Patent Publication Number: US-5158534-A

Title: Automated gas delivery system for blood gas exchange devices

Description:
A portion of the disclosure of this patent document contains material to which a claim of copyright protection is made. The copyright owner has no objection to the facsimile reproduction by any one of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent file or records, but reserves all other rights in the copyrighted work. 
     BACKGROUND 
     1. The Field of the Invention. 
     The present invention relates to methods and apparatus for performing extrapulmonary blood gas exchange wherein blood receives oxygen and releases carbon dioxide. More particularly, the present invention relates to systems used to deliver ventilatory gases to extrapulmonary blood gas exchange devices. 
     2. The Prior Art 
     Thousands of patients in hospitals suffer from inadequate blood gas exchange, which includes both inadequate blood oxygenation and inadequate removal of carbon dioxide (CO 2 ). These conditions are commonly caused by varying degrees of respiratory inadequacy usually associated with acute lung illnesses such as pneumonitis, atelectasis, fluid in the lung, or obstruction of pulmonary ventilation. Various heart and circulatory aliments such as heart disease and shock can adversely affect the flow of blood and thereby also reduce the rate of blood gas exchange. 
     Currently the most widely used methods of treating these types of blood gas exchange inadequacies involve increasing the flow of oxygen through the lungs by either increasing the oxygen concentration of the inspired gases or by mechanically ventilating the lungs. Both methods result in placing further strain on the lungs, which may be diseased and unable to function at full capacity. In order to allow diseased or injured organs to heal it is generally best to allow these organs a period of rest followed by a gradual increase in activity. 
     Various devices have been developed which are capable, at least for a limited period of time, of taking over the gas exchange function of the lungs. Many blood oxygenators are in common use and are employed most frequently during heart surgery. Such commonly available devices are capable of providing blood oxygenation and carbon dioxide removal sufficient to carry the patient through the surgical procedure but are not intended to provide pulmonary support for more than the hours required to perform the surgery. These oxygenators include devices which bubble oxygen into the blood as the blood flows through the device. This is usually followed by a portion of the device which removes the bubbles in the blood to make it acceptable for reintroduction into the patient. 
     Another group of blood oxygenators employ gas permeable membranes. These devices take many different shapes and configurations; however, the basic concept of operation is the same in all of these devices. Blood flows on one side of the gas permeable membranes while a ventilatory gas, i.e., oxygen, flows on the other side of the membrane. As the blood flows through the device the oxygen diffuses, on a molecular level, across the gas permeable membrane and enters the blood. Likewise, carbon dioxide present in the blood diffuses across the gas permeable membrane and enters the gas phase. 
     Of the available blood oxygenators, those incorporating gas permeable membranes may be best used in long term applications (e.g., one to seven days) as a pulmonary assist device for a patient suffering from acute respiratory failure. In the case of cardiopulmonary bypass where all of the patient&#39;s gas exchange needs must be supplied by the oxygenator, constant attention by a trained perfusionist is necessary to guard the welfare of the patient. 
     The use of a blood oxygenator as a pulmonary assist device also requires constant vigilance if maximum blood gas transfer is to take place. As will be appreciated, the condition of the patient may change from hour to hour, or minute to minute. Such changes often require a change in the operation of a blood oxygenator in order to maintain efficient blood gas transfer. Significantly, some changes in a patient&#39;s condition can lead to serious consequences if corresponding changes are not made in the oxygenator&#39;s operation. For example, &#34;outgassing,&#34; or the forcing of undissolved gas through the membrane into the blood as bubbles where they can form gas emboli, may occur if the blood phase pressure drops dramatically and the gas phase pressure at the permeable membrane is allowed to remain above the blood phase pressure. 
     In general, perfusionists are able to satisfactorily control the operational characteristics of blood oxygenators using manual control techniques over the duration of a surgical procedure lasting many hours with an acceptably low incidence of operator and equipment related accidents. It will, however, be appreciated that as the length of time a patient is undergoing pulmonary support increases to several days, the likelihood of operator error greatly increases. Furthermore, many of the parameters which must be considered in order to maximize patient welfare are not easily ascertainable using manual techniques. All of these considerations must be addressed when planning to use a pulmonary assist device on a long term basis. 
