Introduction to ECGs

Before interpreting an ECG, always confirm the basics: ✅ Correct patient name and hospital/NHS number ✅ Correct date and time ✅ Speed (usually 25 mm/s) and gain (10 mm/mV) are clearly marked ✅ Ensure the ECG is technically adequate (no major artefact, all leads visible) ✅ Always review in clinical context — symptoms, medications, electrolytes, and comparison with previous ECGs.

Normal ECG showing sinus rhythm with standard calibration.

Another normal trace, demonstrating upright P waves and progressive R-wave amplitude across the precordial leads.

✅ Features of a Normal ECG

A normal ECG (above) demonstrates regular sinus rhythm, rate around 82 bpm, and normal intervals throughout. All deflections and segments are within physiological limits.

🧭 Stepwise ECG Interpretation (UK Clinical Method)

Adopt a structured, reproducible approach each time you interpret an ECG:

1. 👤 Check the patient details — name, date of birth, hospital/NHS number, and time recorded. Mislabelled ECGs can lead to catastrophic error.
2. ⚙️ Check calibration — 25 mm/s speed and 10 mm/mV gain (1 mV = 10 mm vertically).
3. 💓 Rate — Regular rhythm: 300/150/100/75/60/50 big-box method. Irregular rhythm: count QRS in 10 s ×6.
4. 🔄 Rhythm — Is there a P wave before every QRS? Constant PR interval? Sinus rhythm = P upright in I, II, aVF and inverted in aVR.
5. 🧭 Cardiac axis — Positive I and aVF → normal axis (−30° to +90°). I positive, aVF negative → left axis. I negative, aVF positive → right axis.
6. 📏 Intervals — PR 120–200 ms; QRS ≤120 ms; QTc <440 ms (men) or <460 ms (women). Correct QT for rate (Bazett or Fridericia).
7. 📊 Waves and segments — Evaluate each: P, QRS, ST, T, and U. Check morphology, amplitude, and concordance.
8. 🩸 Compare — Always compare with old ECGs and correlate with symptoms and troponin results.

📏 Normal ECG Parameters

🔍 Key Waveform Components

1️⃣ P wave: Represents atrial depolarisation. Normal ≤0.11 s; smooth, rounded, upright in I, II, aVF. Bifid P = left atrial enlargement (P mitrale); peaked P = right atrial enlargement (P pulmonale).

2️⃣ PR interval: Measured from start of P to start of QRS. Normal 0.12–0.20 s. Prolonged = delayed AV conduction; short = accessory pathway (e.g. WPW).

3️⃣ QRS complex: Represents ventricular depolarisation. Normal ≤0.12 s. Look for bundle branch block patterns and R-wave progression V1→V6.

4️⃣ ST segment: Represents early ventricular repolarisation. Normally flat (isoelectric). ST ↑ > 1 mm in two contiguous leads → consider STEMI. ST ↓ > 0.5 mm → ischaemia or reciprocal change.

5️⃣ T wave: Represents ventricular repolarisation. Asymmetric, upright in most leads (except aVR, V1). Inverted T = ischaemia, strain, PE, or normal variant.

6️⃣ QT interval: Represents total ventricular depolarisation + repolarisation. Normal ≤0.44 s (M) or ≤0.46 s (F). Prolonged QT → risk of torsades (think hypokalaemia, macrolides, antipsychotics). Short QT → hypercalcaemia, digoxin effect.

⚡ Recognising Abnormal Patterns

• Ischaemia / STEMI ❤️‍🔥 — Localised ST elevation, reciprocal depression, new Q waves, evolving T inversion.
• Pericarditis ♨️ — Widespread concave ST elevation + PR depression; no reciprocal change.
• Hyperkalaemia 🔺 — Tall peaked T, PR prolongation, QRS widening → sine-wave.
• Hypokalaemia 🔻 — Flat T, U waves, prolonged QT.
• LVH / RVH 💪 — High voltage, strain pattern (ST depression, T inversion opposite QRS direction).
• Bundle Branch Blocks — RBBB (rsR′ V1), LBBB (broad notched R V6). LBBB + chest pain → treat as STEMI (until proven otherwise).
• PE (S1Q3T3) 🫁 — S in I, Q and T inversion in III; sinus tachycardia; right axis deviation.

🧠 Common UK Exam & OSCE Tips

• Start with: “This ECG is recorded at 25 mm/s and 10 mm/mV. It shows sinus rhythm at 78 bpm with a normal axis.”
• Describe the abnormality last, and conclude with likely cause: “…features of left ventricular hypertrophy with secondary repolarisation change, likely due to longstanding hypertension.”
• Always state the rate, rhythm, axis, intervals, and key wave/segment findings in OSCEs.
• Look for artefact, baseline wander, or poor lead placement before over-interpreting.
• Document serial ECGs — trend changes over time are diagnostically powerful. 📈

🧩 Rapid ECG Reference (Summary Card)

Checklist: ID → Calibration → Rate → Rhythm → Axis → Intervals → Morphology → ST/T → Comparison.
PR 120–200 ms • QRS ≤120 ms • QTc <440/460 ms • Axis −30°→+90° • ST isoelectric • Sinus rhythm = P upright I, II, aVF; negative aVR.
Always treat the patient, not the ECG.