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A case is described where the basic sinus rhythms is associated with an advanced degree of left anterior hemiblock. The rhythm is complicated by atrial extrasystoles, associated with a lesser degree of left anterior hemiblock. This paradox of 'supernormality' is explained on a critical interplay of differential refractoriness within the divisions of the left bundle-branch.

In 8 cases of left anterior hemiblock, combined with right bundle-branch block in 7, serial sections of the conducting system of the heart were examined histologically. In all cases there were pathological changes in the left bundle-branch but the anterior part of it was predominantly affected in only 2 cases. Acute changes were found in 6 cases of early myocardial infarction, and fibrosis in 2 cases of chronic heart disease. Reversible lesions may have a pathogenetic role in acute hemiblocks. The right bundle-branch was disrupted by fibrosis in 6 of the 7 cases with right bundle-branch block, and minor changes in the AV node were observed in 1. The widespread damage to the left bundle-branch in the majority of the present cases does not seem to be consistent with the limited clinicopathological correlation implied by the terms anterior fascicular block or hemiblock. Other cardiac lesions within the left bundle-branch and outside it may contribute to this electrocardiographic pattern.

The clinical and necropsy findings in 3 patients with tetralogy of Fallot who died suddenly after corrective operation are presented. Early postoperative electrocardiograms of 2 patients showed coexistent right bundle-branch block and left anterior hemiblock; one of these developed complete atrioventricular block and required a pacemaker. In the third a pacemaker. In the third patient, postoperative electrocardiograms showed anterior myocardial infarction. At necropsy, there was gross and histological evidence of myocardial infarction in each patient. After operation, extensive myocardial fibrosis and the development of right bundle-branch block and left anterior hemiblock are possible factors causing cardiac arrhythmias and sudden death.

A case is presented in which transient left anterior hemiblock masked the electrocardiographic signs of coronary insufficiency during work.