ECG Interpretation for Beginners: A Simple Step-by-Step Guide
Reading an ECG feels overwhelming at first, but almost every clinician uses the same simple, repeatable system. If you analyse every strip in the same order, you stop missing things. Here is a beginner-friendly approach you can apply to any 12-lead ECG.
1. Check the rate
On standard paper (25 mm/s), each large square is 0.2 seconds. The quickest method is the 300 rule: count the number of large squares between two R waves and divide 300 by it. Two large squares ≈ 150 bpm, three ≈ 100, four ≈ 75, five ≈ 60. A normal resting rate is 60–100 bpm; below 60 is bradycardia, above 100 is tachycardia.
2. Assess the rhythm
Is it regular or irregular? Mark out R-R intervals on a piece of paper and slide it along. Then ask: is there a P wave before every QRS, and a QRS after every P? Regular with normal P waves usually means sinus rhythm. An irregularly irregular rhythm with no clear P waves suggests atrial fibrillation.
3. Look at the P waves
Normal P waves are upright in lead II and precede each QRS. Absent, abnormal, or extra P waves point to atrial problems, blocks, or ectopic beats.
4. Measure the intervals
- PR interval (start of P to start of QRS): normal 120–200 ms (3–5 small squares). A long PR suggests first-degree heart block.
- QRS duration: normal under 120 ms (3 small squares). A wide QRS suggests a bundle branch block or a ventricular origin.
- QT interval: varies with rate; a prolonged QT raises the risk of dangerous arrhythmias.
5. Check the axis
The cardiac axis is the overall direction of depolarisation. A quick check: if the QRS is positive in both lead I and lead aVF, the axis is normal. Deviations can indicate conduction problems or ventricular hypertrophy.
6. Examine the QRS, ST segment and T waves
Look for tall or pathological Q waves, ST elevation or depression, and T-wave changes. ST elevation in a regional pattern is a red flag for acute myocardial infarction and is a clinical emergency — escalate immediately, do not rely on self-interpretation.
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What is the easiest way to calculate heart rate on an ECG?
Use the 300 rule: divide 300 by the number of large squares between two R waves. For irregular rhythms, count the QRS complexes on a 6-second strip and multiply by 10.
How do I tell if a rhythm is sinus?
Sinus rhythm has a P wave before every QRS, a QRS after every P, a normal PR interval, and upright P waves in lead II.
This article is for education only and is not medical advice. ECG interpretation in a real patient must be done by, or confirmed with, a qualified clinician and your local protocols.