Patent Publication Number: US-2006011195-A1

Title: Method and apparatus for non-rebreathing positive airway pressure ventilation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority under 35 U.S.C. § 119(e) from provisional U.S. patent application No. 60/587,781 filed Jul. 14, 2004, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to method of ventilating a patient, and, in particular, to a method of ventilating a patient that provides improved fluid exchange in the lungs, and to a medical ventilator that implements such a mode of ventilation.  
      2. Description of Related Art  
      It is the goal of medical ventilation to safely and effectively ventilate a patient in accordance with the patient&#39;s physiological needs. Accordingly, various methods of operating a medical ventilator in accordance with the various needs of patients have been devised. For example, it is well known to ventilate a patient such that a desired pressure, flow, or volume of fluid is delivered to the patient during inspiration and fluid is allowed to exhaust from the patient during expiration. Conventional ventilator provide a variety of modes for ventilating a patient and allow control over the trigger, the cycle, and the limit for the delivery of fluid to the patient. U.S. Pat. No. 5,868,133 to DeVries discusses such conventional ventilators and their operation.  
      In the treatment of acute lung injuries or adult respiratory distress syndrome (ARDS), a mode of ventilation known as airway pressure release ventilation (APRV), described hereinafter, has gained acceptance. This mode of ventilation is similar to a continuous positive airway pressure (CPAP) mode of pressure support in which a flow of fluid at a constant pressure is delivered to the airway of the patient, with regular, brief, intermittent releases in airway pressure imposed on the CPAP pressure.  
      Although the APRV mode of ventilation has gained acceptance in many circles, the conventional mode of ventilation, which is also described hereinafter, is still preferred, because it is more natural to patients than the APRV mode of ventilation.  
      What is needed, however, and not found in the prior art, is a mode of ventilation, and a medical ventilator that implements a mode of ventilation that combines the advantages of the conventional mode of ventilation and the APRV mode of ventilation in a manner that minimizes patient discomfort, while, at the same time, increases the amount of fluid, i.e., gas or liquid, exchanged from the patient&#39;s lungs and increases the amount of fresh fluid introduced into the patient&#39;s lungs during each ventilator cycle.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an object of the present invention to provide a method of ventilating a patient that satisfies this need. This object is achieved according to one embodiment of the present invention by providing a method of ventilating a patient that includes (a) delivering fluid to a patient during an exhalation/quiescent phase, in which a pressure, a flow, or a volume of fluid is provided at a baseline level, (b) releasing the flow of fluid from such a patient during a release phase following the exhalation/quiescent phase, in which the pressure, flow, or volume of fluid is decreased from the baseline level to a release level that is less than the baseline level. After the release phase, the pressure, flow, or volume of fluid delivered to such a patient is increased during a delivery phase to a peak level above the baseline level. Thereafter, the pressure, flow, or volume of fluid is allowed to return from the peak level to the baseline level.  
      It is a further object of the present invention to provide a medical ventilator that includes a patient circuit, a pressure generator for delivering pressurized fluid to a patient via the patient circuit, and means for controlling the pressure generator. More specifically, the pressure generator is controlled by the controlling means so as to supply the flow of fluid to the patient in accordance with the method set forth above.  
      It is a still further object of the present invention to provide a method of ventilating a patient and a system for ventilating a patient according to this method, wherein the method that includes (a) providing a fluid to a patient at a first pressure; (b) providing the fluid to the patient at a second pressure after terminating the provisioning of the fluid at the first pressure, wherein the second pressure is less than the first pressure; and (c) providing the fluid to the patient at a third pressure after terminating the provisioning of the fluid at the second pressure, wherein the third pressure is greater than the first pressure. Steps (a) through (c) are repeated over each respiratory cycle.  
