Gas Transfer of an Implatable Artificial Lung

Abstract

The water flow characteristics in the implantable artificial lung (IAL) were evaluated by in vitro experiments. The 50-fiber modules were of different lengths but contained well-spaced fibers; and the 300-fiber module had a volume fraction of 0.9. A lower number of tired hollow fibers produce a higher water-side film coefficient of oxygen transfer at a constant velocity of water-side flow. The area of the water flow decreases with increasing hollow fiber packing density, thus if water is induced to flow uniformly in the IAL module, the oxygen transfer rate should monotonously increase with the packing density. The water flow in the IAL is not uniform and depends on the hollow fiber packing density. The stagnation of the water flow occurs in the bundle of the hollow fibers at the water inlet. Intravascular oxygenation represents an attractive, alternative support modality for patients with ARDS. The concept of intravascular oxygenation as an alternative ARDS therapy originated with Mortensen (7), who developed an intravenous oxygenator (IVOX) consisting of a bundle of crimped hollow fiber positioned in the vena cava. In phase I clinical trials, the IVOX provided an average of 28% of basal gas exchange requirements for patients with severe ARDS (8-14). The clinical study, however, concluded that more gas exchange was needed for intravascular oxygenation to be clinically effective in ARDS treatment. We are developing an intravenous membrane oxygenator (IMO) with a design goal of 50% of basal oxygen and carbon dioxide exchange requirements for end-stage ARDS patients. Like the IVOX, the IMO consists of a bundle of manifolded hollow fiber, and is intended for intravenous placement within the superior and inferior vena cava. The target level of gas exchange in the IVOX, and consequently, the IMO, incorporates a polyurethane balloon concentric with the fiber bundle, which rhythmically inflates and deflates to provide active blood mixing, and thus enhances gas exchange. Our current efforts focus on device improvements intended to provide the target levels of gas exchange, given the constraints imposed by intravenous placement on fiber bundle size and hence fiber area for gas exchange. Although critical care techniques have been improved, the high mortality of severe ARDS has not significantly changed (10-15). In an implantable artificial lung (IAL), the greater part of the oxygen transfer resistance is located in the blood-side laminar film (16), and various methods have been attempted to make the laminar film thin and improve the oxygen transfer rate (17, 18). In the present study, the water flow characteristics in the IAL were evaluated by in vitro experiments. The effect of hollow fiber packing fraction on water flow condition was evaluated to produce effective constant of water with the hollow fibers. Oxygen transfer rates were evaluated, and the optimum hollow fiber packing fraction was determined at an outside diameter of hollow fibers of 380 μm. II. THEORYT The Reynolds number (NRe =Lv/υ) characterizes the flow regime and is the ratio of inertial force to viscous force. The Schmidt number (NSc=υ/D), analogous to the Prandtl number in heat transfer, characterizes the fluid properties and is the ratio of momentum transport to diffusive transport. The Peclet number (NPe=Lv/D), which is the product of NRe, characterizes the relative importance of convective and diffusive processes and is the ratio of bulk mass transport to diffusive mass transport. The Sherwood number (NSh=KL/D), also known as the mass transfer Nusselt number, likewise characterizes the relative importance of convective and diffusive transport; it is the ratio of total transport to diffusive transport. The mass transfer Stanton number (NSt=K/v) is proportional to the ratio of the actual mass flux to the mass flux capacity of the flow, i.e. the amount of mass potentially transferable per unit time and cross-sectional area.