Mechanism and significance of widened QRS complexes during complete atrioventricular block in acute inferior myocardial infarction

Twelve of 35 consecutive patients admitted with complete, atrioventricular (A-V) block complicating acute inferior myocardial infarction manifested widened QRS complexes. The escape beats had the pattern of left bundle branch block in four patients, right bundle branch block in five patients and both left and right bundle branch block in three patients.

His bundle recordings in five patients with escape beats that had a left bundle branch block configuration revealed a His bundle potential preceding the widened QRS complex at His-V intervals of 45 to 60 msec. Bradycardia-dependent left bundle branch block was demonstrated in two patients by His bundle pacing. In three patients the conducted beats had a left bundle branch block configuration after critical lengthening of the R-R interval during second degree A-V block before or after the episode of complete A-V block. In six patients whose escape beats had a right bundle branch block configuration, His bundle recordings did not reveal a His bundle potential preceding these beats.

Our observations suggest that widened QRS complexes with a left bundle branch block configuration could be due to an A-V junctional escape rhythm with phase 4 left bundle branch block. Alternatively in association with a right bundle branch block configuration it is possible that the widened QRS complexes represent a ventricular or fascicular escape rhythm.

Two of 12 patients with widened QRS complexes died. There were no significant differences in immediate mortality, 6 month mortality or mean peak serum glutamic oxaloacetic transaminase (SGOT) values between patients with narrow and widened QRS complexes. This finding suggests that widened QRS complexes during complete A-V block in acute inferior myocardial infarction have no prognostic significance.