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

The relationship between cardiac output (CardOut) and oxygen consumption (V̇O₂) 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 V̇O₂ and CardOut at each stage were fit to a quadratic expression y = a + (b·V̇O₂) + (c·V̇O₂²), 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 V̇O₂/kg (median 37.9 vs. 32.4 ml·min^-1·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. V̇O₂ 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. V̇O₂ relationship was secondary to curvature in HR vs. V̇O₂. We conclude that the CardOut vs. V̇O₂ relationship is not always linear, and negative curvature in the relationship is associated with higher fitness levels in normal, non-elite-athletic subjects.