Abstract:
A piston ventilator is disclosed herein which uniquely includes an oxygen blending module which supplies oxygen enhancement for aiding patients requiring respiratory treatment. The oxygen blending module supplies a preselected enrichment of oxygen to a piston and cylinder assembly wherein the amount of oxygen permitted to flow therein is continuously monitored and controlled according to the volume above the piston in the cylinder as calculated by a controller. The oxygen blending module includes a flow sensor which monitors the flow of supplemental oxygen into the piston and cylinder assembly, and further includes a primary oxygen control valve which restricts the flow of oxygen through the flow sensor to the piston and cylinder assembly depending on the difference between the calculated desired amount of oxygen and the target amount of oxygen provided to the controller.

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 09/007,853, filed Jan. 15, 1998, issued Jun. 20, 2000, as U.S. Pat. No. 6,076,523. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for blending oxygen, in a piston-type ventilator which permits constant and accurate supply of supplemental oxygen to the user through a closed loop pressure support ventilation system. More particularly, it is concerned with a piston ventilator used to ventilate patients having difficulty respirating, which adjusts the amount of oxygen supplied both to the piston cylinder and to the patient circuit for ensuring a satisfactory oxygen content throughout the air in the circuit inhaled by the patient. 
     2. Description of the Prior Art 
     Ventilation systems are designed to supply air at a pressure greater than atmospheric pressure to assist in breathing, and may include systems which provide supplemental oxygen which is blended with air to enrich the gas which is inhaled. Such systems are typically employed for patients with respiratory ailments wherein the oxygen-enriched air is provided by blending bottled air or less often atmospheric air with supplemental oxygen in a controlled manner for each breath. 
     Another system for mixing the oxygen with air or another gas in a ventilation system in predetermined proportions involves the use of separate inlets into a pressure vessel up to respective first and second pressures is described in U.S. Pat. Nos. 4,022,234 and 4,023,587. The system shown therein operates in alternating withdrawal and mixing cycles. A feedback control of the rate of flow and pressure of breathing gas to a patient by an inspiration servounit is described in U.S. Pat. No. 3,741,208. U.S. Pat. No. 5,383,449 provides for control of oxygen concentration in a breathable gas by calculation of the mole ratios and pressure in the containment vessel, and by sequentially injecting oxygen and another gas to desired pressure values. These so-called batch mixing ventilators represent one system for patient ventilation. 
     While such systems are very useful in hospitals and other health care facilities, smaller and more confined devices not requiring connection to pressurized air are often more appropriate for home care. Piston and bellows types of ventilators allow delivery of a predetermined volume of breathing gas at a desired pressure responsive to the initiation of inspiratory efforts by a patient. Piston based ventilators can typically be made to be more compact than bellows based ventilators, but piston ventilators typically blend pressurized air and oxygen in a high pressure blender. The resultant mixture is then drawn by a piston through a valve that reduces the pressure of the mixture. Such systems typically do not permit the use of room air and pressurized oxygen, and can result in some risk of overpressurization in the event of failure of a high pressure gas delivery valve controlling introduction of one of the breathing gas components into the high pressure blender. 
     Another system for blending oxygen in a ventilator is shown in International Publication No. WO 96/24402 published Aug. 15, 1996. This system is designed for mixing gases at approximately ambient atmospheric pressure, such as oxygen and air. The mixing apparatus includes a piston disposed within a pump chamber. A flow limiting inlet controls introduction of oxygen for mixing with air, and the pressure of the oxygen is limited to an acceptable maximum pressure whereby even if the oxygen valve fails, the breathing gas will not be provided at an excessive pressure. A demand valve is alternately provided for reducing the pressure of the oxygen supplied before mixing, and a pressure sensor is also provided downstream of the demand valve for detecting failure of the demand valve to shut off the supply of the oxygen to prevent overpressurization. 