      These and other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in the specification and in the claims, the singular form of “a”, an and “the” include plural referents unless the context clearly dictates otherwise. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic block diagram of ventilation system, including a patient, a patient circuit, and a ventilator operative for implementing a mode of ventilation in accordance with the principles of the present invention;  
       FIGS. 2A-2C  are waveforms associated with ventilating a patient in a conventional mode of ventilation;  
       FIGS. 3A-3C  are waveforms associated with ventilating a patient in an airway pressure release ventilation (APRV) mode of ventilation;  
       FIGS. 4A-4C  are waveforms associated with ventilating a patient in accordance with the present invention; and  
       FIGS. 5-7  are pressure waveforms illustrating triggering and cycling techniques using the ventilation mode of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS  
      With reference to  FIG. 1 , a medical ventilator  2  typically includes a pressurized fluid source  4  and a pressure regulator  6  connected to receive pressurized fluid from pressurized fluid source  4 . The source of pressurized fluid can be gas from a conventional pressurized tank, gas in which the pressure is elevated by a pressure generator, such as bellows, piston, or blower, or a combination thereof. Pressure regulator  6  regulates the pressure of the pressurized fluid, i.e., a gas or a liquid, supplied to a circuit  8 , which conveys the pressure regulated fluid to a patient  10  via an interface  12 . It can be appreciated that the pressurized fluid source and the pressure regulator can be combined into a common component that is collectively referred to as the pressure generator. For example, it is known to control the operation of the blower or compressor to produce a desired output pressure for the flow of gas exiting the pressure generating system in combination with a gas flow/pressure control valve either upstream or downstream of the blower or compressor. As used herein, the term “ventilator” or “ventilation system” refers to a device or system that delivers a flow of pressurized fluid to an airway of a patient. These terms include life support ventilators (operated either invasively or non-invasively) and a pressure support system, such as a CPAP, a bilevel system, which delivers a flow of gas at an inspiratory positive airway pressure (IPAP) during inspiration and an expiratory positive airway pressure (EPAP) during expiration.  
      Medical ventilator  2  includes a flow sensor  14  for detecting a flow and/or volume of fluid supplied to circuit  8  from pressure regulator  6 , and to provide to a controller  16  a signal indicative of the detected fluid flow, volume, or both. If desired, however, flow sensor  14  can be omitted.  
      A pressure sensor  18  is also typically provided to detect a pressure of the flow of fluid in circuit  8  and, more particularly, at interface  12 . The pressure signal from sensor  18  is provided to controller  16 , where can be used to monitor the pressure of the flow of fluid delivered to the patient, and, in particular, to detect patient inhalation and exhalation. In response to the one or more signals from flow sensor  14 , pressure sensor  18 , or both, controller  16  causes pressure regulator  6  to supply pressurized fluid to patient  10  in the manner described in detail hereinafter. Alternatively, as shown by the dashed line between controller  16  and pressurized fluid source  4 , controller  16  can control pressurized fluid source  4  to supply pressurized fluid to patient  10 .  
      As noted above, pressurized fluid source  4  can include a source, e.g., a piston, a bellows or a blower, of fluid, e.g., breathing gas, gas, air, oxygen, helium-oxygen or breathing liquid. Pressure regulator  6  can include a poppet valve, solenoid, butterfly valve or sleeve valve. If pressurized fluid source  4  includes a piston or a blower, controller  16  can control the pressure and/or flow of fluid from pressurized fluid source  4  by controlling the speed of the piston or the speed of the blower.  
      In one embodiment of the present invention, circuit  8  includes a first tube or conduit  20  connected between pressure regulator  6  and interface  12 , i.e., a single-limb circuit. First tube or conduit  20  includes an exhaust assembly  22 , which can be an active exhaust or a passive exhaust. The active exhaust can include a modified poppet valve that operates to block or limit the flow of exhaust gas from the patient circuit via exhaust assembly  22  when the flow of fluid is supplied to patient  10  during inhalation and to open or increase the flow of exhaust gas from the patient circuit when patient  10  exhales. The passive exhaust can be a hole or other aperture defined in first tube or conduit  20 , in patient interface  12 , or both. The size, shape, number, and location of the hole or holes can be selected as a compromise between enabling patient  10  to exhale naturally so as to remove exhaust gasses from the patient circuit, and enabling the flow of fluid to be supplied to patient  10  during inhalation.  
      Alternatively, circuit  8  can exclude exhaust  22  and can include a second tube or conduit  24 , i.e., a two-limb circuit, wherein second tube or conduit  24  enables fluid exhaled by patient  10  to be conveyed to ventilator  2 , where the exhaled fluid is exhausted to atmosphere. Typically, an exhaust flow control system is provided in the ventilator that controls the exhausting of fluid from the second limb.  
      Although not shown, it is also known to provide a supplemental gas, such as oxygen, to the patient circuit. The source of supplemental gas can be any conventional gas source, such as an oxygen tank, liquid oxygen storage vessel, hospital oxygen wall supply, or oxygen concentrator. The flow of supplemental gas can be introduced into the primary gas flow at any location, including in the ventilator, upstream or downstream of the pressurized fluid source, the pressure regulator, the flow sensor, or the pressure sensor. The flow of supplemental gas can also be introduced into conduit  24  or interface  12 .  
