[Tachycardia-dependent paroxysmal block in the bundle of His. Electrophysiological demonstration]

The authors report the case of a patient who presented with angina of effort followed by transient loss of consciousness or syncope. Clinical examination yielded little information. The electrocardiogram showed signs of an old antero-septal infarct, and a slightly prolonged PR interval at 0.22 s. A recording of the activity of the bundle of His showed a double H potential, whose two components were separated by an interval of 80 ms. This interval increased progressively under the influence of atrial stimulation. At a critical frequency of 125/mn, a complete block below the bundle of His was produced; this only reverted after slowing or cessation of stimulation. The same phenomenon occurred at rates between 125/mn and 150/mn. By contrast stimulation at 170/mn was followed by a 2:1 block below the bundle of His. It is to be presumed that the complete A-V block was occasioned by latent conduction of the impulse in the injured area. If the rate was increased yet again, the level of the block became displaced to the upper limit of this area. The absence of latent conduction then allowed the abnormal fibres to recover. The present study thus demonstrates the existance of a special type of paroxysmal A-V block: a bundle block which is dependant upon tachycardia.