ObjectiveTo compare measurement of oxygen consumption (&OV0312;o2) by spirometry and the reversed Fick method.DesignWithin-patient comparison using simultaneous measurements by the two methods, one previously calibrated on a metabolic simulator.PatientsTwenty sets of observations on eight patients (57 to 83 yrs) requiring mechanical ventilation in a critical care unit.InterventionsNone during or immediately before the measurements.Measurements and Main ResultsDuplicate pairs of measurements of &OV0312;o2were made with a previously validated spirometric technique and the reversed Fick method (&OV0422;t[Cao2– C&OV0456;o2]), where &OV0422;t is cardiac output, Cao2is arterial oxygen content, and C&OV0456;o2is mixed venous oxygen content. The coefficient of variation of the difference between duplicate measurements by the former technique was only 2.53% compared with 10.4% for the latter. The mean &OV0312;o2measurement by the spirometric method was 285.7 ± 40.7 (SD) mL/min standard temperature and pressure, dry (STPD) and for the reversed Fick method, the mean &OV0312;o2measurement was 249.3 ± 38.5 mL/min STPD. The mean difference was 36.4 ± 28.5 mL/min STPD (p<.001).ConclusionsThe repeatability of the spirometric method was four times better than the reversed Fick method. The latter gave a significantly lower value that probably, in part, reflects the &OV0312;o2of the lung, which is included in the spirometric method but not in the reversed Fickmeasurement.