ECG Localisation of Myocardial Infarction
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Direct answer
ECG localisation of myocardial infarction matches ST-segment changes to the lead groups that face each coronary territory, so the leads showing elevation reveal the culprit artery. Anterior leads V1 to V4 reflect the left anterior descending artery, inferior leads II, III and aVF reflect the right coronary artery in most people, and lateral leads I, aVL, V5 and V6 reflect the circumflex, with reciprocal depression in opposing leads confirming the pattern.
What you must remember
- Anterior: V1 to V4 reflect the left anterior descending artery, with V1 and V2 covering the septum and V3 and V4 the anterior wall; proximal LAD occlusion adds elevation in I and aVL.
- Lateral: I, aVL, V5 and V6 reflect the circumflex or diagonal branches; circumflex occlusion can show lateral elevation with reciprocal change in V1 to V3.
- Inferior: II, III and aVF reflect the right coronary artery in right-dominant circulation; depression in I and aVL, reciprocal to inferior elevation, strongly favours the RCA.
- Right ventricular: ST elevation of 1 mm or more in V4R suggests right ventricular infarction, typically from proximal RCA occlusion, and nitrates and diuretics are then avoided.
- Posterior: ST depression in V1 to V3 with tall R waves; posterior leads V7 to V9 confirm the diagnosis by showing ST elevation.
- Wellens syndrome: deep symmetrical or biphasic T-wave inversion in V2 and V3 in a pain-free patient signals critical proximal LAD stenosis; stress testing is avoided and angiography is urgent.
- de Winter pattern: upsloping ST depression of 1 mm or more at the J point across V1 to V6, with tall peaked T waves, signals proximal LAD occlusion and is a STEMI equivalent demanding emergency PCI.
Reading a culprit from the leads, step by step
Consider a 60-year-old man with central pain for an hour, ST elevation of 2 mm in II, III and aVF, and 1 mm of ST depression in I and aVL.
- Name the territory: the inferior leads share one wall, and the reciprocal depression in lead I and aVL points to the right coronary artery.
- Consider the circumflex: in a left-dominant system the circumflex can supply the same territory, so the pattern is not always decisive. Lateral reciprocal change helps, but angiography settles it.
- Record V4R: inferior infarction from proximal RCA occlusion frequently extends to the right ventricle, and that finding changes management more than any other.
- Examine V7 to V9: an isolated posterior infarct produces no elevation in the standard leads and is easily missed.
- Compare with the old tracing: a new Q wave or altered T-wave pattern can date an infarct in a patient who presents late.
Each step alters the plan: right ventricular involvement makes nitrates dangerous, the territory identifies the artery for angiography, and posterior involvement changes how the trace is read.
Where the exam tries to catch you
Examiners set traps around reciprocity and lead placement.
- Trap: reading ST depression in V1 to V3 as anterior ischaemia. Correct: it actually points to posterior infarction, so record V7 to V9.
- Trap: treating ST elevation in aVR as a culprit pattern for one artery. Correct: widespread depression with aVR elevation suggests left main or three-vessel disease and demands urgent angiography.
- Trap: forgetting V4R in a crowded emergency department. Correct: V4R must be recorded deliberately, since it is easily left out when the department is busy.
- Trap: calling a de Winter pattern a non-STEMI on a hurried read. Correct: upsloping precordial depression with tall T waves is never benign, even without elevation, and it is a STEMI equivalent.
In a district hospital without a cardiologist, these subtle patterns are where a trainee's reading carries the greatest consequence.
Frequently asked questions
Which coronary artery supplies the territory of leads II, III and aVF?
The right coronary artery supplies the inferior wall in right-dominant circulation, which is the majority. In left-dominant systems the circumflex artery can supply the inferior wall instead, so the territory alone does not name the artery with certainty.
How is posterior myocardial infarction recognised on a standard ECG?
Posterior infarction shows ST depression in V1 to V3 with tall R waves, a mirror image of posterior ST elevation that a standard ECG cannot display directly. Posterior leads V7 to V9 confirm the diagnosis by showing ST elevation in those leads.
What does ST elevation in lead V4R indicate?
ST elevation of 1 mm or more in V4R indicates right ventricular infarction, usually from proximal RCA occlusion. Nitrates and diuretics are avoided in this setting because the right ventricle depends on preload.
What is a de Winter pattern on the ECG?
A de Winter pattern is upsloping ST depression of 1 mm or more at the J point across the precordial leads, accompanied by tall, symmetrical T waves. It indicates proximal LAD occlusion and is treated as a STEMI equivalent requiring emergency reperfusion.