Supply Dependency of Oxygen Uptake

Abstract

A puzzling and disturbing finding in certain patients with adult respiratory distress syndrome (ARDS) or sepsis is that their oxygen (O2) uptake appears to vary directly with total O2 delivery (total O2 delivery is defined as the product of cardiac output and arterial O2 content). The more usual situation is that O2 extraction in the periphery varies inversely with O2 delivery so that O2 uptake remains approximately constant. In these certain patients, if delivery to the periphery is increased, O2 uptake will increase or, if delivery decreases, O2 uptake falls. This is puzzling because it is not the normal response of O2 uptake when cardiac output and arterial O2 content are both in acceptable ranges, as they often are in such patients. This is disturbing because when abnormal supply dependency of O2 uptake is found, it appears to be a dire prognostic sign. In this paper, I will expand on the essential differences between normal or physiological O2 supply dependency and its abnormal or pathological manifestation. Following that, I will present some evidence that the pathological form is a very bad prognostic sign. Finally, I will report the results from some animal experiments in which we tried to reproduce the defect in peripheral O2 extraction that is the hallmark of pathological O2 supply dependency and speculate on why it may occur.