Oxygen consumption and oxygen delivery

This review has compared the use of the reverse Fick method and indirect calorimetry for measuring O₂ consumption and energy expenditure. Although both are comparable, measurement of V̇O₂ by indirect calorimetry tends to provide somewhat higher values than those derived by the Fick equation. This may be due, in part, to the fact that lung parenchymal O₂ use is ignored by the Fick method while included in indirect calorimetry. In situations where there is lung injury, the difference may be substantial. Open- and closed- circuit methods of indirect calorimetry each have their advantages which should be considered vis-a-vis the intended application. Estimates of O₂ requirements and energy expenditure based on body size, type of injury, or physiologic stress have been shown to be uniformly unsatisfactory in critically ill patients. Thus, for accurate knowledge of V̇O₂ in a particular patient, direct measurement is mandated. Indirect calorimetry- determined V̇O₂ is more valid than that determined by the Fick equation when examining the relationship between ḊO₂ and V̇O₂ because indirect calorimetry obviates the problem of 'mathematical coupling.' At the whole- body level, much work remains to be done to elucidate the relationship between ḊO₂ and V̇O₂, particularly in the setting of critical illness. One area that needs to be addressed is the area of true cellular O₂ delivery. Another is the need to identify and measure the tissues most susceptible to ischemia or an imbalance between O₂ supply and demand, which requires that whole-body methods be supplemented by measures of regional O₂ metabolism.