A potentially attractive support device for patients with acute respiratory failure is an intravenous membrane oxygenator. One problem, however, is that the membrane surface area required for sufficient gas exchange can unduly increase vena caval pressure drop and impede venous return. The purpose of this study was to design and develop an intravenous oxygenator that would offer minimal venous flow resistance in situ. The device uses a constrained fiber bundle of smaller cross sectional size than the vena cava so as to effect an intentional shunt flow of venous blood around the fiber bundle and reduce the venous pressure drop caused by the device. A pulsating balloon within the fiber bundle redirects part of this shunt flow into reciprocating flow in and out of the fiber bundle. This offers dual advantages: 1) Blood flow through the fiber bundle is mainly perpendicular to the fibers; and 2) the requisite energy for driving flow comes largely from the pneumatic system pulsating the balloon, not from a venous pressure drop. In this mode a full length device with a 2 cm fiber bundle in a 2.5 cm blood vessel would offer a pressure drop of only a few millimeters of mercury. The use of constrained fiber bundles requires good uniformity of fiber spacing for effective gas exchange. Several prototypes have been fabricated, and CO2 and O2 exchange rates of up to 402 and 347 ml/min/m2 have been achieved during acute animal implantation.