Patent Publication Number: US-11033700-B2

Title: Leak determination in a breathing assistance system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 14/980,052, (now U.S. Pat. No. 10,463,819), entitled “LEAK DETERMINATION IN A BREATHING ASSISTANCE SYSTEM,” filed on Dec. 28, 2015, which application is a continuation application of U.S. patent application Ser. No. 14/570,106 (now U.S. Pat. No. 9,254,369), filed on Dec. 15, 2014, which application is a continuation application of U.S. patent application Ser. No. 14/059,193 (now U.S. Pat. No. 8,939,150), filed on Oct. 21, 2013, which application is a continuation application of U.S. patent application Ser. No. 12/769,876 (now U.S. Pat. No. 8,707,952), filed Apr. 29, 2010, which application claims the benefit of U.S. Provisional Application No. 61/303,093, filed Feb. 10, 2010, the complete disclosures of which are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to the field of breathing assistance systems, e.g., systems and methods for estimating leak flow in a breathing assistance system (e.g., a ventilator). 
     BACKGROUND 
     The present description pertains to breathing assistance devices, such as ventilators, for example. Modern ventilators commonly employ positive pressure to assist patient ventilation. For example, after determining a patient-initiated or timed trigger, the ventilator delivers a specified gas mixture into an inhalation airway connected to the patient to track a specified desired pressure or flow trajectory, causing or assisting the patient&#39;s lungs to fill. Upon reaching the end of the inspiration, the added support is removed and the patient is allowed to passively exhale and the ventilator controls the gas flow through the system to maintain a designated airway pressure level (PEEP) during the exhalation phase. Other types of ventilators are non-triggered, and mandate a specified breathing pattern regardless of patient effort. 
     Modern ventilators typically include microprocessors or other controllers that employ various control schemes. These control schemes are used to command a pneumatic system (e.g., valves) that regulates the flow rates of breathing gases to and/or from the patient. Closed-loop control is often employed, using data from pressure, flow, and/or other types of sensors. 
     Certain ventilation configurations provide the potential for leaks occurring at various locations in the ventilation system, e.g., at connection interfaces between system components or at the interface between a breathing mask or tube and the patient&#39;s mouth or face. The magnitude of leaks in the system can vary from setting to setting, and/or dynamically within a particular setting, dependent upon a host of variables. Leaks can impair triggering (transition into inhalation phase) and cycling (transition into exhalation phase) of the ventilator, and thus cause problems with patient-device synchrony, undesirably increase patient breathing work, degrade advisory information available to treatment providers, and/or otherwise compromise the desired respiratory therapy. 
       FIGS. 1A and 1B  provide an example pressure waveform and corresponding flow waveform to illustrate one effect of system leaks.  FIG. 3A  illustrates an example desired pressure waveform in a “Pressure Support” ventilation mode.  FIG. 3B  illustrates two different flow waveforms that provide the desired pressure waveform: one waveform in a configuration with no leaks, and one waveform in a configuration with leaks. As shown, during inspiration more flow is required in the leak configuration to achieve the same pressure level as compared to no-leak configuration. Further, during exhalation, a portion of the volume exhaled by the patient may exit through the leak and be missed by the ventilator exhalation flow measurement subsystem. In many cases, the goal of the control system is to deliver a controlled pressure or flow profile or trajectory (e.g., pressure or flow as a function of time) during the inspiratory phase of the breathing cycle. In other words, control is performed to achieve a desired time-varying pressure or flow output from a pneumatic system, with an eye toward causing or aiding the desired tidal breathing (e.g., the desired pressure profile shown in  FIG. 3A ). 
     Improper leak accounting can compromise the timing and magnitude of the control signals applied to the pneumatic system, especially during volume delivery. Also, inaccurate (or inadequate) leak compensation can jeopardize spirometry and patient data monitoring and reporting calculations. For example, the pressure applied from the pneumatic system to the patient may cause leakage of breathing gas to atmosphere. This leakage to atmosphere may occur, for example, at some point on the inspiratory limb or expiratory limb of the patient circuit, or at the connection between the breathing circuit and pneumatic system or between the breathing circuit and the patient interface (e.g., facial mask or endotracheal tube). 