     In view of the foregoing, it would be an advance in the art to provide a blood oxygenator gas delivery system which is safer to use than previous available ventilatory gas delivery systems and which can be used with a membrane oxygenator to more efficiently transfer oxygen to, and carbon dioxide from, the blood. It would also be an advance in the art to provide a blood oxygenator gas delivery system wherein the gas phase is always maintained at a low enough pressure to ensure that formation of gas emboli in the blood does not occur and wherein the flow of the gas is precisely and automatically controlled. It would be yet another advance in the art to provide a blood oxygenator gas delivery system which may be easily set up and operated for long periods of time without constant attention from a technician. 
     OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
     In view of the above described state of the art, the present invention seeks to realize the following objects and advantages 
     One primary object of the present invention is to provide a blood oxygenator gas delivery system which is safer to use than previous available manual or automatic ventilatory gas delivery systems. 
     It is another object of the present invention to provide a blood oxygenator gas delivery system which maintains more efficient oxygen and carbon dioxide transfer with the blood than previously known devices. 
     It is also an object of the present invention to provide a blood oxygenator gas delivery system wherein the gas phase is always maintained at a pressure below the blood phase pressure to ensure that formation of gas emboli in the blood does not occur. 
     It is a further object of the present invention to provide a blood oxygenator gas delivery system wherein the flow of the gas is precisely and automatically controlled. 
     It is another object of the present invention to provide a blood oxygenator gas delivery system which may be easily set up and operated for long periods of time without constant attention from a technician. 
     These and other objects and advantages of the invention will become more fully apparent from the description and claims which follow, or may be learned by the practice of the invention. 
     Briefly summarized, the foregoing objects are achieved by a system for controlled delivery of ventilatory gases which includes means for adjusting the pressure of a ventilatory gas which is delivered to the gas permeable membrane of a blood gas exchange device, such as an extracorporeal or intracorporeal blood gas exchange device. The means for adjusting the pressure of the gas ensures that the pressure of the ventilatory gas present at the gas permeable membrane is maintained at a pressure which is at least below the central venous pressure of the patient in the case of an intracorporeal gas exchange device and at least below the ambient atmospheric pressure in the case of an extracorporeal gas exchange device. Preferably, the gas phase pressure at the gas permeable membrane is maintained below the ambient atmospheric pressure at all times. By keeping the pressure of the gas phase at the gas permeable membrane to such a low value, outgassing and formation of gas emboli in the patient&#39;s blood is avoided. 
     Also included in the present invention is a means for regulating the mass of the gas flowing to the gas permeable membrane to ensure that sufficient gas flows through the pulmonary assist device to maintain proper gas transfer. Importantly, in order to support a patient&#39;s metabolism a minimum amount of carbon dioxide must pass out of the patient&#39;s blood and oxygen must pass into the red blood cells. The means for regulating the mass of the gas flowing to the gas permeable membrane ensures that the gas flow is sufficient to ensure a minimum amount to oxygen is present at the gas permeable membrane and that the gas flow is sufficient to remove the carbon dioxide which passes out of the blood. 
     By including a means for adjusting the pressure of the gas at the gas permeable membrane, safety is assured. By adjusting the gas phase pressure at the gas permeable membrane to a value which is at least less than the patient&#39;s central venous pressure in the case of an intracorporeal gas exchange device, or preferably less than the ambient atmospheric pressure in the case of all gas exchange devices, outgassing of oxygen into the blood and the formation of gas emboli is avoided. The formation of gas emboli is potentially life threatening. 
     While the means for adjusting the pressure of the gas safely provides that outgassing and air emboli are avoided, the means for regulating the mass of the gas flowing to the gas permeable membrane safely ensures that sufficient oxygen and carbon dioxide transfer will occur across the gas permeable membrane. Thus, gas exchange is carried out with the greatest safety and effectiveness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a presently preferred embodiment of the gas delivery system for blood oxygenators of the present invention. 
     FIGS. 2A-2B are a flow chart showing the steps carried out by the system represented in FIG. 1. 
     FIG. 3 is a diagram showing another apparatus which may be used to carry out the method of the present invention. 
