Background: The Fick principle (cardiac output = oxygen uptake ( O2)/systemic arterio-venous oxygen difference) is used to determine cardiac output in numerous clinical situations. However, estimated rather than measured 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 O2 was measured via the Douglas bag technique in adult patients undergoing right heart catheterization. Resting O2 was estimated by each of 3 published formulae. Agreement between measured and estimated 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/m2; 53% women; 64% non-white. Mean (±standard deviation) measured O2 was 241 ± 57 ml/min. Measured 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 (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/m2), but were not affected by sex or age.
Conclusions: Estimates of resting O2 derived from conventional formulae are inaccurate, especially in severely obese individuals. When accurate hemodynamic assessment is important for clinical decision-making, O2 should be directly measured.
Keywords: cardiac output; catheterization; hemodynamics.