     In the case of non-invasive ventilation, it is typical for some amount of breathing gas to escape via the opening defined between the patient interface (e.g., facial mask) and the surface of the patient&#39;s face. In facial masks, this opening can occur at a variety of locations around the edge of the mask, and the size and deformability of the mask can create significant leak variations. Due to the elastic nature of these masks and the movement of the patient, a leak compensation strategy assuming a constant size leak orifice (and thus constant leak flow) may be inadequate. 
     Accurately estimating and/or accounting for the magnitude of the leak flow may provide significant advantages. In order for a controller to command the pneumatic system to deliver the desired amount of volume/pressure to the patient at the desired time and measure/estimate the accurate amount of gas volume exhaled by the patient, the controller needs knowledge of the extent of the leak flow over time. The fact that the leak magnitude changes dynamically during operation of the ventilator introduces additional complexity to the problem of leak modeling. 
     Triggering and cycling (patient-ventilator) synchrony may also be compromised by sub-optimal leak estimation. In devices with patient-triggered and patient-cycled modalities that support spontaneous breathing efforts by the patient, it can be important to accurately detect when the patient wishes to inhale and exhale. Detection commonly occurs by using accurate pressure and/or lung flow (flow rates into or out of the patient&#39;s lung) variations. Leaks in the airway may cause an error in the signals to the sensors of the pneumatic system. This error may impede the ability of ventilator to detect the start of an inspiratory effort, which in turn compromises the ability of the controller to drive the pneumatic system in a fashion that is synchronous with the patient&#39;s spontaneous breathing cycles. 
     Accordingly, attempts have been made in existing control systems compensate for ventilation system leaks, which first involves estimation of the leak. However, prior techniques for determining leaks have had limited success, particularly in the case of patient-triggered breathing (i.e., patient-triggered inspiration). 
     SUMMARY 
     In some embodiments, a method for estimating a leak flow in a breathing assistance system including a ventilation device connected to a patient is provided. The method may include accessing data of a flow waveform indicating the flow of gas between the ventilation device and the patient, identifying a specific portion of the flow waveform, and performing a linear regression of the identified portion of the flow waveform to determine an estimated leak flow in the breathing assistance system. 
     In some embodiments, a method for controlling breathing assistance provided to a patient includes providing breathing assistance to a patient according to one or more operational parameters, estimating a leak flow, and controlling at least one of the operational parameters based at least on the estimated leak flow. Estimating the leak flow includes accessing data of a flow waveform indicating the flow of gas between the ventilation device and the patient, identifying a specific portion of the flow waveform, and performing a linear regression of the identified portion of the flow waveform to determine an estimated leak flow in the breathing assistance system. 
     In some embodiments, a breathing assistance system for providing breathing assistance to a patient includes a pneumatic system for providing breathing assistance to a patient, and a controller configured to control one or more operational parameters of the pneumatic system. The controller is configured to estimate a leak flow in a breathing assistance system including a ventilation device connected to a patient by: accessing data of a flow waveform indicating the flow of gas between the ventilation device and the patient, identifying a specific portion of the flow waveform, and performing a linear regression of the identified portion of the flow waveform to determine an estimated leak flow in the breathing assistance system. 
     In some embodiments, a controller for a breathing assistance system includes logic instructions embodied in tangible computer readable media and executable by one or more processors. The logic instructions are executable to access data of a flow waveform indicating the flow of gas between the ventilation device and the patient, identify a specific portion of the flow waveform, and perform a linear regression of the identified portion of the flow waveform to determine an estimated leak flow in the breathing assistance system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show example pressure and flow waveforms, respectively, to illustrate certain effects commonly caused by leaks in a breathing assistance system, such as a ventilator. 
         FIG. 2  illustrates an example ventilation system including a controller for estimating system leaks, according to certain embodiments. 
