Anatomic studies of the cardiac conducting system in acute myocardial infarction

Complete heart block with normal QRS duration occurring distal to the his bundle in acute inferior myocardial infarction

A patient with acute inferior myocardial infarction developed on the second day complete heart block with normal QRS duration, and the block was found to be distal to the His bundle. To our knowledge, this is the first such documented case in the English literature. Failure of the preceding Wenckebach periods and of complete heart block to respond to intravenous atropine may be a clinical clue to the nature of this type of heart block. The implications and possible explanations are discussed in the light of recent knowledge concerning the nature of heart block in inferior wall myocardial infarction.

Electrophysiological properties of canine Purkinje cells in one-day-old myocardial infarction

Electrophysiological mechanisms which underlie the ectopic ventricular beats occurring 1 day after occlusion of the anterior descending coronary artery of dogs were explored. In intact dogs, bipolar electrograms were recorded in the infarcted and normal zones. Both Purkinje and ordinary myocardial potentials were recorded from the endocardial surface of the normal zone, but only Purkinje potentials were recorded from the infarcted zone. The Purkinje potentials in the infarcted zone were diminished in amplitude and rapidity. The threshold for endocardial pacing was higher in the infarcted zone than it was in the normal zone. The refractory periods of Purkinje cells were longer in the infarcted zone than they were in the normal zone. Excised specimens of endocardial surface containing the infarcted zone were superfused with Tyrode's solution. No ordinary myocardial cells were electrically active at the endocardial surface of the infarcted zone. Purkinje cells showed diminished resting and action potentials, reduced upstroke velocity, enhanced automaticity and phase 4 depolarization, and long action potentials with prolonged phase 3. Excitability was depressed within the infarcted zone. During superfusion, even with Tyrode's solution poor in oxygen and free of dextrose, Purkinje cells recovered toward normal with respect to amplitude of resting and action potentials, upstroke velocity, automaticity, and excitability. The prolonged action potentials persisted. Repetitive firing was easily elicited within the infarcted zone. Various types of pacemaker activity were detected within the infarcted zone. Thus Purkinje cells in the infarcted zone showed distinctive alterations in electrophysiological properties which were probably related to deleterious factors that accumulated in that zone. It is likely that the arrhythmia which occurred in the intact dogs at this time originated within the altered Purkinje cells of the infarcted zone.

Histopathology of heart block complicating acute myocardial infarction. Correlation with the His bundle electrogram

Histopathological studies of the conduction system were related to His bundle electrogram recordings in seven patients with acute myocardial infarction and atrioventricular (A-V) conduction disturbances. The three patients with inferior infarctions had normal width QRS complexes and delay or block of the impulses above the His bundle. Recent ischemic changes were present in the A-V node in two cases and in the distal conduction system in all three cases. In the four patients with antero-septal infarction and right bundle branch block (RBBB), either the H-V interval was prolonged or block was present below the H spike. The A-H interval was normal in each of these cases, and the A-V node was not affected by the recent infarction. The right bundle branch was involved in all four of these cases and two patients also had involvement of the left bundle branch. In general there was a good correlation between the sites of heart block as defined by the His bundle electrogram and the histopathological analysis of the cardiac conduction system.

Cardiac arrhythmias. 1. Anatomy of the conduction system

Some, but not all, cardiac arrhythmias are related to pathologic lesions of the cardiac conduction system. Common atrial dysrhythmias and first-degree atrioventricular (A-V) block rarely are explained on the basis of anatomic lesions in specific sites of the conduction system or its blood supply. Second-degree A-V block of Mobitz type II, which may be a precursor of complete (third-degree) heart block, commonly is associated with fibrotic lesions of uncertain etiology in the branching part of the bundle of His or the bundle branches. Ischemic lesions are found less often, and other pathologic processes rarely are present. Chronic complete heart block most often results from nonspecific, fibrotic interruption of the distal bundle of His, or of the first parts of the bundle branches after their origins. Ischemic lesions are uncommonly the cause of chronic block. High-grade A-V block complicating acute myocardial infarction may be associated with infarction of the A-V conduction system, but often morphologic evidence of ischemia cannot be identified. Congenital variants in anatomy of the conduction system are responsible for some relatively uncommon arrhythmias.