ST elevation questions are the closest thing Step exams have to a reaction-time sport: you either recognize the pattern in 5 seconds or you start negotiating with yourself about “maybe it’s pericarditis.” This post is a quick-hit, shareable set of STEMI localization patterns + culprit arteries + key ECG clues, with a simple mental “map” you can pull up fast.
The “5‑Second Rule” for ST Elevation: Look, Localize, Link
In 5 seconds, do these three moves:
- Look: Is there ST elevation in contiguous leads (same anatomic region)?
- Localize: Infer the wall (inferior, anterior, lateral, posterior, right ventricle).
- Link: Match to the likely culprit artery (and the classic “gotchas”).
USMLE rule of thumb: ST elevation in ≥2 contiguous leads suggests acute transmural injury (STEMI) until proven otherwise (clinical context matters).
Visual/Mnemonic Device: “Smile, Frown, Mirror”
A fast mental picture that works on test day:
- Smile = Inferior (II, III, aVF look like a “smile” along the bottom of the 12‑lead layout)
- Frown = Anterior (V1–V4 arch across the front like a “frown”)
- Mirror = Posterior (posterior MI is a mirror image on V1–V3: ST depression + tall R)
One-liner: Inferior smiles, anterior frowns, posterior hides in the mirror.
STEMI Patterns at a Glance (High-Yield Table)
| Territory | ST Elevation Leads | Reciprocal Changes (common) | Culprit Artery (most likely) | One-liner |
|---|---|---|---|---|
| Inferior | II, III, aVF | ST ↓ in I, aVL | RCA (most), sometimes LCX | “Bottom leads light up; look for lateral reciprocals.” |
| Lateral (high lateral) | I, aVL | ST ↓ in III, aVF | LCX or diagonal of LAD | “High lateral = I/aVL; inferior reciprocals help.” |
| Lateral (low lateral) | V5–V6 | Variable | LCX | “V5–V6 are your lateral chest leads.” |
| Anterior / Septal | V1–V4 (V1–V2 septal, V3–V4 anterior) | ST ↓ in inferior leads | LAD | “Front of the heart = LAD until proven otherwise.” |
| Extensive anterior | V1–V6 + I + aVL | Often ST ↓ inferior | Proximal LAD (big deal) | “Widespread precordial elevation = proximal LAD = high risk.” |
| Posterior | ST ↓ in V1–V3, tall R in V1–V3, upright T | (True ST ↑ in V7–V9 if placed) | RCA or LCX | “Posterior MI shows up as anterior ST depression with a big R.” |
| Right ventricular (RV) infarct | ST ↑ in V4R (right-sided), often with inferior MI | — | Proximal RCA | “Inferior MI + hypotension? Check V4R and avoid preload killers.” |
The 5-Second “Localization Script” (What to Say in Your Head)
1) Inferior STEMI (II, III, aVF)
Pattern: ST ↑ in II, III, aVF
Quick artery clue: If III > II, think RCA.
One-liner: “Inferior elevation—check I/aVL for reciprocals, suspect RCA if III > II.”
USMLE add-ons
- RCA also supplies AV node → may see bradycardia or AV block.
- Inferior MI + hypotension + clear lungs → suspect RV infarct.
2) Anterior/Septal STEMI (V1–V4)
Pattern: ST ↑ in V1–V4
Culprit: LAD
One-liner: “V1–V4 is LAD—big territory, high risk of pump failure.”
USMLE add-ons
- LAD infarcts can lead to acute LV failure, cardiogenic shock, and malignant ventricular arrhythmias.
- V1–V2 elevation can be septal (LAD septal perforators) and is also where some “mimics” show up—use clinical context.
3) Lateral STEMI (I, aVL, V5–V6)
Pattern: ST ↑ in I, aVL, V5–V6
Culprit: LCX (often), sometimes diagonal branch of LAD
One-liner: “Lateral elevation—think LCX (or diagonal LAD) and look for inferior reciprocals.”
4) Posterior MI (the “Mirror MI”)
Pattern on standard 12‑lead:
- ST depression in V1–V3
- Tall R waves in V1–V3 (mirror of Q waves)
- Often upright T waves
Confirm (if they mention extra leads): ST ↑ in V7–V9
One-liner: “Posterior MI hides as anterior ST depression with tall R—flip the tracing in your head.”
5) Right Ventricular MI (Don’t Miss This One)
When to suspect: Inferior STEMI + hypotension + clear lungs ± JVD
Key lead: V4R (right-sided ECG)
One-liner: “Inferior MI with shock? Get right-sided leads—RV infarct needs preload.”
USMLE management pearl
- Avoid nitrates and aggressive diuresis in RV infarct (they drop preload and can worsen hypotension).
- Treat with IV fluids (carefully) + reperfusion.
“Reciprocal Changes” = Free Points
Reciprocal ST depression strongly supports true STEMI (vs benign early repolarization or pericarditis).
Common pairings:
- Inferior STEMI (II, III, aVF) ↔ reciprocal ST ↓ in I, aVL
- High lateral STEMI (I, aVL) ↔ reciprocal ST ↓ in III, aVF
- Posterior MI: reciprocal idea in reverse—posterior ST ↑ becomes anterior ST ↓ (V1–V3)
STEMI Mimics (How Not to Get Tricked)
You don’t need to diagnose every mimic—just know the most testable differentiators.
| Mimic | ECG Hint | Clinical Hint |
|---|---|---|
| Acute pericarditis | Diffuse concave ST ↑ + PR depression; no true reciprocal changes except aVR/V1 | Pleuritic chest pain, better leaning forward |
| Early repolarization | Concave ST ↑, prominent T waves, stable over time | Young, healthy, asymptomatic |
| LVH / LBBB | ST-T “discordance” (repolarization abnormalities) | Hx HTN, wide QRS (LBBB) |
| Hyperkalemia | Peaked T waves → QRS widening → sine wave | Weakness, renal failure, meds |
USMLE shortcut: Regional, contiguous ST elevation + reciprocal depression + ischemic symptoms = STEMI until proven otherwise.
Ultra-High-Yield “Culprit Artery” Hooks
- LAD: anterior wall, septum (V1–V4) → pump failure
- RCA: inferior wall (II, III, aVF) + AV node → brady/heart block
- LCX: lateral wall (I, aVL, V5–V6) (and can do posterior)
5-Second Practice Drill (Do This Every Time)
When you see ST elevation, force this checklist:
- Which contiguous leads are up? (Inferior vs anterior vs lateral)
- Are there reciprocal changes?
- Could this be posterior? (V1–V3 ST depression + tall R)
- Inferior MI present? If yes, consider RV infarct → check V4R
Mini Summary (Screenshot-Friendly)
- II, III, aVF = Inferior (usually RCA; III > II favors RCA)
- V1–V4 = Anterior/Septal (LAD)
- I, aVL, V5–V6 = Lateral (LCX/diagonal)
- V1–V3 ST ↓ + tall R = Posterior (confirm with V7–V9)
- Inferior MI + hypotension + clear lungs = RV infarct (get V4R, avoid nitrates)