         FIG. 3  illustrates an example control system and method that may be employed in the ventilation system of  FIG. 2 , according to certain embodiments. 
         FIG. 4  illustrates an example flow waveform along with graphics illustrating certain techniques for estimating system leaks, according to certain embodiments. 
         FIG. 5  illustrates an example method of estimating a leak flow in a ventilation system (e.g., the ventilation system of  FIG. 2 ), according to certain embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     As described below, a ventilation system may be provided with control schemes that provide improved leak estimation and/or compensation. In general, these control schemes estimate the system leak based on a linear regression of flow during an end portion of the exhalation phase (referred to herein as the “end exhalation steady phase”). The present discussion focuses on specific example embodiments, though it should be appreciated that the present systems and methods are applicable to a wide variety of ventilator and other breathing assistance systems. 
       FIG. 2  illustrates an example breathing assistance system  10  according to certain embodiments of the present disclosure. Breathing assistance system  10  includes a ventilator  20  connected to a patient  24  by an airway  26 . Ventilator  20  includes a pneumatic system  22 , a controller  50 , and a display  59 . Pneumatic system  22  circulates breathing gases to and/or from patient  24  via airway  26 , which couples patient  24  to pneumatic system  22  via physical patient interface  28  and breathing circuit  30 . Breathing circuit  30  may include any conduits for communicating gas to and/or from patient  24 , e.g., a two-limb or one-limb circuit for carrying gas to and/or from patient  24 . A wye fitting  36  may be provided as shown to couple patient  24  interface to breathing circuit  30 . 
     As used herein, the term “gas” may refer to any one or more gases and/or vaporized substances suitable to be delivered to and/or from a patient via one or more breathing orifices (e.g., the nose and/or mouth), such as air, nitrogen, oxygen, any other component of air, CO 2 , vaporized water, vaporized medicines, and/or any combination of two or more of the above, for example. As used herein, the term “patient” may refer to any person or animal that may receive breathing assistance from system  10 , regardless of the medical status, official patient status, physical location, or any other characteristic of the person. Thus, for example, patients may include persons under official medical care (e.g., hospital patients), persons not under official medical care, persons receiving care at a medical care facility, persons receiving home care, etc. 
     The present systems and methods have proved particularly advantageous in non-invasive 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 patient interface  28  may be invasive or non-invasive, and of any configuration suitable for communicating a flow of breathing gas from patient circuit  30  to an airway of patient  24 . Examples of such patient interfaces  28  include nasal masks, nasal/oral masks (e.g., as shown in  FIG. 2 ), full-face masks, nasal prongs, tracheal tubes, endotracheal tubes, nasal pillows, etc. 
     Pneumatic system  22  may be configured in a variety of ways. In the illustrated 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 . A gas flow source  44  (e.g., a compressor, one or more tanks of compressed gas, a wall outlet through which pressurized air may be supplied (e.g., in a hospital or clinic), etc.) is coupled with inspiratory module  42  to provide a gas source for ventilatory support via inspiratory limb  32 . Pneumatic system  22  also includes one or more sensors  46  for measuring various parameters such as pressure, flow, temperature, etc. Although sensors  46  are illustrated as being located within pneumatic system  22 , sensors  46  may be located at any suitable location or locations in breathing assistance system  10 , e.g., within a housing of pneumatic system  22 , along breathing circuit  30 , coupled to patient interface  28 , etc. Pneumatic system  22  may also include a variety of other components, e.g., sources for pressurized air and/or oxygen, mixing modules, valves, tubing, accumulators, filters, etc. 
