The determination of cardiac output by the Fick principle requires the quantitation of the arteriovenous oxygen (O2) content difference across the pulmonary bed and the O2 consumption. Since a correct measurement of O2 consumption may be difficult, an assumed value is sometimes substituted into this equation. Despite this practice, there is no appropriate reference standard for O2 consumption applicable to the adult catheterization laboratory. Therefore, we determined O2 consumption in 108 patients (69 men and 39 women, mean age, 49 years) at the time of catheterization by multiplying the arteriovenous content difference by the indicator dilution cardiac output, thereby eliminating errors related to the collection and analysis of the timed collection of expired air. O2 consumption index (O2 consumption/m2 of body surface area) averaged 126 +/- 26 ml/min/m2 (mean +/- SD), but there was wide variability(65-250 ml/Min/m2). O2 consumption was not affected by patient age or sex but was lower in those who were mildly sedated before catheterization (sedated, 121 +/- 22 ml/Min/m2; nonsedated, 132 +/- 28 ml/min/m2; p = 0.023). Thus, O2 consumption varies greatly among adults at the time of cardiac catheterization, so that any attempt at estimating this variable is fraught with major potential error.