The ECG showing new RBBB should prompt a primary percutaneous coronary intervention strategy in patients with ongoing symptoms consistent with myocardial ischemia.2 It often suggests a proximal LAD lesion with larger infarct size, which is related to many complications, including heart failure and increased mortality rate.3 The Figure, A shows the ST-segment elevation MI (STEMI) pattern associated with RBBB and LAFB resembling proximal LAD occlusion. However, there is absence of ST-segment elevation in lead V1. A plausible explanation may be an acute total LMT occlusion without collateral circulation.
In patients with MI caused by LMT obstruction, there are 2 prominent electrocardiographic presentations. First, when a subtotal occlusion or acute total LMT occlusion accompanying well-developed collateral circulation occurs, there may be a non-STEMI (NSTEMI). The presence of ST-segment depression in 8 or more surface leads, coupled with ST-segment elevation in lead aVR and/or lead V1, suggests multivessel ischemia or LMT obstruction.2 Second, the electrocardiographic manifestation is a STEMI due to an acute LMT occlusion without collateral circulation. In 2012, Fiol et al1 first reported the STEMI pattern in conjunction with RBBB and LAFB of proximal LAD occlusion without ST-segment elevation in lead V1, which had been recorded in patients with acute total occlusion of LMT without collateral circulation. In this condition, the absence of ST-segment elevation in lead V1 may be related to acute occlusion of the left circumflex artery, which results in ST-segment depression in the right precordial leads, counteracting ST-segment elevation in lead V1 induced by the occlusion of LAD.1,4 Therefore, the presence of ST-segment elevation in lead V1 indicates isolated proximal LAD occlusion, whereas its absence supports acute total occlusion of the LMT without collateral circulation or its equivalent.4