Transient changes in ventilation and cardiac output at the start and end of exercise

In order to clarify the nature of the regulatory mechanism of the hyperventilation observed at the transition from rest to exercise, cardiac output and ventilation changes in response to submaximal bicycle exercise given as a step function were studied in five healthy men. Cardiac output was determined non-inversively by using a type of impedance plethysmography. It was found that (1) both the cardiac output and ventilation started to increase during the first 10 sec after the start of exercise; (2) the rate of increase in the initial response was abrupt in ventilation but rather gradual in cardiac output: (3) the fast response was followed by a slow response with an exponential time course, the half response time of which ranged between 10 and 45 sec in cardiac output and between 5 and 80 sec in ventilation. The half time increased in proportion to an increase in the work rate and (4) end-tidal pressure of carbon dioxide monitored simultaneously showed a slight decrease at 10 to 15 sec after the start of exercise. It was therefore concluded from the above observation that the initial phase of the exercise-induced hyperpnea is probably of a neurogenic origin.