Inaccuracy of Estimated Resting Oxygen Uptake in the Clinical Setting

BACKGROUND—The Fick principle (cardiac output = oxygen uptake ((Equation is included in full-text article.)O2)/systemic arterio-venous oxygen difference) is used to determine cardiac output in numerous clinical situations. However, estimated rather than measured (Equation is included in full-text article.)O2 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 (Equation is included in full-text article.)O2 was measured via the Douglas bag technique in adult patients undergoing right heart catheterization. Resting (Equation is included in full-text article.)O2 was estimated by each of 3 published formulae. Agreement between measured and estimated (Equation is included in full-text article.)O2 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; 53% women; 64% non-white. Mean (±standard deviation) measured (Equation is included in full-text article.)O2 was 241 ± 57 ml/min. Measured (Equation is included in full-text article.)O2 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 (P25% 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), but were not affected by sex or age. CONCLUSIONS—Estimates of resting (Equation is included in full-text article.)O2 derived from conventional formulae are inaccurate, especially in severely obese individuals. When accurate hemodynamic assessment is important for clinical decision-making, (Equation is included in full-text article.)O2 should be directly measured.