Oxygen overpressure protection system for membrane-type blood oxygenators

Abstract

A safety system is provided for a membrane-type blood oxygenator to prevent the possibility of gas embolism through the membranous barrier which separates the blood and the oxygen. The safety system includes a blood reservoir positioned at a higher horizontal level than the blood inlet of the oxygenator, so that a gravity head is maintained. In addition, a manometer is provided which has a fluid level to permit venting of the oxygen if the oxygen pressure exceeds a predetermined gas pressure, which gas pressure is lower than the minimum pressure of the blood in the oxygenator.

Description

BACKGROUND OF THE INVENTION

This invention relates to a safety device for a mass transfer system, and more particularly, to a system for preventing an excessive amount of gas from transferring to a liquid on the opposite side of a membranous barrier.

The system of the present invention is particularly useful as a gas embolism protection system for extracorporeal oxygenators of blood in which both oxygen and carbon dioxide are transferred across a membranous barrier separating the blood and the oxygen. An exemplary oxygenator with which the present invention can be effectively utilized is disclosed in the United States patent application in the name of Ronald J. Leonard, Ser. No. 170,163, filed Aug. 9, 1971 and now U.S. Pat. No. 3,757,955. It is to be understood, however, that the present invention may be utilized with many different types of mass transfer devices, particularly those using a porous hydrophobic membranous barrier separating a liquid and a gas.

The advent of controlled pore size, nonwetting, microporous membranes has made the construction of high transfer rate membrane oxygenators possible. The membranes have open pores which permit relatively rapid transfer of oxygen, yet the nonwetting properties prevent blood loss from the system. During operation of the oxygenator, it is important that the blood pressures exceed the oxygen pressures, because accidental reversal of oxygen and blood pressures might result in large amounts of oxygen rapidly entering the blood spaces of the oxygenator. In high flow rate oxygenators, the rapid oxygen accumulation would overwhelm any reservoir or bubble trap and allow gas to enter the patient. The sizes of reservoirs or bubble traps are limited as a result of the need to limit priming volume.

Extracorporeal oxygenators generally require a relatively high gas volumetric flow rate, and it is important for the gas spaces to be compact, with good mixing, in order to ensure effective gas transfer through the microporous membrane. Since this results in some gas pressure drop in the oxygenator, gas working pressures are generally greater than atmospheric. It can be seen that if the blood pressure were reduced to zero, the gas pressure would be greater than the blood pressure. Such a reversal of gas and blood pressures could easily occur at idle condition when there is no blood flow in the oxygenator.

It is extremely difficult, if not impossible, for an operator to maintain the variable pressures in an oxygenator in the proper direction. It is thus an object of the present invention to provide an automatic system of pressure control for a mass transfer system such as an oxygenator.

It is a further object of the present invention to provide a system for preventing accidental reversal of gas and liquid pressures in a mass transfer system without utilizing devices which have moving parts, springs, small orifices, or diaphragms which can become disabled or plugged up, thereby causing system failure.

Other objects and advantages of the present invention will become apparent as the description proceeds.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention, a safety system is provided for a mass transfer system of the type wherein a membranous barrier separates a liquid and a gas, and including a liquid inlet, a liquid outlet, and a gas inlet and outlet. The improvement comprises means for maintaining at all times a liquid pressure which is higher than the gas pressure. A gas pressure sensing device is coupled to the gas inlet with the gas pressure sensing device comprising means for venting the gas, to prevent the pressure of the gas from exceeding the pressure of the liquid.

In the illustrative embodiments of the invention, the liquid pressure maintaining means comprises a liquid reservoir positioned at a higher horizontal level than the liquid inlet, whereby gravity liquid pressure is maintained. A first pump is located downstream of the reservoir for drawing liquid therefrom, with the reservoir being collapsible to prevent a negative pressure on the mass transfer device if the pumping action is excessive.

In the illustrative embodiments of the invention, the gas pressure sensing device comprises a manometer having a fluid level that prevents gas from venting unless the gas pressure exceeds a predetermined maximum gas pressure. The fluid level of the manometer is such that it permits gas to vent if the gas pressure exceeds the maximum gas pressure, with the maximum gas pressure being a pressure that is lower than the minimum pressure of the liquid in the oxygenator.

In one embodiment of the invention, another manometer is provided, and is operable in response to a pressure of the liquid for variably adjusting the first-mentioned manometer.

A more detailed explanation of the invention is provided in the following description and claims, and is illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a safety fluid flow system for mass transfer devices in accordance with the principles of the present invention.

