation  304  is as follows. Because the ventilator does not know the patient&#39;s mechanics when the PRVC breath type is selected, it performs a startup routine to obtain initial data. In an embodiment, upon startup the ventilator delivers some number (e.g., two, four, etc.) of consecutive pressure-based breaths. One or more of these initial breaths given in the startup period may also include an end-inspiratory or other maneuver that yields estimates of the patient&#39;s lung compliance. Using four training breaths for the initialization operation  304  as an example, the first breath is delivered using a predicted resistance for the artificial airway and conservative estimates for patient lung compliance. The predicted values may be determined based on known characteristics of the patient, such as based on the patient&#39;s ideal body weight (IBW), height, gender, age, physical condition, etc. Each of the following three pressure-based breaths averages stepwise decreased physiologic values with the estimated lung compliance values from the previous breaths, weighting earlier estimates less with each successive breath, and yielding more reliable estimates for lung compliance. Other methods may be used to find a first estimate of the lung compliance and update it on an ongoing basis as ventilation continues. 
     In an embodiment of the method  300 , a leakage estimate may also be done prior to the initialization operation  304 . Prior determination of leak parameters allows for estimates of respiratory mechanics to be made. This may include delivering pressure-regulated breaths with specific settings or performing specific “leak maneuvers”, that is a specified set of controlled actions on the part of the ventilator that allow leakage parameters to be identified and quantified, such as interrupting the therapeutic delivery of respiratory gas and holding or changing the pressure and flow, so that data concerning the leakage of the system during the controlled actions may be obtained. For example, a leak maneuver may include periodically holding the pressure and flow in the circuit constant while determining (based on a comparison of the measured flow into the inspiratory limb and the measured flow out of the expiratory limb via the exhalation valve) the net leakage from the system. In an embodiment, such a leak maneuver may be performed during specific parts of the respiratory phase, e.g., at the end of the expiratory phase. In yet another embodiment, a sequence of pressure-based test breaths is delivered with specific settings to determine leak parameters prior to execution of test breaths for respiratory mechanics determinations. 
     After the initialization operation  304 , the ventilator provides ongoing PRVC ventilation to the patient in a PRVC ventilation operation  306 . As discussed above, during PRVC ventilation the ventilator calculates a target pressure to be delivered to the patient during inspiration based on the desired lung volume to be delivered and the leak-compensated lung compliance of the patient. When in an inspiratory phase, the ventilator raises the pressure in the ventilator tubing system so that the target inspiratory pressure is applied. During exhalation, the pressure may be dropped to some pre-selected positive end expiratory pressure (PEEP) level or to atmospheric level depending on the desires of the operator. The duration of the inspiratory and expiratory phases may be determined based on patient effort or based on a preselected inspiratory time. 
     As described above, the target pressure delivered during each inspiratory phase is determined based on the desired lung volume to be delivered and leak-compensated lung compliance of the patient. In order to compensate for leakage in the circuit, in the method  300  shown the PRVC ventilation operation  306  includes the ongoing calculation of leakage while providing ventilation, as illustrated by the leakage calculation/compensation operation  307 . As discussed in greater detail below with reference to  FIG. 4 , the leakage is calculated and the leak-compensated values for lung flow and current lung volume (i.e., the volume of gas in the lung at that moment) are determined taking into account the calculated leakage. 
     The method  300  also includes determining the leak-compensated delivered lung volume in a delivered volume calculation operation  308 . The delivered volume calculation operation  308  uses the leak-compensated lung flow and net lung volume to determine how much gas was delivered to the lungs of the patient during the breath. In an embodiment, this operation  308  is an ongoing operation in which the volume of gas delivered to the patient is accumulated over the course of the inspiratory phase, so that upon completion of the inspiratory phase the accumulated volume is the total delivered volume for the breath. Alternatively, this operation  308  may be performed as soon as the inspiratory phase ends, any time during the expiratory phase or at the beginning of the next inspiratory phase using leak-compensated lung flow or volume data collected during the breath. 