     Controller  50  is operatively coupled to pneumatic system  22  and an operator interface  52  enabling an operator (e.g., a physician) to interact with ventilator  20  (e.g., to change ventilator settings, select operational modes, view monitored parameters, etc.). Controller  50  may include memory  54  (e.g., RAM or any other suitable volatile and/or non-volatile memories), one or more processors  56 , data storage  58 , and/or other components of the type commonly found in command and control computing devices. As described in more detail below, controller  50  issues commands to pneumatic system  22  in order to control the breathing assistance provided to patient  24  by ventilator  20 . The specific commands may be based on inputs received from patient  24 , pneumatic system  22  (e.g., pressure and/or flow sensors  46 ), operator interface  52 , and/or other components of breathing assistance system  10 . In the illustrated example, operator interface  52  includes a display device  59 . In some embodiments, display device  59  includes a touch-sensitive interface or other input devices, enabling display device  59  to serve both as an input and output device. 
       FIG. 3  illustrates an example control system and method that may be employed in ventilation system  10  of  FIG. 2 , according to certain embodiments. As shown, controller  50  issues control commands  60  to drive pneumatic system  22  and thereby circulate breathing gas to and/or from patient  24 . The illustrated 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 patient  24  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 and/or flow sensors  46 ) and/or sensed from patient  24 . 
     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 lungs and the volume of the gas exhaled by the patient are measured or determined, and the system leak is estimated are accounted for to ensure accurate ventilation support and data reporting and monitoring. Accordingly, the more accurate the leak estimation, the better the baseline calculation of delivered and exhaled gas volume, as well as event detection (e.g., triggering and phase transitions). 
     In some embodiments, controller  50  may calculate a “leak factor” K that may be used by controller  50  for compensating for system leaks and/or for other functionality. The leak factor K may be estimated by Equation (1):
 
 K=Q leak{circumflex over ( )}2/PEEP  (1)
 
     where:
         Qleak=estimated flow at the end of exhalation obtained by linear regression interpolation over an “end exhalation steady phase” (“EESP phase”), and   PEEP=Patient pressure over the exhalation steady phase. PEEP may be determined using any known techniques. For example, PEEP for each breath may be calculated by an averaging of patient pressure measurements over the EESP phase for that breath. Alternatively, PEEP could be estimated using a linear regression technique similar to the techniques disclosed herein for estimating leak flow (Qleak).       

     Thus, the estimation of leak flow (Qleak) may be achieved by a technique including a linear regression of measured exhalation flow during a determined end portion of the exhalation phase, referred to herein as the “end exhalation steady phase” (or “EESP phase”). In general, the leak flow estimation technique includes identifying the end of the exhalation phase of a breath cycle (i.e., the beginning of the subsequent inhalation phase), analyzing the exhalation flow waveform during the exhalation phase to determine an “end exhalation steady phase” (“EESP phase”), performing a linear regression on the determined EESP phase to calculate an estimated end-of-exhalation leak flow at exhalation pressure, calculating a leak factor based on the estimated leak flow, and using the calculated leak factor to compensate for the leak and/or for other functionality. The leak flow estimation technique is discussed in greater detail below, with reference to  FIG. 4 . 
       FIG. 4  illustrates an example flow waveform  100  (i.e., flow vs. time) for an exhalation phase  102  of a breath cycle, followed by the beginning of the inspiratory phase  104  for the next breath cycle, for graphically illustrating the technique for calculating the estimated leak flow according to the technique proved herein. As discussed above, the technique includes identifying the end of exhalation phase  102  (i.e., the beginning of inhalation phase  104 ), analyzing the flow waveform during exhalation phase  102  to determine an “end exhalation steady phase” (“EESP phase”)  106 , performing a linear regression (indicated by line  108 ) on the determined EESP phase  106  to calculate an estimated end-of-exhalation leak flow  110 . The leak flow  110  may then be used to calculate a leak factor, which may then be used by controller  50  for compensating for the leak and/or for other functions of ventilator  20 . 
     In some embodiments, the beginning and/or end of EESP phase  106  depends on whether the next breath (which includes inhalation phase  104 ) is a controlled breath or a patient-triggered breath. Thus, as shown in  FIG. 4 , EESP phase  106   a  refers to an EESP phase when the next breath is a controlled breath, and EESP phase  106   b  refers to an EESP phase when the next breath is a patient-triggered breath. 