     FIGS. 4A-4F are a detailed schematic circuit diagram of presently preferred implementation of central controller portion of the system represented in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings wherein like structures will be provided with like reference designations. 
     Proper oxygenation of a patient&#39;s blood is critical to the survival of a patient. In accordance with the current state of the art, during a surgical procedure involving pulmonary bypass, a highly trained perfusionist attends to the oxygenation devices. The duties of the perfusionist includes making sure that sufficient blood gas transfer takes place and that bubbles formed in the blood do not enter the patient&#39;s circulatory system where they might form gas emboli in vital organs causing blockage of blood flow to critical areas. 
     Due to the attentive care provided by perfusionists, the number of injuries and fatalities occurring due to perfusion errors during cardiopulmonary bypass is very low. Nevertheless, the human errors of a perfusionist do result in fatal mistakes during cardiopulmonary bypass procedures. Surgical procedures involving cardiopulmonary bypass may last many hours. 
     In contrast to the duration of many surgical procedures involving cardiopulmonary bypass, the use of pulmonary assist devices during acute respiratory failure is necessary for long periods of time (e.g., days or weeks rather than hours). The longer duration of providing pulmonary assist during acute respiratory failure makes the likelihood of human error much more significant. 
     Moreover, when using intracorporeal gas exchange devices such as those described in U.S. Pat. Nos. 4,583,969 and 4,850,958 it is more difficult to accurately control the flow of gas, and control the pressure of the gas, where the gas permeable membrane is wholly hidden in the patient&#39;s body. U.S. Pat. Nos. 4,583,969 and 4,850,958 are incorporated herein by reference. 
     A further concern which is encountered when dealing with intracorporeal, e.g., intravenous, gas exchange devices is that conditions within the body may change relatively quickly. The changes which occur within a patient&#39;s body may go unnoticed using prior art techniques and appropriate corrections can go unmade. The fact that the gas permeable membrane is within the patient&#39;s body requires that corrections be made synchronously with changing conditions to avoid the occurrence of outgassing. When using intravenous gas exchange devices, any gas bubbles which do form due to outgassing will immediately travel to the patient&#39;s blood stream without any opportunity to be noticed and eliminated. 
     As will be better appreciated after examination of this disclosure, the embodiments and methods of the present invention provide the greatest possible safety for a patient during complete cardiopulmonary bypass or whose life functions are being supported by a pulmonary assist device. First, the present invention assures that the gas phase pressure present at the gas permeable membrane of the membrane oxygenator is low enough that outgassing does not occur. Second, the present invention assures that sufficient gas flows past the gas permeable membrane so that adequate oxygen and carbon dioxide transfer takes place. 
     Referring now to FIG. 1, a block diagram of the presently preferred embodiment of the present invention is provided. 
     As represented in FIG. 1, the embodiments of the present invention are intended to be used with a membrane gas exchange device, represented at 14 and having an inlet and an outlet, which may be either an extracorporeal device or an intracorporeal (intravenous or intra-arterial) device. As explained, the need for the present invention is greatest when a gas exchange device is implanted into a patient where outgassing may have disastrous results and the relatively long term use of the device makes constant attention by an attendant unsuitable. Also represented in FIG. 1 is a gas source 10 and a vacuum source 12. The gas source 10 may be a commonly available source of ventilatory gas, i.e., oxygen, such as a pressurized tank, which can be incorporated into the embodiment of the present invention or independent thereof. Alternatively, the gas source 10 may be the oxygen distribution system of a medical facility such as a hospital. 
     Also represented in FIG. 1 is a tank pressure regulator 16 which is commonly known in the art. Those skilled in the art will understand the advantages of regulating the gas source 10 to stabilize the pressure which is supplied and also how to carry out the regulation. 
     The vacuum source 12 represented in FIG. 1 may be an independent source of vacuum such as that supplied by the vacuum distribution system of a medical facility or, preferably, a dedicated source of vacuum which is incorporated into the embodiment of the present invention. In the case of a dedicated vacuum source, it is preferred that one commercially available from KNF Neuberger, Inc. model no. PV392-726-12.89 be used. It will also be appreciated that regulation of the source of vacuum, to some extent, may be desirable so that the pressure exerted is relatively constant. 