FIG. 2 is a schematic flow diagram of a modified safety fluid flow system for mass transfer devices according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Referring to FIG. 1, a mass transfer system is shown therein in the form of an extracorporeal oxygenator system, including a main console 10 to which a venous reservoir 12 and an arterial reservoir 14 are attached. Console 10 has an oxygen outlet 15 to which a conduit 16 is connected to feed a regulated flow of oxygen to the inlet 18 of an oxygenator 20. After passing through oxygenator 20, the spent gas exits via outlet 19. Main console 10 contains a gas flow rotometer, an oxygenator shim pressure control, a temperature readout meter, and the necessary selector buttons and switches, all as is well-known in the art. Console 10 is located to be within the operator's easy reach, but out of the way of possible fluid contamination.

Oxygenator 20 is a typical oxygenator wherein oxygen and carbon dioxide are transferred in opposite directions across a membranous barrier separating the blood and the oxygen. Such oxygenators are disclosed in the United States patent application in the name of Ronald J. Leonard, Ser. No. 170,163, filed Aug. 3, 1971.

It is to be understood that the system of the present invention is particularly suitable for use with any oxygenator using a microporous hydrophobic membrane. A typical suitable membrane material is polytetrafluoroethylene sheeting having a pore size of less than 0.5 micron and being about 0.005 inch thick. Another exemplary membrane is formed of polypropylene sheeting approximately 0.001 inch thick and having a pore size of about 0.1 micron. The membranes can be laminated to screening for strengthening support.

A blood conduit 22 is connected from an outlet 24 of venous reservoir 12 to blood inlet 26 of oxygenator 20. The blood and oxygen flow through oxygenator 20 on opposite sides of the membrane contained therewithin and the blood exits via conduit 28 to a heat exchanger 30, which regulates the blood temperature. A typical heat exchanger which could be utilized with the system of the present invention is disclosed in Leonard et al. U.S. Pat. No. 3,640,340, issued Feb. 8, 1972. The blood is then returned via conduit 32 to an inlet 34 of arterial reservoir 14.

An arterial pump 36 is utilized to pump oxygenated blood from arterial reservoir 14 via conduit 38 for flow to the patient's artery. A venous pump 40 is utilized to pump the blood from venous reservoir 12 to blood inlet 26. Line 43 provides blood from the patient's venous supply to venous reservoir 12. The two pumps (venous pump 40 and arterial pump 36) aid to protect the oxygenator and heat exchanger from overpressurization. Venous pump 40 draws blood from venous reservoir 12 and propels it through oxygenator 20 and heat exchanger 30, and into arterial reservoir 14. Arterial pump 36 draws blood from arterial reservoir 14 and propels it back to an artery of the perfused subject.

Since the exact matching of the pumping rate of the two pumps is difficult, if not impossible, the venous pump 40 is set to run at a slightly greater speed than the arterial pump 14. A recirculation line 42 between the arterial reservoir 14 and venous reservoir 12 allows the extra flow generated by venous pump 40 to return to the venous reservoir. This assures that the arterial reservoir 14 has blood in it at all times while protecting the oxygenator 20 from over-pressure due to blood accumulation.

Venous reservoir 12 and arterial reservoir 14 are preferably formed of a medical grade polyvinylchloride plastic, or silicone rubber, and are collapsible. Thus in the event the output of either venous pump 40 or arterial pump 36 exceeds the input into a reservoir, the respective reservoir collapses to restrict outflow, thereby preventing a reduced pressure from forming upstream of the reservoir. This is particularly important with respect to arterial reservoir 14 because it is necessary to maintain a minimum blood pressure on the oxygenator so long as there is blood in the system by maintaining a blood pressure head in conduits 22, 32, and 28.

It is important that reservoir 14, and preferably also reservoir 12, is supported so that its lower edges are above the upper port of the oxygenator generally by at least about 3 inches. In this manner, a gravity-induced liquid pressure head is always exerted on the oxygenator by the blood in the reservoir. The gravity head of the blood is arranged as described below to be always greater than the gas pressure in the oxygenator, to guard against the possibility of gas bubbles passing through the microporous membrane.