     It would be desirable, however, to provide a piston ventilator where oxygen can be blended with gas or air where the piston causes air to be provided to the patient in a cycle which more closely approximates the patient&#39;s inhalation and expiration profile. A disadvantage of using a constant rate of piston movement within the cylinder to produce ventilation flow is that the flow is affected by changes in gas density and altitude, and thus requires the use of barometric pressure monitoring and input to control the piston movement rate. In turn, it would be desirable to monitor the flow of the gas breathed by the patient and provide oxygen blending based on the flow and feedback controls based on: the flow and the volume of gas in the piston cylinder to permit use of less expensive valve controls. It is also desirable to provide oxygen blending in such a piston system where oxygen enrichment can be provided for air remaining in the ventilator system downstream from the piston system after exhalation by the patient. 
     SUMMARY OF THE INVENTION 
     These and other objects are largely met by the oxygen blending system of the present invention. That is to say, the oxygen blending system hereof uses a piston ventilator which is sufficiently compact for home use, controls the operation of the piston to provide oxygen blending in a non-constant flow rate of breathing gas to the patient, and provides enriched oxygen to the patient side of the ventilator circuit, i.e. downstream from the piston, during piston retraction to optimize the oxygen content of all of the air inhaled by the user. 
     The piston ventilator of the present invention broadly includes an oxygen blending module, a primary piston-driven pressurization assembly for providing positive pressure flow of breathable gas to the patient, a secondary make-up gas module, a controller, an exhalation control system, and a patient circuit for delivering air to the patient for inhalation and exhausting exhaled air. The oxygen blending module includes a connection to a source of pressurized oxygen, a first control valve which regulates the flow of oxygen to the piston, and a flow sensor for monitoring the flow of oxygen to the piston. In addition, the oxygen blending module includes a second control valve for regulating the amount of oxygen delivered to the patient circuit to enrich the gas remaining in the patient circuit during the retraction stroke of the piston. The valves are current sensitive orifice valves responsive to signals from the controller, which preferably includes a microprocessor. The flow sensor is operatively connected to the controller to provide signals corresponding to the flow of oxygen to the primary piston-driven pressurization system. 
     The primary piston-driven pressurization system receives oxygen from the oxygen blending module and air or another breathable gas and is operated by a motor, gear drive and cam arm to provide a flow of blended gas therefrom. That is to say, the primary system receives a low volumetric flow of blended gas at the beginning of its retracting intake stroke building to a maximum volumetric flow of blended gas during the intermediate portion of its retracting stroke and then reducing to a low volumetric flow of air at the end of its retracting stroke before beginning the protracting stroke. When relatively large volumes of breathable gas are delivered during protraction of the piston within the cylinder, the primary system delivers a low volumetric flow at the beginning of protraction building to a maximum volumetric flow of blended gas during the intermediate portion of its protracting stroke and then reducing to a low volumetric flow of gas at the end of its protracting stroke. When the volume of gas to be provided to the patient circuit is relatively low, the flow will increase abruptly and then reduce to a smaller flow at the end of the protracting stroke. Alternatively, the movement of the motor and thus the piston may be controlled to expel the blended gas more constantly to provide a flow to the patient of sustained and substantially constant pressure. 
     Because the increase of the volume above the piston in the cylinder is non-linear but rather sinusoidal during intake and blending, the flow of the oxygen into the cylinder is similarly non-linear. The motor driving the piston preferably provides a virtually continuously updated signal to the controller corresponding to the position of the piston, which permits the microprocessor to calculate by integration the volume of gas in the cylinder during the retraction stroke and similarly the volume of added oxygen which should have passed by the flow sensor and should be present in the cylinder. By continuously updating the comparison between the calculated amount of oxygen in the cylinder with the actual amount of oxygen delivered to the cylinder during the retraction stroke, the controller can substantially continuously signal the first control valve to open or close to provide the desired amount of oxygen enrichment to the cylinder. Preferably, the motor is a motor capable of bi-directional movement so that an adjustable end-of-travel sensor can be provided to initialize the position of the motor with the controller and thereafter acts as a safety limiter, with the controller providing a signal to the motor to reverse direction between protraction and retraction and thereby accommodate users of different lung capacities. 
