Cardiac output measured by acetylene rebreathing technique at rest and during exercise

Cardiac output (\(\dot Q_c \)) was measured by a rebreathing technique, using acetylene and a mass-spectrometer for analyzing. In addition the rate of pulmonary uptake of O₂ (\(\dot VO_2 \)) during the rebreathing period and during a preceding steady-state period were determined. Measurements were made on 8 adult humans at rest and at different levels of exercise up to maximum at two occasions. The ratio (\(\dot VO_2 \)) during steady-state/\(\dot VO_2 \) during rebreathing) was found to be significantly below 1 when the\(\dot VO_2 \) was below about 21·min⁻¹ and to be about 0.55 for subjects at rest. This indicates that\(\dot VO_2 \), and hence\(\dot Q_c \) is increased by the rebreathing procedure when this involves deeper and more frequent respirations than those of the preceding period. Accordingly, when\(\dot VO_2 \) was below about 21·min⁻¹, the\(\dot Q_c \) value, calculated exclusively from acetylene concentrations recorded during rebreathing, was multiplied by the above-mentioned\(\dot VO_2 \)-ratio. It is shown that this correcting procedure gives more reasonable values than those obtained by acetylene data alone. It is pointed out in what respects this correcting procedure of calculation deviates from that originally used by Grollman, and it is shown that there are only moderate differences between the results obtained by the two procedures.