Non-invasive measurement of cardiac output by a carbon dioxide rebreathing method at rest and during exercise

Cardiac output was measured in 11 patients undergoing routine cardiac catheterization using a carbon dioxide rebreathing technique and compared with cardiac output measured by direct Fick and thermodilution. The carbon dioxide rebreathing technique gave consistently lower values for cardiac output than the other two methods (mean difference -0.73, 95% CI -0.95 to -0.51 l.min-1 with the direct Fick and -0.72, 95% CI -1.19 to -0.26 l.min-1 with thermodilution). The direct Fick and thermodilution methods gave similar results (mean difference -0.08, 95% CI -0.32 to 0.16 l.min-1). Cardiac output was also measured in 10 healthy subjects at rest and during two steady-state levels of exercise using the carbon dioxide rebreathing technique. Measurements were made in triplicate on 3 separate days. The technique gave reproducible results between replicates at rest (coefficient of variation 9.1%) and became more reproducible on exercise (coefficients of variation 5.6% and 5.4% respectively at each exercise level). There was a good correlation between cardiac output and oxygen consumption (r = 0.98). The carbon dioxide rebreathing technique is a feasible non-invasive way of measuring cardiac output. It tends to underestimate cardiac output at rest but is reproducible and becomes more so on exercise which is where it should be of most value.