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

In the clinical assessment of chest pain, electrocardiography is an essential adjunct to the clinical history and physical examination. A rapid and accurate diagnosis in patients with acute myocardial infarction is vital, as expeditious reperfusion therapy can improve prognosis. The most frequently used electrocardiographic criterion for identifying acute myocardial infarction is ST segment elevation in two or more anatomically contiguous leads. The ST segment elevation associated with an evolving myocardial infarction is often readily identifiable, but a knowledge of the common “pseudo” infarct patterns is essential to avoid the unnecessary use of thrombolytic treatment. In the early stages of acute myocardial infarction the electrocardiogram may be normal or near normal; less than half of patients with acute myocardial infarction have clear diagnostic changes on their first trace. About 10% of patients with a proved acute myocardial infarction (on the basis of clinical history and enzymatic markers) fail to develop ST segment elevation or depression. In most cases, however, serial electrocardiograms show evolving changes that tend to follow well recognised patterns.