ST elevation is the pattern everyone is taught and the one that causes least difficulty. The infarcts that are missed are the ones whose ECG does not meet the elevation threshold, or whose elevation is in leads nobody recorded. This is an account of both.
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The sequence of change
Occlusion produces hyperacute T waves within minutes — tall, broad-based and symmetrical, easy to dismiss as normal variant and the single most useful early sign. ST elevation follows within the first hour and is convex upwards in true infarction. Pathological Q waves appear over hours as myocardium is lost. T wave inversion develops as the ST segment returns towards baseline, and may persist for months. ST elevation that persists beyond several weeks in the territory of an established infarct should raise left ventricular aneurysm.
The practical consequence is that a single trace is a snapshot of an evolving process. In a patient with a convincing history and an unconvincing ECG, repeating the trace at fifteen minutes is worth more than any single additional investigation.
Territory and culprit vessel
- Anterior — V1 to V4: left anterior descending artery. Elevation extending to V5, V6, I and aVL indicates a proximal lesion and a larger territory.
- Inferior — II, III and aVF: right coronary artery in about eight cases in ten, otherwise a dominant circumflex. Elevation greater in III than in II, with reciprocal depression in I and aVL, points to the right coronary.
- Lateral — I, aVL, V5 and V6: circumflex or a diagonal branch.
- Posterior: no elevation anywhere on the standard twelve leads. Look instead for tall R waves and horizontal ST depression in V1 to V3, and confirm with posterior leads V7 to V9.
- Right ventricular: suspected in any inferior infarct, confirmed with V4R. This matters because these patients are preload-dependent and nitrates can produce profound hypotension.
Two additional leads take thirty seconds to record and change management in both of the last two categories. They are the most under-used investigation in acute cardiology.
The elevation thresholds
New elevation at the J point in two contiguous leads: 1 mm in all leads other than V2 and V3; in V2 and V3, 2 mm in men aged forty and over, 2.5 mm in men under forty, and 1.5 mm in women. The sex and age differences are not a technicality — applying the male threshold to a woman is one documented route to a missed anterior infarct.
Six patterns that are not STEMI but need the laboratory
- De Winter T waves. Upsloping ST depression at the J point in the praecordial leads with tall symmetrical T waves, and often slight elevation in aVR. This is proximal left anterior descending occlusion, and it does not evolve into a classical STEMI before the myocardium is lost.
- Wellens’ pattern. Deep symmetrical anterior T wave inversion, or a biphasic pattern in V2 and V3, in a patient who is currently pain-free with preserved R waves and no Q waves. It signifies critical proximal LAD stenosis. Exercise testing such a patient is dangerous.
- Left main or severe three-vessel disease. Widespread ST depression across six or more leads with elevation in aVR exceeding that in V1.
- Posterior infarction. Tall R and horizontal depression in V1 to V3, confirmed on posterior leads.
- New left bundle branch block with a convincing history, assessed with the Sgarbossa criteria — concordant elevation of 1 mm, concordant depression of 1 mm in V1 to V3, or excessively discordant elevation.
- Hyperacute T waves alone, before any elevation has developed. Repeat the trace rather than accept it.
Mimics worth knowing
Pericarditis produces widespread concave elevation with PR depression and no reciprocal change. Early repolarisation shows notched J points, is stable across old traces and occurs in a well patient. Takotsubo cardiomyopathy can be electrocardiographically indistinguishable from anterior infarction at presentation and is separated only in the laboratory. Left ventricular aneurysm gives persistent elevation with established Q waves. Hyperkalaemia produces tall tented T waves with a widening QRS, and the clinical context is usually obvious once considered.
The reading sequence that prevents most of these being missed is set out in the systematic ten-step approach. Acute management pathways are in the interventional and acute care reading list, worked presentations in the fifty emergency cases, and live discussion at the Monday interventional case review.