      Two prior art modes of ventilating patient  10 , namely a conventional mode and an APRV mode, utilizing ventilator  2  will now be described, followed by a description of a mode of ventilating patient  10  utilizing ventilator  2  in accordance with the present invention, namely an NRPAP mode. In the following descriptions, ventilation of patient  10  will be described as occurring in synchronization with the breathing or breath cycle of the patient. However, this is not to be construed as limiting the invention, because the ventilation of patient  10 , especially in the APRV and NRPAP modes of operation, can occur independent of the patient&#39;s breathing or breath cycle.  
      Ventilation of patient  10  independent of the patient&#39;s breathing or breath cycle is often referred to in the art as “mandatory breathing” or “machine timed breathing”. However, this is not to be construed as limiting the invention.  
      With reference to  FIGS. 2A-2C  and with continuing reference to  FIG. 1 , when operated in a conventional assist-control mode that is pressure limited, ventilator  2  causes the flow of fluid to be provided to patient  10  at a pressure P 2  during a first period  30  and causes the flow of fluid to be provided to patient  10  at a pressure P 1 , less than pressure P 2 , during a second period  32  and a third period  34  of each ventilation cycle  28  applied to patient  10 . In this conventional mode of ventilation, which is patient triggered, first period  30  of ventilation cycle  28  coincides with inhalation by patient  10 , second period  32  coincides with exhalation by patient  10 , and third period  34  coincides with a quiescent period of breathing by patient  10 .  
      In  FIGS. 2A-2C , third period  34  is shown on either side of first period  30  and second period  32  for illustration purposes in order for the latter two periods to be in the central part of the figure. However, this is not to be construed as limiting the invention since, as one skilled in the art would appreciate, the relative position of periods  30 - 34  in each x-y chart of  FIGS. 2A-2C  can be adjusted as desired. For example, the beginning of first period  30 , second period  32  or third period  34  (immediately following second period  32 ) can be positioned on the ordinate axis of each x-y chart of  FIGS. 2A-2C .  
      As shown in  FIG. 2A , during third period  34  leading up to first period  30 , the flow of fluid is provided to patient  10  at pressure P 1 , desirably a pressure between 0-10 centimeters of water (cm H 2 O) (10.1973 cm H 2 O=1000 Pascals=1000 N/m 2 ). At or near the onset of first period  30 , ventilator  2  increases the pressure of the flow of fluid from pressure P 1  to pressure P 2 , desirably between 10-60 cm H 2 O. At or near the onset of second period  32 , ventilator  2  decreases the pressure of the flow of fluid from pressure P 2  back to pressure P 1 . Thereafter, the pressure of the flow of fluid is maintained at pressure P 1  until at or near the onset of the next first period  30 . The process of changing from pressure P 1  to pressure P 2  and then back to pressure P 1  in this conventional mode of operation of ventilator  2  is desirably repeated for each breath cycle of patient  10  according to when patient  10  commences inhaling during each first period  30  and exhaling during each second period  32 . However, this is not to be construed as limiting the invention.  
       FIG. 2B  illustrates the fluid flow (velocity of fluid) entering patient  10  during first period  30 , the fluid flow exiting patient  10  during second period  32 , and the absence of fluid flow entering or exiting patient  10  during third period  34 . The area shown in crosshatch represents the fresh fluid that flows into the patient&#39;s lungs during first period  30 .  
       FIG. 2C  illustrates the volume of fluid that enters and exits patient  10  during first period  30  and second period  32 , respectively, as well as the absence of fluid entering or exiting patient  10  during third period  34 . In  FIG. 2C , V T  represents the overall tidal volume of fluid entering and exiting the patient&#39;s lungs during each breath cycle, while V TF  represents the fresh fluid volume entering the patient&#39;s lungs during inhalation first period  30 . V TF  corresponds to the crosshatched area in  FIG. 2B . V T  is greater than V TF  because some of the volume in V T  is used to ventilate the dead space of the total patient respiratory system.  
      While  FIGS. 2A-2C  illustrate a pressure limited mode of conventional ventilation, it is to be understood that other conventional modes of ventilation are known.  
      For example, the flow of fluid to the airway of a patient during inhalation can be limited based on the flow rate or volume. In addition, the triggering and cycling can be based on patient effort or it can be time based.  
