Patent Publication Number: US-11638796-B2

Title: Methods and systems for exhalation control and trajectory optimization

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/456,892, entitled “METHODS AND SYSTEMS FOR EXHALATION CONTROL AND TRAJECTORY OPTIMIZATION,” filed on Mar. 13, 2017, which is a continuation application of U.S. patent application Ser. No. 13/098,130, entitled “METHODS AND SYSTEMS FOR EXHALATION CONTROL AND TRAJECTORY OPTIMIZATION,” filed Apr. 29, 2011, the disclosures of which are hereby incorporated herein by reference. 
    
    
     INTRODUCTION 
     Medical ventilator systems have long been used to provide supplemental oxygen support to patients. These ventilators typically comprise a source of pressurized air and oxygen, and which is fluidly connected to the patient through a conduit or tubing. The amount of pressure in the gas mixture delivered to the patient may be controlled during ventilation including during inspiration and exhalation. 
     Patients on a ventilator system are more comfortable when the delivered volume of inspired gas is allowed to be exhaled in the shortest amount of time possible. Current exhalation modes are designed to reduce pressure in the tubing as fast as possible. Other exhalation modes reduce the pressure in the patient tubing to a preset positive end-expiratory pressure (PEEP) level as fast as possible and then maintain this PEEP level through the remainder of the exhalation period. These exhalation modes are based on the assumption that achieving the highest pressure gradient across the flow restriction promotes the greatest lung flow at any point in time, and the fastest rate of lung emptying. 
     SUMMARY 
     This disclosure describes systems and methods for controlling pressure and/or flow during exhalation. The disclosure describes novel exhalation modes for ventilating a patient. 
     In part, this disclosure describes a method for controlling exhalation during ventilation of a patient on a ventilator. The method includes: 
     a) determining at least one determined pressure profile based on at least one received criterion for an exhalation by a patient being ventilated on a ventilator; 
     b) selecting a pressure profile for delivery to the patient from the at least one determined pressure profile; and 
     c) controlling at least one of airway pressure and flow based on the selected pressure profile during the exhalation by the patient. 
     Yet another aspect of this disclosure describes a method for optimizing a pressure profile delivered to a patient during exhalation on a ventilator including: 
     a) delivering at least one of airway pressure and flow based on a pressure profile during a current exhalation to a patient during ventilation on a ventilator; 
     b) monitoring at least one parameter during the current exhalation by the patient; 
     c) modifying the pressure profile based at least in part on the monitored at least one parameter; and 
     d) delivering at least one of a modified airway pressure and a modified flow based on the modified pressure profile to the patient during at least one of the current exhalation and the next exhalation. 
     Further, the modified pressure profile maintains a received PEEP. 
     The disclosure further describes a computer-readable medium having computer-executable instructions for performing a method controlling exhalation during ventilation of a patient on a ventilator. The method includes: 
     a) repeatedly determining at least one determined pressure profile based on at least one received criterion for an exhalation by a patient being ventilated on a ventilator; 
     b) repeatedly selecting a pressure profile for delivery to the patient from the at least one determined pressure profile; and 
     c) repeatedly controlling at least one of airway pressure and flow based on the selected pressure profile during the exhalation by the patient. 
     The disclosure also describes a ventilator system including means for determining at least one determined pressure profile based on at least one received criterion for an exhalation by a patient being ventilated on a ventilator; means for selecting a pressure profile for delivery to the patient from the at least one determined pressure profile; and means for controlling at least one of airway pressure and flow based on the selected pressure profile during the exhalation by the patient. 
     The disclosure further describes a ventilator system including means for delivering at least one of airway pressure and flow based on a pressure profile during a current exhalation to a patient during ventilation on a ventilator; means for monitoring at least one parameter during the current exhalation by the patient; means for modifying the pressure profile based at least in part on the monitored at least one parameter; and means for delivering at least one of a modified airway pressure and a modified flow based on the modified pressure profile to the patient during at least one of the current exhalation and the next exhalation. Further, the modified pressure profile maintains a received PEEP. 
     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. 
    
    
     
       EXHALATION CONTROL AND TRAJECTORY OPTIMIZATION 
       The following drawing figures, which form a part of this application, are illustrative of embodiments, systems and methods described below and are not meant to limit the scope of the invention in any manner, which scope shall be based on the claims appended hereto. 
         FIG.  1    illustrates an embodiment of a ventilator. 
         FIG.  2    illustrates an embodiment of an exhalation module. 
         FIG.  3    illustrates an embodiment of a schematic model of a lung demonstrating the pressure and resistance relationship within the two compartments of the lung. 
         FIG.  4    illustrates an embodiment of a pressure profile. 
         FIG.  5    illustrates an embodiment of a method for controlling exhalation during ventilation of a patient on a ventilator. 
         FIG.  6    illustrates an embodiment of a method for optimizing exhalation during ventilation of a patient on a ventilator. 
         FIG.  7    illustrates an embodiment of a graph of the effect of different patient&#39;s tubing exhalation pressure profiles on the time required to passively exhale 50% of a given inspired tidal volume in simulations 
         FIG.  8    illustrates an embodiment of a graph of the effect of different patient&#39;s tubing exhalation pressure profiles on the time required to passively exhale 90% of a given inspired tidal volume in simulations. 
     
