VO2 and cardiac output during rest-exercise and exercise-exercise transients.

The dynamics of external gas exchange during exercise is influenced by pulmonary, cardiovascular, and metabolic factors. During rest-exercise transitions the interaction of these factors is more complex than during exercise-exercise transients. In the present study, we directly compared oxygen uptake (VO2) and cardiac output (CO) responses to step changes in exercise intensity. Nine students performed the following step changes on a bicycle ergometer in upright body position: rest-20W, 20W-80W, 20-140W. VO2 was measured breath-by-breath while CO was determined beat-by-beat by means of a Doppler device. The probe was positioned in the suprasternal notch and directed towards the aortic root. When starting from the 20 W baseline the VO2 responses showed a first, mainly cardio-vascular component over the first 30 s, followed by a second, metabolic component at constant CO. The rest-exercise transients displayed two additional early features. First, there was a very rapid VO2 increase during the initial breathing cycles of the on-transient. This was probably caused by events in the lungs or the pulmonary circulation. Second, the CO attained its steady-state level in about 10 s. This suggests a sudden increase in venous return. The present results generally show that both the VO2 and CO kinetics are faster when exercise in the upright body position is started from rest.