Evaluation of a method for estimating cardiac output from a single breath in humans

We have modified the single-breath method of Kim et al. (J. Appl. Physiol. 21: 1338-1344, 1966) for estimating cardiac output and arterial and mixed venous carbon dioxide tensions (PCO2). We assessed this using 30 normal subjects and 23 cardiac patients. The procedure was performed satisfactorily in all but two patients. The random errors, from 60 pairs of estimates of cardiac output in normal subjects and 50 pairs in patients, were +/- 12.8 and +/- 19.6% (95% tolerance limits; i.e., coefficient of variation multiplied by 2 for n greater than 50). The systematic error was assessed in 15 patients from comparisons with results obtained by the direct Fick method. There was no significant difference except in two patients with large intracardiac shunts. Mean values of cardiac output by single-breath and direct Fick estimates were 3.80 and 3.83 l/min. Arterial and mixed venous PCO2 were estimated by the single-breath method with random errors of +/- 1.5 and +/- 1.4 Torr, respectively, and no significant systematic errors. We conclude that our modification of the single-breath method is reliable in humans at rest, although the procedures for delivering the breath and processing the data are of critical importance.