Indirect Calorimetry has Better Reproducibility than the Reverse Fick Method in Measurement of Oxygen Uptake

Purpose: To compare the reproducibility of measurement of oxygen uptake made using the reverse Fick and indirect calorimetry methods. Methods: A custom designed system was constructed for the measurement of oxygen uptake from an anaesthetic breathing system using indirect calorimetry based on the Haldane transformation. In a series of 42 patients undergoing coronary artery surgery in the pre-cardiopulmonary bypass period, two reverse Fick measurements were made using bolus thermodilution and paired arterial and mixed venous blood sampling. Simultaneous measurements of oxygen uptake by the Haldane-based method were made. The within-patient standard deviation (WPSD) and variance (WPV) of repeat measurement by each method was determined using 1-way ANOVA for repeated measures. Results: The median WPSD for the reverse Fick method was 12.5 mL/min and for indirect calorimetry was 8.5 mL/min, giving a WPV of 157.0 mL/min and 72.4 mL/min respectively. The difference between the two methods was significant using the Wilcoxon signed rank test in both WPV (p < 0.01) and WPSD (p < 0.02). Conclusions: The reproducibility of measurement of O2 uptake in patients during cardiac surgery is clinically and statistically superior using indirect calorimetry to that obtainable by the reverse Fick method.