Optimal hematologic variables for oxygen transport, including P50, hemoglobin cooperativity, hematocrit, acid-base status, and cardiac function.

The two important blood properties that affect O2 delivery are the O2 equilibrium curve (OEC) and blood viscosity with its subsequent effect on flow (cardiac output). To quantitate these properties blood OEC's were analyzed in terms of the Adair 4-step oxygenation model and the resulting parameters were used to construct a computer nomogram to reproduce the OEC at any combination of effectors that regulate P50 (pH, PCO2, and 2,3-DPG). In this way, the P50 could be changed systematically and the effects on overall O2 transport could be studied. Hematocrit-viscosity-cardiac output relationships were taken from the literature and validated using data from human subjects with various pathological states and high-altitude natives. A model was then developed, using the Bohr integration, to predict the O2 transport function of blood under a variety of conditions including exercise and hypoxia. The results indicate that the optimal hematocrit is about 43-45%, even in hypoxia. The optimal P50, however, depends on the availability of O2: a high P50 is not necessarily beneficial in hypoxia and high cardiac output states. This model and general approach should prove useful in the design of blood substitutes.