      With reference to  FIGS. 3A-3C  and with continuing reference  FIGS. 1 and 2 A- 2 C, when operated in an airway pressure release ventilation (APRV) mode, ventilator  2  causes the flow of fluid to be provided to patient  10  at a pressure P 3  during first period  30  and during third period  34  of each ventilation cycle  28  applied to patient  10 .  
      However, during second period  32  of each ventilation cycle  28 , ventilator  2  causes the flow of fluid to be provided to patient  10  at a pressure P 4  which is less than pressure P 3 .  
      In  FIGS. 3A-3C , third period  34  is shown on either side of first period  30  and second period  32  for illustration purposes only and is, therefore, not to be construed as limiting the invention for at least the reason discussed above for third period  34  in  FIGS. 2A-2C . The APRV mode of ventilation is described, for example, in U.S. Pat. No. 4,773,411 to Downs.  
      As shown in  FIG. 3A , during third period  34  leading up to second period  32 , the flow of fluid is provided to patient  10  at pressure P 3 , desirably a pressure between 20-40 cm H 2 O. At or near the onset of second period  32 , ventilator  2  decreases the pressure of the flow of fluid from pressure P 3  to pressure P 4 , desirably between 0-10 cm H 2 O. At or near the onset of first period  30 , ventilator  2  increases the pressure of the flow of fluid from pressure P 4  back to pressure P 3 . Thereafter, the pressure of the flow of fluid is maintained at pressure P 3  until at or near the onset of the next second period  32 . The process of changing from pressure P 4  to pressure P 3  and then back to pressure P 3  in the APRV mode of operation of ventilator  2  is repeated for each ventilation cycle  28  of patient  10  which desirably occurs independent of each breath cycle of patient  10 , i.e., machine timed breathing.  
      If desired, however, each ventilation cycle  28  in the APRV mode of operation can be synchronized with each breath cycle of patient  10  whereupon, it is envisioned that patient  10  would exhale during second period  32  and/or just prior to the onset of second period  32 , and inhale during first period  30 . However, this is not to be construed as limiting the invention.  
      In the APRV mode of operation, the onset of second period  32  is delayed until just before the next first period  30 . This is in contrast to the conventional mode of operation shown in  FIG. 2B , wherein second period  32  immediately follows first period  30 .  
      Comparing  FIGS. 2C and 3C , it can be seen that the patient&#39;s functional residual capacity (FRC), in the APRV mode of operation is higher or greater than the patient&#39;s FRC in the conventional mode of operation. In contrast to  FIG. 2C , where the volume of fluid in the patient&#39;s lungs returns to FRC during third period  34 , in the APRV mode of operation shown in  FIG. 3C , maintaining the flow of fluid at pressure P 3  after first period  30  increases the patient&#39;s FRC. Thus, except during first and second periods  30  and  32  in the APRV mode of operation, the volume of fluid in the lungs of patient  10  is maintained at the higher FRC than the patient&#39;s FRC in the conventional mode of operation.  
      The area shown in crosshatch in  FIG. 3B  represents the velocity of fluid flow entering patient  10  during first period  30 . V TF  in  FIG. 3C  represents the fresh fluid volume entering the patient&#39;s lungs during first period  30  and corresponds to the crosshatched area in  FIG. 3B . Comparing  FIGS. 2C and 3C , it can be seen that V TF  in the APRV mode of operation is greater than V TF  in the conventional mode of operation, and that V TF  equals V T  in the APRV mode of operation. Thus, in the APRV mode of operation more fluid will be exchanged in the lungs of patient  10  during each ventilation cycle  28  than in the conventional mode of operation.  
      Because the flow of fluid is provided to patient  10  at pressure P 3  during first period  30  and third period  34 , and because subjecting patient  10  to the flow of fluid at pressure P 4  during third period  34  may not be comfortable for some patients  10 , it may be necessary to sedate patient  10  when ventilator  2  is operated in the APRV mode of operation, especially when ventilator  2  utilizes machine timed breathing, to avoid patient discomfort. Notwithstanding, it has been observed that the APRV mode of operation is clinically significant in the treatment of certain respiratory diseases or disorders.  