     The technique operates according to a set of rules, executed by controller  50 , for determining the beginning and the end of EESP phase  106   a / 106   b . In certain embodiments, the rules specify: 
     If the Next Breath a Controlled Breath: 
     1. The beginning of exhalation phase  102  is determined in any suitable manner, e.g., by the I/E ratio and R-Rate (implying a fixed inspiratory time) in a PRESS A/C mode or based on the “E sens” setting in PSV mode. 
     2A. The end of exhalation phase  102  is defined by the breath rate (“R-Rate”) set in controller  50  (e.g., automatically set based on the current ventilation mode, or manually set by the physician). 
     3A. The end of EESP phase  106  is defined as the end of exhalation phase  102   
     4. The duration of EESP phase  106  is some multiple of 100 ms and cannot exceed 600 ms. Thus, EESP phase  106  is either 100, 200, 300, 400, 500, or 600 ms. 
     5. The duration of EESP phase  106  cannot exceed “exhalation duration/2”. “Exhalation duration” is defined as the duration from the beginning of exhalation phase  102  (Rule 1) to the end of exhalation phase  102  (Rule 2A). 
     6. Subject to the limitations set forth in Rules 4 and 5, the duration of EESP phase  106  is determined by applying a “≤5% variance test” to successive 100 ms periods going backwards in time from the end of exhalation phase  102  (determined by Rule 2) to identify a string of zero, one, or more 100 ms periods that qualify as EESP phase  106 . A particular 100 ms period qualifies for EESP phase  106  if the average flow rate varies by ≤5% from the average flow rate of the final 100 ms period of exhalation phase  102 . 
     More specifically, the average flow rate of the 100 ms period that ends at the end of exhalation phase  102  (defined as P final ) is determined and defined as AFR Pfinal . The average flow rate of each successive 100 ms period (P final-1 , P final-2 , . . . P final-5 ) going backwards from P final  (defined as AFR Pfinal-1 , AFR Pfinal-2 , . . . AFR Pfinal-5 ) is determined and compared to AFR Pfinal , until a 100 ms period (P final-1 , P final-2 , . . . P final-5 ) is identified for which the average flow rate (AFR Pfinal-1 , AFR Pfinal-2 , . . . AFR Pfinal-5 ) varies from AFR Pfinal  by more than 5%. The duration of EESP phase  106  is defined as P final  plus each previous 100 ms period (P final-1 , P final-2 , . . . P final-5 ) that meets the “≤5% variance test” (i.e., qualifying periods), but not the time period that failed the “≤5% variance test” (i.e., the non-qualifying period) and any previous time periods. If the average flow rate of the 100 ms period immediately preceding P final  (i.e., P final-1 ) fails the “≤5% variance test”, then EESP phase  106  is defined as P final . 
     As mentioned at the beginning of Rule 6, the duration of EESP phase  106  as determined by the “≤5% variance test” is subject to the limitations of Rules 4 and 5. Thus, where the “≤5% variance test” process described above would result in a EESP phase  106  duration that would exceed the limits prescribed by Rule 4 and/or 5, the EESP phase  106  duration is instead capped by the limits of Rule 4 and/or 5. The limits of Rules 4 and 5 may be applied during or after the “≤5% variance test” process described above. For example, in some embodiments, the process of Rule 6 may be completed and the resulting EESP phase  106  duration is limited by Rule 4 and/or 5, if applicable. In other embodiments, the process of Rule 6 terminates before testing any 100 ms period (P final-1 , P final-2 , . . . P final-5 ) that would, if found to be a qualifying period, exceed the limits of Rule 4 and/or 5. 
     If the Next Breath a Patient-Triggered Breath: 
     1. Same as Rule 1 above for the scenario of a subsequent controlled breath. 
     2B. The end of exhalation phase  102  is defined by the detection of an inspiratory trigger. 
     3B. The end of EESP phase  106  is defined as predetermined duration, 100 ms, before the end of exhalation phase  102 . 
