Changes in blood P50. Effects on oxygen delivery when arterial hypoxemia is due to shunting.

The theoretic effect of increased values for the oxygen pressure at an oxygen saturation of 50 percent on oxygen delivery in arterial hypoxemia due to right-to-left shunting was analyzed using a mathematical model of the oxygen-hemoglobin equilibrium curve. We found that, regardless of the size of the shunt, a rightward shift of the curve resulted in increased mixed venous oxygen tension, increased arterial oxygen pressure, and, hence, a decreased alveolar-arterial oxygen pressure difference compared to the standard curve (hemoglobin level, cardiac output, and oxygen consumption remaining constant).