Noninvasive determination of cardiac output by a modified acetylene rebreathing procedure utilizing mass spectrometer measurements.

A modified acetylene rebreathing technique for measuring cardiac output has been developed utilizing a mass spectrometer for measuring gas concentrations. Helium serves as an indicator of mixing and system volume. The disappearance rate of acetylene is proportional to pulmonary capillary blood flow, which is taken as an index of cardiac output. The method also provides an estimate of pulmonary tissue volume. The technique was assessed by comparing 22 simultaneous acetylene and dye-dilution cardiac outputs measured at rest and during exercise in six healthy subjects. There was no significant difference between the mean acetylene and dye-dilution cardiac output values (11.5 l/min vs. 11.24 l/min, respectively). The correlation coefficient was 0.94 and the S.E.M. difference was 0.53 l. The effect of the procedure on cardiac output was evaluated in a separate study of six subjects. There was a significant positive correlation between respiratory rate and cardiac output; but the magnitude of this effect was small, indicating that accurate measurements may be made at spontaneous respiratory rates.