Introduction

CLICK HERE FOR AN INTERACTIVE VIEW OF THIS DIAGRAM. Figure 1 - Oxygen-hemoglobin dissociation curve. Note that the oxygen saturation drops only minimally as the PaO2 drops from 100 to 60 mm Hg. This enables the body to maintain a high oxygen content in the blood despite gas exchange abnormalities in the lungs. At PaO2 below 60 mm Hg, however, the oxygen saturation, and blood oxygen content, drops quickly. Remember that the oxygen content of the blood is comprised of oxygen bound to hemoglobin and oxygen dissolved in blood. The blood can carry far more oxygen bound to hemoglobin than it can in the dissolved state. Thus, once the oxygen saturation of hemoglobin exceeds 90%, further increases in PaO2 have relatively small effect on oxygen content. Oxygen content = 1.34 (Hgb) (%sat) + 0.003 (PaO2) For example: Point A: PaO2 = 60, O2 sat = 90%, oxygen content = 16.5 ml O2/100 ml blood Point B: PaO2 = 75, O2 sat = 95%, oxygen content = 17.4 Point C: PaO2 = 95, O2 sat = 99%, oxygen content = 18.2 To go from a PaO2 of 60 to 95 results in only a 10% increased in oxygen content. However, to go from a PaO2 of 45 (point D) to a PaO2 of 60 (point A) results in a change in O2 content from 13.1 t o 16.5 ml O2/100 ml of blood, a 26% increase in oxygen content. The shape of the oxygen-hemoglobin dissociation curve provides humans with the ability to maintain nearly fully saturated blood despite significant drops in PaO2 from a variety of disease states. Hemoglobin saturation does not fall below 90% until PaO2 is less than 60 mm Hg. Except for patients who are having active myocardial ischemia, it is not clear that maintenance of oxygen saturation much above 90-93% offers any advantage – oxygen content in the blood does not rise significantly with further increases in partial pressure of oxygen since the blood is nearly fully saturated.