[Which factors determine the critical hematocrit as an indication for transfusion?]

The question as to what extent the hematocrit (Hct) is a strong indicator for or against the need for transfusion of whole blood or blood products is still controversial. In order to enable the clinician to make a definite decision, a number of aspects have to be taken into consideration. The human organism has only limited oxygen reserves, and these are even more limited under pathological conditions. Oxygen flux - the amount of oxygen transported by the blood in 1 min - is a critical factor in the oxygenation of the human body. Another critical factor is oxygen consumption, which is highly variable depending on the presence of conditions such as rest, shivering, seizures, hypothermia, etc. Furthermore, different organ systems have different oxygen consumption rates. The ratio of oxygen consumption to oxygen flux is referred to as the oxygen extraction rate or oxygen utilization. Under normal conditions oxygen uptake is independent of oxygen flux, and thus independent of blood flow. Under conditions of organ dysfunction, however, oxygen deficiency may be present without being recognized on standard clinical diagnostic parameters. The normal human organism has a number of possibilities to compensate for acute or chronic anemia, i.e., increases in cardiac output, organ perfusion, 2,3-DPG content, a shift in the oxygen dissociation curve, etc. These compensatory mechanisms may, however, be restricted or cease to function under conditions of acute or chronic disease. Arterial and mixed-venous PO2 and oxygen content are some of the parameters used to assess the oxygen reserves available to the organism even under critical conditions. Although oxygen content is the most significant of these parameters, accurate measurement of this parameter remains a problem of laboratory medicine. PVO2 is of only limited importance under conditions of anemia. Minimum oxygen content or minimum oxygen flux values should under no conditions be approximated during anesthesia or intensive care. The critical Hct as an indicator for or against transfusion of blood or blood products is considerably modified by restricted organ function, anesthesia, intensive care treatment, resuscitation, etc.(ABSTRACT TRUNCATED AT 400 WORDS)