Summary:The determination of cardiac output by the Fick principle requires the quantitation of the arteriovenous oxygen (02) content difference across the pulmonary bed and the 0 2 consumption. Since a correct measurement of 0 2 consumption may be difficult, an assumed value is sometimes substituted into this equation. Despite this practice, there is no appropriate reference standard for 0 2 consumption applicable to the adult catheterization laboratory. Therefore, we determined 0 2 consumption in 108 patients (69 men and 39 women, mean age, 49 years) at the time of catheterization by multiplying the arteriovenous 0 2 content difference by the indicator dilution cardiac output, thereby eliminating errors related to the collection and analysis of a timed collection of expired air. 0 2 consumption index (02 consumption/m2 of body surface area) averaged 126f26 ml/min/m2 (meanfSD), but there was wide variability (65-250 ml/min/m2). 0 2 consumption was not affected by patient age or sex but was lower in those who were mildly sedated before catheterization (sedated, 121 f 2 2 ml/min/m2; nonsedated, 132f28 ml/min/m2; p = 0.023). Thus, 0 2 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.