    
    
     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 for use in providing ventilation support 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. 
     Medical ventilators are used to provide a breathing gas to a patient who may otherwise be unable to breathe sufficiently. In modern medical facilities, pressurized air and oxygen sources are often available from wall outlets. Accordingly, ventilators may provide pressure regulating valves (or regulators) connected to centralized sources of pressurized air and pressurized oxygen. The regulating valves function to regulate flow so that respiratory gas having a desired concentration of oxygen is supplied to the patient at desired pressures and rates. Ventilators capable of operating independently of external sources of pressurized air are also available. 
     While operating a ventilator, it is desirable to control the percentage of oxygen in the gas supplied by the ventilator to the patient. Further, it is desirable to control the amount of pressure delivered to the patient during inspiration. In some ventilators, it is desirable to control the amount of pressure delivered to the patient during exhalation. 
     The pressure control provided during exhalation is based on the assumption that patients find it more comfortable to exhale unimpeded. For example, modes of exhalation have been designed to reduce the pressure in the patient tubing to a preset positive end exhalation pressure (PEEP) level or to atmospheric pressure as fast as possible. If a preset PEEP is utilized, the exhalation mode must also maintain this pressure at the set PEEP level throughout exhalation. This exhalation approach is justified by the belief that achieving the highest pressure gradient across the flow restriction promotes the greatest lung flow at any point in time, and the fastest rate of lung emptying. This approach is correct if the resistance of the airways is independent from the actual pressure in the airways and lungs. 
     However, the resistance of lung airways may not always be independent from the actual pressure in the airways and lungs. It is suspected that a nonlinear character of resistance of internal lung airways causes the exhalation lung flow to decay more rapidly than normal thereby preventing complete lung emptying, causing patients discomfort, suboptimal ventilation, etc. For example, the physiology of the lung and airways associated with different disease states has been identified as a significant contributor to the impairment of the normal lung emptying process during exhalation. Thus, this nonlinear dependency of the airways resistance to the lung and airway pressure may result in a non-intuitive relationship between the optimum tubing pressure profile and the exhalation lung flow. Accordingly, it is desirable to modify the exhalation mode to obtain a faster rate of lung emptying or to decrease the amount of time it takes the patient to passively expire an inspired volume of gas to provide for faster and/or complete lung emptying. 
       FIG.  1    is a diagram illustrating an embodiment of an exemplary ventilator  100  connected to a human patient  150 . Ventilator  100  includes a pneumatic system  102  (also referred to as a pressure generating system  102 ) for circulating breathing gases to and from patient  150  via the ventilation tubing system  130 , which couples the patient  150  to the pneumatic system  102  via an invasive (e.g., endotracheal tube, as shown) or a non-invasive (e.g., nasal mask) patient interface  180 . 
     Ventilation tubing system  130  (or patient circuit  130 ) may be a two-limb (shown) or a one-limb circuit for carrying gases to and from the patient  150 . In a two-limb embodiment, a fitting, typically referred to as a “wye-fitting”  170 , may be provided to couple a patient interface  180  (as shown, an endotracheal tube) to an inspiratory limb  132  and an expiratory limb  134  of the ventilation tubing system  130 . 
     Pneumatic system  102  may be configured in a variety of ways. In the present example, pneumatic system  102  includes an expiratory module  108  coupled with the expiratory limb  134  and an inspiratory module  104  coupled with the inspiratory limb  132 . Compressor  106  or other source(s) of pressurized gases (e.g., air, oxygen, and/or helium) is coupled with inspiratory module  104  and the expiratory module  108  to provide a gas source for ventilatory support via inspiratory limb  132 . 
     The inspiratory module  104  determines the pressure profiles delivered during inspiration. The expiratory module  108  determines the pressure profiles delivered during exhalation. In one embodiment, the inspiratory module  104  and the expiratory module  108  determine the pressure profiles during ventilation by controlling valves and gas flow within the ventilator  100 . As used herein, the term “pressure profile” refers to how pressure is delivered for the entire period of exhalation, such as the amount of pressure per second or millisecond of the exhalation time period. In an alternative embodiment, the inspiratory module  104  and the expiratory module  108  determine the pressure profiles during ventilation by sending instructions to the controller  110  to control the valves and gas flow within the ventilator  100  during ventilation. 
     Previously utilized systems provided pressure control during exhalation based on the assumption that patients find it more comfortable to exhale fast and on the belief that achieving the highest pressure gradient across the flow restriction promotes the greatest lung flow at any point in time, and the fastest rate of lung emptying. This previously utilized approach is correct if the resistance of the airways is independent from the actual pressure in the airways and lungs. However, the resistance of lung airways may not always be independent from the actual pressure in the airways and lungs. It is suspected that a nonlinear character of resistance of internal lung airways causes the exhalation lung flow to decay more rapidly than normal thereby preventing complete lung emptying, causing patients discomfort, suboptimal ventilation, etc. 
     Accordingly, the expiratory module  108  determines the pressure profile delivered during exhalation based on at least one received criterion. The at least one received criterion does not include a received or set PEEP. However, the pressure profile determined by the expiratory module  108  may include a received or a set PEEP in addition to the received at least one criterion. 
     For the example, the at least one criterion may include a nonlinear relationship between airway resistance and the lung and airway pressure within a patient. A relationship between the airway resistance to lung and airway pressure exists because the lungs are essentially divided into two compartments: 1) the upper airways; and 2) the lower airways. These two compartments of the lung create a relationship, which is nonlinear, between airway resistance and lung and airway pressure. In one embodiment, this nonlinear relationship is modeled by the following equation:
 
 R   L   =f ( P   L1   ,P   L2 ).
 