     The leak-compensated delivered lung volume for the breath is then used in a lung compliance calculation operation  310  to calculate a leak-compensated lung compliance for the patient. In an embodiment, the lung compliance calculation operation  310  includes using the following equation to determine lung compliance:
 
Leak-Compensated Lung Compliance= V /(EIP−EEP)
 
in which V is the leak-compensated delivered lung volume for the breath, EIP is the pressure at the end of the inspiratory phase of the breath, and EEP is the pressure at the end of the expiratory phase of the breath. As mentioned above, these pressure values are determined from the monitoring of pressure and/or flow during the PRVC ventilation operation  306 .
 
     The above equation is but one example of a method of determined lung compliance from parameters monitored by a ventilator such as pressure, flow and volume. Any suitable method may be used as long as the monitored parameters are compensated for the leakage identified in the leakage calculation/compensation operation  307  so that a leak-compensated lung compliance is obtained. 
     In an embodiment, the delivered volume calculation operation  308  and lung compliance calculation operation  310  may be performed as a single operation at the same time instead of separately as shown. 
     Based on the leak-compensated lung compliance, the system then calculates a target pressure for the next breath in a calculate target pressure operation  312 . This calculation takes into account the leak-compensated volume of gas delivered to the patient&#39;s lungs during the last breath to determine if sufficient volume is being delivered relative to the desired volume identified by the operator. This calculation also may take into account the leak-compensated volume of gas delivered during earlier breaths depending on the time period over which the desired volume is to be delivered. Depending on the comparison of the leak-compensated delivered lung volume and the desired delivered lung volume, the target pressure to be used for the next breath may be raised or lowered relative to the current pressure. The amount the target pressure is raised or lowered is a function of the leak-compensated lung compliance, the predicted or known inspiratory phase duration and ventilator settings as well as patient safety precautions. 
     The newly determined values of lung compliance and lung resistance may be averaged, low-pass filtered or otherwise combined with the previously determined values. These revised values are then stored for use in later delivery of PRVC ventilation. 
     In an embodiment, the delivered volume calculation operation  308 , lung compliance calculation operation  310  and calculate target pressure operation  312  may be performed as a single operation at the same time instead of separately as shown. 
     After calculating the revised target pressure, upon the next inspiration the ventilator then provides the revised target pressure to the patient, illustrated in  FIG. 3  by the flow returning to the PRVC ventilation operation  306 . The method  300  is then repeated from that point until the delivery of PRVC ventilation is terminated by some outside action, such as, change of mode setting or failure to converge on an acceptable lung compliance value. 
       FIG. 4  illustrates an embodiment of a method for calculating leak-compensated parameters while providing PRVC ventilation to a patient. In an embodiment, the method  400  corresponds to the operations performed during the leakage determination operation  306  discussed with reference to  FIG. 3 . In the embodiment of the method  400  illustrated, the operations occur repeatedly while the ventilator is providing PRVC ventilation, such as once a sample period or computation cycle, while the ventilator is providing either the target pressure (during the inspiratory phase) or an expiratory pressure such as PEEP (during the expiratory phase). 
     During PRVC ventilation, the pressure and flow and other parameters of the system are monitored, illustrated by the monitoring operation  402 . In an embodiment, the monitoring operation  402  collects data including the instantaneous pressure and/or flow at or indicative of one or more locations in the ventilation tubing system. Depending upon how a particular leak model is defined, the operation  402  may also include making one or more calculations using data from pressure and flow measurements taken by the sensors. For example, a model may require a flow measurement as observed at the patient interface even though the ventilation system may not have a flow sensor at that location in the ventilation tubing system. Thus, a measurement from a sensor or sensors located elsewhere in the system (or data from a different type of sensor at the location) may be mathematically manipulated in order to obtain an estimate of the flow observed at the patient interface in order to calculate the leak using the model. 
     The data obtained in the monitoring operation  402  is then used to calculate leakage from the ventilator tubing system in a leakage calculation operation  404 . In an embodiment, the leakage calculation operation  404  uses the data obtained in the monitoring operation  402 , e.g., some or all of the instantaneous pressure and flow data collected during the monitoring operation  402  as well as information about the current respiratory phase (inhalation or exhalation). 
     The leakage calculation operation  404  calculates an instantaneous leakage flow or volume for the sample period. The instantaneous leakage is calculated using a mathematical formula that has been previously determined. In an embodiment, the mathematical formula is a leakage model that separates the leak into the sum of two leak components, inelastic leak and elastic leak, in which each component represents a different relationship between the quantity of leakage from the ventilation system and the measured current/instantaneous pressure and/or flow of gas in the ventilation system. As discussed above, the inelastic leak may be modeled as the flow through a rigid orifice of a fixed size while the elastic leak may be modeled as the flow through a different orifice of a size that changes based on the pressure (or flow) of the gas in the ventilation system. 
