HOLLOW FIBER TYPE OXYGENATOR

Abstract: In a hollow fiber type oxygenator comprising a housing, a bundle of a plurality of hollow fibers axially entending through the housing and each presenting a gas-exchange membrane, partitions engaged in the housing at its opposite end portions and retaining the opposite ends of the hollow fibers without blocking the opening of the fibers, a gas inlet and a gas outlet both in fluid communication with the interior space of the hollow fibers, a blood chamber defined by the partitions, the outer surface of the hollow fibers, and the inner surface of the housing, and a blood inlet and a blood outlet located in the housing wall in fluid communication with the blood chamber, the bundle of the hollow fibers has packing density d₁ of not more than 30% near the partitions and a packing density d₂ of 40 to 50% near an axial mid-point, the ratio of d₁/d₂ being not more than 0.6.

Description: TITLE OF THE INVENTION Hollow Fiber Type Oxygenator BACKGROUND OF THE INVENTION This invention relates to a hollow fiber type oxygenator for use in extracorporeal blood circulation which oxygenates blood passing therethrough while removing carbon dioxide. Conventional oxygenators may be divided into two classes, that is, a bubble type oxygenator and a membrane type oxygenator. The membrane type oxygenators including flat membrane type and hollow fiber type oxygenators cause less damages to blood such as hemolysis, denaturation of proteins, blood coagulation, blood adhesion and the like as compared with the bubble type oxygenators, and are generally recognized to have a mechanism much closer to a living lung. In spite of the superiority of the membrane type oxygenators to the bubble type oxygenators, the bubble type oxygenators have been widely used in open heart surgery. In membrane type oxygenators now available, blood stream layers must be thin to obtain a higher oxygenating capacity. Narrow flow paths cause high flow resistance to blood. These factors prevent blood perfusion by gravity wherein blood is perfused through an oxygenator by the head between the patient and the oxygenator. Then, an extracorporeal blood circuit having a membrane type oxygenator must employ a pump upper stream of the oxygenator, namely on the venous side of the oxygenator. This invites a problem that the pressure near the outlet of the pump exceeds the sum of pressure losses across the blood return catheter and the oxygenator, which results in an increased pressure in the blood-returning line. If the pressure is extremely increased, a tube in a roller pump can be expanded to burst. An additional problem is that two oxygenators are required when extracorporeal circulation is separately carried out for the brain and the lower half body during an open heart sugery. To abate postoperative complications, the use of a pulsatile pump is recently recommended for the extracorporeal circulation of blood which is close to the blood in vivo. The pressure loss across conventional oxygenators are too high to provide pulsatile flow. European Patent Publication No. 103899 proposes a process for exchanging oxygen with carbon dioxide in a hollow fiber type oxygenator by passing oxygen through the interior of the hollow fiber membranes and blood along the exterior of the hollow fiber membranes, that is, the space defined by the housing and the hollow fiber membranes. However, there is no oxygenator which allows for blood drainage by gravity with a sufficient gas exchangeability. SUMMARY OF THE INVENTION It is, therefore, an object of the invention to provide an improved hollow fiber type oxygenator of the type wherein blood is passed between the inner wall of a housing and the outer surface of hollow fibers and oxygen is passed through the interior of the hollow fibers, which allows blood to reach the oxygenator only by the head between the patient and the oxygenator, while providing a sufficient gas exchangeability even with a reduced membrane area as compared with oxygenators of the other type wherein blood is passed through the interior of hollow fiber membranes. It is another object of the invention to provide a compact hollow fiber type oxygenator as mentioned above by determining optimum dimensional parameters to achieve smooth blood passage effective gas exchange. According to the present invention, there is provided a hollow fiber type oxygenator comprising a housing having opposite end portions, a bundle of a plurality of hollow fibers axially extending through the housing and each presenting a gas-exchange membrane, partitions engaged in the housing at its opposite end portions