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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.
Indications for thrombolytic treatment
ST elevation >1 mm in two contiguous limb leads or >2 mm in two contiguous chest leads
Posterior myocardial infarction
Left bundle branch block
ST segment depression or enzymatic change are not indications for 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.
Hyperacute T waves
The earliest signs of acute myocardial infarction are subtle and include increased T wave amplitude over the affected area. T waves become more prominent, symmetrical, and pointed ("hyperacute"). Hyperacute T waves are most evident in the anterior chest leads and are more readily visible when an old electrocardiogram is available for comparison. These changes in T waves are usually present for only five to 30 minutes after the onset of the infarction and are followed by ST segment changes.
ST segment changes
In practice, ST segment elevation is often the earliest recognised sign of acute myocardial infarction and is usually evident within hours of the onset of symptoms. Initially the ST segment may straighten, with loss of the ST-T wave angle. Then the T wave becomes broad and the ST segment elevates, losing its normal concavity. As further elevation occurs, the ST segment tends to become convex upwards. The degree of ST segment elevation varies between subtle changes of <1 mm to...