Inaccuracy of Estimated Resting Oxygen Uptake in the Clinical Setting

Background— The Fick principle (cardiac output = oxygen uptake ( V ˙ O 2 )/systemic arterio-venous oxygen difference) is used to determine cardiac output in numerous clinical situations. However, estimated rather than measured V ˙ O 2 is commonly used because of complexities of the measurement, though the accuracy of estimation remains uncertain in contemporary clinical practice. Methods and Results— From 1996 to 2005, resting V ˙ O 2 was measured via the Douglas bag technique in adult patients undergoing right heart catheterization. Resting V ˙ O 2 was estimated by each of 3 published formulae. Agreement between measured and estimated V ˙ O 2 was assessed overall, and across strata of body mass index, sex, and age. The study included 535 patients, with mean age 55 yrs, mean body mass index 28.4 kg/m 2 ; 53% women; 64% non-white. Mean (±standard deviation) measured V ˙ O 2 was 241 ± 57 ml/min. Measured V ˙ O 2 differed significantly from values derived from all 3 formulae, with median (interquartile range) absolute differences of 28.4 (13.1, 50.2) ml/min, 37.7 (19.4, 63.3) ml/min, and 31.7 (14.4, 54.5) ml/min, for the formulae of Dehmer, LaFarge, and Bergstra, respectively ( P <0.0001 for each). The measured and estimated values differed by >25% in 17% to 25% of patients depending on the formula used. Median absolute differences were greater in severely obese patients (body mass index > 40 kg/m 2 ), but were not affected by sex or age. Conclusions— Estimates of resting V ˙ O 2 derived from conventional formulae are inaccurate, especially in severely obese individuals. When accurate hemodynamic assessment is important for clinical decision-making, V ˙ O 2 should be directly measured.