Reading 12-Lead ECGs: A Practical Framework for Junior Doctors

Confident ECG interpretation is one of the most useful skills a junior doctor can develop. A reproducible, step-by-step framework prevents missed diagnoses and helps you communicate findings clearly during ward rounds and handover. This guide walks through a practical seven-step approach that you can apply to any 12-lead ECG.

Why a structured approach matters

Pattern recognition alone breaks down under pressure. A structured method ensures that subtle but life-threatening findings — such as a posterior STEMI, Wellens’ pattern, or hyperkalaemia — are not missed because you jumped to the most obvious feature. The framework below should take less than two minutes once you have practised it.

Step 1: Confirm patient details and calibration

Check the name, date and time, and whether standard calibration applies (10 mm/mV, 25 mm/s). Mis-calibrated tracings can mimic LVH or low voltage. Always compare with prior ECGs when available.

Step 2: Rate

For regular rhythms, count large squares between two R waves and divide 300 by that number (300/1 = 300, 300/2 = 150, 300/3 = 100, 300/4 = 75, 300/5 = 60, 300/6 = 50). For irregular rhythms, count QRS complexes on the rhythm strip and multiply by six.

Step 3: Rhythm

• Is the rhythm regular or irregular?
• Is there a P wave before every QRS, and a QRS after every P wave?
• Are P waves upright in lead II (suggesting sinus origin)?
• Is the QRS narrow (<120 ms) or broad?

Step 4: Axis

A quick axis check uses leads I and aVF. Both upright = normal axis. Lead I up, aVF down = possible left axis deviation. Lead I down, aVF up = right axis deviation. Both negative = extreme axis. Marked left axis deviation suggests left anterior fascicular block; right axis deviation can indicate pulmonary embolism, RVH, or lateral MI.

Step 5: Intervals

• PR interval (normal 120-200 ms): prolonged in first-degree AV block; short with pre-excitation.
• QRS duration (normal <120 ms): broadened by bundle branch block, hyperkalaemia, ventricular rhythms.
• QT/QTc: prolongation increases risk of torsades. Use Bazett’s formula and watch for drug effects.

Step 6: Morphology — P, QRS, ST, T

P waves

Tall, peaked P waves in lead II suggest right atrial enlargement; broad, notched P waves suggest left atrial enlargement.

Q waves

Pathological Q waves are >40 ms wide or >25% of the R-wave amplitude in the same lead, and indicate prior infarction.

ST segment and T waves

Look for ST elevation (acute injury), ST depression (ischemia, posterior MI when in V1-V3), T-wave inversion, and hyperacute T waves. Always check reciprocal changes — for example, inferior STEMI with reciprocal depression in I and aVL.

Step 7: Compare and conclude

Compare with the patient’s previous ECG if available, then write a one-line summary: rate, rhythm, conduction, axis, and any acute findings. Example: “Sinus rhythm at 88 bpm, normal axis, normal intervals, 2 mm ST elevation in II/III/aVF with reciprocal depression in I/aVL — inferior STEMI.”

Worked example

A 64-year-old man presents with central chest pain. The ECG shows a regular rhythm at 72 bpm, sinus P waves, normal PR and QRS, and 1.5 mm ST elevation in V2-V4 with biphasic T waves. Recognising this Wellens-like pattern as a sign of critical proximal LAD stenosis is what changes management — early cardiology review rather than waiting for a troponin to rise.

Practising on real cases

The fastest way to build pattern recognition is to review hundreds of real tracings with explanations. Our ECG Atlas app contains a structured library of real ECG cases with interpretations designed for medical students and junior doctors.

Key takeaways

• Always use the same seven-step approach: details, rate, rhythm, axis, intervals, morphology, conclusion.
• Compare with prior tracings whenever possible.
• Look for reciprocal changes when you see ST elevation.
• Consider posterior MI when V1-V3 show ST depression with tall R waves.
• Practise on real cases until the framework is automatic.