Defining the low limit of hematocrit for surgical patients

The question of what constitutes the safe hematocrit (Hct) limit in surgical patients is discussed. Man has limited oxygen reserve. The reserve is further limited by acute or chronic pathological conditions. One approach to answering the question is to examine the oxygen delivery (ḊO₂)-oxygen consumption (V̇O₂) relationship. When oxygen delivery decreases, oxygen consumption does not decrease until the critical point of oxygen reserve is reached. The components of ḊO₂ include Hct, cardiac output and arterial oxygen saturation. In a given situation, Hct level that constitutes critical oxygen delivery is the critical level of Hct. The value is, therefore, influenced by the oxygen requirement and whether or not a compensatory increase in cardiac output is caused by a low Hct. The relationship between ḊO₂ and VO₂ is not the same for each organ. Thus, significant modification of the critical level of Hct must be made for patients with organ specific disease.