Localising an MI from the ECG
Maps ST-elevation lead groups to the myocardial territory and culprit coronary artery, with the reciprocal-change, posterior and right-ventricular traps that catch candidates.
The leads tell you where — and which artery
A STEMI is diagnosed by ST elevation in two contiguous leads. Which leads localises the infarct, and the territory names the culprit vessel.
| Territory | ST-elevation leads | Culprit artery | Reciprocal change |
|---|---|---|---|
| Anteroseptal | V1–V2 | LAD (proximal / septal branches) | — |
| Anterior / apical | V3–V4 | LAD | — |
| Anterolateral | V3–V6, I, aVL | LAD (or diagonal) | II, III, aVF |
| Extensive anterior | V1–V6, I, aVL | Proximal LAD | II, III, aVF |
| Lateral (high) | I, aVL | LCx or diagonal | II, III, aVF |
| Inferior | II, III, aVF | RCA (~80%) or LCx | I, aVL |
| Posterior | V7–V9 (tall R + ST↓ V1–V3) | RCA or LCx | Anterior leads |
| Right ventricle | V4R (ST↑) | Proximal RCA | — |
Reading the culprit in inferior MI: ST elevation greater in lead III than lead II, with reciprocal depression in I and aVL, points to the RCA. If lead II ≥ III with lateral involvement, suspect the left circumflex.
The three traps
1. Reciprocal changes are a clue, not a second infarct
ST depression in the leads electrically opposite the infarct (e.g. I/aVL depression in an inferior STEMI) confirms a true transmural event — it increases specificity. Do not misread it as a separate ischaemic zone.
2. Posterior MI hides as anterior ST depression
There are no standard posterior leads. Horizontal ST depression in V1–V3 with tall R waves and upright T waves is the mirror image of posterior ST elevation. Confirm with posterior leads V7–V9. Often accompanies an inferior or lateral MI.
3. Right-ventricular infarct — the one that makes nitrates dangerous
Suspect it in any inferior STEMI. Record right-sided leads; ST elevation in V4R is the most specific sign. The RV-infarct patient is preload-dependent — nitrates and diuretics cause profound hypotension. Treat with fluids, not vasodilators.
Pearl: Every inferior STEMI earns a V4R and a blood pressure check before any nitrate — miss the RV and the first GTN spray drops the patient.
Anchor
Anterior = LAD, lateral = LCx/diagonal, inferior = RCA. Inferior ST elevation → look right (V4R) and look behind (V7–V9) before you treat.
Sources
- Harrison's Principles of Internal Medicine, 21e
- Braunwald's Heart Disease, 12e