1. Field of the Invention
This invention relates to a blood oxygenator of the outside perfusion type using a hollow-fiber membrane.
2. Description of the Prior Art
A number of blood oxygenators using hollow fibers as the gas exchange membrane have already been proposed, for example, in U.S. Pat. Nos. 2,972,349, 3,794,468, 4,239,729 and 4,374,802.
In these blood oxygenators, hollow fibers made of a homogeneous membrane of gas-permeable material such as silicone or hollow fibers made of a microporous membrane of hydrophobic polymeric material such as polyolefins are used to bring blood into contact with gas through the medium of the hollow-fiber membrane and effect gas exchange therebetween. There are two types of blood oxygenators: the inside perfusion type in which blood is passed through the bores of the hollow fibers while gas is passed on the outside of the hollow fibers and the outside perfusion type in which, conversely, gas is passed through the bores of the hollow fibers while blood is passed on the outside of the hollow fibers.
In blood oxygenators of the inside perfusion type, no channeling of the blood occurs if the blood is uniformly distributed and fed to the large number of hollow fibers. However, since the blood flowing through the bores of the hollow fibers moves in a perfect laminar flow, the internal diameter of the hollow fibers needs to be reduced in order to increase the oxygenation rate (i.e., the oxygen transfer rate per unit area of membrane). For this purpose, hollow fibers having an internal diameter of 150 to 300 .mu.m have actually been developed for use in blood oxygenators.
Nevertheless, even if the internal diameter is reduced, the laminar flow phenomenon of the blood passing through the hollow fibers is not mitigated and the oxygenation rate of a blood oxygenator of this type is not greatly enhanced. Moreover, as the internal diameter becomes smaller, clotting (i.e., blockade of the bore due to the coagulation of blood) may occur more frequently, thus posing a serious problem for a practical point of view. Furthermore, a blood oxygenator generally uses ten thousand to forty thousand hollow fibers made into a bundle or bundles and it is very difficult to distribute and feed the gas uniformly to the external surfaces of such a large number of hollow fibers, so that special consideration must be given to achieve the desired end. If the gas is not distributed uniformly, the carbon dioxide desorption rate (i.e., the carbon dioxide transfer rate per unit area of membrane) will be reduced. On the other hand, in blood oxygenators of the outside perfusion type, the gas can be distributed uniformly and the blood can be expected to move in a turbulent flow. However, they have the disadvantage of being subject to insufficient oxygenation due to channeling of the blood or blood coagulation at the sites of stagnation. Thus, no blood oxygenator having satisfactory performance has been realized as yet.
In most of the conventionally known blood oxygenators, a cylindrical housing is simply packed with a large number of hollow fibers for gas exchange use in such a way that the hollow fibers are parallel to the longitudinal axis of the cylindrical housing. However, blood oxygenators of this construction have low gas exchange rate per unit area of the hollow-fiber membrane. As an improved form of the outside perfusion type, U.S. Pat. No. 3,794,468 has proposed a blood oxygenator in which hollow tubular conduits of semipermeable membrane are wound about a hollow, cylindrical core having a large number of pores in the wall and then contained in a housing, and blood is allowed to flow out of the cavity of the core through its pores while gas is passed through the bores of the hollow tubular conduits. However, this blood oxygenator is disadvantageous in that the priming blood volume is unduly large and the manufacture thereof requires a complicated procedure because of its intricate structure. Thus, it has not yet been put to practical use.
The conventionally known blood oxygenators in which the hollow fibers are disposed so as to be substantially perpendicular to the direction of blood flow can produce more marked turbulences of the blood flow and hence an improvement in oxygenation rate, as compared with those in which the hollow fibers are disposed so as to be parallel to the direction of blood flow. However, if the size of such a blood oxygenator is magnified or the flow rate of blood is increased in order to treat large volumes of blood, there arise such problems as an increase in pressure loss, channeling of the blood and blood coagulation at the sites of stagnation. The prior art has been unable to solve these problems.