Medical reference
12-lead ECG basics.
What each lead sees, where they go, and how to localize a STEMI in the field.
Anatomical view by lead group
| Wall | Leads | Likely artery | Field note |
|---|---|---|---|
| Inferior | II, III, aVF | Right coronary artery (RCA) | Watch for RV involvement and bradycardia; nitro cautiously. |
| Septal | V1, V2 | Left anterior descending (LAD) | Often paired with anterior changes. |
| Anterior | V3, V4 | LAD | Large anterior MI carries a high mortality — 'widow maker'. |
| Lateral | I, aVL, V5, V6 | Left circumflex (LCX) | Often subtle; look for reciprocal inferior changes. |
| Posterior | V7–V9 (or reciprocal V1–V3) | RCA / LCX | Tall R waves and ST depression in V1–V3 suggest posterior MI. |
Precordial lead placement
- V1 — 4th intercostal space, right sternal border.
- V2 — 4th intercostal space, left sternal border.
- V4 — 5th intercostal space, left midclavicular line.
- V3 — midway between V2 and V4.
- V5 — anterior axillary line, same horizontal plane as V4.
- V6 — midaxillary line, same horizontal plane as V4.
Right-sided leads (V4R) go in the mirror-image location on the right chest — useful when inferior STEMI is present to detect RV infarct.
STEMI recognition in 5 steps
- Confirm rate, rhythm, and axis are interpretable — check for artifact first.
- Scan for ST elevation in ≥ 2 contiguous leads.
- Group the elevated leads by anatomical region to name the wall.
- Look for reciprocal ST depression opposite the affected wall.
- Correlate with symptoms and transport to a PCI-capable facility.
Common trap
Left bundle branch block, LVH, early repolarization, and pericarditis all mimic STEMI. When in doubt, transmit the ECG and let the receiving physician confirm activation.
FAQ
How is a 12-lead ECG different from a 3-lead?
A 3-lead monitors rate and rhythm from a limited view. A 12-lead adds precordial (chest) leads and augmented limb leads so you can localize ischemia and injury to a specific wall of the heart.
What defines a STEMI on a 12-lead?
New ST-elevation ≥ 1 mm in two or more contiguous limb leads, or ≥ 2 mm in two contiguous precordial leads (V2–V3 has sex-specific thresholds), plus a clinical picture consistent with ACS.
Where do the precordial leads go?
V1 = 4th intercostal space, right sternal border. V2 = 4th ICS, left sternal border. V4 = 5th ICS, midclavicular line. V3 sits between V2 and V4. V5 = anterior axillary line at V4's level. V6 = midaxillary line at V4's level.
What are reciprocal changes?
ST depression in leads opposite the injured wall. Inferior STEMI often shows reciprocal depression in I and aVL. Posterior MI shows reciprocal depression in V1–V3.
When should EMS get a 12-lead?
Any adult with chest pain, dyspnea, syncope, weakness, or an ACS-equivalent presentation (especially diabetics, elderly, and women). Repeat serial ECGs if symptoms persist or change.
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