Left bundle branch block in the setting of acute coronary syndrome

Beatriz López Barbeito1,2, Matilde González Tejera2,3

The management of patients with suspected acute coronary syndrome (ACS) and left bundle branch block (LBBB) in emergency departments represents a highly complex clinical challenge. Traditionally, European clinical practice guidelines have established an indication for an emergent reperfusion strategy in patients with LBBB (right bundle branch block or paced rhythm) in the presence of clinical suspicion of ongoing ischemia. This recommendation is based both on the technical difficulty of interpreting the baseline electrocardiogram and on the fact that acute LBBB is often a marker of extensive ischemia and hemodynamic compromise. However, in most cases, LBBB is a preexisting condition, and only approximately 30% of these patients ultimately present with acute coronary occlusion. Therefore, it is necessary to balance the presence of instability and specific electrocardiographic criteria requiring immediate intervention against clinical stability, which allows reliance on high-sensitivity troponin assessment. This review evaluates the pathophysiological basis of LBBB, electrocardiographic criteria for detecting acute ischemia in the presence of LBBB, and the role of echocardiography in evaluating these patients.