     In the above equation R L  is the amount of resistance in the lungs, f(⋅) is a nonlinear function of two values, P L1  and P L2 , where, P L1  is the amount of pressure in the first compartment of the lungs, and P L2  is the amount of pressure in the second compartment of the lungs. As would be known by a person of skill in the art, the above nonlinear equation may be a function of more than two values.  FIG.  3    illustrates this model by showing the pressure and resistance relationships between the upper airways or lung compartment  1  (C L1 ), the lower airways or lung compartment  2  (C L2 ), and the trachea.  FIG.  3    also illustrates the flow (q L ) within the lungs and the external positive pressure (P p ) acting upon the lungs. Further, the physiology of the lung and airways associated with different disease states may be a significant contributor to the impairment of the normal lung emptying process. Accordingly, a pressure profile taking into account a nonlinear relationship, such as the example nonlinear relationship shown above, may provide for more comfortable and faster exhalation for some patients. 
     The nonlinear relationship of the lung illustrated above is just one example of a non-linear pressure flow relationship that may exist in a patient. Other nonlinear relationships, between airway resistance and lung and airway pressures may exist within a patient and vary between patients based on their measured parameters and diseases. For, example, the resistance can be a nonlinear function of more than two different local pressures measured in different parts of the lung and airways. Further, not all patients exhibit a measureable nonlinear relationship between airway resistance and lung and airway pressure. Accordingly, the expiratory module  108  determines the pressure profile to deliver during exhalation based on at least one received criterion, such as ventilator data, predetermined nonlinear pressure profiles, pressure profile trajectory equations, operator determined pressure profiles, and/or measured, derived, inputted, and/or selected patient parameters to determine how to provide a pressure profile with the fastest rate of lung emptying. As discussed above, the at least one criterion does not include a received/set PEEP. However, the at least one criterion may be any suitable criterion for controlling or effecting the pressure profile to provide for a faster rate of lung emptying, such as a percent of inspired volume/elapsed time, a flow as a function of delta P (estimated lung pressure and circuit pressure), an AutoPEEP, measurements of patient resistance and/or compliance, a diagnosis (e.g., chronic obstructive pulmonary disease), an inner diameter of artificial airway, a type of patient interface (e.g., mask or tube), an ideal body weight, carbon dioxide levels in exhaled gas and/or blood, an end expiratory flow, a patient assessment of comfort/dyspnea, a percentage of volume exhaled within a given period of time after the start of exhalation, a mean expiratory flow, a peak expiratory flow, a time to exhale a predetermined percentage of inspired volume, a time to reach a predetermined level of expiratory flow, a functional residual capacity (FRC), a ratio of functional residual capacity to total lung capacity (FRC/TLC), a breath rate, a ratio of inspiratory to expiratory time, a tidal volume, a forced expiratory volume in 1 second (FEV 1 ), an expiratory lung volume, and/or an instantaneous level of flow. 
     As used herein, any parameters/criteria that are “received” are input by the clinician, selected by the clinician, or provided by the ventilator. The ventilator may derive the “received” parameter/criteria based on patient parameters, ventilator parameters, and/or input or selected clinician data. In some embodiments, the ventilator contains stored default values that are “received” or utilized by the ventilator when the clinician does not input or select a parameter or a criterion. As used herein, the term “predetermined” designates that a value was set by a clinician and/or determined by the ventilator prior to use of the value. 
     The pneumatic system  102  may include a variety of other components, including mixing modules, valves, sensors, tubing, accumulators, filters, etc. Controller  110  is operatively coupled with pneumatic system  102 , signal measurement and acquisition systems, and an operator interface  120  that may enable an operator to interact with the ventilator  100  (e.g., change ventilator settings, select operational modes, view monitored parameters, etc.). Controller  110  may include memory  112 , one or more processors  116 , storage  114 , and/or other components of the type commonly found in command and control computing devices. In the depicted example, operator interface  120  includes a display  122  that may be touch-sensitive and/or voice-activated, enabling the display  122  to serve both as an input and output device. 
     The memory  112  includes non-transitory, computer-readable storage media that stores software that is executed by the processor  116  and which controls the operation of the ventilator  100 . In an embodiment, the memory  112  includes one or more solid-state storage devices such as flash memory chips. In an alternative embodiment, the memory  112  may be mass storage connected to the processor  116  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  116 . That is, computer-readable storage media includes non-transitory, 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. For example, computer-readable storage media includes 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. 
