Paroxysmal bundle branch block converted into two-to-one bundle branch block by means of atropine

The question whether or not the main branches of the His-Tawara bundle are subject to the influence of the vagus nerves is as yet unsettled. Drury and Mackenzie 1 were unable to demonstrate any effect on branch conduction, either by the use of acetylcholine or by stimulation of the vagi in the neck, in normal dogs. If, however, either the right or the left branch was injured mechanically, bundle branch block complexes appeared in the electrocardiogram for a short period of time, after which what appeared to be full recovery occurred. At a critical period following this recovery, bundle branch block complexes could be made to reappear by stimulation of the vagi in the neck. The authors attributed this effect to action of the vagus on the junctional tissues, rather than on the bundle branches. They thought that vagal stimulation increased the "decrement" through the A-V node and the main bundle, and consequently the impulse arrived at the still partially damaged branch too weak to pass, whereas it passed the undamaged branch readily. This report deals with a patient, one of whose bundle branches was damaged but was still capable of fluctuation in conductivity.