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In eleven patients in whom atrial premature stimulation elicited functional block in the conduction system distal to the atrioventricular (A-V) node, the influence of atrial driving rate was studied. Functional conduction disturbances in the left bundle branch-Purkinje system were produced by driving the atria at a rate just above the spontaneous sinus rate and by introducing premature atrial stimuli. In five patients this was associated with block or delay of conduction in the right bundle branch-Purkinje system and in one with block within the His bundle. In these patients it was shown that the ultimate QRS configuration of the premature beat was determined by different conduction delays in the three parts of the bundle branch system. In three patients complete block of impulse conduction could be produced below the level of the His bundle, in one within the His bundle. In these four patients conduction delay of earlier atrial premature beats within the A-V node with resumption of conduction in the more peripheral pathways, gave rise to the phenomenon of the A-V gap. In two patients functional conduction delay in the contralateral bundle branch caused pseudo-normalization of a pre-existent complete right bundle branch block pattern, indicating that this pattern may not be caused by complete block, but by delayed conduction in the right bundle branch.In ten of the eleven patients functional block or delay of atrial premature beats in the peripheral conduction system became less pronounced as the driving rate was increased. In nine of these ten the phenomenon disappeared completely at higher rates; in six this occurred with an increase in rate of only 20 beats/min. The A-V gap phenomenon present in four cases at the lowest rate disappeared at higher rates in all four. It was concluded that functional conduction disturbances in the peripheral conduction system are favored by a low basic heart rate and that this rate dependency is caused by a differential influence of heart rate upon the functional properties of the A-V node and of the more peripheral parts of the conduction system.

Rate-Dependency of Functional Block in the Human His Bundle and Bundle Branch-Purkinje System