       FIG.  2    illustrates an exhalation module  200 . The exhalation module  200  may include memory, one or more processors, storage, and/or other components of the type commonly found in command and control computing devices as described above. The exhalation module  200  further includes a pressure profile module  202  and a parameters module  204 . 
     The pressure profile module  202  determines the pressure profile delivered during exhalation based on at least one received criterion. This pressure profile affects the amount of time a patient takes to exhale the inspired amount of inspiratory gas and rate of lung emptying. 
     The pressure profile module  202  utilizes at least one criterion for reducing the amount of time it takes the patient to exhale an inspired volume of gas and/or for increasing the rate of lung emptying as would be known by a person of skill in the art. As discussed above, the at least one criterion does not include a received PEEP. However, the pressure profile module  202  may utilize a received PEEP in addition to the received at least one criterion for determining the pressure profile. The at least one criterion may include ventilator data, predetermined pressure profiles, pressure profile trajectory equations, operator determined pressure profiles, a nonlinear relationship between airway resistance to the lung and airway pressure, and/or measured, derived, inputted, and/or selected patient parameters. 
     For example, the at least one criterion may include patient parameters, such as height, heart rate, weight, diseases, ideal body weight, etc. The criterion may further include ventilator data, such as flow rate, respiration rate, ventilation modes, expiration time, etc. The pressure profile module  202  may receive the at least one criteria from other ventilator components (e.g., a sensor, user interface, and/or controller) and/or may calculate/derive the desired criteria from received criteria or parameters. The ventilator may determine the desired pressure profile for each breath, over a fixed number of breaths (e.g., take data over a fixed number of breaths), or for a predetermined period of time. Further, the ventilator may repeat this calculation periodically as determined by a ventilator or as selected by the operator. 
     In some embodiments, the at least one criterion is ventilator data or patient parameters, such as a current ventilation mode and/or a diagnosed patient condition. In other embodiments, the criterion is a group of pressure profiles provided by the ventilator, which the operator may select the pressure profile from. Any suitable pressure profile for shortening the amount of time to exhale the delivered amount of inspiratory gas or for increasing the rate of lung emptying based on the at least one received criterion may be utilized by the exhalation module  200 . 
     In one embodiment, the at least one criterion includes an exhalation pressure fall time parameter (EPFTP). The EPFTP is the amount of time it takes for the pressure to drop from the inspiratory pressure level to the set PEEP level during exhalation. In one embodiment, the actual rate of pressure decay in the ventilator is defined by the EPFTP. In another embodiment, the pressure profile module  202  determines the pressure profile based on the actual rate of pressure decay as defined by the EPFTP. 
     In a further embodiment, pressure profile module  202  determines the pressure profile based on the at least one criterion of an actual rate of pressure decay in the ventilator  100  as defined by predetermined parameters that are settable and modified by operators or the ventilator automatically. In another embodiment, the at least one criterion is a family of different pressure profiles that is utilized by the pressure profile module  202  to determine the pressure profile. The family of different pressure profiles may be defined by a predetermined set of criteria, such as EPFTP and exhalation time. 
     In an additional embodiment, the pressure profile module  202  utilizes at least one parameter that is repeatedly monitored during an exhalation to optimize the pressure profile. In this embodiment, the pressure profile is adjusted or modified during the current exhalation and/or before the next exhalation according to the monitored parameter in order to achieve a smaller time required to exhale the delivered amount of inspiratory gas and/or to achieve a faster rate of lung emptying in the next exhalation. 
     In another embodiment, the pressure profile module  202  utilizes repeated measurements of the time required to exhale the delivered amount of inspiratory gas to optimize the pressure profile. In this embodiment, different parameters of the pressure profile are adjusted after each exhalation according to the measured time required to exhale the delivered amount of inspiratory gas in order to achieve a shorter time required to exhale the delivered amount of inspiratory gas in the next exhalation. 
     In one embodiment, the at least one criterion is the pressure at the patient wye-fitting or the trajectory of the pressure profile. In some embodiments, the trajectory or the patient wye-fitting is determined based on the following equation:
 
