[Perioperative and postoperative normovolemic anemia. Physiological compensation, monitoring and risk evaluation]

We accept increasingly lower haemoglobin concentrations in order to avoid erythrocyte transfusions in surgical patients. Knowledge about the physiological adaptation to anaemia, and how this is affected by disease and anaesthesia, are necessary in order to foresee when compensation may be inadequate. Only a few methods are available at present for monitoring tissue oxygenation i.e. mixed venous oxygen saturation, systemic lactacidosis and ECG. In acute haemodilution, cardiac output increases by about 30%, mainly because of a drop in peripheral resistance. Increased contractility and heart rate may contribute to the hyperdynamic circulation and elevate myocardial oxygen demand. In acute anaemia the maximal oxygen extraction ratio in humans is approximately 0.5. Erythrocyte transfusion carries a small, but not negligible risk of infectious disease transmission and immunological incompatibility. Whether allogen transfusions increase the rate of postoperative infections and recurrence of cancer is still a debated issue. The low rate of complications associated with erythrocyte transfusions indicates that the margins of safety should not be too narrow, and that the transfusion trigger should be individualized between 5 and 10 g/100 ml.