     The secondary make-up module uses a low-pressure blower to provide make-up air or other breathable gas to the system to compensate for leakage, in particular the leakage from around tracheal tubes inserted into the patient&#39;s windpipe or mouth. The controller provides a speed control signal to the blower to maintain the appropriate amount of pressure in the patient circuit based upon the amount of flow out of the patient circuit. The controller senses the amount of flow out of the patient circuit, which in turn operates an oxygen valve to maintain a satisfactory oxygen enriched gas in the patient circuit. The speed control signal, together with the amount of total oxygen specified to the controller, operates the second oxygen control valve to permit greater or smaller flows of oxygen to flow to the patient side of the ventilator and thus into the patient circuit for maintaining satisfactory oxygen enrichment in the patient circuit during retraction of the piston in the cylinder preparatory to initiation of inhalation. The primary flow sensor provides a signal corresponding to the volume of mixed gas leakage in the patient circuit (V T ). The volume of oxygen (O 2 ) introduced is known by introducing O 2  gas of known pressure upstream of a known orifice size for a specific period of time, yielding an oxygen volume (V O2 ). 
     The concentration of oxygen in the make-up gas is then known by the equation: 
     
       
         
           V 
           T 
           =V 
           AIR 
           +V 
           O2 
         
       
     
     
       
         O 2 %=79( V   O2   −V   T )+21 
       
     
     where V T  is obtained by a measurement by the primary flow sensor and V O2  is known by how long the primary oxygen control valve is open, the orifice size, and the upstream pressure. 
     The exhalation control system is positioned on the patient side of the ventilator for connection to the patient circuit which is connected to the ventilator and includes a flow sensor for monitoring the flow of breathable gas to the patient, pressure sensors for detecting the pressure in the patient circuit during inhalation and exhalation, and a positive end expiratory pressure control valve. The patient circuit is attached to the ventilator for connection to the exhalation control system and includes and a pneumatically controlled exhalation valve. The positive end expiratory pressure (PEEP) control valve regulates the amount of gas delivered to a diaphragm, preferably an inflatable balloon diaphragm, in the exhalation valve by selectively venting gas from the diaphragm in the exhalation control system prior to delivery to the patient circuit. By such venting, a pneumatic signal is provided through a signal conduit as the pressure on the diaphragm increases relative to the pressure in the patient circuit and the resistance of the exhalation valve to the flow of gases from the patient circuit increases. Excess gas from the diaphragm is exhausted through the PEEP control valve to the atmosphere. An increase in the pressure on the diaphragm thereby increases the amount of PEEP, that is, the pressure remaining in the patient&#39;s airway after exhalation, which in turn resists collapse of the patient&#39;s lungs and enhances the rapidity with which the patient may begin effective inhalation of the next breath. 