and fluid tightly retaining the opposite ends of the hollow fibers without blocking the opening of the fibers, a gas inlet and a gas outlet both in fluid communication with the interior space of said hollow fibers, a blood chamber defined by the partitions, the outer surface of said hollow fibers, and the inner surface of said housing, and a blood inlet and a blood outlet located in the housing wall at the opposite end portions in fluid communication with said blood chamber, characterised in that the bundle of hollow fibers has a end packing density d1 of not more than 30% near the partitions and a central packing density d2 of 40 to 50% near an axial mid-point, the ratio of d1/d2 being not more than 0.6. Several preferred embodiments of the present invention are described below. (i) The ratio of fiber packing density dl/d2 is in the range from 0.4 to 0.6. (ii) The hollow fibers bundle has the packing density dl in the range from 20 to 30%. (iii) The hollow fiber membrane comprises a porous hollow fiber membrane made of polyolefin having an inner diameter of about 100 to about 1,000 pm, a wall thickness of about 10 to about 200 Vm, an average pore diameter of about 200 to about 2,000 , and a porosity of 20 to 80%. Polypropylene is the preferred polyolefin. (iv) The wall of the housing is tapered toward an axial mid-point such that the wall has a minimum inner diameter approximately at the mid-point and is flared therefrom toward the opposite end portions. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features, and advantages of the present invention will be readily understood from the following description taken in conjunction with the accompanying drawings, in which: FIG. 1 is an elevational cross section of a hollow fiber type oxygenator according to one embodiment of the present invention; FIGS. 2, 3 and 4 illustrates different circuit arrangements wherein the oxygenator of the present invention is used; FIG. 5 is a diagram showing the volume of oxygen migrated and the pressure loss AP in relation to the end packing density dl; FIG. 6 is a diagram showing the volume of oxygen migrated and the pressure loss dP in relation to the central packing density d1; FIG. 7 is a diagram showing the volume of oxygen migrated and the pressure loss AP in relation to the ratio of d1/d2; FIG. 8 is an elevational cross section of a hollow fiber type oxygenator according to another embodiment of the present invention; and FIG. 9 is a perspective view of a hollow fiber type oxygenator similar to that shown in FIG. 8. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there is schematically illustrated a hollow fiber type oxygenator according to one embodiment of the present invention. The oxygenator generally designated at 1 includes a hollow cylindrical housing 2 defining an interior space and being open at axially opposite end portions. The housing 2 receives therein a bundle or assembly 35 of a plurality of axially extending hollow fibers 3 each presenting a gas exchange membrane. The opposite ends of the hollow fibers 3 are fluid tightly retained by opposed partitions 41 and 42 which are engaged in the housing 2 at its opposite ends, respectively. For brevity of description, the partitions 41 and 42 and the corresponding housing ends are referred to as upper and lower partitions and upper and lower housing ends. Upper and lower conical headers 21 and 22 are fixedly secured to the upper and lower ends of the housing 2. The inner surface of the upper header 21 and the outer surface of the upper partition 41 define a gas incoming chamber 23 in fluid communication with the interior space of hollow fibers 3. The header 21 has a gas inlet 24 at its apex. The inner surface of the lower header 22 and the outer surface of the lower partition 42 define a gas outgoinig chamber 25 in fluid communication with the interior space of hollow fibers 3. The header 22 has a gas outlet 26 at its apex. In the oxygenator 1 of this arrangement, a gas such as oxygen and air can be passed from the inlet 24 through the interior space of the hollow fibers 3 to the outlet 26. The lower header 22 may not necessarily be provided, that is, the outgoing chamber 25 and outlet 26 may be omitted. Then the outer side of the lower partition 42 becomes a gas discharge end and the gas coming out of the hollow fibers 3 is directly released into the atmosphere. A blood chamber 5 is defined by the inner surfaces of the partitions 41, 42, the inner wall of the housing 2, and outer wall of the hollow fibers 3. The housing 2 is provided near its lower and upper ends with a blood inlet 61 and a blood outlet 