Time courses of pulmonary gas exchange and heart rate changes in supine exercise

The time courses of ventilation (VE), O2 uptake (VO2), CO2 elimination (VCO2), respiratory exchange ratio (R), end-tidal PO2 and PCO2 and heart rate (HR) were studied in seven subjects performing light dynamic leg exercise in the supine position. Individual and group mean time courses in response to step changes in work load were computed and displayed graphically. A computer-based method was also used to fit mono- or bi-exponential mathematical functions to the recorded responses. The over-all rate of HR change in response to the transition from 0-load pedalling to exercise (on-response) was faster (mean response time, MRT = 31 s) than the corresponding VO2 response (MRT = 45 s) while VE responded considerably slower (MRT = 86 s). During the reverse transition (off-response), VO2 and VE changed with the same rate as in the on-response, while the HR-change was slower than during the on-response (MRT = 50 s). During the initial 15-sec period, VO2 changed only slightly, which contrasts to previous results in the sitting position, where 50% of the final change in VO2 has been reported to occur within the first 15-sec period, and where changes in blood distribution and stroke volume are known to be more pronounced than in the supine position. Our results emphasize the importance of central circulatory changes for the time course of VO2 at the start and end of exercise.