A safe, positive, direct method of pressure control is provided by coupling to oxygen inlet 18 a gas pressure sensing means 60. Gas pressure sensing means 60 comprises a manometer including an open container 62 having liquid 64, such as water, filled to a predetermined level. A venting conduit 66 is coupled from oxygen inlet 18, to the inside of container 62, passing downwardly through the top of container 62, to form the manometer construction. Fluid 64 is filled to that level which requires enough back pressure in venting conduit 66 to thereby prevent the gas from venting unless the gas pressure exceeds the predetermined maximum gas pressure, and to permit the gas to vent if the gas pressure exceeds such maximum gas pressure. The maximum gas pressure is selected to be a pressure that is lower than a pressure of the blood in the oxygenator created by the pressure head in line 34, that is, lower than the pressure of the blood in the oxygenator at the vertically highest point of the blood flow path therein. Thus, the vertical distance between outlet 34 of reservoir 14 and oxygen inlet 18 of oxygenator 20 must be greater than the vertical distance between lower end 68 of conduit 66 and surface 70 of fluid 64. In that manner, the gas pressure must always be lower than the blood head, which typically is about 18-19 inches minimum at inlet 26. Thus gas cannot bubble through the membrane to enter the blood spaces. Typically, a 14 inch pressure head of water exists in manometer 60 when the gas pressure is sufficient to cause flow through line 66. When gas is not flowing, the pressure head is slightly less, since then water resides within line 66, lowering the liquid level in container 62.

The system is fail-safe because if the fluid 64 were to evaporate, the gas would be vented at a lower pressure than before evaporation. Thus, evaporation of fluid 64 only permits the gas to vent at a lower pressure, and the oxygenator remains safe from the possibility of a gas embolism passing through the membrane.

A line 43 connected to the patient's venous blood supply feeds blood to venous reservoir 12.

The system also may include a cardiotomy reservoir 44, such as is shown in U.S. Pat. No. 3,507,395, the inlet of which is coupled to the incision site of the patient via suction line 46. Blood spilled in the incision site of the patient is sucked by means of a suction pump 48 to which line 46 is connected. Conduit 50 couples the outlet of cardiotomy reservoir 44 to venous reservoir 12 through an optional auxiliary filter 52 which filters out any remaining clots and other gross particles in the blood, and then passes the blood to the venous reservoir. The cardiotomy reservoir is usually also located above the venous and arterial reservoirs to assist in providing a gravity head of blood.

Where coronary perfusion or other localized perfusion of an organ is desired, a perfusion conduit 54 is coupled to an outlet of arterial reservoir 14, and the fluid is pumped through line 54 by means of a perfusion pump 56.

A modified gas pressure sensing means is illustrated in FIG. 2. As the remainder of the system may be identical to the FIG. 1 system, like reference numerals have been used for like structure. The gas pressure sensing means 60' of FIG. 2 comprises a manometer formed by container 62' and having an open top and bottom. Container 62' contains fluid, such as water 64', and has oxygen line 66 inserted therein in a manner similar to the previous embodiment. The manometer formed by container 62', fluid 64', and line 66 operates similarly to gas sensing means 60 of the FIG. 1 embodiment. However, the gas sensing means 60' of the FIG. 2 embodiment permits the maximum gas pressure to be raised if the blood pressure is raised, due to a change in blood flow rate or the like. However, it is still mandatory that the gas pressure be limited and remain less than the blood pressure. To this end, a closed blood manometer 70 is provided. Manometer 70 contains an amount of blood 72, which is dependent on the pressure in line 22, to which it is connected. This provides a variable gas pressure in the space 74 above blood 72 which also depends on the pressure in line 22. Hence, the height of fluid 64' is thereby dependent upon the pressure in space 74, tube 76, and line 22. The outlet of tube 76 communicates with sealed container 78, into which container 62' is positioned. A microporous plug 77 prevents blood from entering the control manometer 60' and provides a sterile barrier through which only the gas in space 74 and container 78 can pass. Plug 77 can be made of the same porous, hydrophobic membrane material as can be used in oxygenator 20.

It can be seen that a shift in the level of blood 72 will cause a pressure shift in space 74 and tube 76, thereby creating a fluid shift with respect to fluid 64'. Assuming that the blood pressure in line 22 is increased by an increased flow rate of other reason, the level of blood 72 will rise, thereby increasing the pressure in closed container 78. This will cause fluid 64 to rise in container 62', thereby permitting a higher oxygen pressure before venting from line 66 will occur. On the other hand, if the blood pressure in line 22 is decreased, the level of blood 72 in manometer 70 will be lowered, thereby decreasing the pressure upon fluid 64 and causing a drop in the height of fluid 64' within container 62'. Thus the gas will vent at a lower pressure as is required.

The above system provides additional efficiency coupled with safety, in that higher gas pressures may be used when higher blood pressures exist, but upon a sudden drop in blood pressure, the limiting maximum gas pressure will also drop to safe levels.

