ELECTROPHYSIOLOGICAL AND CLINICAL OBSERVATIONS IN PATIENTS WITH ALTERNATING BUNDLE-BRANCH BLOCK

Electrophysiological studies (His bundle recordings and atrial stimulation) were performed in 9 patients who manifested periods of both right and left bundle branch block (RBBB and LBBB). In 7 of the patients, alternating BBB appeared to reflect intermittent or chronic BBB superimposed on incomplete (but ECG complete) block of the contralateral bundle branch. In 3 of these 7, shift from BBB pattern to the other was associated with reproducible change in H-V (mean change 30 ms), and could be induced by alteration of cardiac rate with carotid massage, coupled atrial stimulation and rapid atrial pacing. In 1 of the 7, RBBB with a P-R of 0.20 s preceded chronic LBBB with a P-R of 0.24 s, implying that RBBB was incomplete. In 3 of the 7, although a definite mechanism of alternation could not be demonstrated, transient contralateral BBB occurred superimposed on chronic ipsilateral BBB, implying that the ipsilateral block was incomplete. Patients (2) manifested periods of narrow QRS, LBBB, RBBB and paroxysmal atrialventricular (A-V) block. Based upon pathological data (1 case), this pattern appeared to reflect a lesion involving the distal His bundle and proximal bundle branches. In the total group of patients, the clinical course was primarily determined by the severity of heart disease and not by occurrence of A-V block. The conduction defect in the majority of patients was surprisingly benign.