Evaluation of a modified acetylene rebreathing method for the determination of cardiac output

Summary. In order to evaluate a computerized modified acetylene rebreathing method for the determination of cardiac output, 15 healthy subjects were studied at different levels of their maximal oxygen uptake (V?o_2max). Submaximal exercise was performed on a cycle ergometer and maximal exercise on a treadmill. Oxygen uptake, heart rate, and cardiac output (acetylene method) were determined in all test situations. In seven subjects simultaneous determinations of cardiac output were made by a modified acetylene rebreathing method (Q?_A) and a dye dilution method (Q?_D) Furthermore, a new resting rebreathing technique was used. The methodological error for (Q?_A)(means of double samples) was 0·37 litre min^-1 (2·8%) in the same individual at 150 W. The corresponding values between individuals were 0·71 (rest), 0·41 (50 W), 0·69 (150 W), and 0·40 litre min^-1 (V?o_2max). Thus the methodological error of the modified acetylene method was very low. There was a significant difference (P<0·01), however, between the acetylene method and the dye dilution method, which showed a lower value for Q?_A at all levels. This was probably due to the long response time of the mass spectrometer combined with anatomical and physiological arteriovenous shunt effects in the lungs during exercise. When these factors were considered the correcting formula was:Q?_Ac=Q?_A+0·005·Q²_A There was no significant difference between the corrected cardiac output values (Q?_Ac) and the corresponding Q?_D values. In conclusion, this modified acetylene rebreathing method is a very useful non-invasive method for measuring cardiac output at rest as well as during heavy exercise.