[Preoperative haemodilution: basis adaption mechanism and limitation of clinical application (author's transl)].

Basically the adaptational mechanism in acute, normovolaemic dilutional anaemia with dextran-60 is an increased flow rate to organs and tissues due to decreased viscous flow resistance. There is no decrease in systemic oxygen transport and no increase in oxygen extraction ratio unless the hematocrit drops well below 20%. Thus adequate tissue oxygen supply is guaranteed in limited dilution as shown by direct measurements of tissue PO2. The feasibility of a new concept of acute preoperative haemodilution with subsequent autotransfusion of the shed blood is demonstrated in 34 patients. Within the safe limits of clinical dilution in the haematocrit range of 25-20% adequate tissue oxygenation is provided by a rise in cardiac output, whereas heart rate, blood gases and acid base status are essentially unchanged. Autotransfusion during surgery and postoperatively helps to save donor blood, is easy to perform, gives better tissue perfusion at the microcirculatory level and prevents postoperative thromboembolic complications.