     As a result, the movement of the piston directly corresponds to the inhalation and exhalation of the patient pneumatically connected to the ventilator, with the desired amount of oxygen enrichment provided to aid the patient&#39;s respiration. The enrichment is provided notwithstanding characteristics of the intake flow of breathable gas into the piston and cylinder assembly whether it be linear or sinusoidal because the amount of oxygen added to the cylinder is constantly monitored and controlled in a closed loop system. Furthermore, the make-up air or other gas provided to the patient is oxygen enriched, and provided in connection with a positive end expiratory pressure control to enhance the rapidity with which the next breath may be inhaled. These and other advantages will be readily apparent to those skilled in the art with reference to the drawings and description set forth below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a ventilator system including an oxygen blending module, with pneumatic connections shown in solid lines and electric connections shown in dashed lines; 
     FIG. 2 is a block diagram of various components of the controller shown in FIG. 1 in operative relationship to controlled components of the ventilator system; and 
     FIG. 3 is a block diagram of other components of the controller shown in FIG. 1 in operative relationship to controlled components of the exhalation control system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawing, a ventilator system  10  particularly useful for assisting respiration in medical patients is shown schematically in FIG.  1  and broadly includes an oxygen blending module  12 , a primary piston-driven pressurization system  14 , a secondary make-up gas module  16 , a controller  18 , a patient circuit  20  and an exhalation control system  21  having a positive end expiratory pressure (PEEP) control valve  22 . The patient circuit is detachably mounted to the exhalation control system  10  and thus is located outside an enclosure and in immediate proximity to the patient  26 . The patient circuit  20  includes an exhalation control valve  24 . The ventilator system  10  is particularly useful in delivering air with enhanced oxygen content in a patient-assist mode to a medical patient  26  with incomplete respiratory capabilities, but it may be appreciated that the ventilator system  10  may employ other breathable gases, for instance helium or nitrogen which may be blended with oxygen, as circumstances dictate. 
     The oxygen blending module  12  is connected to a source of pressurized oxygen  28  such as bottled oxygen or a connection to a central oxygen source by an oxygen fitting  30 . The oxygen received from the source  28  passes through a check valve  32  and a filter  34 , e.g. 40 micron mesh filter, and is divided into first and second paths  36  and  38 . The first path  36  leads to the primary pressurization system  14 , while the second path  38  leads to the exhalation control system  21 . 
     The oxygen provided by the first path  36  passes from a pressure regulator  40  for providing the supplied oxygen at a substantially constant desired pressure, and then to primary oxygen control valve  42 . Preferably, the regulator  40  limits pressure of the oxygen downstream from the regulator to 55 psi to maximize output, although lower settings are permissible. Advantageously, the primary oxygen control valve station  42 , illustrated schematically in FIG. 1 as a single valve, is provided as two parallel current-controlled voltage sensitive orifice (VSO) valves. Current rather than voltage is used to control the VSO valves because it is less subject to variations due to temperature. The primary oxygen control valve station  42  is electrically coupled to controller  18  for receiving control signals and transmitting valve position information. A gas flow sensor  44  is provided downstream from valve  42  to monitor the flow of oxygen therepast. The gas flow sensor  44  is preferably provided as a low flow gas sensor in parallel with a flow inducing restrictor to provide a larger full scale flow capacity than the low flow gas sensor alone. The gas flow sensor  44  is electrically connected to the controller  18  for providing a signal thereto. 
     The primary piston-driven ventilation system  14  includes a cylinder  46  and a reciprocating piston  48  presenting a chamber  50  between the piston  48  and the cylinder  46 . The piston  48  is coupled to an arm  52  which functions as a cam to move the piston  48  vertically within the cylinder  46  at the greatest rate at the middle of the stroke and at the smallest rate at the beginning and end of the stroke. The speed of the piston  48  during retraction is thus sinusoidal corresponding to the cosine of the angle between the arm and the horizontal as illustrated in FIG.  1 . The arm  52  is coupled to a gear box  54  driven by a bidirectional motor  56 . That is to say, the motor is capable of both clockwise and counterclockwise movement, whereby the arm  52  does fully rotate but rather reverses direction during transition between the protracting and retracting stroke and vice versa. An end of travel sensor  58  is adjustably mounted for operative engagement with the piston  48  at the bottom of the retracting stroke and electrically coupled to the controller  18  to signal the controller  18  to initialize the position of the motor  56  and thus the piston  48  at start up and as a safety device during continued operation. By initializing the position of the motor  56 , the controller  18  controls the operation of the motor  56  to change its direction of rotation and thus the length of the stroke of the piston  48  within the cylinder  46  at the appropriate position based on the motor position and a preselected total volume setting provided by the operator to the controller  18 . The adjustment feature permits the amount of enriched oxygenated air or other gas supplied to the patient  26  to be varied based on the patient&#39;s lung capacity. The motor  56  is preferably a brushless direct current motor which is electrically coupled to the controller  18  to provide a continuous signal corresponding to the number of revolutions and position of the motor drive and thus the arm  52 . Air or other breathable gas is supplied to the primary piston-driven ventilation system  14  from the ambient air inlet  59  which passes through a filter  60 , e.g. a 0.3 micron filter, and through a check valve  62 . The air is mixed with the oxygen supplied from the oxygen blending module  12  in the chamber  50 , and then discharged through check valve  64  before passing through another filter  66 , such as a 50 by 250 mesh filter leading to the exhalation control system  21 . 
