Relationships between increased automaticity and depressed conduction in the main intraventricular conducting fascicles of the human and canine heart.

Escapes from the injured fascicle (EIF) were investigated in 281 cases of bundle branch block (BBB), and during 35 experiments in which rate-dependent BBB was provoked in the intact canine heart. During vagal stimulation, EIF occurred in 27 of the 35 canine experiments, in seven of 24 patients with phase 4 (bradyeardia-dependent) BBB, and in nine of 31 patients with fixed BBB. Changes in the degree of fascicular injury and phase 4 BBB were accompanied by correlative changes in the frequency and coupling interval of the EIF, indicating the existence of a close relationship between degree of injury, phase 4 BBB and EIF or enhanced automaticity within the affected fascicle. Therapeutic doses of isoproterenol and lidocaine were tested and were shown to have a simultaneous and sometimes concordant effect on the BBB and the EIF. Occasionally in the acute experiments on dogs, commonly in chronic patients, or at times in patients under the effects of lidocaine, a dissociation or desynchronization between the phase 4 BBB and the EIF was documented. This dissociation implies the existence of other physiologic factors, which may eventually cause the occurrence of concealed or abortive escapes. The fact that phase 3 (tachycardia dependent) and phase 4 BBB can be identified in patients or provoked experimentally in the intact canine heart, with or without EIF, provides with a model of great potential value for studying effects of antiarrhythmic drugs.