The site of the cardiac lesion in two instances of intraventricular heart block

The hearts of two cases were examined in order to determine the site of the lesion associated with electrocardiograms suggesting partial block of either the right or the left branch of the auriculoventricular system. As the main deflections in the two sets of electrocardiograms were in opposite directions, it was to be expected that the lesions, if any, in the two instances, would be found on opposite sides of the heart. There has been considerable theoretical discussion as to which type of electrocardiogram is associated with right-sided and which with left-sided block. The first case showed electrocardiographically a main deflection inverted in lead I, upright in leads II and III, a marked widening of the foot-points of the Q.R.S. complex, and only moderate voltage; in addition there was auricular fibrillation. Microscopic examination of serial sections of the A.-V. system, showed that the right bundle branch became attenuated almost immediately after its origin from the main stem, and was surrounded by connective tissue. This diminution became more pronounced until at a distance of 7.5 mm. from the bifurcation, scarcely one or two doubtful muscle fibers could be seen. Below this the right branch increased in size again until at 4 cm. below the bifurcation it was of normal dimensions. There was marked fibrous myocarditis of the septum, involving chiefly the left side, especially the sub-endocardial region. The left branch presented no lesion. The second case showed, on two examinations at an interval of six weeks, electrocardiograms in which the main deflection was upright in lead I, inverted in leads I1 and 111, was notched, and its foot-points abnormally separated. Wave P was present throughout. Serial sections showed the A.-V. node, stem and right branch intact.