Dissociation with interference between pacemakers located within the A-V conducting system

Dissociation with interference occurred between pacemakers located, one at the uppermost region of the A-V junction, and the other, either above or below the bifurcation of the common bundle. A prolonged A-V conduction time of interference beats permitted the assumption that some degree of A-V block accompanied this form of dissociation. When both foci discharged at a high enough rate (over 70), it was then possible to make the diagnosis of simultaneous, independent, coexisting A-V tachycardias, a form of double tachycardia not previously described. In some records of Case 1, occasional stimuli which had their origin in the uppermost pacemaker were stopped at different levels of the auriculoventricular conduction system; the effect of this was revealed only by the unexpected failure of the succeeding beat from the same pacemaker to appear. The varying extent of penetration depended on diverse causes, which were discussed and analyzed. This physiologic phenomenon, known as concealed conduction, had been described by Langendorf, and is observed when dealing with normal as well as with depressed conductivity. The most important consideration from the clinical standpoint was the production of the arrhythmia by digitalis toxicity in old persons with arteriosclerotic heart disease and congestive heart failure. Prompt withdrawal of the drug is emphasized.