 P   y =PEEP+(EIP−PEEP) e   −αt .
 
     In this equation, P y  is the pressure at the patient wye-fitting, PEEP is the set positive end exhalation pressure, EIP is the measured end inspiratory pressure, α is greater than zero and denotes the EPFTP, and t is the amount of time measured from the onset of an exhalation phase. This equation illustrates an exponential decay from EIP to PEEP, with alpha being varied to create different trajectories. For example, alpha may be varied (e.g., 0.1, 0.2, 0.3, . . . ∞) and then optimized based upon a predetermined parameter, such as the amount of time it takes the patient to exhale 50% of an inspired volume. 
     In some embodiments, the at least one criterion utilized by the pressure profile module is an operator-determined pressure profile. The operator may select or input a desired pressure profile or input various different parameters for modifying a pressure profile as desired. Allowing the operator to adjust, change, and input a pressure profile provides the operator with several benefits. For instance, the operator may select a pressure profile based on patient comfort. For example, the operator may deliver two different pressure profiles to a patient, ask the patient which profile the patient prefers, and then select a pressure profile based on the patient&#39;s answer. 
     In other embodiments, the at least one criterion utilized by the pressure profile module is a nonlinear relationship between airway resistance to the lung and airway pressure. In another embodiment, the pressure profile module  202  utilizes a predetermined nonlinear pressure profile as the at least one criteria, such as a nonlinear pressure profile with a fast initial decay in pressure followed by an increase of pressure to provide a shorter amount of time required to exhale the delivered amount of inspiratory gas. For example, an embodiment of a predetermined nonlinear pressure profile for the pressure profile module  202  is illustrated in  FIG.  4   .  FIG.  4    illustrates a pressure profile with a fast initial decay of pressure that actually drops below a set PEEP, then increases in pressure to above the set PEEP at time 0.50 seconds followed by a gradual pressure reduction after the increase in pressure at time 0.8 seconds back down to the set PEEP. 
     The at least one criterion may be any suitable criterion for controlling or effecting the pressure profile to provide for a faster rate of lung emptying, such as a percent of inspired volume/elapsed time, a flow as a function of delta P (estimated lung pressure and circuit pressure), an AutoPEEP, measurements of patient resistance and/or compliance, a diagnosis (e.g., chronic obstructive pulmonary disease), an inner diameter of artificial airway, a type of patient interface (e.g., mask or tube), an ideal body weight, carbon dioxide levels in exhaled gas and/or blood, an end expiratory flow, a patient assessment of comfort/dyspnea, a percentage of volume exhaled within a given period of time after the start of exhalation, a mean expiratory flow, a peak expiratory flow, a time to exhale a predetermined percentage of inspired volume, a time to reach a predetermined level of expiratory flow, a functional residual capacity (FRC), a ratio of functional residual capacity to total lung capacity (FRC/TLC), a breath rate, a ratio of inspiratory to expiratory time, a tidal volume, a forced expiratory volume in 1 second (FEV 1 ), an expiratory lung volume, and/or an instantaneous level of flow. 
     The embodiments for determining the pressure profiles by the pressure profile module  202  as described and discussed above are exemplary only and may be utilized alone or in various combinations. It is understood by a person of skill in the art that any suitable pressure profile based on the received PEEP and the at least one received criterion may be utilized by the pressure profile module  202 . 
     In some embodiments, if more than one pressure profile is determined by the pressure profile module  202 , then the pressure profile module  202  selects the pressure profile with the fastest rate lung emptying to be sent to the parameters module  204 . Further, the pressure profile module may present some or all of the unselected pressure profiles to the operator for selection depending on how the system is implemented and the degree of operator control desired. In a further embodiment, if more than one pressure profile is determined by the pressure profile module  202 , then the pressure profile module  202  presents all of the pressure profiles to the operator for selection and continues to deliver the previously utilized pressure profile during exhalation until a new pressure profile is selected by the operator. 
     The ventilator may utilize various different methods to determine different pressure profiles. For example, the ventilator may utilize different criteria to determine different pressure profiles. In this embodiment, the ventilator may automatically select the pressure profile calculated with a specific criterion, such as a measurement of patient compliance and/or resistance, and only utilize the other calculated pressure profiles based on other criterion if a predetermined threshold is met by other measured patient or ventilator parameters. If the threshold is met, the ventilator may select a pressure profile calculated based on exhalation time. In some embodiments, the ventilator may have a predetermined nonlinear pressure profile stored, such as the one displayed in  FIG.  4   , in addition to a calculated pressure profile. In this embodiment, the ventilator may automatically select the calculated pressure profile unless a predetermined threshold is met by patient or ventilator measured parameters. For example, the predetermined threshold may be related to work of breathing or arterial blood gas saturation. The selection parameters listed above are merely exemplary. The ventilator may utilize any suitable means for selecting a pressure profile from a family of pressure profiles as would be known by a person of skill in the art for ventilating a patient. 
     In some embodiments, the family of pressure profiles is created utilizing the following equation:
 
 P   y ( t )=PEEP+(EIP−PEEP) e   −αt  
 
which is described in detail above. In this embodiment, different values for alpha ranging from 0.1 to 100 and/or to infinity may be applied over a time period to generate several different pressure profiles. The ventilator may select one of these calculated pressure profiles utilizing various different techniques. For example, the ventilator may compare the calculated pressure profiles to previously delivered pressure profiles and choose the calculated pressure profile closest to a previously utilized pressure profile that obtained the fastest rate of lung emptying. In another embodiment, the ventilator delivers a different calculated exhalation profile in each breath for at least two consecutive breaths. The ventilator in this scenario may then select which pressure profile to deliver based on a predetermined parameter, such as the amount of time it takes the patient to exhale 50% of an inspired volume, which was measured during the delivery of the pressure profile.
 
