Time Courses of Pulmonary Gas Exchange and Heart Rate Changes in Supine Exercise

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Abstract
The time courses of ventilation (VE), 02 uptake (V̇O2), CO2 elimination (V̇co2), 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 V̇o2 response (MRT = 45 s) while V̇E responded considerably slower (MRT = 86 s). During the reverse transition (off‐response), V̇o2 and V̇E 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, V̇O2, changed only slightly, which contrasts to previous results in the sitting position, where 50% of the final change in V̇o2, 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 V̇o2, at the start and end of exercise.