Membrane Oxygenator with a Built-In Hemoconcentrator

In order to facilitate the handling of cardiopulmonary bypass (CPB) and simplify the circuit, we have developed a new membrane oxyganator with a hemofiltration function. The hollow fiber units for gas exchange and hemofiltration were combined in concentric circles in a cylindrical housing. The total priming volume was 190 ml. Because we used a silicon-coated hollow fiber membrane for gas exchange, this oxygenator was completely resistant to serum leakage. The gas exchange and hemofiltration sections both have a blood-outside flow configuration. All blood flows in a radial direction from around the central core to the surrounding hollow fiber units, first to the hemofiltration portion and then to the gas exchange section. Filtered fluid was easily collected through a stopcock mechanism. The oxygen transfer rate was 312 ml/min at a blood flow rate of 6 L/min, and the ultrafiltration rate was 3.5 L/hour at a blood flow rate of 4 L/min with 25% hematocrit and 200 mmHg transmembrane pressure in an in vitro study. The pressure drop was 62 mmHg at a blood flow rate of 4 L/min. We found no adverse effects in an in vivo study using a mongrel dog. In conclusion, this durable combined device could achieve excellent and simplified hemoconcentration by having all the blood in the unit flow through the hemofiltration portion, and may be useful not only in CPB during open heart surgery, but also in extracorporeal membrane oxygenation. Address correspondence to: Hiroshi Nishida, MD Department of Cardiovascular Surgery The Heart Institute of Japan Tokyo Women's Medical College 8-1 Kawadacho, Shinjuku-ku, Tokyo 162, Japan Volume 29, Number 4, December 1997 189 THE jOURNAL OF EXTRA-CORPOREAL TECHNOLOGY INTRODUCTION tained at 65%. The ultrafiltration rate was measured at a hematocrit of 25% and 35%, while altering the blood flow rate from 2 L/min to 4 L/min. IN VIVO ASSESSMENT OF PERFORMANCE After completing the developmental stage and in vitro evaluation, we conducted a preclinical in vivo assessment in a canine model. A mongrel dog weighing 14 kg underwent cardiopulmonary bypass with a closed circuit containing Prototype 4 for three Although screening methods for homologous blood used during surgery have significantly improved, the danger of various blood-related infectious diseases has not been completely overcome. Therefore, there is still a great demand for open heart surgery without the need for homologous blood transfusion. A hemoconcentrator is one of the most important devices to achieve this goal and has been widely used to raise the hematocrit of blood diluted with priming solution and crystalloid cardioplegic solution. Because both the membrane oxygenator and hemoconcentrator are devices made of hollow fibers, we have been developing a combined device: an oxygenator with a hemofiltration function (I, 2). In this article, we will describe the specifications and the in vitro and in vivo performance of the final prototype, together with a brief description of the development process and the various advantages of this new device and concept. Table 1: Specifications of Prototype 4 MATERIALS AND METHODS We have developed four prototypes, each improving and solving the problems of the previous design. We believe the performance of Prototype 4 is satisfactory for clinical application (Table 1). The hollow fiber units for gas exchange (effective membrane surface area: 2.23 m) and hemofiltration (0.61 m) were combined in concentric circles in a cylindrical housing (outer diameter 95 mm, length 140 mm). The [Housing] Outer diameter: 95 mm; Length: 140 mm [Hollow Fiber Bundle] Outer diameter: 80 mm; Length: 85 mm [Central Core] Diamter: 35 mm [Priming Volume] 190 ml Oxygenator Hemoconcentrator Module Cross-Wind Cross-Wind Material Polypropylene Polypropylene (silicone coated) Outer Diameter 300J.lm 300J.lm Wall Thickness 30J.lm 30J.lm Porosity 30% 40% Surface Area 2.23 m2 0.61 m2 Packing Density 55% 50% total priming volume is only 190 ml. In Prototype 4, we combined a longitudinal flow direction and radial direction by making a small gap between the central core and the surrounding hollow fiber bundle. All blood entering into this combined device first flows in a longitudinal diFigure 1: Diagram of Prototype 4 rection to the small gap between the central core and surrounding hollow fibers for hemofiltration (Figure I). The blood simultaneously radiates centrifugally into the adjacent hollow fiber units. This unique blood path enables the coexistence of excellent performance and a small pressure drop. IN VITRO EVALUATION OF PERFORMANCE In vitro performance was evaluated in a mock circulation using bovine blood. During the measurement of the oxygen transfer rate and pressure drop, which lasted for three hours, the hemoglobin concentration was maintained at 12 g/dl and the temperature was maintained at 3TC. Oxygen saturation of the venous blood was main190 Prototype IV filtered fluid outlet