Extensive research has been done over the past two decades looking at the role of oxygen delivery (DO2) and oxygen utilization (VO2) in critical illness. VO2 depends on cardiac output, arterial oxygen content, and the body's ability to extract oxygen effectively from the blood. Oxygen demand rises in critical illness as the body goes into a catabolic state, and lower VO2 has been associated with higher lactate levels and with poorer outcomes. Although increasing DO2 will often raise VO2, Hayes et al found that a subset of critically-ill patients failed to demonstrate a rise in VO2 in spite of achieving supranormal values of cardiac index (CI) and DO2. This group, in contrast to patients whose VO2 rose with the increase in CI and DO2, had exceedingly poor outcomes, suggesting that an inability to extract oxygen from the blood confers a poorer prognosis.(1)