It is seen that an automatic system of pressure control has been provided for a mass transfer system, such as an oxygenator. The system is operative to prevent accidental reversal of gas and liquid pressures in a mass transfer system, without utilizing devices having moving parts, springs, small orifices, or diaphragms. The invention not only provides a safety system, but also permits effective operation of an oxygenation system at high altitudes, since manometers 60, 60' permit the safe use of gas pressures in an oxygenator which may exceed the ambient atmospheric pressure.

Furthermore, the use of manometers 60, 60' permit the continued lifesaving oxygenation of a patient even in the event of a gas delivery pressure valve failure or the like causing excess pressure, since the excess gas pressure is simply bled off by manometers 60, 60', while the oxygenator remains exposed to whatever predetermined maximum gas pressure has been selected.

Although two illustrative embodiments of the invention have been illustrated and described, it is to be understood that various modifications and substitutions may be made by those skilled in the art without departing from the novel spirit and scope of the present invention .

Claims

That which is claimed is:

1. In an extracorporeal oxygenator system wherein oxygen and carbon dioxide are transferred across a porous, hydrophobic membranous barrier separating the blood and the oxygen, said oxygenator having a blood inlet, a blood outlet, and an oxygen inlet and outlet; the improvement comprising, in combination: a blood reservoir coupled to said blood outlet downstream therefrom, said blood reservoir being positioned at a higher level than said blood outlet to provide to blood adjacent the membranous barrier a pre-determined minimum pressure; and manometer means which comprises an open liquid container being at least partially filled with a liquid; an oxygen supply line coupled to said oxygen inlet, venting conduit means coupled with said oxygen line and communicating with said liquid within said container and having an outlet therein, said liquid having a level that is selected to provide a pressure at said venting conduit means outlet but to permit venting of the oxygen in order to prevent the pressure of the oxygen from exceeding said minimum pressure of said blood.

2. An extracorporeal oxygenator system as described in claim 1, wherein the further improvement comprises said blood reservoir having an inlet and outlet for blood, and further including a first pump located downstream of and operatively connected to said outlet of said blood reservoir for drawing blood from said reservoir, said blood reservoir being collapsible to prevent a negative pressure upon the blood side of the oxygenator membranous barrier if the pumping action is excessive; and a second pump located upstream of and operatively connected to said blood inlet of said oxygenator for propelling blood to said blood inlet of said oxygenator said second pump being operated to pump at a greater flow rate than said first pump, and means for recirculating the extra flow of blood from upstream of said first pump to upstream of said second pump.

3. In an oxygen and blood delivery system for use in conjunction with a membrane-type blood oxygenator having a blood inlet and outlet and an oxygen inlet and outlet for diffusion therebetween across a porous, hydrophobic membrane, the improvement comprising:

blood and oxygen conduit means for operatively communicating with said respective blood and oxygen inlets and outlets of said oxygenator, and for conveying such materials to and from said oxygenator;

means for carrying a flexible blood reservoir in a position elevated above the position of said oxygenator,

receptacle means, open to the atmosphere, for containing a liquid;

an oxygen line having one end thereof disposed within said receptacle means, for immersion in liquid disposed in said receptacle means to create a predetermined pressure head at said one end, said oxygen line communicating with said oxygen conduit means, whereby the oxygen pressure in said conduit means is limited in a manner dependent upon said predetermined pressure head.

4. The system of claim 3 in which means are provided for assuring a continuous minimum blood pressure comprising the further improvement in said oxygenator.

5. The system of claim 4 comprising the further improvement in which said oxygen line communicates with said gas conduit means upstream from said oxygenator.

6. In an extracorporeal oxygenation system in which oxygen and carbon dioxide are transferred across a membranous barrier separating the blood and the oxygen, said oxygenator having a blood inlet, a blood outlet, and an oxygen inlet and outlet; the improvement comprising, in combination: a blood reservoir coupled to said blood outlet downstream therefrom, said blood reservoir being positioned at a higher level than said blood outlet to provide to blood adjacent the membranous barrier a predetermined minimum pressure; and manometer means which comprises a liquid container, open to the atmosphere, being at least partially filled with a liquid; an oxygen supply line coupled to said oxygen inlet, venting conduit means coupled with said oxygen line and communicating with said liquid within said container and having an outlet therein, said liquid having a level that is selected to provide a pressure at said venting conduit means outlet but to permit the venting of the oxygen in order to prevent the pressure of the oxygen from exceeding said minimum pressure of said blood, and further including second manometer means coupled to the blood inlet line, said second manometer means being also operatively connected to said liquid in the container, and responsive to the pressure of the blood in said blood inlet line to automatically adjust the level of said liquid in said container.