Using the 12-lead ECG to diagnose acute myocardial infarction in the presence of left bundle branch block

The 12-lead ECG is a powerful clinical tool used to risk stratify patients presenting to the emergency department with chest pain. In particular the ECG is used as the diagnostic tool to instigate reperfusion therapy in patients with acute coronary syndromes. The ECG features of acute myocardial infarction (AMI) may be masked by the presence of left bundle branch block (LBBB) and the ECG may be difficult to interpret. Invariably this results in delays to the provision of thrombolysis to these patients despite the mounting body of evidence which demonstrates that patients with AMI who present with LBBB have greater in-hospital mortality than those who do not. Difficulties in interpreting the ECG in these patients can therefore delay treatment and compromise their prognosis. The utility of the ECG for the diagnosis of AMI in the presence of LBBB has recently received renewed attention. ECG criteria have been identified which have a high association with AMI in patients with LBBB and two ECG tools have been evaluated in clinical practice which utilise these ECG criteria. The use of these simple algorithmic tools is recommended for clinical practice.