62 both in fluid communication with the blood chamber 5. The oxygenator permits blood to pass through the blood chamber 5 in a turbulent flow around hollow fiber membranes 3. That portion of the wall of the housing 2 where the blood inlet 61 is provided is radially dilated as compared with the intermediate portion. An annular space is thus defined between the outer periphery of the bundle 35 of hollow fibers 3 and the radially dilated housing wall, providing an annular blood flow path. The annular flow path circumscribes the bundle 35 of hollow fibers 3 so that blood is smoothly and evenly distributed among all the hollow fibers 3. Also, that portion of the wall of the housing 2 where the blood outlet 62 is provided is radially dilated as compared with the intermediate portion. An annular space is thus defined between the outer periphery of the bundle 35 of hollow fibers 3 and the radially dilated housing wall, providing an annular blood flow path. The annular flow path circumscribes the bundle 35 of hollow fibers 3 so that blood flowing around the hollow fibers 3 is smoothly directed to the blood outlet 62 from the entire periphery of the fiber bundle 35. The wall of the housing 2 is tapered toward an axial mid-point. The housing wall has a minimum inner diameter approximately at the mid-point and is flared or divergent therefrom toward the upper and lower ends. The wall of the housing 2 is tapered toward the mid-point to squeeze or throttle the bundle or assembly 35 of hollow fibers at the axial mid-point. The throttling of the fiber assembly 35 by the housing 2 ensures a uniform blood flow in a transverse cross-section of the assembly 35 and changes the flow velocity of blood in axial direction of the fiber assembly 35, promoting the occurrence of turbulent flow to improve the gas exchangeability of the oxygenator. In additon to the tapering of the housing wall toward the mid-point as previously described, preferably the tapered wall of the housing 2 is connected to the radially dilated wall defining the annular blood flow path by a tapered surface (not shown). Then air to be purged upon priming is smoothly driven off alone the inner surface of the housing 2 and released into the atmosphere without remaining in the blood chamber 5. The hollow fiber membranes 3 used in the oxygenator of the invention may be microporous membranes. The porous hollow fibers may preferably be formed of a polyolefin such as polypropylene and polyethylene. Polypropylene is most preferred. Each hollow fiber (membrane) used in the present invention has many micropores which communicate the interior and the exterior of the membrane. The fiber has an inner diameter of about 100 to about 1,000 um, a wall thickness of about 10 to 200 Um, an average pore diameter of about 200 to about 2,000 A, and porosity of 20 to 80%. When the hollow fibers 3 of the microporous membrane type are used, gas migration takes places as a volume flow. The resistance of the membrane to gas migration is reduced and the gas exchangeability of the membrane is improved. Silicone membranes which provide for gas migration through dissolution and diffusion process may also be employed as well as the microporous membranes. The partitions 41 and 42 serve for the important funciton of isolating the interior from the exterior of the hollow fibers 3. In general, the partitions 41 and 42 are formed by casting a highly polar, high molecular weight potting compound, for example, polyurethane, silicone and epoxy resins by a centrifugal casting process to the inner wall surface of the housing end portions where it is cured, More illustratively, first a number of hollow fibers 3 longer than the axial length of the housing 2 are prepared, opposite open ends of the fibers are sealed with a viscous resin, and then the fibers are placed in the housing 2 in mutually juxtaposed relationship. The opposite ends of the fiber bundle are completely covered. The housing 2 is rotated about its longitudinal axis while a polymeric potting compound is introduced from the sides of blood inlet 61 and outlet 62 where it is cured. The edge portions of the fiber bundle bonded with the potting compound are severed with a sharp knife to expose freshly cut openings of the hollow fibers 3 at their opposite ends. The partitions 41 and 42 are thus formed. Those surfaces of the partitions 41 and 42 facing the blood chamber 5 are circular and concave as shown in FIG. 1. In order that the oxygenator thus prepared may exert satisfactory performance, the bundle 35 of hollow fibers must meet certain limitations with respect to fiber packing density. First, the