Estimation of cardiac output by an N2O rebreathing method in adults and children

A nitrous oxide rebreathing method was used to measure recirculation time and cardiac output (as pulmonary blood flow) in adults and children during exercise. The method was compared with the indirect (CO2) Fick rebreathing method in almost simultaneous measurements of cardiac output. Recirculation time varied from about 15 s at rest to 8.5 s at high levels of exercise, and was slightly longer in children than adults. Cardiac output estimated by both methods was within the normal range for children and adults, being lower for a given oxygen consumption in children. For the adults the mean intrasubject variability of the N2O rebreathing method, measured as the coefficient of variation of cardiac output, was in the range of 6-8% at all levels of work. This was lower than reported for other N2O rebreathing methods, and similar to values based on the indirect (CO2) Fick, direct (O2) Fick, and dye-dilution methods of estimating cardiac output. The method is therefore proposed as a noninvasive technique suitable for rapid, accurate, and reproducible measurements of cardiac output.