     The secondary make-up gas module  16  primarily includes a low-pressure/low-volume blower  68  delivering air through a check valve  70  into the patient circuit  20  downstream from filter  66  and is prevented from flowing back into the chamber  50  by check valve  71 . The blower  68  typically operates constantly to supply a flow of breathable gas such as air obtained from the ambient air and at higher speeds may generate as much as 20 cm H 2 O peak pressure to make-up air lost through leakage of up to 10 liters per minute of air in the patient circuit, e.g., around a tracheal tube or the like. The operating speed of the blower  68  is controlled by the controller  18  in response to a signal corresponding to the pressure in the exhalation control system  21  provided to the controller as sensed by a pressure sensor  98 . The blower  68  thus not only compensates for leaks in the system but also maintains PEEP levels and provides a flow, when a patient initiates a breath, for which to trigger a breath. 
     The controller  18  includes a microprocessor  72  which is programmed with operating instructions. As shown in FIG. 2, the controller  18  also includes a flow sensor signal conditioner  74  which receives input from the flow sensor  44 , with the signal being delivered to the microprocessor  72  via an analog to digital converter  76 . Similarly, the microprocessor  72  provides a signal to a blender valve drive  78  which provides sufficient current to operate the VSO valves of the primary oxygen control valve station  42  through a digital to analog converter  80 . Also as shown in FIG. 2, the microprocessor  72  receives a signal from a motor position monitor  82  operatively connected to motor  56  which in turn senses the number of revolutions and position of the motor  56  and thus the arm  52  and piston  48  and in turn signals the motor  56  to protract or retract the piston  48 . As shown in FIG. 3, the controller  18  includes other components to perform functions relating to sensing the PEEP and operating the PEEP valve  22  in response thereto. The controller also includes a pressure sensor signal conditioner  112  which receives input from the first pressure sensor  98 , with the signal being delivered to the microprocessor  72  via an analog to digital converter  114 . Similarly, the microprocessor  72  provides a signal to a PEEP valve controller  116  mounted on PEEP valve  22  through a digital to analog converter  118 . The PEEP valve  22  is then in turn connected to the exhalation valve  24  by a signal conduit  91  as shown in FIG. 1 to provide resistance based on PEEP by providing a pneumatic signal and thereby controls the PEEP at the end of the exhalation of the patient&#39;s breath. 
     The exhalation control system  21  receives blended air from filter  66  and make up air from blower  68  through check valve  70 . Excess pressure may be relieved by actuating manual pressure relief valve  84 . The second path  38  is provided as a conduit which advantageously may be connected to regulator  40  to obtain oxygen at 55 psi instead of directly from the supply  28  which is typically at 80 psi. The routing of the second path  38  from regulator  40  permits the use of a pressure regulator  86  which steps down the pressure from 55 psi to about 15 psi. Oxygen is provided through second path  38  through pressure regulator  86  and then to make-up oxygen control valve  88  for delivery through a flow restrictor  128  to the exhalation control system  21  for delivery to the patient circuit  20 . Make-up oxygen control valve  88  may be a circuit-controlled voltage sensitive orifice valve or alternatively a digital valve which is electrically connected to controller  18  as is blower  68 . The amount the make-up oxygen control valve  88  is permitted to open to admit oxygen from second path  38  is determined by the volume of air flowing across the primary flow sensor, and thus proportional to the flow of air or other breathable gas delivered by the blower  68  as controlled by the controller  18  based on the desired total amount of oxygen to be delivered to the patient. It is intended that the pressure of the oxygen supplied through regulator  86  will exceed the pressure generated by the blower  68  to ensure that the gas delivered to the patient circuit  20  will be suitably oxygen enriched. 
