Accuracy of estimating resting oxygen uptake and implications for hemodynamic assessment

The Fick principle (cardiac output [Q _c] = oxygen uptake [Vo ₂]/arteriovenous oxygen difference) can be used to calculate Q _c, with VO ₂ frequently estimated by derived equations. To compare the accuracy of measured versus estimated VO ₂, data were analyzed from 2 studies in which VO ₂ at rest was measured using the Douglas bag technique. One study comprised adults with diabetes, and the other was an exercise study of healthy adults. VO ₂ at rest was estimated as VO ₂ (ml/min) = 125 ml/min/m ² × body surface area (m ²), with sensitivity analyses evaluating 2 other commonly used equations. Mean absolute difference (milliliters per minute) and ordinary least products regression were used to assess agreement between measured and estimated VO ₂. Overall, mean measured versus estimated VO ₂ differed significantly (307.2 ± 75.2 vs 259.9 ± 36.7 ml/min, p <0.0001), with a mean absolute difference of 52.9 ± 43.2 ml/min (p <0.0001); 20% of the estimates differed by >25% from the measured VO ₂. Mean absolute difference increased from 36.7 ml/min in the lowest body mass index group (<25 kg/m ²) to 91.7 ml/min in the highest group (<40 kg/m ²) (p for trend = 0.001) and was significantly higher in men than in women (65.6 vs 33.9 ml/min, p = 0.001); error was similar by median-split age (p = 0.65) and race (p = 0.34). Similar results were obtained when evaluating each of the other 2 estimating equations. Estimation of VO ₂ at rest is inaccurate, especially in men and with increasing adiposity. In conclusion, when clinical hemodynamic assessment is performed, VO ₂ should be measured, not estimated.