hollow fiber bundle 35 must have a packing density dl of not more than 30% at the location of partitions 41 and 42. The term packing density used in the present invention is the sum of the outer diameter areas of the hollow fibers 3 divided by the area defined by the envelope of the fiber bundle 35 as expressed in percentage. With a packing density dl of above 30%, the amount of oxygen permeated through the membranes is reduced below a practically required level. Also an increased pressure loss prevents passage of blood by gravity. More illustratively, a head AH of 90 to 120 cm, preferably 100 cm, is set between an operating table 0 and a floor level F in order to enable passage pf blood by gravity as shown in FIG. 2. This means that the passage of blood by gravity may not be conducted unless the pressure loss is kept less than 60 mmHg. The oxygenator must accommodate a maximum blood flow rate of approximately 6.0 1/min. The oxygenator must have such an oxygenating capacity that the amount of oxygen migrated through the membranes be at least 240 ml/min. at a blood flow rate of 6.0 1/min. These requirements are satisfied when the packing density dl is equal to or less than 30%. More preferable results are obtained when the packing density dl is from 20% to 30%. When the packing density dl is less than 20%, the oxygenator is no longer compact and undesirably increases the volume of blood contained therein. Secondly, the hollow fiber bundle 35 must have a packing density d2 of 40% to 50% near its axial mid-point or the throat of the housing 2. A packing density d2 of more than 50% makes impossible the passage of blood by the head or gravity, whereas a packing density d2 of less than 40% produces a practically unacceptable oxygenating capacity. The ratio of fiber packing density dl/d2 should be kept equal to or below 0.6. Ratios of dl/d2 in excess of 0.6 make impossible the passage of blood by the head or gravity and result in a low oxygenating capacity. For the compactness of the oxygenator, the ratio of dl/d2 may preferably be between 0.4 and 0.6. Further, the radius r2 of the hollow fiber bundle 35 at the housing throat may preferably be not less than 60 mm, when the blood flow rate QB is 6.0 l/min. Then the blood flow rate solely caused by the head is increased. For compactness purpose, r2 may preferably be less than about 90 mm. In order to obtain an improved oxygenating capacity, it is desirable that the total membrane surface area may preferably be at least 1.5 square meter as calculated on the basis of the inner diameter of the hollow fibers. A total membrane area of about 2.0 to 3.0 m2 is recommened for compactness. Another embodiment of the hollow fiber type oxygenator according to the present invention is shown in FIG. 8. FIG. 8 shows an oxygenator similar to the previously described oxygenator shown in FIG. 1 with the exception that the blood outlet 62 is replaced with blood drain means in the form of a plurality of blood drain openings 65 and blood reserving chamber 7 is further provided in fluid communication with the blood drain means. The blood reserving chamber 7 is provided at the bottom with an outlet 75. The oxygenator 1 shown in FIG. 8 permits the line between the blood outlet and a blood reservoir to be omitted so as to reduce the volume of blood retained in the overall circuit. This is particularly effective when the oxygenator is used for a neonate with a low absolute volume of blood. Another advantage is ease of handling due to simplified circuit. As shown in FIG. 9, the blood reserving chamber 7 may be provided with a heat exchanger 85, with the advantage of omitting a line connecting the blood storage chamber to a heat exchanger. EFFECT OF THE INVENTION FIG. 2 shows a circuit arrangement wherein the oxygenator 1 of the invention is used in combination with a roller pump 7 placed downstream of the oxygenator, whereby blood is passed through the oxygenator by gravity or the head between the patient and the oxygenator. A pressure loss of less than 60 mmHg is achieved with the present oxygenator at a head AH of approximately 100 cm so that a sufficient blood flow rate is secured. The amount of oxygen migrated can be at least 240 1/min. at a blood flow rate of 6.0 1/min. The oxygenator of the invention has another benefit over prior art oxygenators of the type wherein blood is passed inside hollow fiber membranes, that an equivalent oxygenating capacity is obtained with an about one half membrane area, which results in size, weight and cost reductions. FIG. 3 shows another circuit arrangement wherein the oxygenator 1 of the invention is