     As shown in FIG. 1, a gas flow or gas stream is established from the gas source 10 through an gas exchange device 14 to the vacuum source 12. In FIG. 1, the gas flow path is indicated by heavier lines (102, 108, 114, 120, and 124) with arrows showing the direction of the flow and lighter lines (132) being used to represent electrical control/data signal paths. For example, it is preferred that the heavier lines represent 1/8 inch inner diameter tubing which is suitable for use in medical applications. 
     The structures of the preferred embodiment described herein are intended to ensure the greatest possible safety to the patient both by providing adequate gas flow at the gas permeable membrane and by preventing any incidents of outgassing. 
     Shown in FIG. 1 is a connector 100 which functions as a means for receiving a gas under pressure from a gas source. In the illustrated embodiment, the gas source is an external tank of oxygen. Other supplies of ventilatory gases as described above and as known in the art can also serve as a gas source and the means for receiving a gas under pressure is intended to include any structure performing an equivalent function to that performed by the connector 100. Also represented in FIG. 1 is another connector 104 which is the presently preferred example of the means for connecting to a source of vacuum. 
     Represented in FIG. 1 are two pressure sensors 116 (P2) and 126 (P3). Each of the pressure sensors 116 and 126 are preferably those which are commercially available from Sensyn with pressure sensor 116 preferably being model no. 142SC01D and pressure sensor 126 being model no. 142SC05D. Another pressure sensor (referred to as P1 in the programming code appended hereto) which is not represented in FIG. 1 can be positioned to sense the pressure of the gas in line 102. 
     It will be appreciated that the pressure sensed by the pressure sensor 126 will be less than the pressure sensed by the pressure sensor 116, depending on the mass flow rate of the ventillatory gas. Thus, a significant pressure drop occurs across the gas exchange device 14. Each of the pressure sensors 116 and 126 outputs an electrical signal output corresponding to the pressure which is sensed. 
     As shown in FIG. 1, two mass flow controllers 106 and 128 are positioned in the gas flow. The mass flow controllers are preferably those which are commercially available from MKS Instruments, Inc. of Andover, Mass. utilizing apparatus and methods described in U.S. Pat. No. 4,464,932 which is incorporated herein by reference. It is preferred that mass flow controller 106 be model no. 1159B-05000RB-SP sensing a flow range of from 0 to 5000 standard cubic centimeters per minute (sccm). It is also preferred that mass flow controller 128 be model no. 1159B-05000B-SP sensing a flow range of from 0 to 3000 sccm. The preferred mass flow controllers are capable of sensing and controlling the mass of the gas flowing therethrough with a high degree of precision. 
     Each of the described mass flow controllers are an example of a flow control means or a means for regulating the mass of the gas flowing to the gas permeable membrane. As taught herein, the present invention may be carried out in other forms including only one mass flow controller or an equivalent functioning device. Thus, any structure performing functions which are equivalent to those carried out by one of the mass flow controllers described herein is intended to fall within the scope of the means for regulating the mass of the gas flowing to the gas permeable membrane of the gas exchange device. 
     Also represented in FIG. 1 is a pressure valve 112 and a vent 110. The function of pressure valve 112 and vent 110 is to adjust the pressure of the gas present within the gas exchange device. Since the avoidance of outgassing is of crucial importance in the embodiments of the present invention, the pressure of the gas within the gas exchange device must be kept at least as low as the patient&#39;s central venous pressure in the case of an intracorporeal gas transfer device and, as is done in the case of the described embodiment, preferably at least as low as the ambient atmospheric pressure. 
     The pressure valve 112 is preferably one also available from MKS Instruments, Inc. as model no. 0248A-50000RV. The pressure valve 112 is adapted to receive an electrical signal command and adjusts its output pressure accordingly. 
     With pressure valve 112 commanded to maintain a subatmospheric pressure, preferably -15 mm Hg, the vacuum exerted by the vacuum source 12 downstream from the pressure valve 112 causes gas to be drawn through the pressure valve 112. In order to ensure enough flow through the gas exchange device 14, the flow rate through mass flow controller 106 is set higher than the flow rate through mass flow controller 128. In the described embodiment, it is preferred that the flow through mass flow controller 106 is set at about 20 per cent higher than the flow through mass flow controller 128. 