     The exhalation control system  21  includes the PEEP control valve  22 , signal conduit  91 , a flow restrictor  94 , a flow sensor  96 , a first pressure sensor  98 , a second pressure sensor  102 , and a filter  104 . The PEEP control valve  22  serves to vent excess air to the atmosphere through vent  90  to regulate the pressure on the diaphragm side of the exhalation control valve  24 , which in turn regulates the pressure in the patient circuit  20  to the PEEP setting entered by the operator. The PEEP valve  22  is electrically coupled to the controller  18  to regulate the amount of air permitted to be vented through vent  90 . When the PEEP valve is at full bypass or open, an inflatable diaphragm in the exhalation control valve  24  of the patient circuit will be fully deflated giving nearly a 0 cm H 2 O PEEP. When the PEEP valve  22  is fully closed, the exhalation valve  24  is closed to prevent gas from passing through the exhalation valve  24  to exhaust  92  and thus gas cannot escape during inhalation. Under these conditions, pressure in the patient circuit  20  and the exhalation control system  21  can be arbitrarily large depending on the amount of gas contained in the available volume until pressure relief valve  84  is activated. Similarly, the PEEP control valve  22  is pneumatically coupled to exhalation control valve  24  to decrease the pressure in the patient circuit  20  when the PEEP pressure rises above the predetermined valve. On the other hand, when the PEEP pressure falls below the predetermined value, the controller  18  signals the blower  68  to increase its speed and thereby increase the pressure in the patient circuit. As a safety measure the PEEP valve  22  is normally open to deflate the inflatable diaphragm and allow the patient to breathe in the event of a failure. The PEEP control valve  22  is pneumatically connected to the exhalation control valve  24  by a signal conduit  91 . The PEEP control valve  22  is pneumatically connected to the exhalation control valve  24  and includes a body defining therein a enclosure and the inflatable diaphragm is located within the enclosure which regulates the restriction through which the patient  26  must exhale air delivered to exhaust  92  to the atmosphere. One exhaust valve  24  which has been found to be acceptable for use is commercially available as Part No. 6350 which is a component of either patient circuit Model No. 6462 or 6461 available from Nellcor Puritan Bennett of Pleasonton, Calif. 
     Oxygen enriched air is delivered to PEEP valve  22  through a flow restrictor  94 , with the main flow of air delivered by the primary piston-driven ventilation system through a flow sensor  96  which monitors the delivery of air to the patient. The size of the opening of the PEEP valve  22  and the pressure in the patient circuit  20  as determined by first pressure sensor  98  determine the rate of flow of gases out of the PEEP valve  22 . Because the PEEP valve  22  and the restrictor  94  are in series, the flow of gas out of the PEEP valve  22  is also the flow through the restrictor  94 , as well as any minimal amount of gas flowing to or from the diaphragm through the signal conduit. The rate of flow of the gases through the restrictor  94  induces a pressure drop across the length of the restrictor  94 . This causes pressure on the backside or signal conduit side of the diaphragm to be less than the pressure at the patient side (i.e the front side) of the diaphragm. This pressure difference reduces the resistance of the exhalation valve  24 , allows gases in the patient circuit  22  escape out of the exhalation valve  24 , and ultimately decreases the pressure in the patient circuit  20 . By controlling the rate of flow of gases through the restrictor  94 , the pressure in the patient circuit  20  and PEEP may also be controlled. 
