Site of Heart Block in Acute Myocardial Infarction

Bundle of His electrograms were recorded in eight patients with acute myocardial infarction and heart block. Three patients with diaphragmatic myocardial infarction (DMI) and one with subendocardial infarction were characterized by slowing or block above the bundle of His and A-V junctional escape rhythms during periods of advanced or complete block. An additional patient with DMI had block in the His bundle itself. Intraventricular conduction in the above patients was characterized by normal H-Q intervals (35 to 60 msec) and absence of widened QRS. In contrast, three patients with anterior infarction (AMI) manifested complete block below the bundle of His and idioventricular escape. P-H intervals were normal (80 to 140 msec) and A-V conduction was considered unaffected. Our electrophysiologic observations coupled with previous clinical, anatomic, and pathologic findings suggest that the heart block in DMI is usually due to an ischemic lesion of the A-V node, while heart block in AMI is due to necrosis involving both bundle branches.