Cardiac output and oxygen release during very high-intensity exercise performed until exhaustion
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.