1. A new CO2-rebreathing method for estimating cardiac output is described, and compared with a method employing N2O performed at the same time.

2. The subject inhales from a reservoir of 30% O2 in N2 and rebreathes into and out of an empty bag for 10s.

3. Oxygenated mixed venous Pco2 is then determined by rebreathing 7–15% CO2 in O2, the mixture being selected to obtain a plateau of CO2 concentration.

4. Pco2 rises exponentially towards the plateau value during the rebreathing of 30% O2. Cardiac output is calculated from the rate of change of the alveolar—mixed venous Pco2 difference by a differential version of the Fick equation employing published CO2 dissociation curves for whole blood in vitro.

5. The slope of the regression of cardiac output on V̇o2 is similar to that obtained in other studies employing direct Fick measurements. The slope is some 15% greater than obtained with N2O but the difference is significant only when Oz consumption is greater than 2 litres/min.

6. The CO2 dissociation slope of blood does not differ during pulmonary gas exchange in vivo from that determined at equilibrium in vitro.

7. The volume of pulmonary blood available for CO2 exchange may rise to about 1 litre in heavy exercise, with a transit time of 1–2 s in the lungs.

8. The method can be employed for estimating pulmonary blood flow during physiological studies in subjects with normal lungs.

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