     4. Same as Rule 4 above for the scenario of a subsequent controlled breath. 
     5. Same as Rule 5 above for the scenario of a subsequent controlled breath. 
     6. Same as Rule 6 above for the scenario of a subsequent controlled breath. 
     In the example Rules presented above, only Rules 2 and 3 differ depending on whether the next breath is a patient-triggered breath. Thus, only Rules 2 and 3 are distinguished by 2A/2B and 3A/3B notations. 
     It should be understood that the various values set forth in the preceding Rules are examples only, and any other suitable values may be used. For example, the following values are examples only and any other suitable values may be used:
         the 600 ms limit for EESP phase  106  set forth in Rule 4,   the number of possible EESP phase  106  durations (6) set forth in Rule 4,   the “exhalation duration/2” formula set forth in Rule 5,   the 100 ms period duration set forth in Rules 3A/3B and 6, and   the 5% variance threshold set forth in Rule 6.       

     It should also be understood that depending on the particular embodiment, one, some, or all of the Rules set forth herein may be used. Further, any one or more additional Rules may be used in conjunction with any or all of the Rules set forth above. 
     After determining the beginning and the end (and thus the duration) of EESP phase  106   a / 106   b , controller  50  may perform a linear regression interpolation of flow waveform  100  over the determined EESP phase  106   a / 106   b , and determine Qleak as the leak flow at the end of exhalation phase  102  according to the linear regression interpolation. Controller  50  may use any known techniques for performing the linear regression analysis. 
     After determining the end of exhalation flow, Qleak, controller  50  may calculate the leak factor K using Equation (1) above. Controller  50  may then use the calculated leak factor K for compensating for the leak and/or for other functionality. The leak factor K may be used for various purposes. For example, controller  50  may use the leak factor K for evaluating the real patient flow (e.g., by subtracting the inspiration leak flow Qleak′=(K*Pressure) 0.5 =Qleak*(Pressure/PEEP) 0.5  at the current pressure from the flow measured by flow sensor  46 ). Then controller  50  may calculate the tidal volume by integrating Qleak′ over the inspiratory phase. As another example, Qleak′ can also be used for determining the inspiration phase ending (e.g., using an Esens function based on a peak flow % threshold) that is corrupted by any leak occurring if no leak compensation is implemented. As another example, Qleak may be used to display a leak value on display  59  for the user to track the leak variations of the interface. In some embodiments, Qleak is used in pressure controlled/support breath. However, in other embodiments, Qleak may be used for driving directly an inspiratory flow in a volume controlled breath. 
     Any or all of Qleak, PEEP, and leak factor K may be recalculated/determined after each breath, at regular intervals (e.g., after every x breaths or after every y seconds), upon a predetermined triggering event (e.g., a patient-triggered breath, or a change in Qleak or PEEP greater than a preset threshold value), or according to any other frequency or timing. In addition, any or all of Qleak, PEEP, and leak factor K may be utilized by controller  50  as individual calculated values, as average values of multiple iterations (e.g., a rolling average of the last five calculated Qleak values or K values), or according to any other suitable algorithm. 
     Any or all of the Rules, calculations, estimations, and other functions set forth herein may be implemented in any suitable hardware, software, firmware, or any combination thereof. For example, the Rules and equations set forth above may be embodied in firmware or software stored in memory or other computer readable medium associated with or accessible by controller  50 . 
       FIG. 5  illustrates an example method  200  for determining a leak factor, and regulating one or more functions of ventilator  20  based on the determined leak factor, according to certain example embodiments. More specifically, method  200  incorporates the Rules and techniques discussed above, according to certain example embodiments. 
     At step  202 , ventilator  20  provides breathing assistance to patient  24 , e.g., as controlled by controller  50  based on the current ventilation mode and/or settings. 
     At step  204 , controller  50  records a flow vs. time waveform, e.g., which may be similar to the waveform shown in  FIG. 1B . 
     At step  206 , controller  50  identifies the exhalation phase for the next breath cycle. 
