Endurance exercise improves heart rate complexity in the presence of vagal withdrawal in young adults

Sudden cardiac death (SCD) has been reported during and following physical activity. SCD may be due to vagal withdrawal and/or sympathetic dominance associated with the exercise occurring at any time during, immediately following, or up to several days after exercise. Heart rate variability (HRV) describes the influence of the autonomic nervous system on heart rate. We assessed the immediate post-exercise influence of endurance training on HRV in young adults following the morning exercise bout and also on the same day in the afternoon. Linear domain parameter root mean square of successive RR interval differences (RMSSD) showed vagal withdrawal when analysed both immediately after the AM session and also when pre exercise HRV was compared to post exercise HRV during the early evening (median average change 6.6%) suggesting a possible increased risk of adverse cardiac events. However multiscale Rényi entropy indicated either no change immediately following the exercise for all scaling factors or an increase in HRV complexity of the heart rate for the afternoon recordings. Despite decreased vagal influence, endurance training may be protective for some individuals that retain a higher heart rate complexity in the presence of vagal withdrawal.