     In order to prevent a build up of pressure on the upstream side of the pressure valve 112, the vent 110 is provided. The vent 110 has a cross sectional area which is sufficient to allow the necessary amount of gas to escape without undue resistance. During normal operation, gas is continually exhausted to some extent through the vent 110. Thus, the entry of contaminants through the vent 110 against the flow of the gas is not a significant concern. 
     The pressure valve 112 is one presently preferred example of a pressure control means or a means for adjusting the pressure of the gas received from the gas source 10 so as to prevent outgassing. As will be explained shortly and as appreciated by those skilled in the art, other arrangements and devices can perform functions equivalent to those performed by the described pressure valve. It is intended that such other arrangements and devices be included within the scope of the means for adjusting the pressure of the gas included in the present invention. 
     Still referring to FIG. 1, a bacteriological filter 118, such as is commercially available in the art, is present in the gas flow immediately before the gas exchange device 14. A liquid trap 12 is also present in the gas flow immediately after the gas exchange device 14. The liquid trap 120 is used to remove any liquid which has appeared in the gas flow after passing through the gas exchange device and which might otherwise interfere with the operation of the mass flow controller 128. 
     Also represented in FIG. 1 is a central controller 130. The components which generate, or respond to, electrical control signals are connected to the central controller 130 by various control lines as represented at 132. The central controller 130 preferably includes a microprocessor 130A, an analog to digital converter 130B, and a digital to analog converter 130C, if necessary, to communicate with the other components. The central controller 130 is the presently preferred example of a control means of the present invention. 
     A user interface, 136 in FIG. 1, may comprise visual and/or audio signals and displays to indicate to medical personnel the operational status of the system. For example, in the described embodiment, six LEDs 136A-136F, or abnormal operating indicators, are provided to indicate to medical personnel that portions of the system are operating in a normal state or that a problem is present. Also represented in connection with the user interface 130 is a digital display H, or a flow rate display, and a user operable control 135G with which the user can select the flow rate through the gas exchange device. The user interface 136 communicates with the central controller 130 by way of a bus represented at 134. 
     A detailed schematic diagram of the presently preferred configuration for the central controller 130 is provided in FIGS. 4A-4F. The programming code for the microprocessor included in the central controller 130 is provided in the programming code appendix attached hereto. 
     After examining the structure of the described embodiment, those skilled in the art will appreciate that other equivalent arrangements may be used to accomplish the objectives of the present invention. For example, a single mass flow controller (functioning as a means for regulating the mass of the gas stream) could be located upstream from the gas exchange device, a pressure sensor positioned immediately at the inlet to the gas exchange device, and a pressure valve (functioning as a means for regulating the pressure) located downstream from the gas exchange device. 
     In another example of a potential embodiment within the scope of the present invention, a single mass flow controller could be positioned in the flow stream downstream from the gas exchange device with a vent and pressure valve positioned upstream from the gas exchange device as represented in FIG. 1. Furthermore, a single mass flow controller could be positioned downstream from the gas exchange device and a variable vacuum pump utilized to vary the pressure exerted thereon. While such arrangements are possible, they are not presently preferred because of the present potentially unstable operation using presently available components. Since it is an objective to provide the safest possible implementation of the present invention, the described embodiment is preferred. It is within the scope of the present invention, however, to utilize other arrangements of the described structures to accomplish the same or equivalent functions. 
     The operation of the structures represented in FIG. 1 is controlled principally by microprocessor 130A and the presently preferred programming code for the microprocessor is attached hereto. It is to be appreciated that devices other than the described microprocessor and its associated devices can function as the control means of the present invention. 
     Reference will now be made to the flow chart of FIGS. 2A-2B and to the block diagram of FIG. 1 to describe the presently preferred method of the present invention. Beginning at Start 200 in FIG. 2A, the flow through the gas exchange device is set to zero as represented at step 202. A zero flow control command is read at step 204 by the controller 130 and the apparatus waits until the flow sensed by the mass flow controller is actually zero. 
