Optimal hematocrit 30%: Farewell to a fallacy

Background: In the early 1970s the existence of an optimal hematocrit or Hb concentration under normovolemic or isovolemic hemodilution as well as hemoconcentration was found experimentally in vivo for animals. This was highly conform to a theoretically predicted optimum in that range. Thus, the maximum of systemic oxygen availability as the product of cardiac output and arterial oxygen content at about 110% relative to the control coincided with the hematocrit value of about 30%, corresponding to a Hb concentration of about 10 g/dl. As a result of decreased blood viscosity, the cardiac output increased more than necessary for compensation. A few years later, other authors could not reproduce these results and additionally mentioned that the conclusions were not compatible with their experimental data. Aim and Sources: Relevant literature data of the last 35 years were analyzed critically provided that they comprised quantitative data concerning the oxygen availability as a function of oxygen or Hb concentration or that these values could be derived in part from the published data of hematocrit. Results: As a result of critical analysis of a great number of relevant literature data, the above statement now is obsolete: During normovolemic hemodilution as well as hemoconcentration, the maximum of oxygen availability is reached at the physiological hematocrit and Hb concentration, respectively, decreasing at both sides. Conclusion: Only one group has published an optimal hematocrit of 30%, a result which is not compatible with the data they obtained experimentally. Therefore, this revised conclusion must be interpreted as a farewell to the fallacy of an optimal hematocrit of 30%.