Inaccuracy of Estimated Resting Oxygen Uptake in the Clinical Setting
Background—The Fick principle (cardiac output (Qc) = oxygen uptake (VO2) / systemic arterio-venous oxygen difference) is used to determine Qc in numerous clinical situations. However, estimated rather than measured V•O2 is commonly used due to complexities of the measurement, though the accuracy of estimation remains uncertain in contemporary clinical practice.
Methods and Results—From 1996 to 2005, resting VO2 was measured via the Douglas bag technique in adult patients undergoing right heart catheterization. Resting VO2 was estimated by each of 3 published formulae. Agreement between measured and estimated VO2 was assessed overall, and across strata of body mass index, sex, and age. The study included 535 patients, with mean age 55 yrs, mean BMI 28.4 kg/m2; 53% women; 64% non-white. Mean (+ standard deviation) measured VO2 was 241 ± 57 ml/min. Measured VO2 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 (BMI > 40 kg/m2), but were not affected by sex or age.
Conclusions—Estimates of resting VO2 derived from conventional formulae are inaccurate, especially in severely obese individuals. When accurate hemodynamic assessment is important for clinical decision-making, VO2 should be directly measured.
- Received April 25, 2013.
- Revision received September 9, 2013.
- Accepted September 23, 2013.