THE CONTEXT
The 12-lead electrocardiogram is central to confirming myocardial infarction and deciding immediate management. Serial changes help estimate the approximate age of an infarct.
Initial assessment and repeat recordings
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Bundle branch block or previous myocardial infarction can make the ECG difficult to interpret.
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The initial ECG may be normal or non-diagnostic in one-third of cases.
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Repeated ECGs are important when the diagnosis is uncertain or symptoms recur or persist.
Sequence in transmural infarction
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The earliest ECG change is usually ST-segment deviation.
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Proximal occlusion of a major coronary artery initially produces ST-segment elevation or new bundle branch block.
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Later changes include diminution of the R wave and, with transmural or full-thickness infarction, development of a Q wave.
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Subsequent T-wave inversion reflects a change in ventricular repolarisation and persists after the ST segment returns to normal.
Approximate timing of serial changes
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Acute ST elevation generally appears within minutes.
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Progressive R-wave loss, a developing Q wave, resolution of ST elevation and terminal T-wave inversion generally appear within hours.
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Deep Q waves and T-wave inversion generally appear within days.
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An old or established infarct pattern develops after several weeks or months: the Q wave tends to persist while T-wave changes become less marked.
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The rate of evolution is very variable, so these changes provide only an approximate estimate of infarct age.
Non-ST segment elevation acute coronary syndrome
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There is usually partial occlusion of a major vessel or complete occlusion of a minor vessel, causing unstable angina or partial-thickness, subendocardial infarction.
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Associated ECG findings include ST-segment depression and T-wave changes.
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When infarction is present, some R-wave loss may occur without Q waves.
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The illustrated recent anterior non-ST elevation infarction shows deep symmetrical T-wave inversion with reduced R-wave height in leads V1–V4.