An unusual bundle-branch block

We report a case of right bundle-branch block (RBBB) showing a QRS configuration typical for left bundle-branch block (LBBB) in leads V(5) and V(6). The QRS axis was at +90 degrees, and the QRS duration was 0.14 second. There were wide S waves in leads I and aVL, suggesting at first glance an RBBB, but the QRS morphology in the inferior leads (monophasic R wave with secondary ST-T changes) was more consistent with an LBBB. Lead V(1) suggested an RBBB, whereas leads V(5) and V(6) showed a monophasic R wave as in LBBB; moreover, a negative T wave, typical of LBBB, was present in lead V(5). Placement of the electrodes of leads V(4), V(5), and V(6) 2 intercostal spaces above restored in these leads a QRS configuration suggestive of RBBB. The diagnostic problem was mainly caused by the inferior direction of the QRS axis. Because the electrode of V(6) is normally placed below the electrical center of the heart, namely, on a plane that is not orthogonal to the sagittal plane, a vector directed mainly inferiorly and slightly to the right does not project on the negative part but on the positive of the lead line. For this reason, the S waves normally observed in the left precordial leads with RBBB disappear. The superior displacement of the electrodes "normalizes" the plane upon which the lead lines lie, thereby restoring the expected QRS configuration.