used in combination with two roller pumps 71 and 75 placed downstream of the oxygenator. Separate extracorporeal circulation can be smoothly carried out, because of the capacity of the oxygenator. FIG. 4 shows a further circuit arrangement wherein the oxygenator 1 of the invention is used in combination with a pulsatile pump 8. According to embodiments (i) and (ii), the size of the oxygenator is reduced and the volume of blood retained therein is accordingly reduced. An oxygenator according to embodiment (iii) achieves a remarkably high gas exchangeability. An oxygenator according to embodiment (iv) offers a uniform flow of blood resulting in an improved gas exchangeability. The oxygenator thus constructed was tested to confirm the effect of the invention. One of such experiments is described hereinafter. Example An oxygenator as shown in FIG. 1 was manufactured using hollow microporous fibers of polypropylene which were formed by axially stretching to the following specifications: length 85 mm, inner diameter 200 um, outer diameter 240 um, average pore diameter 650 , and porosity 45%. The fiber bundle had a total membrane area of 2.5 m2 as calculated on the basis of the inner diameter of hollow fibers, and a radius r2 at the throat of 76 mm. The fiber bundles having varying fiber packing densities dl and d2 were assembled in housings. Bovine 0 blood having a Hematocrit value Ht of 35% at 37 C was passed through the oxygenators at a blood flow rate QB of 6 1/min., while oxygen gas was passed at a flow rate V of 6 1/min. As described above, the lower limit of the volume of oxygen migrated through the membranes is 240 ml/min. and the upper limit of the pressure loss AP during passage of blood by gravity is 60 mmHg. FIG. 5 shows the volume of oxygen migrated and the pressure loss AP in relation to the end packing density d when the central packing density d2 is set to 46%. The end packing density dl should be 30% or less in order to clear the critical points. FIG. 6 shows the volume of oxygen migrated and the pressure loss dP in relation to the central packing density d2 when the end packing density dl is set to 24%. The central packing density d2 should range between 40% and 50 % (about 53% under the experimented conditions). FIG. 7 shows the volume of oxygen migrated and the pressure loss dP in relation to the ratio of packing density dl/d2. The ratio dl/d2 should be 0.6 or less. The data in FIGS. 5, 6 and 7 reveal that the packing densities defined in the present invention are critical.

Claims: WHAT IS CLAIMED IS: 1. A hollow fiber type oxygenator comprising a housing having opposite end portions, a bundle of a plurality of hollow fibers axially extending through the housing and each presenting a gas-exchange membrane, partitions engaged in the housing at its opposite end portions and fluid tightly retaining the opposite ends of the hollow fibers without blocking the opening of the fibers, a gas inlet and a gas outlet both in fluid communication with the interior space of said hollow fibers, a blood chamber defined by the partitions, the outer surface of said hollow fibers, and the inner surface of said housing, and a blood inlet and a blood outlet located in the housing wall at the opposite end portions in fluid communication with said blood chamber, characterised in that the bundle of hollow fibers has a packing density dl of not more than 30% near the partitions and a packing density d2 of 40 to 50% near an axial mid-point, the ratio of dl/d2 being not more than 0.6. 2. A hollow fiber type oxygenator according to claim 1 wherein the ratio of dl/d2 is in the range from 0.4 to 0.6. 3. A hollow fiber type oxygenator according to claim 1 wherein the packing density dl of the hollow fiber bundle ranges from 20 to 30%. 4. A hollow fiber type oxygenator according to claim 1 wherein each said hollow fiber is a porous hollow fiber made of a polyolefin having an inner diameter of about 100 to about 1,000 iim, a wall thickness of about 10 to about 200 iim, an average pore diameter of about 200 to about 2,000 A, and a porosity of 20 to 80%. 5. A hollow fiber type oxygenator according to claim 4 wherein the polyolefin is polypropylene. 6. A hollow fiber type oxygenator according to claim 1 wherein the wall of the housing is tapered toward an axial mid-point such that the wall has a minimum inner diameter approximately at the mid-point and is flared therefrom toward the opposite end portions.

Inventors: DEGUCHI, HIROMI, NAGAYAMA, KIYOTAKA
Applicants: TERUMO KABUSHIKI KAISHA TRADING AS TERUMO CORPORATION
Application year: 1985
Main classification: A61M1
IPC codes: A61M1