12-Lead ECG Interpretation
The 12 leads photograph the heart from twelve angles. Learn what each group sees so you can localise ischaemia and infarction at a glance.
The 12 leads: limb and precordial
The 12-lead ECG combines 6 limb leads (I, II, III, aVR, aVL, aVF) that view the heart in the vertical (frontal) plane and 6 precordial leads (V1–V6) that view it in the horizontal plane. Together they give a 3-D-ish picture from twelve directions.
Lead groups and their territory
| Lead group | Wall | Coronary artery |
|---|---|---|
| II, III, aVF | Inferior | RCA (usually) |
| V1–V2 | Septal | LAD |
| V3–V4 | Anterior | LAD |
| I, aVL, V5–V6 | Lateral | Circumflex |
| V1–V2 (tall R, ST depression) | Posterior (reciprocal) | RCA / circumflex |
Reciprocal changes
An area of ST elevation is often mirrored by ST depression in the opposite leads — a reciprocal change. Inferior STEMIs (II, III, aVF) show reciprocal depression in I and aVL; true posterior MIs show tall R waves and ST depression in V1–V2. Reciprocal changes increase confidence that ST elevation is a real infarct.
Reading the axis from the limb leads
| Lead I | Lead aVF | Axis | Think |
|---|---|---|---|
| Up | Up | Normal | — |
| Up | Down | Left axis deviation | LVH, LAFB, inferior MI |
| Down | Up | Right axis deviation | RVH, PE, lateral MI |
| Down | Down | Extreme axis | VT, hyperkalemia |
A quick 12-lead reading routine
- Confirm calibration, rate, and rhythm.
- Scan each lead group for ST elevation or depression.
- Check the axis with leads I and aVF.
- Look for Q waves, bundle branch blocks, and T-wave changes.
- Compare with a prior ECG if available.
Ground the basics first in the step-by-step ECG interpretation guide.
Summary
- 6 limb + 6 precordial leads view the heart from 12 angles.
- Memorise the lead-group → wall → artery map.
- Reciprocal ST depression supports a true STEMI.
- Axis: I and aVF both up = normal.
Practise 12-lead concepts
Test ECG and MI-localization questions with instant feedback.
Practise ECG →Frequently asked questions
What does each ECG lead show?
Limb leads (I, II, III, aVR, aVL, aVF) view the frontal plane; precordial leads (V1–V6) view the horizontal plane. Grouped, they localise disease: inferior (II, III, aVF), septal (V1–V2), anterior (V3–V4), lateral (I, aVL, V5–V6).
How do you localise a STEMI on a 12-lead ECG?
Find the lead group with ST elevation: inferior = II, III, aVF (RCA); anteroseptal = V1–V4 (LAD); lateral = I, aVL, V5–V6 (circumflex).
What are reciprocal changes?
ST depression in leads opposite the area of ST elevation; they support a genuine infarction.
How do you read ECG axis quickly?
Look at leads I and aVF: both upright is a normal axis; I up with aVF down is left axis deviation; I down with aVF up is right axis deviation.
Sources & further reading
- Cardiovascular Credentialing International (CCI)
- American College of Cardiology
- American Heart Association
- MedlinePlus (U.S. National Library of Medicine)
External links are provided for reference; always confirm current details with the official source.