     For example, a pressure profile with an alpha of 1 may be delivered in a first breath and a pressure profile with an alpha of 2 may delivered in a second breath. During the delivery of these pressure profiles the ventilator may measure the amount of time it takes a patient to exhale 90% of the volume of an inspired breath. In this example, the ventilator compares each of these measured times and then delivers in the next exhalation the pressure profile with the shortest measured time. 
     The parameters module  204  receives the pressure profile for the current or next exhalation from the pressure profile module  202 . The parameters module  204  determines the necessary ventilator settings for delivering airway pressure and/or flow based on the received pressure profile. In one embodiment, the parameters module  204  sends the necessary ventilator settings to a controller for implementation. In an alternative embodiment, the parameters module  204  sends the instructions directly to the necessary component or components (e.g., to the exhalation valve) for implementing the desired pressure profile during exhalation. 
     In some embodiments, the exhalation module  200  is part of the pressure generating system  102 , as illustrated in  FIG.  1   . In alternative embodiments, the exhalation module  200  is part of the controller  110 . In some embodiments, the pressure profile module  202  and/or the parameters module  204  are separate from the exhalation module and are contained within the controller  110 . 
       FIG.  5    illustrates an embodiment of a method  500  for controlling exhalation during ventilation of a patient on a ventilator. As illustrated, method  500  includes a determination operation  502 . The ventilator in determination operation  502  determines at least one pressure profile by utilizing at least one received criterion for an exhalation by a patient being ventilated on a ventilator. 
     The at least one received criterion includes ventilation data, a nonlinear relationship between airway resistance and lung and airway pressure, predetermined pressure profiles, operator designed pressure profiles, pressure profile trajectory equations, and/or input, selected, measured, and/or derived patient parameters. As discussed above, the received criterion does not include a received PEEP. However, the determination operation  502  may utilize a received PEEP in addition to the received at least one criterion to determine the pressure profile. In some embodiments, the at least one criterion includes a current ventilation mode and/or a diagnosed patient condition. In some embodiments, the at least one criterion is a monitored parameter, such as a patient or ventilator parameter, from previous delivered exhalations. For example, the measured at least one parameter may be obtained for each breath, over a period of more than one breath (e.g., take data over a fixed number of breaths), or over a period of time. Any suitable pressure profile for shortening the amount of time to exhale the delivered amount of inspiratory gas and/or for increasing the rate of lung emptying may be utilized by the ventilator during method  500 . 
     In one embodiment, the at least one criterion utilized by the ventilator in the determination operation  502  to determine the pressure profile is an EPFTP. In another embodiment, the at least one criterion utilized by the ventilator in the determination operation  502  to determine the pressure profile is the actual rate of pressure decay. The actual rate of pressure decay may be defined by the EPFTP. In a further embodiment, the at least one criterion utilized by the ventilator in the determination operation  502  to determine the pressure profile is the actual rate of pressure decay in the ventilator as defined by predetermined parameters, which are settable and modified by operators or the ventilator automatically. The parameters may be predetermined and may include the set PEEP and a measured end inspiratory pressure. In another embodiment, a family of different pressure profiles is utilized by the ventilator in the determination operation  502  to determine the pressure profile. The family of different pressure profiles is based on a set of received predetermined criteria, such as EPFTP and exhalation time. In an additional embodiment, the at least one received criterion utilized by the ventilator in the determination operation  502  is at least one parameter, such as a ventilation or patient parameter, that is repeatedly measured to optimize the pressure profile. For example, the ventilator in the determination operation  502  may repeatedly measure the time it takes the patient to exhale the delivered amount of inspiratory gas to optimize the pressure profile. The ventilator in the determination operation  502  may utilize any suitable means for determining the shortest time to expire the delivered amount of inspiratory gas and/or for determining the faster rate of lung emptying based on a received at least one criterion. 
     In some embodiments, the received at least one criterion is the pressure at the patient wye-fitting or the trajectory of the pressure profile. In one embodiment, the pressure at the patient wye-fitting or the trajectory of the pressure profile is determined by the ventilator in the determination operation  502  by utilizing the following equation:
 
 P   y ( t )=PEEP+(EIP−PEEP) e   −αt .
 
     In this equation, P y  is the pressure at the patient wye-fitting, PEEP is the set positive end exhalation pressure, EIP is the measured end inspiratory pressure, α is greater than zero and denotes the EPFTP, and t is the amount of time measured from the onset of exhalation phase. This equation illustrates an exponential decay from EIP to PEEP, with alpha being varied to create different trajectories. For example, alpha may be varied (e.g., 0.1, 0.2, 0.3, . . . ∞) and then optimized based upon a predetermined parameter, such as the amount of time it takes the patient to exhale 50% of an inspired volume. 
     In another embodiment, the at least one criterion utilized by the ventilator in the determination operation  502  to determine the pressure profile is a predetermined pressure profile. For example, the predetermined pressure profile may have a fast initial decay in pressure followed by an increase in pressure to provide a shorter amount of time required to exhale the delivered amount of inspiratory gas as illustrated in  FIG.  4   . In some embodiments, the determination operation  502  is performed by an exhalation module, a pressure profile module, pneumatic system, and/or a ventilator controller. 
     The at least one criterion may be any suitable criterion for controlling or effecting the pressure profile to provide for a faster rate of lung emptying, such as a percent of inspired volume/elapsed time, a flow as a function of delta P (estimated lung pressure and circuit pressure), an AutoPEEP, measurements of patient resistance and/or compliance, a diagnosis (e.g., chronic obstructive pulmonary disease), an inner diameter of artificial airway, a type of patient interface (e.g., mask or tube), an ideal body weight, carbon dioxide levels in exhaled gas and/or blood, an end expiratory flow, a patient assessment of comfort/dyspnea, a percentage of volume exhaled within a given period of time after the start of exhalation, a mean expiratory flow, a peak expiratory flow, a time to exhale a predetermined percentage of inspired volume, a time to reach a predetermined level of expiratory flow, a functional residual capacity (FRC), a ratio of functional residual capacity to total lung capacity (FRC/TLC), a breath rate, a ratio of inspiratory to expiratory time, a tidal volume, a forced expiratory volume in 1 second (FEV 1 ), an expiratory lung volume, and/or an instantaneous level of flow. 
     The embodiments as discussed above for determining the pressure profile in the determination operation  502  by the ventilator may be utilized alone or in various combinations. 
     Next, method  500  includes a selection operation  504 . The ventilator in selection operation  504  selects a pressure profile from the at least one determined pressure profile. In one embodiment, the ventilator of method  500  selects the pressure profile predicted to provide the shortest amount of time to expire the delivered amount of inspiratory gas and/or to provide the fastest rate of lung emptying based the received at least one criterion. In an alternative embodiment, the operator selects a pressure profile from the at least one determined pressure profile. 
     As discussed above, the ventilator may utilize various different methods to determine different pressure profiles. For example, the ventilator may utilize different criteria to determine different pressure profiles for faster lung emptying. In this embodiment, the ventilator may automatically select the pressure profile calculated based on a specific predetermined criteria, such as such as a measurement of patient compliance and/or resistance, and only utilize the other calculated pressure profiles if a predetermined threshold is met by other measured patient or ventilator parameters. If the threshold is met in this embodiment, the ventilator may automatically select a pressure profile determined based on exhalation time. In some embodiments, the ventilator may have a stored predetermined nonlinear pressure profile, such as the one displayed in  FIG.  4   , in addition to a calculated pressure profile. In this embodiment, the ventilator may automatically select the calculated pressure profile unless a predetermined threshold is met by patient or ventilator measured parameters. For example, the predetermined threshold may be related to work of breathing or arterial blood gas saturation. The selection parameters listed above are merely exemplary. The ventilator may utilize any suitable means for selecting a pressure profile from a family of pressure profiles as would be known by a person of skill in the art for ventilating a patient. 
     In some embodiments, the family of pressure profiles is created utilizing the following equation:
 
