His bundle (BH) electrograms were recorded in 123 patients. The electrocardiogram showed only left axis deviation (LAD) in ten, right bundle branch block (RBBB) with a normal axis in thirty, RBBB and LAD with or without old myocardial infarction in sixty-eight and RBBB with right axis deviation (RAD) in fifteen. Thirteen of these patients were restudied at intervals of six months to two years. The conduction times through the atrioventricular (A-V) node (A-H) and His-Purkinje system (H-V) were measured during normal sinus rhythm and atrial pacing. In all but four patients with sinus rhythm in whom second degree A-V block developed only during atrial pacing at rapid rates (range 100 to 180/minute) the block was localized in the A-H interval (A-V node). In the other four with sinus rhythm the second degree block was localized distal to the BH deflection (H-V). Most of the patients with RBBB (normal axis) and first degree A-V block had a normal H-V time with delay localized in the A-V node (A-H), indicating that this combination in the electrocardiogram does not necessarily mean bilateral bundle branch block. In none of the ten patients with only LAD was the H-V time abnormal (normal range = 35 to 45 msec). In seven (23 per cent) of the thirty patients with RBBB (normal axis) the H-V time was abnormal. Forty-nine (72 per cent) of the sixty-eight patients with RBBB and LAD with or without myocardial infarction had an abnormal H-V time. Furthermore, patients with similar electrocardiographic patterns (RBBB and LAD) may or may not have an abnormal H-V time. All fifteen patients (100 per cent) with RBBB and RAD had abnormal H-V times. This study demonstrates that (1) the surface electrocardiographic criteria for assessing quantitative damage in the His-Purkinje system are frequently of limited value, and (2) the majority of the patients (72 per cent) with RBBB and LAD, besides RBBB and anterior hemiblock, have additional damage in the remaining fascicles of the His-Purkinje system. This would suggest partial bilateral bundle branch block or trifascicular block.
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