      With reference to  FIGS. 4A-4C , a Non-Rebreathing Positive Airway Pressure (NRPAP) mode of operation in accordance with the principles of the present invention will now be described. In the NRPAP mode of operation, ventilator  2  causes flow of fluid to be provided to patient  10  at a pressure P 5  during a quiescent phase  44  of each ventilation cycle  28 . The quiescent phase corresponds to a state of ventilation in which the pressure, flow, or volume of fluid delivered to the patient is maintained at a baseline level P 5 . In the illustrated exemplary embodiment, baseline level P 5  corresponds to ventilating the patient such that the pressure, flow, or volume of fluid delivered to the patient is substantially constant.  
      The quiescent phase can be thought of as an extension of an exhalation phase  46 . It can also be appreciated that the quiescent phase may be very short or even not present for some patient and under certain circumstances. That is, the quiescent phase can be replaced by the exhalation phase in cases when there is no clear period of constant pressure, flow, or volume of fluid at the end of an exhalation phase, e.g., during rapid breathing. Thus, the present invention refers to the period between a delivery phase  40  and release phase  42  collectively as an exhalation/quiescent phase, in which the presence of the quiescent phase may or may not be present.  
      During a release phase  42  of each ventilation cycle  28 , ventilator  2  allows the flow of fluid to be released from the patient therefore removing the used gas from the patient&#39;s respiratory system that results in elimination or reduction of the dead space of the total patient respiratory system, i.e., patient lungs, airways, and part of the patient circuit. This is accomplished, for example, by reducing the pressure of the flow of fluid provided to patient  10  to a release pressure level P 6  that is less than baseline pressure level P 5 . During a delivery phase  40  of each ventilation cycle  28 , ventilator  2  delivers the flow of fresh fluid to the patient by increasing the pressure of the flow of fluid from the release pressure level P 6  to a peak pressure level P 7  that is greater than baseline level P 5 . Thereafter, the flow of fluid is controlled so as to allow the pressure, the flow, or the volume of fluid to return from peak level P 7  to the baseline level P 5  during an exhalation phase  46 .  
      During the quiescent phase  44 , which, as noted above, can be considered an extension of exhalation phase  46 , ventilator  2  causes the flow of fluid to be provided to patient  10  at baseline pressure level P 5 . In  FIGS. 4A-4C , quiescent phase  44  is shown on either side of release phase  42 , delivery phase  40 , and exhalation phase  46  for illustration purposes only, and is, therefore, not to be construed as limiting the invention for at least the reason discussed above for third period  34  in  FIGS. 2A-2C .  
      As shown in  FIG. 4A , during quiescent phase  44  leading up to release phase  42 , the flow of fluid is provided to patient  10  at baseline pressure P 5 , desirably between 5-30 cm H 2 O, typically about 15 cm H 2 O. The present invention contemplates that baseline pressure P 5  is set to the positive end expiratory pressure (PEEP) or is set based on the PEEP, such as PEEP plus a constant. At or near the onset of release phase  42 , ventilator  2  decreases the pressure of the flow of fluid from pressure P 5  to pressure P 6 , desirably between 0-15 cm H 2 O, typically about 5 cm H 2 O. At or near the onset of delivery phase  40 , ventilator  2  increases the pressure of the flow of fluid from pressure P 6  to pressure P 7 , desirably between 15-60 cm H 2 O, typically about 30 cm H 2 O. At or near the onset of exhalation phase  46  after delivery phase  40 , ventilator  2  decreases the pressure of the flow of fluid from pressure P 7  back to pressure P 5 . Thereafter, the pressure of the flow of fluid is maintained at pressure P 5  until at or near the onset of the next release phase  42 .  
      The process of changing from pressure P 5  to pressure P 6 , from pressure P 6  to pressure P 7 , and then from pressure P 7  back to pressure P 5  in the NRPAP mode of operation of ventilator  2  is repeated for each ventilation cycle  28  applied to patient  10 , which desirably occurs independent of each breath cycle of patient  10 . However, if desired, each ventilation cycle in the NRPAP mode of operation can be synchronized with each breath of patient  10  whereupon, it is envisioned that patient  10  would exhale during release phase  42  and/or just prior to the onset of release phase  42 , and inhale during delivery phase  40  and exhale during the exhalation phase  46 . However, this is not to be construed as limiting the invention.  
      Comparing  FIGS. 2B and 4B , it can be seen that in the NRPAP mode of operation, the onset of release phase  42  is delayed until just before the next delivery phase  40 . This is similar to the APRV mode of operation discussed above where the onset of second period  32  is delayed until just before the next first period  30 . Moreover, like the APRV mode of operation, maintaining the flow of fluid at pressure P 5  during quiescent phase  44  in the NRPAP mode of operation increases the patient&#39;s FRC above the patient&#39;s FRC in the conventional mode of operation.  
