Sino-auricular block, interference dissociation, and different recovery rates of excitation in the bundle branches.

Disturbances in the propagation of the heart beat between the auricle and the two ventricles are of frequent occurrence and well known under the terms A-V dissociation or heart block. The term sinoauricular block is the standard name for an apparent block between the sinus node and the auricle, a hypothetical incident to explain transient auricular standstill for a period of time that is a multiple of the length of the regular sinus cycle. Interference dissociation is commonly associated with a nodal rhythm with retrograde block, the slower sinus excitation and auricular beat being conducted intermittently to the ventricle. Less commonly, interference dissociation is seen in cases of near complete A-V dissociation, with occasional sinus beats being conducted in a favourable time period, a phenomenon usually attributed to the likelihood that junctional tissues have a supernormal phase of excitation and conductivity following their refractory period. Some of the best examples of this phenomenon are the first case of Lewis and Master (1924), the cases of Wolferth (1928), of Luten and Pope (1930), of Pareja (1933), and the first case of Burchell (1942). Electrocardiographic records obtained recently from a woman, aged 60, exhibited the recurring phenomenon of nodal rhythm with interference dissociation in which the ventricular beat conducted from the auricle showed the presence of either left or right bundle branch block. The unique feature was the fact that the type of bundle branch block exhibited was determined by the time relationship between the P wave that was to be followed by a ventricular response and the preceding R wave of the idioventricular (nodal) beat. The explanation seemed related to either different rates of recovery in conduction in the two bundle branches or a supernormal phase either of conduction in the left bundle or of excitation in the left ventricle. As there were, in addition, frequent cardiographic sequences showing the picture of sino-auricular block, the possible relationship of this abnormality to the disturbance in A-V and intraventricular conduction invited some speculation.