 P   y ( t )=PEEP+(EIP−PEEP) e   −αt  
 
which is described in detail above. In this embodiment, different values for alpha ranging from 0.1 to 100 and/or to infinity may be applied over a time period to generate several different pressure profiles. The ventilator may select one of these calculated pressure profiles utilizing various different techniques. For example, the ventilator may compare the calculated pressure profiles to previously delivered pressure profiles and choose the calculated pressure profile closest to a previously utilized pressure profile that obtained the fastest rate of lung emptying. In another embodiment, the ventilator delivers a different calculated exhalation profile in each breath for at least two consecutive breaths. The ventilator in this scenario may then select which pressure profile to deliver based on a predetermined parameter, such as the amount of time it takes the patient to exhale 50% of an inspired volume, which was measured during the delivery of the pressure profile.
 
     For example, a pressure profile with an alpha of 1 may be delivered in a first breath and a pressure profile with an alpha of 2 may be delivered in a second breath. During the delivery of these pressure profiles the ventilator may measure the amount of time it takes a patient to exhale 90% of the volume of an inspired breath. In this example, the ventilator compares each of these measured times and then delivers in the next exhalation the pressure profile that results in the shortest measured time. 
     Next, method  500  includes a control operation  506 . The ventilator in control operation  506  controls airway pressure and/or flow based on the selected pressure profile during the exhalation by the patient. The ventilator in control operation  506  delivers the airway pressure and/or flow based on the selected pressure profile by modifying valve settings and/or flow rates during exhalation. In some embodiments, the control operation  506  is performed by an exhalation module, a parameters module, pneumatic system, and/or a ventilator controller. 
     Method  500  may also include an inspiration operation. In the inspiration operation, the ventilator delivers a volume of gas to the patient for inspiration during ventilation on the ventilator. The exhalation by the patient includes exhaling the volume of gas inhaled by the patient from the volume of delivered gas. 
     In one embodiment, method  500  is performed by the systems illustrated in  FIGS.  1  and  2   , which are described above. 
     In some embodiments, a microprocessor-based ventilator that accesses a computer-readable medium having computer-executable instructions for performing the method of controlling exhalation during ventilation of a patient is disclosed. This method includes repeatedly performing the steps disclosed in method  500  and as illustrated in  FIG.  5   . 
     In some embodiments, a ventilator system that includes: means for determining at least one determined pressure profile based on at least one received criterion for an exhalation by a patient being ventilated on a ventilator; means for selecting a pressure profile for delivery to the patient from the at least one determined pressure profile; and means for controlling airway pressure and/or flow based on the selected pressure profile during the exhalation by the patient. 
       FIG.  6    illustrates an embodiment of a method  600  for optimizing exhalation during ventilation of a patient on a ventilator. As illustrated, method  600  includes a delivery operation  602 . The ventilator in delivery operation  602  delivers airway pressure and/or flow based on a pressure profile during an exhalation to a patient during ventilation on a ventilator. The pressure profile delivered during exhalation may be any suitable exhalation pressure profile. The ventilator either delivers pressure in accordance with a user-determined pressure profile or a ventilator-determined pressure profile. In some embodiments, the ventilator utilizes a nonlinear relationship between airway resistance to the lung and airway pressure to determine the pressure profile. In some embodiments, the ventilator utilizes a received at least one criterion to determine the pressure profile. 
     Next, method  600  includes a monitor operation  604 . In the monitor operation  604 , the ventilator monitors at least one parameter during the exhalation by the patient. The at least one parameter may be any suitable ventilator or patient parameter for determining a pressure profile for providing faster lung emptying. For example, the monitors at least one parameter may include ventilation data, data relating to a nonlinear relationship between airway resistance and lung and airway pressure, data relating to pressure profile trajectory equations, and/or measured/derived patient parameters. For example, the ventilator in the monitor operation  604  may monitor an exhalation time based on the amount of time it takes the patient to exhale at least a portion of gas inspired by the patient. 
     The monitor operation  604  may also include storing or calculating a pressure, flow and/or volume profile that describes the exhalation of the patient. Such a profile may be stored as a series of measured patient parameters taken during the exhalation phase. The ventilator may also or instead perform one or more mathematical analyses on the measured data in order to create a mathematical or model description of one or more parameter profiles during the exhalation phase. 
     Further, method  600  includes a modify operation  606 . The ventilator in the modify operation  606  modifies the pressure profile based at least in part on the at least one monitored parameter in order to increase the rate of lung emptying and/or decrease the amount of time it takes the patient to exhale an inspired volume. Additionally, in order to increase the rate of lung emptying and/or to decrease exhalation time, the ventilator in the modify operation  606  may further adjust a number of other criteria, such as gas flow, ventilation modes, exhalation time, etc. However, the ventilator during the modify operation  606  does not adjust a received PEEP in the modified pressure profile. If a PEEP was received by the ventilator, the ventilator in the modify operation  606  maintains the received PEEP. For example, if the ventilator does not receive a PEEP, the ventilator during the modify operation  606  provides/determines a modified pressure profile with no PEEP. In some embodiments, the pressure profile is determined based on monitored parameters from a group of previously delivered exhalations. Any suitable pressure profile for shortening the amount of time to exhale the delivered amount of inspiratory gas or that increases the rate of lung emptying during exhalation may be utilized by the ventilator during method  600 . For example, any suitable method for determining the pressure profile for method  600  as would be known by a person of skill in the art as described above in method  500  may be utilized by method  600 . 