      Because the flow of fluid is provided to patient  10  at pressure P 7  during delivery phase  40  in the NRPAP mode of operation, it is believed that the volume of fluid introduced into patient  10  in the NRPAP mode of operation during delivery phase  40  will be greater than the volume of fluid introduced into patient  10  during first period  30  in the APRV mode of operation. As a result, it is believed that V TF , i.e., the fresh fluid tidal volume or fresh fluid volume entering the patient, in the NRPAP mode of operation will be greater than V TF  in both the APRV mode of operation and the conventional mode of operation. Thus, in the NRPAP mode of operation, it is believed that more fluid will be exchanged in the lungs of patient  10  during each ventilation cycle  28  than in either the APRV mode of operation or the conventional mode of operation.  
      The area shown in crosshatch in  FIG. 4B  represents the velocity of fresh fluid that flows into the patient&#39;s lungs during delivery phase  40  and corresponds to V TF  in  FIG. 4C . As shown in  FIG. 4C , V TF  equals V T  in the NRPAP mode of operation. Because providing the flow of fluid at pressure P 5  during exhalation/quiescent phase  46 ,  44  and at pressure P 7  during delivery phase  40  may not be comfortable for some patients  10 , it may be necessary to sedate patient  10  when ventilator  2  is operated in the NRPAP mode of operation. As can be seen, the present invention is a method of ventilating patient  10  that increases the volume of fresh fluid provided to patient  10  and increases the volume of fluid exchanged in the lungs of the patient  10  over ventilating patient  10  with the conventional or APRV modes of operation.  
      In the embodiment described above, the flow of fluid is described as being pressure limited. That is, during the exhalation/quiescent phase, the release phase, and the delivery phase, the flow of fluid is controlled based on the pressure of the fluid flow. It is to be understood, that the present invention also contemplates controlling the flow of fluid during one or more of these phases based on other characteristics. For example, the present invention also contemplates controlling the flow of fluid based on the flow rate or the volume. When operated in a volume limited mode, for example, the ventilator seeks to maintain the volume of fluid in the patient to a baseline level during the exhalation/quiescent phase, to decrease the volume of fluid in the patient during the release phase below the baseline level, increase the volume of fluid in the patient during the delivery phase above the baseline level, and allow the volume of fluid in the patient to return to the baseline level during the next exhalation phase and to reach the quiescent phase.  
      When operated in a flow rate limited mode, for example, the ventilator seeks to maintain the flow of fluid in the patient to a baseline level during the exhalation/quiescent phase, to decrease the flow of fluid in the patient during the release phase below the baseline level, e.g., to remove some used gas from the patient&#39;s airways or lungs, increase the flow of fluid in the patient during the delivery phase above the baseline level, and allow the flow of fluid in the patient to return to the baseline level during the next exhalation phase towards the quiescent phase.  
      The present invention also contemplates that the duration of the quiescent phase, the release phase, the delivery phase, and the exhalation phase, can be controlled based on time, rather than on the pressure, flow, or volume of fluid delivered to the patient. For example, the ventilator can be programmed to initiate the release phase at a predetermined period of time following the end of the previous delivery phase. The duration of the release phase can be set to a predetermined period of time. Similarly, the duration of the delivery phase can be set to a predetermined period of time. Of course, using timed based control of the pressure, flow, or volume changes is best suited for a patient that is not spontaneously breathing.  
      The present invention also contemplates that the shape of the pressure, flow, or volume waveform during the release phase, the delivery phase, and/or the return to the baseline level of the exhalation and quiescent phases following the delivery phase can be controlled so as to have a specific profile or contour. Conversely, the shape of the pressure, flow, or volume waveform during the release phase, the delivery phase, and/or the return to the baseline level of the exhalation/quiescent phase following the delivery phase can be uncontrolled. In other words, in a pressure limited mode, the ventilator can seek to hit a target pressure, such as the baseline, release, and peak pressure during the quiescent phase, the release phase, and the delivery phase, respectively, but the specific shape of the pressure curve can be left uncontrolled. In which case, shape of the pressure curve will be dictated by the mechanical capabilities of the ventilator, the patient effort, and the respiratory mechanics of the patient, such as his or her respiratory compliance and resistance. It is to be further understood that the shape of the pressure, flow, or volume waveforms can be controlled during one phase and uncontrolled during another phase within the same breath cycle.  
