Electrophysiological Significance of First Degree Atrioventricular Block with Intraventricular Conduction Disturbance

His bundle electrograms (H) were recorded in 27 patients with PR prolongation and intraventricular conduction disturbances. The PR interval was subdivided into P-H (normal 80-140 msec), a measure of atrioventricular (A-V) conduction, and H-Q (normal 35-55 msec), a measure of intraventricular conduction. In 12 patients with left bundle-branch block (LBBB), P-H varied from 115-410 msec (mean 173 msec), and was prolonged in eight patients. H-Q varied from 39-125 msec (mean 80 msec), and was prolonged in 10 patients. In eight patients with right bundle-branch block (RBBB), P-H varied from 146-450 msec (mean 206 msec), and was prolonged in all patients. H-Q ranged from 41-65 msec (mean 51 msec), and was slightly prolonged in three patients, all of whom had, in addition, evidence of left anterior or posterior hemiblock. Seven patients with lesser intraventricular conduction defects all had prolonged P-H intervals, with normal H-Q intervals. With atrial pacing at increased heart rates, block proximal to H occurred in all groups. Three patients with LBBB developed block distal to H with atrial pacing, and one additional patient with LBBB and one patient with RBBB and left posterior hemiblock developed block distal to H following atropine administration. In conclusion, most patients with PR prolongation and intraventricular conduction defects had prolongation of P-H, suggesting A-V nodal disease. In addition, the patients with LBBB usually had significant H-Q prolongation, suggesting bilateral bundle-branch disease. In the group with RBBB, H-Q prolongation was less common and less marked, and, when present, occurred in patients with evidence of left anterior or posterior hemiblock.