GAS-EXCHANGE RESPONSES TO CONSTANT WORK RATE EXERCISE IN CHRONIC CARDIAC-FAILURE

Objective-To examine the time course of changes in minute oxygen consu mption and other gas exchange variables and heart rate during constant work rate exercise in patients with chronic cardiac failure. Design-T readmill exercise with on line measurement of gas exchange and a targe t duration of 10 minutes. Subjects-Seven men in New York Heart Associa tion class II, six in class III, and seven controls. Main outcome meas ures-Gas exchange variables and heart rate were averaged for the final two minutes of exercise. Time constants were calculated for the incre ase in all variables. Results-Consumption of oxygen at the end of exer cise (VO2) was similar in class II patients (mean (95% confidence inte rval (95% CI) 14.9 (13.6 to 16.1) ml kg(-1) min(-1)), class III patien ts (13. 2 (11.2 to 15.1) ml kg(-1) min(-1)), and controls (13.3 (12.5 to 14. 2) ml kg(-1) min(-1)). The patients reached this Vo(2) more slo wly with longer exponential time constants of 0.82 (0.59 to 1.04) min in class II and 1.19 (0.86 to 1.51) min in class III, than the 0.49 (0 .35 to 0.64) min in the controls. Time constants of other gas exchange variables and heart rate were also longer in patients. By analysis of covariance, peak accounted for the between group difference in the ti me constant for Vo(2), suggesting that circulatory factors may be an i mportant cause of the delayed kinetics. Conclusions-A delayed rise in Vo(2), in response to exercise may be responsible for subnormal values of Vo(2) early in exercise in patients with chronic cardiac failure.