Time Courses of Cardiac Output and Oxygen Uptake Following Stepwise Increases in Exercise Intensity

The adjustment of pulmonary oxygen uptake (VO₂) following a step increase in work rate has been characterized as consisting of an early "cardiodynamic" component with unchanged mixed-venous O₂-content ("phase 1") and a subsequent "metabolic" component ("phase 2") starting when venous blood from the muscle arrives at the lungs. The aim of the present study was to investigate whether the onset of phase 2 actually indicates the arrival of blood influenced by the altered muscle metabolism. Parallel measurements of cardiac output (Doppler technique) and VO₂ (breath-by-breath measurements at the mouth) were performed in eight subjects during step increases in exercise intensity (from a 20 W baseline to either 80 W, 120 W, 160 W or 200 W). To vary the absolute cardiac output values for given muscle VO₂ the subjects exercised both in upright and supine position. Individual time-courses of the arterio-venous O₂ difference (a-v ΔO₂) were computed from cardiac output and VO₂ data. Independent of body position two clear-cut phases of similar duration were seen both in VO₂ and in the computed a-v ΔO₂. The duration of the first component with unchanged a-v ΔO₂ was about 20 s at the lowest step amplitudes (20 - 80 W and 20 - 120 W). It decreased to about 15 s for the 20 - 160 W and 20 - 200 W steps. At the lower exercise intensities the duration of phase 1 appears too long to be entirely due to the transit time of venous blood from the exercising muscles to the lungs. The present data rather suggest that, at lower exercise intensities, there is no abrupt decrease in venous O_2-content in the muscle so that the arrival of muscle blood at the lungs cannot simply be discerned by means of the respiratory VO₂ measurements or computed a-v ΔO₂ time courses.

Key words

Cardiac output dynamics - oxygen uptake dynamics - cycle exercise - body position - humans