[Critical hematocrit from the viewpoint of the clinician].

The hazards of homologous blood transfusions have been controlled but not eliminated by modern blood banking methods. In addition to the possibility of transmission of infectious diseases, blood transfusion has been reported to induce immunosuppression and thereby impair the host resistance of surgical patients. Experimental and clinical research, particularly in cardiac surgery, have encouraged physicians to accept postoperative anemia. The influence of hemodilution on oxygen supply to the tissues was found to be an increase of local oxygen tension. When hematocrit is progressively lowered under normovolemic conditions, blood cell flux per time and tissue unit is maintained at a hematocrit level as low as 20 vol% as a result of an increase of red blood cell velocity. An increasing number of cardiac operations have been performed upon patients belonging to Jehova's witnesses, inducing an average hematocrit of 11 vol% during extracorporeal circulation. The outcome of these patients was similar to that of patients having received blood transfusion in order to elevate oxygen supply. Some recent studies in critically ill patients demonstrate the importance of oxygen consumption as an important criterion in estimating the effect of oxygen transport. It seems to be proven that, in presence of cardiovascular stability, elevation of the hemoglobin level above 9 g/dl does not result in an increase of oxygen utilization. In accordance with the topical literature we suggest that during the perioperative period a hemoglobin level of 8.5 g/dl will become an accepted transfusion trigger. Particular patients will tolerate even much lower levels.