In extracorporeal lung assist, membrane oxygenators are used to improve gas exchange. Accumulations on the membranes of coagulation end products can increase resistance to blood flow and diffusion distance. Thus, functioning of the system can be impaired and, in extreme cases, lead to malfunction which may necessitate change out of the oxygenator. We describe a method that complements conventional continuous heparinisation with the administration of acetylsalicylic acid to inhibit thrombocyte aggregation.