Electrocardiographic diagnosis of acute myocardial infarction in the presence of left bundle branch block.

It is often stated that the diagnosis of acute myocardial infarction (AMI) is extremely difficult if not impossible in the presence of left bundle branch block (LBBB). Likewise the electrocardiographic diagnosis of previous myocardial infarction is also difficult in this setting. The ST segment and T wave abnormalities associated with uncomplicated LBBB may mimic acute ischaemic changes. A wide range of electrocardiographic abnormalities have been suggested to establish the diagnosis ofAMI in the presence of LBBB but no single set of criteria have been universally accepted nor found to be particularly useful. The diagnosis of AMI in the presence of LBBB is extremely important as these patients have a greatly increased risk of complications and death. New LBBB suggests occlusion of the proximal left anterior descending artery resulting in a large area of jeopardised myocardium; new AMI in the setting of pre-existing LBBB places the patient a extreme risk of cardiogenic shock and death. It has been suggested that the outcome of patients with LBBB and AMI is significantly improved by acute reperfusion therapies, such as a thrombolytic agent or primary angioplasty. It is therefore important to be aware of the changes that may indicate AMI when the electrocardiogram (ECG) is complicated by the presence of LBBB.