Lower pacemaker in high degree atrioventricular block is modulated electrotonically by atrial excitations

To see if the lower pacemaker in patients with atrioventricular (AV) block was modulated electrotonically by atrial excitation, we studied R-R intervals in two groups of AV block patients using a phase response curve (PRC). Group I consisted of 20 patients with high degree AV block, including seven patients in whom complete AV block was transiently observed in the course of acute inferior myocardial infarction. Group II consisted of 19 patients with complete AV block. In every patient, PRC was obtained from the continuous electrocardiogram by plotting each R1-R2 interval (response) on the ordinate as a function of the R1-Px interval (phase, x = 1, 2 ...) on the abscissa. In Group I, the R-R interval was prolonged when the P wave fell in the initial half of the R-R interval, and was abbreviated when the P wave fell in the later half of the cycle. In Group II, the fluctuation of the R-R interval was minimum. In ten group I patients, with the improvement of the AV block, PRC became sharper and transition from prolongation to shortening occurred at shorter R-P intervals. We conclude that, in Group I, lower pacemaker was modulated electrotonically by atrial excitations through decreased electrical coupling along the AV node.