Observations on atrioventricular conduction in patients with bilateral bundle-branch block

Atrioventricular conduction was studied in three patients, each representing a different grade of bilateral bundle-branch block. His bundle activity was recorded by a catheter technic. Two of the three patients had experienced Adams-Stokes attacks. The first patient had right bundle-branch block with left axis deviation at the time of catheterization. Increases in the atrial driving rate resulted in progressive lengthening of the A-H interval, the H-V interval remaining constant and within normal limits. Second degree A-V block occurred at atrial rates higher than 140/min. The site of this block was located above the His bundle. On no occasion was the impulse blocked distally to the His bundle. The second patient had a Mobitz type II A-V block with complete left bundle-branch block and a prolonged P-R interval in the conducted beats. The block of the nonconducted beats could be located distal to the His bundle. H-V block occurred more frequently when the atrial rate was increased. Second degree A-H block occurred at rates above 140/min. At these rates interesting patterns were seen due to conduction impairment at two levels in the A-V conduction system. The third patient presented with complete A-V block. The site of this block could be located at a level lower than the His bundle. A-H conduction studied with increases of atrial rate and with atrial premature beats seemed to be normal. There was evidence for the existence of retrograde V-A conduction in this patient.