     Referring still to FIG. 2A, in the next step 208 the pressure sensors 116 and 126 are read. As explained, it is crucial to maintain the gas phase pressure within the gas exchange device 14 at a value which is at least less than the lowest blood pressure of the patient, in the case of an intracorporeal gas exchange device, and preferably less than the ambient atmospheric pressure in all cases. This is accomplished in the described embodiment by maintaining the pressure in the gas exchange device at a subatmospheric value. Reading pressure sensors 116 and 126 provides a check that the pressure at the gas permeable membrane is within acceptable values. If necessary, the pressure may be regulated by altering the command presented to the pressure valve 112 by the central controller 130 as indicated at step 210. 
     The flow sensors which are integral with the mass flow controllers 106 and 128 are read as shown at step 214 and a dynamics analysis step 216 takes place wherein the described embodiment analyzes the characteristics of the flow of gas through the particular gas exchange device in conjunction with a particular patient and gas exchange device. As an example of a useful type of dynamic analysis which may be carried out is to determine the resistance of the gas exchange device to the flow of gas therethrough by examining the pressure drop across the gas exchange device and the flow therethrough. Other types of dynamic analysis may also be beneficially carried out. 
     Performed next is a series of steps which are included within the dashed box labeled safety check 218. The steps of included within the safety check 218 are intended to find and identify the source of an &#34;out-of-tolerance&#34; value so that corrective action can be taken. 
     At step 220, the pressure at the inlet of the gas exchange device (PIN), sensed by pressure sensor 116, is compared to a tolerance value (PINTOL), and if the tolerance value is exceeded, then a decision is made at step 220 to turn off the pressure valve 112 by the central controller 130 (step 222) and enter into a diagnostic and alarm routine as indicated at step 232. The tolerance pressure for the inlet of the gas exchange device (PINTOL) may be 0 mm Hg, for example, if the target pressure for the pressure valve 112 is set at -15 mm Hg. 
     If the pressure at the inlet of the gas exchange device (PIN) is less than the expected value (PINTOL) at step 220, then the process proceeds to step 224 where it is determined whether the pressure at the outlet of the gas exchange device (POUT) is greater than the tolerance value (POUTTOL) as sensed by the pressure sensor 126. If the pressure at the outlet of the gas exchange device is greater than the tolerance value, then the diagnostic and alarm routine 232 is entered. If the pressure at the outlet of the gas exchange device is less than or equal to the proper value, the process moves on to step 228. 
     Similarly, to the previous steps, in step 228, if the gas flow through mass flow controller 128 (F#2) is greater than or less than (i.e., unequal) to the flow command presented to the mass flow controller 128 (F#2SET), then the diagnostic and alarm routine 232 is invoked. In the operation of the described embodiment, it is the flow through mass flow controller 128 which is of crucial importance since that gas flow is also the precise flow through the gas exchange device. Also, in step 230, if the gas flow through mass flow controller 128 is greater than the gas flow through mass flow controller 106, then the diagnostic and alarm routine 232 is called. 
     If the decisions made at steps 220, 224, 228, and 230 are all &#34;no,&#34; the state of the system is normal as indicated at step 234. If after calling the diagnostic and alarm routine 232 an abnormality is detected in the system (as represented at 236), the decision at step 238 is made to rerun the loop which comprises the steps of the safety check 218 to continue to alert the user of the abnormality which has been detected. 
     If the state of the system is normal, the user interface 136 is checked for the flow command which may have been input (step 240) and the flow command entered thereat is read (step 242). The flow command is entered into the central controller 130 of the described embodiment and is determined by a medical professional in accordance with the needs of the patient and considering the particular gas exchange device being used. When the operational parameters are altered, care must be taken to avoid exceeding the pressure which can be tolerated in the gas exchange device. Occurrences such as &#34;overshoot&#34; which might occur when the pressure valve 112 or the mass flow controllers 106 and 128 are presented with an altered command. Moreover, the embodiment should be designed such that electrical and physical noise does not cause the significant problems. 
     When using the described embodiment, the flow command causes the mass flow controller 128 to be set to the flow rate which will result in that rate of flow through the gas exchange device. The mass flow controller 106, which is positioned upstream from the gas exchange device 14, is set to maintain a flow at specific amount, preferably 20 per cent above that maintained by mass flow controller 128. Once the flow command is read (step 242) the flow is controlled by the system and a loop beginning at step 208 is entered and repeated. The gas flow continues to be controlled (step 244) until the system is shut down as represented at the End step 246 shown in the flow chart. 
