[Gap phenomenon and supranormal conduction during intermittent left bundle-branch block].

The mechanism of intermittent bundle branch block, whether spontaneous or as a result of vagal stimulation, can be studied by reference to the known facts of atrial stimulation. Three cases with intermittent left bundle branch block were studied by means of an intracavitary electrode, which allowed the potential of the bundle of His to be measured, and was also used for the extrastimulus method of study. In case 1, in which the block was independent of the cardiac rate, a type 1 "Gap" phenomenon (of nodal origin) was demonstrated; the coupling zone in which the extrastimulus overcame the block was situated in two areas, one being late, and the other earlier in a region where the conduction was normal. In case 2, in which the block was dependent upon the heart rate, the "recovery" of the blocked branch after early coupling of the extrastimulus seemed to be associated with the phenomenon of "supra-normal" conduction. In case 3, in which the block was again dependent upon heart rate, the two phenomena of "Gap" and "suppranormality" seemed to act successively when the coupling of the extrastimulus is decreased progressively. These observations allow us to distinguish two types of intermittent left bundle branch block; one, which is not related to heart rate, is characterised by a prolonged refractory period of the left bundle branch, which becomes progressively shorter as the heart rate increases; the other, which is dependent upon tachycardia, is characterised on the one hand by an increased left bundle branch refractory period which does not shorten as the heart rate increases, and on the other by the phenomenon of supranormal conduction.