Estimation of Cardiac Output by C0(2) Rebreathing during Incremental Exercise in Patients with Coronary Artery Disease

In 9 patients with stable coronary artery disease, measurements were made during a progressive incremental maximum exercise test of O(2) intake (VO(2)), CO(2) output (VCO(2)), mixed venous PCO(2) by an exponential rebreathing method, and end-tidal PCO(2) (PETCO(2)) to estimate arterial PCO(2) (PaCO(2)). By applying the Fick principle to CO2, these measurements were used to derive cardiac output at several incremental work loads. Each subject underwent two incremental exercise studies to establish the reproducibility of the technique, and also a steady state exercise study to compare steady state responses with unsteady state incremental exercise. The results were also compared to those previously obtained in healthy subjects. PvCO(2) showed a curvilinear increase with increasing VCO(2) with only a small intersubject variation. Cardiac output-oxygen uptake relationships (Q/VO(2)) were similar in the two incremental studies (intercepts 6.27 and 6.641/min; slopes 4.82 and 4.58 1/min), and also similar to that obtained previously in healthy subjects (intercept 6.441/min; slope 4.71 1/min). Low calculated values for cardiac output were obtained at high exercise levels during the incremental test in those subjects in whom PETCO(2) fell at the highest work loads. This effect could be corrected for by using a PETCO(2) obtained 1 min prior to rebreathing, suggesting that the low values of Q were an artifact due to a transient decrease in PaCO(2) secondary to hyperventilation. At any given VO(2) or VCO2, PvCO(2) and Q were the same in both steady state and unsteady state incremental exercise. The exponential CO(2) rebreathing method may be reliably applied to incremental exercise testing to evaluate the cardiac output responses to exercise in patients with cardiac disease.