Membrane oxygenators: current developments in design and application

Cardiopulmonary bypass (CPB) procedures require a blood-gas exchanger (oxygenator) to temporarily replace the respiratory function of the lungs. In the past the majority of CPB procedures have been carried out with bubble oxygenators which effect gas exchange by dispersion of bubbles into the blood. Membrane oxygenators, on the other hand, utilize a hydrophobic gas permeable membrane between the blood and gas phases. Bubble oxygenators are being superseded by membrane types for CPB due to improvements in membrane technology and mass transfer efficiency. These advances are reviewed in this paper and are illustrated by reference to the gas exchange and operating characteristics of a number of clinical oxygenators designed for adult CPB. Membrane oxygenators are also being used for long-term support in the treatment of acute respiratory failure. Operated in a partial bypass circuit, the oxygenator may have to function for several days or weeks. In one particular treatment method, the rate of spontaneous breathing is controlled by the partial or total removal of the metabolic CO2 production by the membrane oxygenator. For this method, known as extracorporeal CO2 removal (ECCO2R), the oxygenator must be optimized for CO2 transfer at low blood flow rates. The suitability of clinical oxygenators for ECCO2R is discussed in terms of gas exchange and functionality over a prolonged operation.