     An example of a method and system for modeling leak in a ventilation system as a combination of an elastic leak component and an inelastic leak component can be found in commonly-assigned U.S. Provisional Patent Application Ser. No. 61/041,070, filed Mar. 31, 2008, titled VENTILATOR LEAK COMPENSATION, which application is hereby incorporated by reference herein. The VENTILATOR LEAK COMPENSATION represents one way of characterizing the leak from a ventilation system as a combination of elastic and inelastic components. Other methods and models are also possible and may be adapted for use with this technology. 
     The mathematical formula used to calculate leakage may contain several parameters that are empirically determined and that may be periodically or occasionally revised in order to maintain the accuracy of the leakage estimate. For example, in an embodiment the parameters of a leakage formula include a first constant associated with the rigid orifice and a second constant associated with the variable-sized orifice. At various times during ventilation, the calculated leakage may be checked against a measured leakage and, if the estimate is significantly different from the measured leakage, the constants may be revised. This revision of the parameters in a leakage formula may be done as part of the leakage calculation operation  404  or may be done as a separate operation (not shown) that may, or may not, be performed every sample period. 
     The term instantaneous is used herein to describe a determination made for any particular instant or sampling period based on the measured data for that instant. For example, if a pressure measurement is taken every 5 milliseconds (sample period), the pressure measurement and the leak model can be used to determine an instantaneous leak flow based on the instantaneous pressure measurement. With knowledge of the length of the sample period, the instantaneous flow may then be used to determine an instantaneous volume of gas leaking out of the circuit during that sample period. For longer periods covering multiple sample periods the instantaneous values for each sample period may be summed to obtain a total leakage volume. If a measurement is also the most recent measurement taken, then the instantaneous value may also be referred to as the current value. 
     After the current leak has been calculated, the method  400  further estimates the leak-compensated instantaneous lung flow to or from the patient in a lung flow estimation operation  406 . The estimated lung flow is compensated for the leak flow calculated in the instantaneous leak calculation operation  404  so that it represents a more accurate estimate of the actual flow into (or out of depending on the point of view and period selected) the lungs of the patient. 
     In the embodiment illustrated, the leak-compensated net and delivered lung volumes are also calculated as part of the lung flow estimation operation  406 . In an embodiment, this may be performed by maintaining a running summation of net flow into/out of the lung over the period of a breath and a running summation of the flow delivered to the patient during the inspiratory phase. For example, upon triggering inhalation, the ventilator may set a variable corresponding to net lung volume to zero and, each sample period, update this net lung volume to include the detected leak-compensated instantaneous lung flow delivered to the patient during that sample period. Likewise, the ventilator may also set a variable corresponding to delivered lung volume to zero and, each sample period during the inspiratory phase, update this net lung volume to include the detected leak-compensated instantaneous lung flow into the patent, if any, during that sample period. 
     In the PRVC ventilation method  400  illustrated, the leak-compensated lung flow or delivered lung volume will ultimately be used to calculate a leak-compensated lung compliance as described in  FIG. 3 . Ultimately, this leak-compensated lung compliance along with other leak-compensated data will be used to determine the target pressure for the next inspiratory phase. 
     The method  400  is then repeated every computational cycle or sample period, as illustrated by the feedback loop, so that the leak-compensated instantaneous lung flow and leak-compensated delivered lung flow are continuously determined during PRVC ventilation. 
     The following is a discussion of two embodiments of methods for compensating the estimation of respiratory mechanics for leaks. The first embodiment is that of applying leak compensation to a static compliance and resistance determination. The second embodiment is that of applying leak compensation to a dynamic compliance determination. 
