Theoretical analysis of altitude tolerance and hemoglobin function.

The potential effect of changes in the position and shape of the oxyhemoglobin dissociation curve (ODC) in modifying altitude tolerance is quantitatively analyzed and discussed. The most frequently used criteria for oxygen affinity, the position or P50 of the ODC (oxygen pressure at 50% saturation) is shown to have slight influence on tissue oxygenation in terms of arteriovenous oxygen difference under steady-state, hypoxic-hypoxia conditions. Conversely, changes in the shape of the curve, also expressed as the cooperativity or heme-heme interaction, could substantially modify oxygen transport under these conditions. Published observations are reviewed which suggest that hemoglobin cooperativity is not normally invariant.