Clarification of the effects of changes in P50 on oxygen transport.

In summary, a shift to the right in the O2-Hb curve in patients with anemia and decreased cardiac output and normal PaO2 will increase PvO2. This shift is mediated through increases in red cell 2,3-DPG. The stimulus for the increase in 2,3-DPG is most likely due to the decrease in SvO2. In hypoxemia caused by a decreased PAO2 (altitude), an increase in P50 may have no effect on PvO2 and in fact if the PaO2 is low enough, a left shift may increase the PvO2. Animals and man most successful at adapting to high altitudes reflect this fundamental physiological effect and have a left-shifted curve. This decrease in P50 is probably related to an intrinsic property of the hemoglobin and not to changes in 2,3-DPG. In hypoxemia caused by shunt, an increase in P50 increase the PvO2 regardless of the PaO2. Patients with congenital cyanotic heart disease have an increased P50 mediated through an increase in 2,3-DPG.