Cardiac acceleration as a marker of vagal withdrawal in heart rate control during exercise in humans

Objective: To investigate whether the time rate of change in heart rate i.e. cardiac acceleration, during aerobic exercise in human subjects could be used to differentiate vagal withdrawal from sympathetic stimulation. Methods: Fifteen male subjects exercised on a bicycle ergometer at 50 Watts (Step 1), then 100 Watts (Step 2), for 2 minutes each. Results: Heart rate (HR) was monitored from a resting value (mean ± SD) of 80.3 ± 12.9 to 113. 6 ± 13.6 beats min-1 in Step 1. In Step 2 exercise, HR increased from 113.8 ± 13.6 to 145 ± 20 beats min-1. At the initiation of Step 1, a rapid acceleration of HR was observed in the form of an overshoot response. In contrast to Step 1, a small overshoot response of cardiac acceleration was observed during Step 2. The difference between the mean cardiac acceleration at 10 seconds in Steps 1 and 2 was significant (2.40 ± 0.19 and 0.71 ± 0.12 beats min-1 sec-1, p<0.0001). Conclusion: The initial vagal withdrawal of exercise-induced tachycardia, as a frontline adaptive mechanism, can be indirectly identified from HR transients using cardiac acceleration as a new marker.