2:1 and phase 4 peri-infarction block.

A patient with acute inferior and anteroseptal myocardial infarction initially developed a 2 : 1 AV block with alternate conducted ventricular complexes showing aberrancy, and later Wenckebach-type, 2nd-degree AV block with aberrancy of the beats following a long diastolic pause. Intracavitary recording suggested that aberrancy was related to an intraventricular block. ECG and VCG recordings excluded the site of block as being in the main bundles or in the fascicles of the left bundle. The patient therefore showed evidence for a 2 : 1 and later a phase 4 peripheral block, defined also as a peri-infarction block because of the underlying etiology of the block. The block could be localized in the posterior wall of the right ventricle, by the marked rightward and posterior orientation of the middle and terminal electrical forces evident in the vectorcardiogram.