A systematic way to read an ECG

When you're handed an ECG, it's tempting to scan for the dramatic finding. The trouble is that your eye goes to whatever stands out and skips the quieter abnormalities. A fixed, systematic order is the best protection against that.

Before you look at the waves

Check the basics first:

• Patient details and the time it was recorded. Is this the right patient, and is it the most recent trace?
• Calibration. Standard paper speed and voltage calibration, so your measurements mean what you think they mean.
• Clinical context. Why was it done? Chest pain, palpitations and a routine pre-operative check call for different levels of urgency.
• Previous ECGs. Comparison is one of the most useful things you can do. A finding that's new matters far more than one that's been there for years.

1. Rate

Estimate the heart rate. On a regular rhythm you can use the number of large squares between R waves; on an irregular rhythm, count the QRS complexes across the whole strip and multiply. Decide whether it's normal, slow (bradycardia) or fast (tachycardia).

2. Rhythm

Ask:

• Is it regular or irregular? If irregular, is it regularly irregular or irregularly irregular?
• Is there a P wave before every QRS, and a QRS after every P wave?
• Are the P waves a normal shape?

This is where you begin to separate sinus rhythm from atrial fibrillation, flutter and heart block.

3. Axis

Look at the overall direction of electrical activity, most simply using leads I and II (some people add aVF). Decide whether the axis is normal, deviated to the left or deviated to the right, then think about what could explain it.

4. P waves

Check their shape and size. Are they present at all? Are there more P waves than QRS complexes?

5. PR interval

Measure from the start of the P wave to the start of the QRS. A prolonged or variable PR interval points you towards conduction problems; a short one has its own differentials.

6. QRS complex

Look at:

• Width. A broad QRS suggests a conduction delay such as a bundle branch block, or a ventricular origin.
• Height. Unusually tall or small complexes can be significant.
• Q waves. Note where they are and whether they look pathological.

7. ST segment

Look for elevation or depression and, importantly, which leads are affected. Changes that follow the territory of a coronary artery mean something very different from widespread changes. ST changes always need to be read alongside the clinical picture.

8. T waves

Check for inversion, flattening or unusually peaked T waves, and again note the leads involved.

9. QT interval

Measure it and correct it for heart rate. A prolonged QT matters because it can be caused by medications and electrolyte disturbances, and it carries a risk of dangerous arrhythmias.

Put it into words

Finish with a short summary in a consistent format: "Sinus rhythm at a normal rate, normal axis, normal intervals, with ST depression in the lateral leads." Then add what you'd do with that information in context.

Make the system automatic

The order matters less than using the same order every time. Practise on normal ECGs as well as abnormal ones, because you can only spot what's unusual once you know what normal looks like. When you work through Sushruta cases that involve investigations, use the same habit: interpret the result in context, then decide whether it changes your diagnosis or plan.

This guide is for study and revision. Always interpret real ECGs with senior support and local guidance.