Estimation of Cardiac Output from the Rate of Change of Alveolar Carbon Dioxide Pressure during Rebreathing

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 O_z 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.