Relationship between cardiac output and oxygen uptake at the onset of exercise

Summary

The purpose of the present study was to assess the relationship between the rapidity of increased gas exchange (i.e. oxygen uptake$\dot VO_2$) and increased cardiac output ($\dot Q_c$) during the transient phase following the onset of exercise. Five healthy male subjects performed multiple rest-exercise or light exercise (25 W)-exercise transitions on an electrically braked ergometer at exercise intensities of 50, 75, or 100 W for 6 min, respectively. Each transition was performed at least eight times for each load in random order. The$\dot VO_2$ was obtained by a breath-by-breath method, and$\dot Q_c$ was measured by an impedance method during normal breathing, using an ensemble average. On transitions from rest to exercise,$\dot VO_2$ rapidly increased during phase I with time constants of 6.8–7.3 s. The$\dot Q_c$ also showed a similar rapid increment with time constants of 6.0–6.8 s with an apparent increase in stroke volume (SV). In this phase I,$\dot VO_2$ increased to about 29.7%–34.1% of the steady-state value and$\dot Q_c$ increased to about 58.3%–87.0%. Thereafter, some 20 s after the onset of exercise a mono-exponential increase to steady-state occurred both in$\dot VO_2$ and$\dot Q_c$ with time constants of 26.7–32.3 and 23.7–34.4 s, respectively. The insignificant difference between$\dot Q_c$ and$\dot VO_2$ time constants in phase I and the abrupt increase in both$\dot Q_c$ and SV at the onset of exercise from rest provided further evidence for a “cardiodynamic” contribution to$\dot VO_2$ following the onset of exercise from rest.