     At step  208 , controller  50  identifies the beginning, end, and duration of the identified exhalation phase. Controller  50  may use any suitable techniques or rules for determining the beginning and end of the exhalation phase, e.g., example Rules 1 and 2A/2B presented above. 
     At steps  210 - 232 , controller  50  analyzes the exhalation phase flow waveform to determine the “end exhalation steady phase” (EESP phase) for the exhalation phase. 
     At step  210 , controller  50  identifies the end of the EESP phase. Controller  50  may use any suitable techniques or rules, e.g., example Rules 3A/3B presented above. 
     At step  212 , controller  50  identifies the 100 ms period that ends at the end of the exhalation phase (P final ) and determines the average flow rate during P final  (AFR Pfinal ). 
     At step  214 , controller  50  includes P final  in the EESP phase being determined. 
     At step  216 , controller  50  resets a counter “n” equal to 0. 
     At step  218 , controller  50  determines whether the EESP phase duration plus 100 ms is greater than one-half of the duration of the identified exhalation phase determined at step  208  (see Rule 5 above). If so, the EESP phase determination is complete, and the method skips to step  232 . If not, the method proceeds to step  220 . 
     At step  220 , controller  50  determines whether the EESP phase duration plus 100 ms is greater than 600 ms (see Rule 4 above). If so, the EESP phase determination is complete, and the method skips to step  232 . If not, the method proceeds to step  222 . 
     At step  222 , controller  50  increases the counter “n” to “n+1”. 
     At step  224 , controller  50  applies the “≤5% variance test” to the period P final-n . Specifically, controller  50  determines whether the average flow rate during the period P final-n  (AFR Pfinal ) varies from the average flow rate during the period P final  (AFR Pfinal ) by more than 5%. If so, P final-n  is identified as a Non-Qualified Period at step  226  and is not included from the EESP phase, the EESP phase determination is complete, and the method skips to step  232 . 
     Alternatively, if AFR Pfinal  varies from AFR Pfinal  by less than or equal to 5%, P final-n  is identified as a Qualified Period at step  228  and included in the EESP at step  230 . The method then returns to step  218  to analyze the next previous 100 ms period. Steps  218 - 230  are repeated until a threshold is exceeded at steps  218  or  220  or until a Non-Qualified Period is identified at step  226 , any of which occurrences completes the EESP determination at step  232 . 
     As indicated as step  232 , the final determined EESP for the exhalation phase being analyzed includes the final period P final  and each Qualified Period determined at step  228 , if any. 
     At step  234 , controller  50  performs a linear regression of flow waveform  100  over the determined EESP phase, and determines leak flow, Qleak, at the end of exhalation phase  102 , e.g., as shown in  FIG. 4 . Controller  50  may use any known techniques for performing the linear regression analysis. 
     At step  236 , controller  50  calculates PEEP using any suitable technique, e.g., by averaging pressure over the EESP. 
     At step  238 , controller  50  calculates a leak factor K using Equation (1) above, based on the Qleak and PEEP values determined at steps  234  and  236 . 
     At step  240 , controller  50  may then use the calculated leak factor K for controlling any one or more functions of ventilator  20 , according to any predetermined rules or algorithms. 
     The method then returns to step  202  for continued ventilation assistance and analysis of the next exhalation phase. 
     The techniques discussed herein may provide one or more various advantages. For example, using the linear regression techniques discussed herein, variation associated with raw measurements regarding leak determination may be reduced. The techniques introduce a level of filtering but without the type/level of error associated with other filtering methods, such as averaging. As another example, the techniques discussed herein may be more robust to patient triggering than known techniques, in which patient triggering often negatively affects the accuracy of the leak determination. 
     It will be appreciated that while the disclosure is particularly described in the context of breathing assistance systems, the apparatuses, techniques, and methods disclosed herein may be similarly applied in other contexts. For example, similar principles may be applied in certain CPAP devices, BiPAP devices, or other suitable types of breathing assistance devices. Additionally, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as illustrated by the following claims.