      It is to be further understood that the present invention contemplates controlling the level of the pressure (P 5 , P 6 , P 7 ), the flow, or the volume delivered during the quiescent phase, the release phase, the delivery phase, and the exhalation phase on an active basis. That is, the level of the pressure, the flow, or the volume delivered during the quiescent phase, the release phase, the delivery phase, and the exhalation phase can automatically titrated by the ventilator system based on the monitored condition of the patient and/or the ventilator system. Techniques for automatically adjusting the pressure of a flow of gas delivered to a patient are well known. The present invention contemplates using these conventional autotitration techniques for setting the level of the pressure, the flow, or the volume delivered during the quiescent phase, the release phase, the delivery phase, and the exhalation phase. This can be done on an ongoing basis, such as on a breath-by-breath basis, or less frequently.  
      As noted above, the present invention contemplates that each ventilation cycle in the NRPAP mode of operation can be synchronized with each patient breath. There are several techniques by which the pressure delivery provided by the NRPAP mode of ventilation can be synchronized with the patient&#39;s spontaneous respiration. These techniques are discussed below with reference to  FIGS. 5-7 , each of which illustrates an exemplary pressure waveform provided to the patient by the pressure generating system. As used herein, the term “trigger” refers to the transition from the expiratory to the inspiratory phase of the breathing cycle, and the term “cycle” refers to the transition from the inspiratory to the expiratory phase of the breathing cycle. The term “trigger” can also refer to transition to release phase initiated by the inspiratory effort or signal. The term “cycle” can also refer to transition to release phase initiated by expiratory effort or signal. It should be noted that the terms “trigger” and “cycle” as used herein, refer to the transition from one phase to the next, and are not intended to imply that the ventilation system necessarily adjusts or changes the pressure, flow, or volume of gas delivered to the patient upon detection of a trigger or cycle event.  
       FIG. 5  illustrates an NRPAP mode of ventilation in which the patient&#39;s spontaneous inspiratory effort is a trigger point  50 , and initiates a delivery phase  52 . The duration of the delivery phase can be cycled based on the patient&#39;s expiratory effort, time, flow, volume, pressure, or any combination thereof. That is, if the patient attempts to exhale, this effort can be sensed using any conventional technique and the ventilator cycles at point  54  from delivery phase  52  to an exhalation/quiescent phase  56 . This cycle can also occur if a predetermined period of time has elapsed, a threshold flow rate is sensed, a threshold volume is reached, a threshold pressure is reached, or any combination thereof. This is why the cycle points in  FIG. 5  are indicated as being optional, i.e., the ventilator need not use the patient&#39;s expiratory effort as a cycle event.  
      The duration of exhalation/quiescent phase  56  can be controlled based on the patient&#39;s effort, time, flow, volume, or pressure. For example, the present invention contemplates waiting a predetermined period of time after cycle point  54  and thereafter initiating a release phase  58 . It is to be understood that the initiation of release phase  58 , i.e., the control of the duration of exhalation/quiescent phase  56 , can be based on monitored parameters other than being purely time based. For example, the initiation of release phase  58  can occur if the flow, pressure, volume, or any combination thereof reach predetermined thresholds. In this embodiment, the system remains in the release phase until a trigger event occurs, for example, the patient spontaneously initiates an inspiration. The trigger causes the system to enter the delivery phase and the process described above repeats.  
       FIG. 6  illustrates an NRPAP mode of ventilation in which the patient&#39;s spontaneous inspiratory effort is a trigger point  60 , and initiates a release phase  62 . That is, just as the patient is beginning to inspire, this is sensed using any conventional technique and release phase  62  is initiated. The duration of the release phase can be controlled based on the patient&#39;s effort, time, flow, volume, pressure, or any combination thereof. For example, if the patient attempts to further inhale, this inspiratory effort can be sensed using any conventional technique, and the ventilator transitions to a delivery phase  64 . The transition from the release phase  62  to delivery phase  64  can also occur if a predetermined period of time has elapsed, a threshold flow rate is sensed, a threshold volume is reached, a threshold pressure is reached, or any combination thereof.  
