Spontaneous beat-to-beat variation of PR interval in normal children

Autonomic innervation of the heart both directly and indirectly influences conduction through the atrioventricular (AV) node.^1-3 Sympathetic stimulation directly decreases cycle length by way of the sinoatrial node and enhances AV conduction through the AV node. Parasympathetic activity directly increases cycle length and prolongs AV conduction. However, changes in cycle length cause opposite changes in AV conduction time (indirect effect), thereby modulating direct effects of autonomic activity on AV conduction.^1-3 This study non-invasively investigated the net influence of these opposing factors on AV conduction during spontaneous variation of cycle length.