Oxygen-Hemoglobin Dissociation Curve. There is a nonlinear relationship between the percentage of total hemoglobin saturated with oxygen (SaO2) and PaO2, as demonstrated by the oxygen-hemoglobin dissociation curve illustrated in Figure 1. Hemoglobin saturations of 95% and higher sustain PaO2 at or above 80 mm Hg, preventing hypoxemia. At 90% saturation, the curve becomes steep, and within a relatively narrow period, the percent hemoglobin saturation and PaO2 decline dramatically. The relationship between SaO2 and PaO2, although in different units, approximates a value of 30 during this rapid decline. Intracellular oxygen tension or that of mixed venous blood is normally 40 mm Hg, so that hemoglobin saturation below 70% would indicate that normal cellular function is compromised.