       FIG. 5  illustrates a functional block diagram of modules and other components that may be used in an embodiment of ventilator that compensates for elastic and rigid orifice sources of leaks when determining the target pressure during PRVC ventilation. In the embodiment shown, the ventilator  500  includes pressure sensors  506  (two are shown placed at different locations in the system), flow sensors (one is shown), and a ventilator control system  502 . The ventilator control system  502  controls the operation of the ventilator and includes a plurality of modules described by their function. In the embodiment shown, the ventilator control system  502  includes a processor  508 , memory  514  which may include mass storage as described above, a leak estimation module  512  incorporating a parametric leak model accounting for both elastic and rigid orifice leak sources such as that described in U.S. Provisional Application 61/041,070 previously incorporated herein, a target pressure calculation module  516 , a pressure and flow control module  518 , a monitoring module  522 , a leak-compensated lung compliance module  524 , and a leak-compensated lung flow and volume estimation module  526 . The processor  508  and memory  514  have been discussed above. Each of the other modules will be discussed in turn below. 
     The main functions of the ventilator such as receiving and interpreting operator inputs and providing therapy via changing pressure and flow of gas in the ventilator circuit are performed by the control module  518 . In the context of the methods and systems described herein, the module  518  will perform one or more actions upon the determination that a patient receiving therapy is inhaling or exhaling. 
     In the embodiment described herein, the control module  518  determines and provides the appropriate pressure to the patient when in PRVC ventilation mode. This may include performing one or more calculations based on leak-compensated lung flow, leak-compensated lung volume, leak-compensated lung compliance and leak-compensated lung resistance. 
     The calculation of the target pressure to provide during the inspiratory phase of a breath is performed by the target pressure calculation module  516 . The target pressure is calculated based on the therapist-selected desired lung volume and the leak-compensated delivered lung volume. The module  516  utilizes one or more respiratory models suitable for determination of target pressure based on monitored parameters and/or leak-compensated respiratory mechanics such as lung compliance. The module  516  uses leak-compensated values for one or both of lung flow and delivered lung volume when calculating the target pressure, depending on the method used by the module. Leak-compensated values may be retrieved if they have already been calculated or may be calculated as needed from leakage information received from the leak-compensated lung flow and net lung volume estimation module  526 . 
     The dynamic calculation of lung compliance is performed by the leak-compensated lung compliance calculation module  524 . The module  524  utilizes one or more dynamic respiratory models, such as that described above with reference to lung compliance calculation operation  310  of  FIG. 3 , to calculate leak-compensated lung compliance. The module  524  uses leak-compensated values for one or both of lung flows and delivered lung volume when calculating lung compliance. Leak-compensated values may be retrieved if they have already been calculated or may be calculated from leakage information received from the leak-compensated lung flow and delivered lung volume estimation module  526 . 
     The current conditions in the ventilation system are monitored by the monitoring module  522 . This module  522  collects the data generated by the sensors  504 ,  506  and may also perform certain calculations on the data to make the data more readily usable by other modules or may process the current data and or previously acquired data or operator input to derive auxiliary parameters or attributes of interest. In an embodiment, the monitoring module  522  receives data and provides it to each of the other modules in the ventilator control system  502  that need the current pressure or flow data for the system. 
     In the embodiment shown, leak-compensated lung flow and delivered lung volume are calculated by the lung flow module  526 . The lung flow module  526  uses a quantitative model for lung flow of the patient during both inhalation and exhalation and from this characterization and pressure and flow measurements generates an estimate for instantaneous lung flow. In an embodiment, lung flow may be simply determined based on subtracting the estimated leak flow and measured outflow via the expiratory limb from the flow into the inspiratory limb, thereby generating a leak-compensated net flow into (or out of) the lung. The lung flow module  526  may or may not also calculate a leak-compensated delivered lung volume for a patient&#39;s breath as described above. Compression in the circuits and accessories may also be accounted for to improve the accuracy of estimated lung flow. 
