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-exercis e transitions the interaction of these factors is more complex than du ring exercise-exercise transients. In the present study, we directly c ompared oxygen uptake (VO2) and cardiac output (CO) responses to step changes in exercise intensity. Nine students performed the following s tep changes on a bicycle ergometer in upright body position: rest-20W, 20W-80W, 20-140W. VO2 was measured breath-by-breath while CO was dete rmined beat-by-beat by means of a Doppler device. The probe was positi oned in the suprasternal notch and directed towards the aortic root. W hen starting from the 20 W baseline the VO2 responses showed a first, mainly cardio-vascular component over the first 30 s, followed by a se cond, metabolic component at constant CO. The rest-exercise transients displayed two additional early features. First, there was a very rapi d VO2 increase during the initial breathing cycles of the on-transient . This was probably caused by events in the lungs or the pulmonary cir culation. Second, the CO attained its steady-state level in about 10 s . This suggests a sudden increase in venous return. The present result s generally show that both the VO2 and CO kinetics are faster when exe rcise in the upright body position is started from rest.