Autonomic Nervous System Effects on the Atrioventricular Conduction Time - Heart Period Relationship During Physical Exercise and Passive Recovery

During physical exercise and the following passive recovery, for a given heart period (i.e. RR interval), the atrioventricular conduction time (i.e. PR interval) is known to be lengthened during recovery (i.e. hysteresis phenomenon). We aimed to determine non-invasively the role of the autonomic nervous system on this hysteresis. 20 healthy participants (10 males, 10 females) performed, on separate days, two identical ramp exercise tests on a cycle ergometer to exhaustion (233±56W). Following exercise, participants remained either seated on the bike or adopted a supine position for 30 min. A 12-lead ECG was monitored to determine RR and PR intervals. An ad-hoc PR estimator was used to limit potential P wave detection error during intense exercise resulting from an overlapping T wave. A PR-RR hysteresis was observed in all participants, which area was >3 times more pronounced (P<0.05) in the supine vs. the upright position. Our results showed that during recovery, the atrioventricular conduction time returned to a faster rate than the heart period toward resting values. This effect was greater in the supine position, when the vagal input to the heart is enhanced. Our findings suggest that the PR-RR hysteresis might be explained by a greater vagal input at the atrioventricular compared to the sinus node during recovery.