ABC of clinical electrocardiography: Acute myocardial infarction—Part I (2024)

ABC of clinical electrocardiography

In the clinicalassessment of chest pain, electrocardiography is an essentialadjunct to the clinical history and physical examination. A rapid andaccurate diagnosis in patients with acute myocardial infarction isvital, as expeditious reperfusion therapy can improve prognosis. Themost frequently used electrocardiographic criterion for identifyingacute myocardial infarction is ST segment elevation in two or moreanatomically contiguous leads. The ST segment elevation associated withan evolving myocardial infarction is often readily identifiable, but aknowledge of the common “pseudo” infarct patterns is essential toavoid the unnecessary use of thrombolytic treatment.

In the early stages of acute myocardial infarction theelectrocardiogram may be normal or near normal; less than half ofpatients with acute myocardial infarction have clear diagnostic changeson their first trace. About 10% of patients with a proved acutemyocardial infarction (on the basis of clinical history and enzymaticmarkers) fail to develop ST segment elevation or depression. In mostcases, however, serial electrocardiograms show evolving changes thattend to follow well recognised patterns.

Hyperacute T waves

The earliest signs of acutemyocardial infarction are subtle and include increased T waveamplitude over the affected area. T waves become more prominent,symmetrical, and pointed (“hyperacute”). Hyperacute T waves aremost evident in the anterior chest leads and are more readily visiblewhen an old electrocardiogram is available for comparison. Thesechanges in T waves are usually present for only five to 30 minutesafter the onset of the infarction and are followed by ST segmentchanges.

ST segment changes

In practice, ST segment elevation is often theearliest recognised sign of acute myocardial infarction and is usuallyevident within hours of the onset of symptoms. Initially the ST segmentmay straighten, with loss of the ST-T wave angle . Then the T wavebecomes broad and the ST segment elevates, losing its normal concavity.As further elevation occurs, the ST segment tends to become convexupwards. The degree of ST segment elevation varies between subtlechanges of <1 mm to gross elevation of >10mm.

Sometimesthe QRS complex, the ST segment, and the T wave fuse to form asingle monophasic deflection, called a giant R wave or“tombstone”

Pathological Q waves

As the acute myocardial infarctionevolves, changes to the QRS complex include loss of R wave height andthe development of pathological Q waves.

Both of these changes develop as a result of the loss ofviable myocardium beneath the recording electrode, and the Q wavesare the only firm electrocardiographic evidence of myocardial necrosis.Q waves may develop within one to two hours of the onset of symptoms ofacute myocardial infarction, though often they take 12 hours andoccasionally up to 24 hours to appear. The presence of pathological Qwaves, however, does not necessarily indicate a completed infarct. IfST segment elevation and Q waves are evident on theelectrocardiogram and the chest pain is of recent onset, the patientmay still benefit from thrombolysis or direct intervention.

When there is extensive myocardial infarction, Q waves act asa permanent marker of necrosis. With more localised infarction the scartissue may contract during the healing process, reducing the size ofthe electrically inert area and causing the disappearance of the Qwaves.

Resolution of changes in ST segment and T waves

As the infarct evolves, the STsegment elevation diminishes and the T waves begin to invert. The STsegment elevation associated with an inferior myocardial infarction maytake up to two weeks to resolve. ST segment elevation associated withanterior myocardial infarction may persist for even longer, and if aleft ventricular aneurysm develops it may persist indefinitely. T waveinversion may also persist for many months and occasionally remains asa permanent sign of infarction.

Reciprocal ST segment depression

ST segment depression in leads remote from the site ofan acute infarct is known as reciprocal change and is a highlysensitive indicator of acute myocardial infarction. Reciprocal changesare seen in up to 70% of inferior and 30% of anterior infarctions.

Typically, the depressed ST segments tend to be horizontal ordownsloping. The presence of reciprocal change is particularly usefulwhen there is doubt about the clinical significance of ST segmentelevation.

Reciprocal change strongly indicates acuteinfarction, with a sensitivity and positive predictive value of over90%, though its absence does not rule out the diagnosis.

The pathogenesis of reciprocal change is uncertain. Reciprocalchanges are most frequently seen when the infarct is large, and theymay reflect an extension of the infarct or occur as a result ofcoexisting remote ischaemia. Alternatively, it may be a benignelectrical phenomenon. The positive potentials that are recorded byelectrodes facing the area of acute injury are projected as negativedeflections in leads opposite the injured area, thus producing a“mirror image” change. Extensive reciprocal ST segment depressionin remote regions often indicates widespread arterial disease andconsequently carries a worse prognosis.

Localisation of site of infarction

The distribution of changes recordedin acute myocardial infarction allows the area of infarction to belocalised, thus indicating the site of arterial disease. Proximalarterial occlusions tend to produce the most widespreadelectrocardiographic abnormalities. The anterior and inferior aspectsof the heart are the areas most commonly subject to infarction.Anteroseptal infarcts are highly specific indicators of disease of theleft anterior descending artery. Isolated inferior infarcts—changes inleads II, III, and aVF—are usually associated with disease in theright coronary or distal circumflex artery. Disease in the proximalcircumflex artery is often associated with a lateral infarctpattern—that is, in leads I, aVL, V5, and V6.

Footnotes