Non-linear cardiac output dynamics during ramp-incremental cycle ergometry

Published literature asserts that cardiac output (Q̇=V̇O2×1/C(a-v)O2) increases as a linear function of oxygen uptake with a slope of approximately 5–6 during constant work rate exercise. However, we have previously demonstrated that C(a-v)O2 has a linear relationship as a function of V̇O2 during progressively increasing work rate incremental exercise. Therefore, we hypothesized that Q̇ may indeed have a non-linear relationship with respect to V̇O2 during incremental, non-steady state exercise. To investigate this hypothesis, we performed five maximal progressive work rate exercise studies in healthy human subjects. Q̇ was determined every minute during exercise using measured breath-by-breath V̇O2, and arterial and pulmonary artery measurements of PO2, hemoglobin saturation, and content. Q̇ was plotted as a function of V̇O2 and the linear and non-linear (first order exponential and hyperbolic) fits determined for each subject. Tests for linearity were performed by assessing the significance of the quadratic terms added to the linear relation using least squares estimation in linear regression. Linearity was inadequate in all cases (group P<0.0001). We conclude that cardiac output is a non-linear function of V̇O2 during ramp-incremental exercise; the pattern of non-linearity suggests that while the kinetics of Q̇ are faster than those of V̇O2 they progressively slow as work rate (and V̇O2) increases.