Relationship between cardiac output and oxygen consumption during upright cycle exercise in healthy humans

The relationship between cardiac output (CardOut) and oxygen consumption (VO2) during exercise has generally been assumed to be linear. To test this assumption, we studied 72 healthy subjects using a graded, 2-min cycle-ergometry exercise test to maximum while measuring gas exchange continuously and CardOut at the end of each stage, the latter using an open-circuit gas technique. Data for VO2 and CardOut at each stage were fit to a quadratic expression y = a + (b.VO2) + (c.VO2(2)), and statistical significance of the quadratic c term was determined in each subject. Subjects were then divided into two groups: those with statistically significant negative quadratic term ("negative curvature group," n = 25) and those with either nonsignificant quadratic term or c significantly > 0 ("non-negative curvature group," n = 47, 2 with c significantly > 0). We found the negative curvature group had significantly higher maximal VO2/kg (median 37.9 vs. 32.4 ml x min(-1) x kg(-1); P = 0.03) higher resting stroke volume (SV; median 77 vs. 60 ml; P = 0.04), lower resting heart rate (HR; median 72 vs. 82 beats/min, P = 0.04), and higher tissue oxygen extraction at maximal exercise (17.1 +/- 2.2 vs 15.5 +/- 2.1 ml/100 ml; P < 0.01), with tendencies for higher maximal CardOut and SV. We also found the HR vs. VO2 relationship to be negatively curved, with negative curvature in HR associated with the negative curvature in CardOut (P < 0.05), suggesting the curvature in the CardOut vs. VO2 relationship was secondary to curvature in HR vs. VO2. We conclude that the CardOut vs. VO2 relationship is not always linear, and negative curvature in the relationship is associated with higher fitness levels in normal, non-elite-athletic subjects.