     It will be appreciated that with mass flow controllers 106 and 128 commanded as described, there will be a continual flow of gas out of the vent 110 and the flow set by the mass flow controller 128 will be the actual gas flow at the gas permeable membrane through the gas exchange device. Once the flow command has been read and implemented, the described embodiment will provide that the sufficient flow through the gas exchange device 14 occurs without interruption. 
     FIG. 3 is a diagram showing the arrangement of another apparatus which may be used to manually carry out the method of the present invention. Represented in FIG. 3 is a tank 300 containing pressurized gas. A pressure regulator 302 is manually changed to increase or decrease the pressure of the gas leaving the tank 300 and the pressure at the inlet of the membrane gas exchange device 312. Flow control valve 304 is manually adjusted to maintain the desired gas flow into the gas exchange device 312. The pressure gauges 306 and 316 measure the pressure at the inlet and the outlet of the membrane gas exchange device 312. Relief valves 308A-B and filter 310 are positioned upstream from the membrane gas transfer device 312. A liquid trap 314 is positioned immediately downstream from the membrane gas exchange device. Another flow control valve 318, positioned immediately upstream from the vacuum pump 320, is adjusted to set the flow through the membrane gas exchange device 312 to the desired value. 
     FIGS. 4A-4F provide detailed schematic diagrams of the presently preferred circuit implementation of the central controller 130 which may be included in the present invention. It will be appreciated that the circuit represented in FIGS. 4A-4F is merely exemplary of the arrangements and devices which can be incorporated into the present invention. Also, the boxed designations shown in FIGS. 4A-4F indicate the circuit interconnections between the figures. 
     Provided below in Table A is a list of the parts referenced in FIGS. 4A-4F. The reference designations included in FIGS. 4A-4F are those which are commonly used in the art in such schematic diagrams. 
     
                       TABLE A                                                     
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Item Quantity  Reference Designation                                      
                                  Part No.                                
______________________________________                                    
1    4         u6, U2, Uda6, u8   74LS374                                 
2    1         U1                 Z8681                                   
3    1         U4                 ADC0808                                 
4    1         U15                74LS138                                 
5    1         U7                 74LS541                                 
6    1         U9                 741                                     
7    1         XSTL               7.3728 mhz                              
8    1         U3                 27C64                                   
9    1         C1                 33                                      
10   4         Psys, f1, f2, Sys                                          
11   7         R33 ,R34, R35, R36, R37, R38,                              
                                  RESISTOR                                
               R40                                                        
12   2         R66, R104          50 k                                    
13   2         U5, Uda5           DAC0800                                 
14   8         C2, C3, C4, C5, Cda5, C6, C7,                              
                                  .1                                      
               C8                                                         
15   2         C9, C10            22 pf                                   
16   2         C11, Cda11         01                                      
17   1         port1                                                      
18   1         C15                1                                       
19   1         JP20                                                       
20   4         R4, R1, R2, R3     10K                                     
21   1         U22                7414                                    
22   1         RDA                2.5K                                    
23   2         R5, R6             2.5 k                                   
24   3         R42, R43, R44      10 k                                    
25   1         ALARM                                                      
26   1         R41                1.5K                                    
27   2         JPALM, VLV                                                 
28   1         RLED                                                       
29   2         P2, P1                                                     
30   1         RPWR                                                       
31   3         RDA2, Rda5, Rda6   5K                                      
32   1         Cda7               0.1                                     
______________________________________                                    
 
    
     In view of the foregoing, it will be appreciated that the present invention provides a blood gas exchange delivery system which is safer to use than previous available manual or automatic ventilatory gas delivery systems and which maintains more efficient oxygen and carbon dioxide transfer with the blood than previously known devices. The present invention also advantageously maintains the gas phase pressure within the gas permeable membrane gas exchange device at a pressure below the central venous pressure of the patient to ensure that formation of gas emboli in the blood does not occur. Moreover, embodiments of the present invention may be easily set up and operated for long periods of time without constant attention from a technician. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. ##SPC1##