     The leak model parameters are generated by the leak estimation module  512  which creates one or more quantitative mathematical models, equations or correlations that uses pressure and flow observed in the ventilation system over regular periods of respiratory cycles (inhalation and exhalation) and apply physical and mathematical principles derived from mass balance and characteristic waveform settings of ventilation modalities (regulated pressure or flow trajectories) to derive the parameters of the leak model incorporating both rigid and elastic (variable pressure-dependent) orifices. In an embodiment, the mathematical model may be a model such as:
 
 Q   inelastic   =R   1   *P   i   x  
 
 Q   elastic   =R   2   *P   i   y  
 
wherein Q elastic  is the instantaneous leak flow due to elastic leaks in the ventilation system, Q inelastic  is the instantaneous leak flow due to inelastic leaks in the ventilation system, R 1  is the inelastic leak constant, R 2  is the elastic leak constant, P i  is the current or instantaneous pressure measurement, x is an exponent for use when determining the inelastic leak and y is an exponent different than x for use when determining the elastic leak. The group R 1 *P i   x  represents flow through an orifice of fixed size as a function of instantaneous pressure P i  and the group R 2 *P i   y  represents flow through a different orifice that varies in size based on the instantaneous pressure. The equations above presuppose that there will always be an elastic component and an inelastic component of leakage from the ventilation system. In the absence of an elastic component or a leak source of varying size, R 2  would turn out be zero.
 
     In the embodiment shown, the current or instantaneous elastic leak is calculated by the leak estimation module  512 . The calculation is made using the elastic leak portion of the leak model developed by the leak estimation module  512  and the pressure data obtained by the monitoring module  522 . The leak estimation module  512  may calculate a new instantaneous elastic leak flow or volume for each pressure sample taken (i.e., for each sampling period) by the monitoring module  522 . The calculated elastic leak may then be provided to any other module as needed. 
     In the embodiment shown, the current or instantaneous inelastic leak is also calculated by the leak estimation module  512 . The calculation is made using the inelastic leak portion of the leak model and the pressure data obtained by the monitoring module  522 . The leak estimation module  512  may calculate a new instantaneous inelastic leak flow or volume for each pressure sample taken (i.e., for each sampling period) by the monitoring module  522 . The calculated inelastic leak may then be provided to any other module as needed. 
     The system  500  illustrated will compensate lung flow for leaks due to elastic and inelastic leaks in the ventilation system. Furthermore, the system may perform a dynamic compensation of lung flow based on the changing leak conditions of the ventilation system and the instantaneous pressure and flow measurements. The system then compensates the lung compliance and target pressure calculations based on the estimated leakage in the system. By compensating for the inelastic as well as the elastic components of dynamic leaks, the medical ventilator can more accurately and precisely a target pressure so that the desired lung volume selected by the therapist is achieved. 
     Furthermore, embodiments of the systems and methods described above may also include checks and balances based on patient type and known characteristics (e.g., Ideal Body Weight, etc.). For example, a calculated pressure target (or change between the current and the newly calculated pressure target to be used in the next inspiration) may be compared against a safety criteria based on Ideal Body Weight, age, gender, patient parameters determined during ventilation or operator provided safety thresholds. 
     If the comparison indicates the newly calculated pressure is unsafe, the ventilator may perform one or more safety actions. In an embodiment, such safety actions may include limiting stepwise changes in desired pressure target and generating alarms or warnings. Delivery of PRVC may also be terminated in situations deemed unsafe for the patient or when acceptable data are not available (e.g, when the process of lung compliance estimation fails to converge to an acceptable value). In such a situation the ventilator may switch to some other mode, such as a pressure support mode or volume control mode, than PRVC. The mode switched may be determined by the operator when setting up the PRVC ventilation or may be a default mode selected by the manufacturer. 
     It will be clear that the systems and methods described herein are well adapted to attain the ends and advantages mentioned as well as those inherent therein. Those skilled in the art will recognize that the methods and systems within this specification may be implemented in many manners and as such is not to be limited by the foregoing exemplified embodiments and examples. For example, the operations and steps of the embodiments of methods described herein may be combined or the sequence of the operations may be changed while still achieving the goals of the technology. In addition, specific functions and/or actions may also be allocated in such as a way as to be performed by a different module or method step without deviating from the overall disclosure. In other words, functional elements being performed by a single or multiple components, in various combinations of hardware and software, and individual functions can be distributed among software applications. In this regard, any number of the features of the different embodiments described herein may be combined into one single embodiment and alternate embodiments having fewer than or more than all of the features herein described are possible. 
     While various embodiments have been described for purposes of this disclosure, various changes and modifications may be made which are well within the scope of the technology described herein. For example, the systems and methods described herein could be adapted to periodically perform a static respiratory mechanics maneuver to obtain a more accurate lung compliance estimate in order to check the dynamically determined leak-compensated lung compliance. Numerous other changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the disclosure and as defined in the appended claims.