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1 Dept. of Physiology, Ohio State University, Columbus,, Ohio, United States
* To whom correspondence should be addressed. E-mail: billman.1{at}osu.edu.
Both a large heart rate (HR) increase at exercise onset and a slow heart rate recovery following the termination of exercise have been linked to an increased susceptibility to ventricular fibrillation (VF). Endurance exercise training can alter cardiac autonomic regulation and thereby restore a more normal HR regulation in high-risk individuals. To test this hypothesis, HR and heart rate variability (HRV, 0.24 to 1.04 Hz frequency component; an index of cardiac vagal activity) responses to submaximal exercise were measured 30, 60 & 120 s after exercise onset and 30, 60 & 120 s following the termination of exercise in dogs with healed myocardial infarctions known to be susceptible (S, n=19) to VF. These studies were then repeated after either a 10-wk exercise program (treadmill running n=10) or an equivalent sedentary period (n=9). After 10-weeks, the response to exercise was not altered in the sedentary animals. In contrast, endurance exercise increased cardiac vagal activity such that HR at exercise onset was reduced (30 s after exercise onset: HR pre-training, 179±8.4 vs. post-training 151.4±6.6 beats/min; HRV pre-training 4.0±0.4 vs. post-training 5.8±0.4 ln ms2) while HR recovery increased (30 s after the termination of exercise, HR, pre-training 186±7.8 vs. post-training 159.4±7.7 beats/min; HRV pre-training 2.4±0.3 vs. post-training 4.0±0.6 ln ms2). Thus, endurance exercise training restored a more normal heart rate regulation in dogs susceptible to VF.
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