ECG Interpretation for GPs: A Systematic Approach to the 12-Lead ECG
From rate and rhythm to STEMI recognition — a practical framework for primary care
The 12-lead ECG is one of the most powerful diagnostic tools in general practice. This systematic guide covers rate, rhythm, axis, P waves, PR interval, QRS complex, ST segments, and T waves — with a focus on the patterns GPs must not miss.
Clinical Decision Support: This article is for educational purposes and supports — not replaces — clinical judgment. Always verify with current national guidelines, BNF, and specialist consultation when needed.
The 12-lead ECG is one of the most information-dense diagnostic tools available to the GP. A systematic approach is essential — random pattern recognition leads to missed diagnoses. The mnemonic RRATE (Rate, Rhythm, Axis, Trace, Extras) provides a reliable framework for every ECG you interpret.
Step 1: Rate
The standard ECG runs at 25 mm/s with 1 mV = 10 mm. Each large square = 0.2 seconds; each small square = 0.04 seconds.
- Regular rhythm: 300 ÷ number of large squares between R waves (e.g., 4 large squares = 75 bpm)
- Irregular rhythm: Count QRS complexes in a 10-second strip and multiply by 6
- Normal: 60–100 bpm; Bradycardia: <60 bpm; Tachycardia: >100 bpm
Step 2: Rhythm
Is the Rhythm Regular or Irregular?
- Regular: Sinus rhythm, SVT, VT, complete heart block
- Regularly irregular: 2nd degree heart block (Mobitz I/II), bigeminy
- Irregularly irregular: Atrial fibrillation, multifocal atrial tachycardia, frequent ectopics
Sinus Rhythm Criteria
- P wave before every QRS
- P wave upright in leads I, II, aVF
- P wave inverted in aVR
- PR interval 0.12–0.20 seconds (3–5 small squares)
- QRS <0.12 seconds (3 small squares)
Step 3: Axis
| Axis | Lead I | Lead aVF | Clinical Significance |
|---|---|---|---|
| Normal (−30° to +90°) | Positive | Positive | Normal |
| Left axis deviation (−30° to −90°) | Positive | Negative | LBBB, left anterior fascicular block, inferior MI |
| Right axis deviation (+90° to +180°) | Negative | Positive | RBBB, right ventricular hypertrophy, PE, lateral MI |
| Extreme axis (−90° to +180°) | Negative | Negative | VT, hyperkalaemia, dextrocardia |
Step 4: P Waves and PR Interval
- Normal P wave: <0.12 s duration, <2.5 mm height, upright in I, II, aVF
- Broad bifid P wave (P mitrale): Left atrial enlargement — mitral stenosis
- Tall peaked P wave (P pulmonale): Right atrial enlargement — COPD, pulmonary hypertension
- PR interval >0.20 s: 1st degree heart block
- Progressive PR lengthening then dropped QRS: Mobitz I (Wenckebach)
- Fixed PR with dropped QRS: Mobitz II (more serious — risk of complete block)
- No relationship between P and QRS: Complete (3rd degree) heart block
Step 5: QRS Complex
Bundle Branch Blocks
| Feature | LBBB | RBBB |
|---|---|---|
| QRS duration | >0.12 s | >0.12 s |
| V1 pattern | rS or QS (deep S) | RSR' (M-shaped, "rabbit ears") |
| V6 pattern | Broad R, no Q, no S | qRS (broad S wave) |
| Clinical significance | New LBBB = treat as STEMI equivalent | Often benign; can indicate RV strain or PE |
New LBBB in the context of chest pain should be treated as a STEMI equivalent — activate the primary PCI pathway immediately. Do not wait for troponin results.
Step 6: ST Segments and T Waves
STEMI Recognition
- ST elevation ≥1 mm in ≥2 contiguous limb leads, or ≥2 mm in ≥2 contiguous precordial leads
- Inferior STEMI: ST elevation in II, III, aVF (right coronary artery)
- Anterior STEMI: ST elevation in V1–V4 (LAD)
- Lateral STEMI: ST elevation in I, aVL, V5–V6 (circumflex)
- Posterior STEMI: ST depression in V1–V3 with tall R waves (mirror image)
Other ST/T Wave Changes
- Widespread ST depression + T wave inversion: NSTEMI, demand ischaemia
- Saddle-shaped ST elevation: Pericarditis (all leads except aVR and V1)
- Deep symmetrical T wave inversion V1–V4: Wellens syndrome (critical LAD stenosis — do not stress test!)
- Tall peaked T waves: Hyperkalaemia (early sign)
- Prolonged QTc (>440 ms men, >460 ms women): Drug-induced, hypokalaemia, congenital LQTS
Key Clinical Takeaways
- Use RRATE: Rate, Rhythm, Axis, Trace, Extras — every ECG, every time
- New LBBB with chest pain = STEMI equivalent — activate PCI pathway
- Irregularly irregular rhythm with no P waves = AF until proven otherwise
- Wellens syndrome (deep T inversion V1–V4) = critical LAD stenosis — do not discharge
- Saddle-shaped ST elevation in all leads = pericarditis (not STEMI)
- Tall peaked T waves = hyperkalaemia — check potassium urgently