The concept of masquerading bundle-branch block; an electrocardiographic-pathologic correlation.

This is a clinicopathologic study of 2 patients who electrocardiographically presented so called "masquerading" bundle-branch block. The conduction systems and the entire hearts of these patients were studied pathologically by methods especially devised by Lev for electro-cardiographic correlation. This report is part of a long-term project attempting to ascertain the anatomic substrate of electrocardiographic abnormalities. THE electrocardiographic complex referred to as "masquerading bundle-branch block, " consisting of the pattern of left bundle-branch block (LBBB) in the limb leads and right bundle-branch block (RBBB) in the unipolar precordial leads, has generated considerable speculation and interest relating to possible mechanisms responsible for its production. It has been designated " mas-querading" by Richman and Wolff1 because they thought that even though the precordial leads suggested RBBB according to Wilson's criteria (right precordial leads showing late R or R' deflections, regardless of the morphology of the limb leads), this complex was actually the result of LBBB. This discrepancy was believed to be the result of the probable transmission of high septal potentials through the infareted free right ventricular wall. These authors' described 4 such cases in which the veetoreardiograms were interpreted as L1BBB. The vectoreardiograms obtained in their cases differed markedly from those usually seen in L1BBB except for the initial vectors, which were thought to be typically suggestive of L1BBB. The marked difference in the remainder of the vectoreardiogram was attributed to the complication of septal infarc-tion. A postmortem examination was reported by these authors on a case in which extensive infarction of the interventricular septum and the posterolateral wall of the left ventricle was found. No histologic study of the conduction system was mentioned. Sodi-Pallares and Rodriguez2 in an earlier study on activation of the interventricular septum and the clinical evaluation of septal damage described this same electrocardi-ographic complex. In an effort to clarify this discrepancy between limb and precordial leads, they studied additional leads taken around the thorax at different levels searchingr for the electrocardiographic evidence of L1BBB in these leads to conform with what the limb leads showed. In some of their cases this was found in leads taken in higher interspaces than the conventional chest leads. However, even here the precordial leads showed delayed intrinsicoid deflections over both the right and left precordial regions. The explanation offered was that the infarct had involved the lower portion of the interventricular septum and invaded the adjacent right and left free ventricular …