Evaluation of the CO2 rebreathing method for the non-invasive measurement of resting cardiac output in man.

We have compared the indirect Fick, a non-invasive CO2 rebreathing method for measuring resting cardiac output, with the thermodilution method in eleven subjects including some with cardiac and pulmonary disease. Three alternative methods for calculating veno-arterial CO2 content difference were used: (i) from end-tidal and rebreathing bag equilibrium PCO2 modified from the equations developed for use during exercise by Jones et al. (1975); (ii) by using the uncorrected difference between end-tidal and equilibrium PCO2 and the standard CO2 dissociation curve; (iii) by direct measurement of arterial PCO2. Each method was satisfactory in that reproducibility was similar to thermodilution (5-10%) and the equations relating thermodilution to indirect Fick cardiac output were linear with slope and intercept, close to 1 and 0, respectively. End-tidal PCO2 accurately predicted arterial PCO2 except in five patients with liver disease. Direct measurement of arterial PCO2 is recommended in such patients. In other subjects there was no advantage in either correcting non-invasive measurements of alveolar gas to obtain veno-arterial CO2 content difference, or in direct measurement. The indirect Fick is an accurate method for measuring cardiac output at rest in normal subjects and those with cardiovascular disease. Its use can be extended to other groups by a single arterial blood sample.