Abstract 188: Abnormal Heart Rate Responses to Exercise and Susceptibility to Ventricular Fibrillation: Effect of Endurance Exercise Training

Abstract

BACKGROUND : 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 enhance cardiac vagal and reduce cardiac sympathetic regulation and could, thereby, improve the exercise HR response and decrease the risk for VF. METHODS : To test this hypothesis, a 2-min coronary occlusion was made during the last min of an 18-min exercise test in dogs with healed myocardial infarctions: 20 had VF (S, susceptible) and 13 did not (R, resistant). On a previous day, 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 before and 30, 60 & 120s after exercise onset and 30, 60 & 120s following the termination of exercise. RESULTS : Larger HR increases and a slower HR recovery (ANOVA, both P<0.01) were noted in the susceptible dogs (HR: 30s after exercise onset, S 179 ± 8.4 vs. R 156.8 ± 3.4 beats/min; 30s post exercise, S 186 ± 7.8 vs. R 164.2 ± 5.7 beats/min). These HR differences were accompanied by corresponding differences in cardiac vagal activity (HRV: 30s after exercise onset, S 4.0 ± 0.4 vs. R 4.8 ± 04 ln ms 2 ; 30s post exercise, S 2.4 ± 0.3 vs. R 3.8 3 0.4 ln ms 2 ). The dogs then received either a 10-wk exercise program (treadmill running; S n = 9; R n = 8) or an equivalent sedentary period (S n = 11; R n = 5). After 10-weeks, the response to exercise was not altered in the sedentary animals. In contrast, exercise training increased cardiac vagal activity such that HR at exercise onset was reduced while HR recovery increased. As a result, differences were no longer noted between the groups (30s after exercise onset: HR, S 151.4 ± 6.6 vs. R 149.5 ± 7.3 beats/min; HRV, S 5.8 ± 0.4 vs. R 5.6 ± 0.4 ln ms 2 ; 30s post exercise: HR, S 159.4 ± 7.7 vs. R 153.5 ± 5.3 beats/min; HRV, S 4.0 ± 0.6 vs. R 4.9 ± 0.5 ln ms 2 ). After training, the exercise plus ischemia tested provoked VF in sedentary but not trained susceptible dogs. CONCLUSION : Endurance exercise training can alter cardiac autonomic balance (increased cardiac vagal activity) to restore a more normal heart rate regulation and could, thereby, prevent ventricular fibrillation in high-risk individuals.