ObjectiveOxygen consumption (VO2) is often measured in critically ill patients using the Fick equation: VO2equals cardiac output times arterial-venous oxygen content difference. To determine if this method is accurate, it was compared with a spirometric technique.DesignProspective study.SettingUniversity laboratory.SubjectsNineteen large adult pigs.InterventionsCardiac output, measured with bolus thermodilution technique, and arterial and venous oxygen content values, determined with the galvanic fuel cell method, were used to determine VO2with the Fick equation. The spirometrically determined VO2was the rate of disappearance of oxygen from a water-sealed spirometer. Dobutamine and labetalol were titrated to vary VO2(range 204 to 584 mL/min).Measurements and Main ResultsThe bias between the Fick and spirometrically determined VO2values was 58 mL/min. The precision (SD of the bias) between the Fick and spirometrically determined VO2was 35 mL/min. Fick-derived VO2was greater than VO2measured spirometrically. The correlation coefficient was 0.90.ConclusionsDespite all attempts to reduce measurement error, there was an unexplained difference in Fick-derived and spirometrically measured VO2. Therefore, I feel that the two methods are not interchangeable, and that calculations of VO sub 2 using the Fick method should be used cautiously when therapeutic maneuvers are based on these data.Crit Care Med 1996; 2491-95)