ST elevation in contiguous leads with reciprocal depression. Lead groups map to coronary territories.
Why this is true
Each ECG lead looks at the heart from a fixed angle, so a transmurally injured wall produces elevation in the leads facing it and depression in those facing the opposite wall. Because coronary supply is territorial, the group of leads showing elevation names the occluded artery. The clinical importance is not academic: an inferior infarct is right coronary in most people, and the same artery supplies the right ventricle and the SA/AV nodes — which is why inferior infarcts cause bradycardia and become dangerously preload-dependent.
Key points
- II, III, aVF → inferior → right coronary artery. Get right-sided leads (V4R) to check for RV involvement.
- V1–V4 → anteroseptal → left anterior descending.
- I, aVL, V5–V6 → lateral → left circumflex.
- Tall R with ST depression in V1–V2 → posterior infarct; confirm with posterior leads V7–V9.
- New left bundle branch block with ischaemic symptoms is treated as a STEMI equivalent.
- Door-to-balloon target is 90 minutes for primary PCI; thrombolyse if PCI is unavailable within 120 minutes.
Cardiac biomarkers
| Marker | Rises | Peaks | Returns to normal |
|---|---|---|---|
| High-sensitivity troponin | 2–3 h | 24–48 h | 7–10 days |
| CK-MB | 4–6 h | 24 h | 48–72 h |
| Myoglobin | 1–2 h | 6–8 h | 24 h |
Common traps
- Nitrates in a right ventricular infarct cause profound hypotension — the RV needs preload. Give fluids first.
- CK-MB is the marker for *re*infarction within 72 hours, because troponin is still elevated from the first event.
Test yourself
A man with inferior ST elevation becomes hypotensive minutes after sublingual nitroglycerin. Lungs are clear and JVP is raised.
High-yield
Inferior MI with hypotension after nitrates suggests RV infarction — give fluids.