Physiological factors affecting brand new haemoglobin-fresh air dissociation bend

In general, a rise in PaOdos (produced, for example, by an increase in the rate or depth of ventilation or by breathing supplemental oxygen) is accompanied by a rise in the arterial oxygen saturation. Since the dissociation curve is relatively flat when the oxygen saturation is >90%, increases in PaO2 have relatively little impact on saturation or content in this range. Clearly, as SaO2 approaches 100% (equivalent to a PaO2 greater than about 17 kPa or 128 mmHg), there can be no further increase in saturation however high the PaO2 rises. However, breathing increasingly higher oxygen concentrations continues to increase the PaO2 and there will also be a small, progressive rise in oxygen content of the blood, due to the small, though increasing, amount dissolved (as, indeed, would also happen with water exposed to a high PO2). Paradoxically, however, a very high PaO2 may reduce oxygen delivery to the tissues due to other physiological effects such as vasoconstriction or reduction in cardiac output .

By contrast, when PO2 falls below 8 kPa (60 mmHg) there is a steep decline in oxygen saturation (fig. 1). This situation has been described as a “slippery slope” as small reductions in PaO2 are accompanied by disproportionately large reductions in oxygen saturation and content and therefore in oxygen delivery. Continue reading