      The duration of delivery phase  64  can be cycled based on the patient&#39;s expiratory effort, time, flow, volume, pressure, or any combination thereof. That is, if the patient attempts to exhale once the delivery phase has begun, this expiratory effort can be sensed using any conventional technique, and the ventilator cycles at point  66  to exhalation/quiescent phase  68 . This cycle can also occur if a predetermined period of time has elapsed, a threshold flow rate is sensed, a threshold volume is reached, a threshold pressure is reached, or any combination thereof. In this embodiment, the system remains in exhalation/quiescent phase  68  until the next trigger event occurs, for example, when the patient spontaneously initiates an inspiration. This next trigger causes the system to enter the release phase and the process described above repeats.  
       FIG. 7  illustrates an NRPAP mode of ventilation in which the release phase is synchronized with the patient&#39;s spontaneous expiratory cycle. At a cycle point  70 , the patient begins exhaling and the ventilator transitions from a previous exhalation phase to a release phase  72 . Sensing the cycle point is accomplished using any conventional technique. The duration of release phase  72  is controlled based on the patient&#39;s effort, time, flow, volume, pressure, or any combination thereof. For example, if the patient attempts to inhale at point  74 , this becomes a trigger point  74  causing the ventilator to transition to a delivery phase  76 .  
      The duration of delivery phase  76  can be controlled based on time, flow, volume, pressure, or any combination thereof. That is, if a predetermined period of time has elapsed, a threshold flow rate is sensed, a threshold volume is reached, a threshold pressure is reached, or any combination thereof, the system ends the delivery phase and transitions to an exhalation/quiescent phase  78 . In this embodiment, the system remains in exhalation/quiescent phase  78  until the next cycle event occurs, for example, when the patient spontaneously initiates an expiration. This next cycle causes the system to enter the release phase and the process described above repeats.  
      It should be noted that the transition from release phase  72  to delivery phase  76  need not be based on the patient&#39;s own inspiratory effort. In which case, if the patient attempt to inspire, the inspiratory effort would have no impact on the operation of the ventilation system. Thus, the triggers shown in  FIG. 7  represent a pressure transition, but need not be based on the patient&#39;s inspiratory effort.  
      It should be emphasized that the patient will be able to breath spontaneously (assisted or not assisted, supported or not supported) during all of the NRPAP phases. In other embodiments of the present invention, the patient will not be able to breath spontaneously in some or all of the NRPAP phases.  
      It should also be emphasized that any or all of the NRPAP phases can be very short or very long in relation to the duration of inspiration or expiration of normal breathing. In another implementations of the NRPAP mode of operation, the NRPAP phases will or will not be related to the respiratory cycles. For example, the pressure can be changed in accordance to the NRPAP principle to control the Functional Residual Capacity (FRC) and allow the patient to breath at different FRC levels for periods of time.  
      The NRPAP mode of operation invention has been described above in the context of using this technique to increase the amount of fresh fluid that is introduced into the lungs of the patient by controlling the volume, pressure, or flow of fluid using tri-level flow/pressure variations. Typically, this is done to augment or replace a patient&#39;s own ventilatory effort. It is to be understood, however, that the present invention contemplates using the NRPAP mode of operation for therapeutic purposes that are not related to ventilation or that are not related to the use of a ventilator. Several examples of these other applications for the NRPAP mode of operation are discussed below.  
      The NRPAP pressure applying technique can be used in the context of applying external pressure to the surface of the patient. For example, the NRPAP type of variations can be used to control the pressure applied externally to a patient during resuscitation or during physiotherapy. In this embodiment, the NRPAP mode of operation is imposed on the pressure variations introduced onto the surface of the patient.  
      The NRPAP technique can be used to control blood pressure or to control the activity of the heart, with or without using the respiratory system. A fluid pump can be operated according to the NRPAP mode of operation. Such an NRPAP pump would augment or replace the function of the heart, for example. The NRPAP mode of operation can also be used to apply pressure to a patient&#39;s circulatory or other physiological systems or portions thereof, such as the arteries, blood vessels or other vessels. In short, the NRPAP or tri-level mode of applying pressure support, including the pressure release phase, can be applied generally in many areas, for instance in physiotherapy, massage, leg or arm pressure cuffs, and abdominal and/or bowl movement stimulation. It is to be understood that these are merely examples of other uses for the NRPAP mode of operation in areas outside the realm of ventilation. The present invention in not intended to be limited to these specific examples for the use of the NRPAP mode of operation.  
      The present invention has been described with reference to the preferred embodiment. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.