     During exhalation, the piston  48  is retracted and thus air or other breathable gas is supplied to both PEEP valve  22  and exhalation control valve  24  by a combination of make-up air from blower  68  and oxygen supplied through patient circuit oxygen control valve  88 , and from the exhalation of the patient: 26 . A first pressure sensor  98  is in immediate proximity to the patient  26  in the conduit delivering oxygen-enriched air to the patient  26 , and is electrically connected to the controller  18  to signal the PEEP valve  22  how much air is to be exhausted to the atmosphere through exhaust  92 , and therefore how much of the make-up air and breath exhaled by the patient is delivered to the exhalation control:valve inflatable diaphragm. The first pressure sensor  98  is electrically connected to the controller  18  and thus provides the controller  18  with a signal corresponding to the pressure of the gas being provided to the patient  26  during inhalation, and PEEP at the end of exhalation. A second pressure sensor  102  is electrically connected to the controller  18  and provides a backup and validation of the first pressure sensor  98  by monitoring the pressure in the exhalation control system  21  and providing a second pressure signal to the controller. 
     The PEEP valve  22  is normally open and indirectly controls the resistance of the exhalation valve  24  to the flow of gas from the breathing circuit to the atmosphere. By increasing or decreasing this resistance, the exhalation valve  24  can decrease or maintain the amount of gas in the patient circuit and thereby decrease or maintain the pressure in the patient circuit  20 . The PEEP valve  22  regulates the pressure applied to the exhalation valve  24  by controlling the amount of air delivered from the patient circuit  20  to the exhaust  92 . The more gas delivered by the PEEP valve  22  to the vent  90 , the lower the pressure on the diaphragm on the exhalation valve  24  that covers the exhalation opening  100  of the exhalation control valve  24 . The greater the restriction in the PEEP valve  22 , and thus the less gas delivered to the vent  90 , the greater the pressure on the diaphragm of the exhalation control valve  24  and thus the greater the expansion of the diaphragm within the enclosure thereby reducing the size of the restriction to increase the resistance provided during exhalation by the patient  26 . The amount of resistance to exhalation is proportional to the pressure of the gas supplied to the diaphragm and thus proportional to the amount the PEEP valve  22  is open. The signal supplied to open or close the PEEP valve  22  by the controller  18  is determined by the first pressure sensor  98 . 
     The primary flow sensor  96  provides a signal corresponding to the volume (V T ) of mixed gas leak in the patient circuit. The volume of oxygen introduced is known by introducing O 2  gas at a known pressure upstream of a known orifice size for a specific period of time. Alternatively, a voltage sensitive orifice (VSO) valve may be used in place of the fixed orifice and a digital valve. The oxygen concentration of the make-up gas is then known via the equation: 
     
       
           V   T   =V   AIR   +V   O2 ; 
       
     
     and 
     
       
         O 2 %=79( V   O2   V   T )+21 
       
     
     The controller  18  in turn sends operating signals to blower  68  and patient circuit control valve  88 . A filter  104 , such as a 40 mesh filter, provides final filtration of the air with enriched oxygen prior to delivery to the patient. 
     The patient circuit  20  is connected to the exhalation control system  21  by flexible conduits  120 ,  122  and  124  which lead to hose connectors  126  which are respectively operatively connected to the first pressure sensor  98 , the filter  104  delivering breathable gas to be inhaled by the patient, and the signal conduit  91  leading to PEEP valve  22 . 
     In use, the ventilator system  10  hereof provides respiratory assistance to patient  26  so that the respiration rate of patent  26  directly corresponds to the rate of reciprocation of the piston  48  within the chamber  50 . Thus, with each protracting movement of the piston  48  within the cylinder  46 , patient inhales, and with each retracting stroke of the piston  48  within the cylinder  46 , the patient exhales. Prior to operation of the ventilator system, the operator selects and sets the controller  18  at a desired level of oxygen enrichment, for example within a range of21% to 100% total oxygen received by the patient and additionally selects a target pressure level for PEEP within a typical range of 0 cm H 2 O providing no resistance in the exhalation valve during the exhale segment of the patient&#39;s breath to 20 cm H 2 O providing full resistance in the exhalation valve  24 . 
