Intravascular lung assist devices (ILADs) must provide sufficient gas transport without excessive pressure drop. In vitro studies of fiber-bundle devices determined the effect of geometric configuration on gas transfer efficiency, the amount of surface area needed to achieve sufficient gas transport, and the relationship between gas transport and blood-side pressure drop. ILADs with cross-flow fibers transfer at least 10 times more O2 than parallel flow ILADs of the same surface area, and six to eight times more CO2. Some tested devices with 0.4-0.6 m2 of surface area can transfer 100 ml/min of both O2 and CO2. Configurations with high gas transport, however, often require moderate pressure drops.