Comparison of oxygen consumption measurements: indirect calorimetry versus the reversed Fick method.

To compare measurement of oxygen consumption (JOURNAL/ccme/04.02/00003246-199111000-00016/ENTITY_OV0312/v/2017-07-20T220806Z/r/image-pngo2) by spirometry and the reversed Fick method. Within-patient comparison using simultaneous measurements by the two methods, one previously calibrated on a metabolic simulator. Twenty sets of observations on eight patients (57 to 83 yrs) requiring mechanical ventilation in a critical care unit. None during or immediately before the measurements. Duplicate pairs of measurements of JOURNAL/ccme/04.02/00003246-199111000-00016/ENTITY_OV0312/v/2017-07-20T220806Z/r/image-pngo2 were made with a previously validated spirometric technique and the reversed Fick method (JOURNAL/ccme/04.02/00003246-199111000-00017/ENTITY_OV0422/v/2017-07-20T220806Z/r/image-pngt[Cao2 – CJOURNAL/ccme/04.02/00003246-199111000-00017/ENTITY_OV0456/v/2017-07-20T220806Z/r/image-pngo2]), where JOURNAL/ccme/04.02/00003246-199111000-00017/ENTITY_OV0422/v/2017-07-20T220806Z/r/image-pngt is cardiac output, Cao2 is arterial oxygen content, and CJOURNAL/ccme/04.02/00003246-199111000-00017/ENTITY_OV0456/v/2017-07-20T220806Z/r/image-pngo2 is 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 JOURNAL/ccme/04.02/00003246-199111000-00016/ENTITY_OV0312/v/2017-07-20T220806Z/r/image-pngo2 measurement 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 JOURNAL/ccme/04.02/00003246-199111000-00016/ENTITY_OV0312/v/2017-07-20T220806Z/r/image-pngo2 measurement was 249.3 ± 38.5 mL/min STPD. The mean difference was 36.4 ± 28.5 mL/min STPD (p <.001). The 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 JOURNAL/ccme/04.02/00003246-199111000-00016/ENTITY_OV0312/v/2017-07-20T220806Z/r/image-pngo2 of the lung, which is included in the spirometric method but not in the reversed Fickmeasurement.