     During each retracting stroke of the piston  48 , the motor position monitor  82  signals the position of the piston  48  within the cylinder  46  and permits the controller  18  to calculate the volume of the chamber  50 . For any desired amount of oxygen enrichment, this produced a corresponding calculation of the amount of added oxygen which should be present in the chamber. The calculation of volume and thus the amount of added oxygen present in the chamber  50  is continuously updated during the retracting stroke of the piston  48 . Because the speed of retraction of the piston  48  is not linear but rather sinusoidal, the volume in the chamber  50  changes sinusoidally. 
     Flow sensor  44  thus determines the flow rate of oxygen delivered to chamber  50  on a continuous basis. Its signal is received by the controller  18  and the flow rates are integrated to yield the amount of oxygen actually delivered to the chamber  50 . If the controller  18  determines that the accumulated oxygen delivered to the chamber is insufficient, it signals the primary oxygen control valve station  42  to open and permit additional oxygen to flow past flow sensor  44 . If this produces oxygen in the chamber  50  in excess of the target amount prior to the end of the retracting stroke as determined by the integrated value, calculated by the controller from the flow rates determined by flow sensor  44 , the controller signals the primary oxygen control valve station  42  to close and thereby reduce the flow of oxygen past flow sensor  44  and delivered to chamber  50   
     During the retraction stroke of the piston  48 , the patient exhales. Exhalation by the patient is primarily delivered through exhalation control valve  24  to exhaust  92 . The inflation of the diaphragm within the exhalation control valve  24  causes the flow of exhaled air through the exhalation opening  100  to be restricted, and the patient must exert some effort to overcome the restriction depending on the pressure of the gas supplied to the inflatable diaphragm. The amount of the restriction may be controlled by the signal supplied by the controller  18  to the PEEP valve  22  to determine the amount of air permitting to vent therefrom. Because of leakage around tracheal tubes in tracheostomies or through conduit connections, the majority of the air supplied to the exhalation control valve  24  is delivered by blower  68 . Because a portion of the air remaining in the inhalation control system  21  will be inhaled when the piston  48  protracts, it is desirable to ensure that the air inhaled by the patient  26  is properly oxygenated. To that end, supplemental oxygen is delivered to the patient circuit  20  through make-up oxygen control valve  88  and its opening and closing as well as the speed of and thus the flow of gas delivered by the blower are determined by the controller  18  based on the signal provided by the first pressure sensor  98 . If a PEEP setting of zero is provided to the controller  18 , the blower  68  will shut off and the make-up oxygen control valve  88  will be closed. In the event of a leak (up to 10 liters per minute) and in the presence of PEEP (up to 20 cm H 2 O), the blower  68  is energized by the control logic to provide the make-up air to fulfill the requirements of the leak. While doing this, PEEP pressure is maintained. The air provided by the blower  68  is non-O 2  enriched air. If O 2  levels are being maintained, supplemental oxygen is provided through the second path  38  as referenced above. 
     Once the piston  48  reaches the end of its retracting stroke as sensed by the motor position monitor and determined by the controller  18  based on motor position and speed and the preset overall volume desired for delivery to the patient  26 , the controller  18  signals the motor  56  to reverse its direction and begin protraction of the piston  48  within the cylinder  46 . As the piston protracts, oxygen-enriched air is expelled from the chamber  50  through the check valve  64  for delivery to the patient circuit  20  and ultimately inhalation by patient  26  after passage through filter  104 . Thus, protraction of the piston  48  corresponds to inhalation  26  by the patient due to the delivery of pressurized air to inflate the patient&#39;s lungs. 
     Although preferred forms of the invention have been described above, it is to be recognized that such disclosure is by way of illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventors hereby states their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of their invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set out in the following claims.