Estimation of true venous and arterial PCO2 by gas analysis of a single breath.

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Abstract
This method is based on the O2 and CO2 analyses of several gas samples collected successively during a single expiration lasting about 10 sec. During that period the alveolar O2 decreases at a fairly constant rate, while the rate of increase in CO2 drops progressively. The R [rate] at which blood passing through the lungs exchanges is given by the relationship between the rate of CO2 increase and the rate of O2 decrease, measured at that time. This instantaneous gas exchange ratio falls rapidly. The linear relationship between PcO2 [CO2 tension] and this instantaneous R, which drops from 0.9 to 0. 2 during a single prolonged expiration, is established in each experiment. The true mixed venous PCO2 is the interpolated value read at R = 0. 32, where arterial (alveolar) and venous PcO2 are equal by virtue of the Haldane effect. The mean alveolar PcO2 is determined by the R obtained from an expired gas analysis prior to the exhalation maneuver. From the CO2 tensions in arterial and venous blood, the CO2 dissociation curve, and the gas exchange of the subject, the cardiac output can be computed.

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