ABC of clinical electrocardiography: Acute myocardial infarction---Part II

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Abstract
This article describes the association of bundle branch block with acute myocardial infarction and the differential diagnosis of ST segment elevation. Appropriate discordance in uncomplicated left bundle branch block (note ST elevation in leads V1 to V3) Acute myocardial infarction in the presence of bundle branch block carries a much worse prognosis than acute myocardial infarction with normal ventricular conduction. This is true both for patients whose bundle branch block precedes the infarction and for those in whom bundle branch block develops as a result of the acute event. Thrombolytic treatment produces dramatic reductions in mortality in these patients, and the greatest benefits are seen in those treated early. It is therefore essential that the electrocardiographic identification of acute myocardial infarction in patients with bundle branch block is both timely and accurate. ### Left bundle branch block Left bundle branch block is most commonly seen in patients with coronary artery disease, hypertension, or dilated cardiomyopathy. The left bundle branch usually receives blood from the left anterior descending branch of the left coronary artery and from the right coronary artery. When new left bundle branch block occurs in the context of an acute myocardial infarction the infarct is usually anterior and mortality is extremely high. The electrocardiographic changes of acute myocardial infarction can be difficult to recognise when left bundle branch block is present, and many of the conventional diagnostic criteria are not applicable. Acute myocardial infarction and left bundle branch block. Note that the ST segments are elevated in leads V5 and V6 (inappropriate concordance) and grossly elevated (> 5 mm) in leads V2, V3, and V4; note also the ST segment depression in leads III and aVF Abnormal ventricular depolarisation in left bundle branch block leads to secondary alteration in the recovery process (see …

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