An ECG looks intimidating because everything arrives at once: twelve leads, dozens of waves and a busy grid. The trick is to stop looking for the answer and follow the same steps every time. Pattern recognition comes later, built on a reliable system.
The ten steps
- Check the details. Right patient, date and time, and standard calibration: 25 mm/s and 10 mm/mV. A wrong speed or gain changes everything you measure.
- Rate. For a regular rhythm, divide 300 by the number of large squares between two R waves. For an irregular rhythm, count the complexes on the 10-second strip and multiply by 6.
- Rhythm. Is it regular? Is there a P wave before every QRS and a QRS after every P? If yes, it is sinus rhythm. An irregularly irregular rhythm without clear P waves suggests atrial fibrillation.
- Axis. If the QRS is mainly positive in lead I and in aVF, the axis is normal. Positive in I but negative in aVF is left axis deviation; negative in I and positive in aVF is right axis deviation.
- P waves. Tall P waves suggest right atrial enlargement; broad, notched P waves suggest left atrial enlargement.
- PR interval. Normal is 120–200 ms (3–5 small squares). Long: first-degree heart block. Short with a slurred upstroke (delta wave): pre-excitation.
- QRS complex. Width over 120 ms suggests a bundle branch block or a ventricular rhythm. Look at voltage (left ventricular hypertrophy) and for pathological Q waves (old infarction).
- ST segments. Elevation or depression, in which leads, and is there a reciprocal change? ST elevation in two contiguous leads with reciprocal depression suggests acute coronary occlusion.
- T waves. Tall, peaked T waves suggest hyperkalaemia; inversion can mean ischaemia, strain or a normal variant in some leads.
- QT interval. Correct it for heart rate (QTc). A long QTc increases the risk of torsades de pointes; check medications and electrolytes.
Map the territory
- Inferior (II, III, aVF): usually the right coronary artery.
- Anterior (V1–V4): the left anterior descending artery.
- Lateral (I, aVL, V5–V6): the circumflex or diagonal branches.
- Posterior: ST depression in V1–V3; confirm with leads V7–V9.
With an inferior infarct, record right-sided leads (V4R) to look for right ventricular involvement before giving nitrates.
Patterns you must not miss
- ST elevation myocardial infarction, including posterior infarction.
- Complete heart block.
- Ventricular tachycardia (a regular broad-complex tachycardia is VT until proven otherwise).
- Hyperkalaemia.
- Pre-excited atrial fibrillation (irregular, broad and very fast).
- Long QT and the Brugada pattern.
Practise it
Read every ECG out loud using the same ten steps until it is automatic. Then test yourself: the [ECG read outline](/library/ecg-read) and the cardiovascular questions in the [question bank](/qbank) put the steps to work.
Written by Medverion Editorial. Educational content only; not medical advice.
