The determination of oxygen consumption (VO2) in critically ill patients is useful to evaluate metabolic expenditure, to calculate cardiac output and to assess the adequacy of oxygen supply to the tissues by the relationship between oxygen delivery (DO2) and VO2. Only the last application requires a high degree of accuracy in measurement. Highly accurate devices have been developed over the last few years although there are still problems with their use during spontaneous ventilation, mechanical ventilation with high FiO2, and treatment with inhaled nitric oxide (NO). Due to Haldane's transformation and the body's large CO2 stores, VO2 should only be measured in steady-state conditions, avoiding changes in ventilatory conditions or cardiac output. Air leaks should be carefully avoided since measurements would be erroneous. There is generally good agreement between Fick-derived VO2 and VO2 obtained by indirect calorimetry; discrepancies between the two methods in the study of changes in VO2 in response to changes in DO2 could be due to errors in both techniques. There is no strong evidence that mathematical coupling of data alone can account for VO2/DO2 dependency, which can sometimes be observed in critically ill patients.
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