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12-lead ECG interpretation diagram
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ECG waveform components P wave QRS T wave labeled diagram
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How to Read a 12-Lead ECG — A Systematic Approach
The ECG Paper
Standard ECG paper runs at 25 mm/sec:
- Small square = 1 mm = 0.04 sec
- Large square = 5 mm = 0.2 sec
- Amplitude: 10 mm = 1 mV
The Waveform Components
| Wave/Segment | Represents |
|---|---|
| P wave | Atrial depolarization |
| PR interval | AV conduction time (start of P → start of QRS) |
| QRS complex | Ventricular depolarization |
| ST segment | Early ventricular repolarization (J point → T wave) |
| T wave | Ventricular repolarization |
| QT interval | Total ventricular electrical activity |
| U wave | Afterpotentials (prominent in hypokalemia) |
The 8-Step Systematic Approach
Step 1 — Rate
Method: Count the number of large boxes between two R waves, then divide into 300.
- 1 box = 300 bpm | 2 boxes = 150 | 3 boxes = 100 | 4 boxes = 75 | 5 boxes = 60
Normal adult HR: 60–100 bpm
Step 2 — Rhythm
- Sinus rhythm: Every QRS is preceded by a P wave; P wave is upright in leads I and aVF
- Normal respiratory variation in R-R interval (sinus arrhythmia) is expected
- Look for: regular vs. irregular, P before every QRS, QRS after every P
Step 3 — Axis
The QRS axis describes the average direction of ventricular depolarization:
| Axis | Leads I & aVF |
|---|---|
| Normal (−30° to +90°) | Both upright |
| Left axis deviation (LAD) | I upright, aVF negative |
| Right axis deviation (RAD) | I negative, aVF upright |
| Extreme / NW axis | Both negative |
Causes of LAD: LBBB, left anterior fascicular block, inferior MI
Causes of RAD: RVH, RBBB, lateral MI, PE, left posterior fascicular block
Causes of RAD: RVH, RBBB, lateral MI, PE, left posterior fascicular block
Step 4 — Intervals
| Interval | Normal (adults) | Abnormal if... |
|---|---|---|
| PR | 0.12–0.20 sec (3–5 small boxes) | Short = pre-excitation (WPW); Long = heart block |
| QRS | < 0.12 sec (< 3 small boxes) | Wide = BBB, ventricular rhythm, hyperkalemia |
| QTc | ≤ 0.44 sec (males), ≤ 0.46 sec (females) | Prolonged = drugs, electrolytes, channelopathies |
QTc formula (Bazett):
QTc = QT (sec) / √(R-R interval in sec)
Step 5 — P Wave Morphology
- Normal: < 0.12 sec wide, < 2.5 mm tall, upright in I, II, aVF; inverted in aVR
- Broad, notched P (P mitrale) → Left atrial enlargement
- Tall, peaked P (P pulmonale > 2.5 mm in II) → Right atrial enlargement
Step 6 — QRS Morphology
R-wave progression (V1→V6):
- R waves should progressively increase, S waves decrease
- R/S = 1 (transition zone) normally at V3–V4
- Poor R-wave progression → anterior MI or LVH
Bundle Branch Blocks (BBB) — requires QRS ≥ 0.12 sec:
- RBBB: rSR' pattern in V1 ("rabbit ears"), wide S in I & V6 → "W in V1, M in V6" (opposite)
- LBBB: broad, notched R in I, V5, V6; deep QS in V1 → "M in V1, W in V6" (opposite)
Step 7 — ST Segment & T Waves
| Finding | Think of... |
|---|---|
| ST elevation (≥ 1 mm) | STEMI, pericarditis (saddle-shaped, diffuse), Brugada, early repolarization |
| ST depression | NSTEMI, ischemia, digoxin effect ("reverse tick") |
| T wave inversion | Ischemia, LBBB, RVH strain, PE (V1–V4), normal in aVR, V1 |
| Tall peaked T waves | Hyperkalemia, hyperacute STEMI |
| Flat/inverted T waves | Ischemia, hypokalemia, hypothyroidism |
Step 8 — Overall Interpretation
After the above steps, ask:
- Is there a chamber enlargement (atrial or ventricular)?
- Is there evidence of infarction (Q waves, ST/T changes by territory)?
- Is there a conduction abnormality (BBB, hemiblock, WPW, AV block)?
- Are there arrhythmia clues?
12-Lead Anatomy — Which Leads Look Where?
| Territory | Leads | Artery |
|---|---|---|
| Inferior | II, III, aVF | RCA |
| Lateral | I, aVL, V5, V6 | LCx |
| Anterior | V1–V4 | LAD |
| Septal | V1, V2 | LAD (septal perforators) |
| Posterior | Reciprocal changes in V1–V3 (tall R, ST depression) | RCA / LCx |
Quick Memory Aid
"Rate, Rhythm, Axis, Intervals, P waves, QRS, ST-T, Overall"
This systematic approach ensures you never miss a finding. — Harriet Lane Handbook / Braunwald's Heart Disease
Want to go deeper into any specific step — e.g., STEMI patterns, bundle branch blocks, axis interpretation, or a specific arrhythmia?
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