Cardiac output determined by the CO2 rebreathing method with correction of lung-bag volume shrinkage.

Abstract

The purpose of this study was to elucidate the effect of lungbag volume shrinkage on P'vco2 estimated by the C02 rebreathing method and to correct its effect in calculation of cardiac output. The P'vco2 was extrapolated with the linear relationship between dPAco2 Jdt and PAc o 2 during rebreathing. The P'vc o 2 was higher than real mixed P002 by 0.82 mmHg at rest, by 1.35 mmHg at 50W, 1.57 mmHg at 100W, and 3.06 mmHg at 150W considering the effect of lung-bag volume shrinkage detected by argon concentration. The cardiac output obtained by the C02 rebreathing method with correction of above differences was in good agreement with that determined by the acetylene method (r=0.930, p<0.001). When the cardiac output was plotted against 02 uptake, our results and the regression line were fairly close to the reported regression lines.Key Words: cardiac output, C02 rebreathing method, lung-bag volume shrinkage, mixed venous C02 pressure. DEFARES (1956) not only introduced the rebreathing technique to estimate oxygenated mixed venous C02 pressure (Pvco2), but also was the first to discuss the effect of lung-bag volume shrinkage on the estimation of the Pvco2. The effect of lung-bag volume shrinkage which derived from 02 uptake being higher than C02 elimination during rebreathing was then judged to be negligible.The recent development of a fast responding gas analyzer makes it possible to use the C02 rebreathing method even at high exercise intensities. At the same time, since exercise requires high 02 uptake, the problem of lung-bag volume shrinkage would be revealed. In fact, LUNDIN and THOMSON (1965) proved such effect on the Pvcp2 estimation in detecting the change of nitrogen concentration. Later, FARHI et al. (1976) proposed the equation to estimate the theoretical difference between the real Pvco2 and the Pvco2 determined by the C02 rebreathing

Fully automatic, noninvasive measurement of cardiac output by means of the CO2 rebreathing method and its clinical application to hypertensive patients.

“…And, there are a number of controversial points in connection with the theory of the equilibrium attamed. 15)- 19) It has been reported that, in the equilibrium method , because a high CO2 content is inhaled into the alveoli , discharge of CO2 from mixed venous blood is impeded and partial pressure exchange between the alveoli and arterial blood increases. Jones et al20) perform downstream corrections to counter this influence, though the theoretical basis for their revisions is in Jpn.…”