     Next, the ventilator during method  600  either continues with or repeats delivery operation  602 . Again, the ventilator during delivery operation  602  delivers airway pressure and/or flow based on a pressure profile during an exhalation to a patient during ventilation on a ventilator. However, during this delivery operation  602 , the ventilator delivers a modified airway pressure and/or a modified flow based on the modified pressure profile to the patient during the current and/or next exhalation. The current exhalation is the exhalation during which the received at least one parameter was monitored and utilized to calculate the modified pressure profile. The next exhalation is the exhalation subsequent to an exhalation where the received at least one parameter was monitored and utilized to calculate the modified pressure profile. The next exhalation may further include every exhalation, a predetermined number of exhalations, or the number of exhalations performed in a predetermined amount of time subsequent to the exhalation where the at least one criterion was monitored or subsequent to the current exhalation. Accordingly, the delivery of airway pressure and/or flow based on this modified pressure profile should reduce the amount of time required by the patient to exhale the delivered volume of gas inspired by the patient and/or should increase the rate of lung emptying during the current and/or next exhalation. 
     In some embodiments, the ventilator may repeat method  600  for every breath, after a predetermined number of breaths, or after a predetermined amount of time expires. In other embodiments, method  600  is performed by the systems illustrated in  FIGS.  1  and  2   , which are described above. 
     In some embodiments, a microprocessor-based ventilator that accesses a computer-readable medium having computer-executable instructions for performing the method of controlling exhalation during ventilation of a patient is disclosed. This method includes repeatedly performing the steps disclosed in method  600  and as illustrated in  FIG.  6   . 
     In other embodiments, a ventilator system that includes: means for delivering at least one of airway pressure and flow based on a pressure profile during a current exhalation to a patient during ventilation on a ventilator; means for monitoring at least one parameter during the current exhalation by the patient; means for modifying the pressure profile based at least in part on the monitored at least one parameter; and means for delivering at least one of a modified airway pressure and a modified flow based on the modified pressure profile to the patient during at least one of the current exhalation and the next exhalation. Further, the modified pressure profile maintains a received PEEP. 
     Example 1 
     During testing with various simulation tools, it was discovered that this nonlinear relationship between airway resistance of the internal lung with lung and airway pressure causes the exhalation lung flow to decay more rapidly than normal, thereby, preventing complete lung emptying when utilizing a pressure profile based on the assumption that achieving the highest pressure gradient across the flow restriction promotes the greatest lung flow at any point in time, and the fastest rate of lung emptying. 
     For example, during simulation, the time it takes for 50% of the inspired tidal volume to be exhaled by the patient increases as the exhalation pressure decay is reduced (or as the EPFTP increases) and target PEEP is increased, as illustrated by  FIG.  7   .  FIG.  7    illustrates a graph of the effect of different patient&#39;s tubing exhalation pressure profiles on the time required to passively exhale 50% of a given inspired tidal volume in simulations. 
     For example, during simulation, the time it takes for 90% of the inspired tidal volume to be exhaled by the patient is reduced by reducing the rate of exhalation pressure decay (or increasing the EPFTP) and reducing the PEEP level, as illustrated in  FIG.  8   .  FIG.  8    illustrates a graph of the effect of different patient&#39;s tubing exhalation pressure profiles on the time required to passively exhale 90% of a given inspired tidal volume in simulations. Further,  FIG.  8    also illustrates that these changes affect the pressure profile performance non-monotonically. 
     Accordingly, these results show that decreasing pressure as fast as possible to the set PEEP rate does not always provide for the fastest exhalation. Further, these results show that a nonlinear relationship between airway resistance and lung and airway pressure exists within the lungs. 
     Those skilled in the art will recognize that the methods and systems of the present disclosure may be implemented in many manners and as such are not to be limited by the foregoing exemplary embodiments and examples. In other words, functional elements being performed by a single or multiple components, in various combinations of hardware and software or firmware, and individual functions, can be distributed among software applications at either the client or server level or both. In this regard, any number of the features of the different embodiments described herein may be combined into single or multiple embodiments, and alternate embodiments having fewer than or more than all of the features herein described are possible. Functionality may also be, in whole or in part, distributed among multiple components, in manners now known or to become known. Thus, myriad software/hardware/firmware combinations are possible in achieving the functions, features, interfaces and preferences described herein. Moreover, the scope of the present disclosure covers conventionally known manners for carrying out the described features and functions and interfaces, and those variations and modifications that may be made to the hardware or software or firmware components described herein as would be understood by those skilled in the art now and hereafter. 
     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. 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 present invention. 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.