Right bundle branch block with left posterior hemiblock
Right bundle branch block is evident as wide QRS (160 msec), slurred S wave in lead I and slurred R in V1. In V1 there is qR pattern suggesting anterior wall infarction with right bundle branch block (qRBBB). The QRS shows right axis deviation (dominant negative deflection in leads I and aVl) with dominant positive deflection in aVf along with rS pattern in lead I and qR pattern leads III and aVf, suggesting left posterior hemiblock. This combination is bifascicular block. The PR interval is borderline at 200 msec. If PR interval is prolonged in this combination, it becomes trifascicular block.
Left posterior hemiblock is the rarest of the fascicular blocks as it is spared in most cases due to the dual blood supply. For the same reason, left posterior hemiblock carries a poorer prognosis. Since the left posterior fascicle is a broad fascicle compared with the anterior fascicle and has dual blood supply, it is involved only in more extensive myocardial infarction. This would mean more severe left ventricular dysfunction. The combination of left posterior hemiblock with posterior hemiblock carries a still worse outcome. The very wide QRS for an RBBB in this case also indicates more extensive myocardial damage. The T wave inversion in leads V1 to V3 could be explained either by the old myocardial infarction or by the right bundle branch block.