Al HaYaT MedicosMany students find ECGs intimidating because they try to "spot" the diagnosis. A better way is to read every ECG in the same order. This systematic approach takes less than two minutes once practised, and it stops you missing important findings.
Before you start: check the basics
- Confirm the patient's name, the date and the time of the ECG.
- Check the paper speed (standard 25 mm/s) and calibration (standard 10 mm per mV).
- Note the clinical context — for example, chest pain or palpitations.
On standard ECG paper at 25 mm/s, one small square is 0.04 seconds and one large square (five small squares) is 0.2 seconds.
Step 1: Heart rate
- For a regular rhythm: divide 300 by the number of large squares between two R waves (or 1500 by the number of small squares).
- For an irregular rhythm: count the QRS complexes on a 10-second rhythm strip and multiply by 6.
A normal adult resting rate is 60–100 beats per minute. Below 60 is bradycardia; above 100 is tachycardia.
Step 2: Rhythm
Ask: is the rhythm regular or irregular? Is there a P wave before every QRS complex, and a QRS after every P wave? Sinus rhythm has a regular rate with an upright P wave before each QRS in lead II. An irregularly irregular rhythm with no clear P waves suggests atrial fibrillation.
Step 3: Cardiac axis
A quick method uses leads I and aVF:
- Both QRS complexes mainly positive — normal axis.
- Lead I positive, aVF negative — possible left axis deviation (check lead II: if it is also negative, the axis is to the left).
- Lead I negative, aVF positive — right axis deviation.
Step 4: P wave
Normal P waves are less than 0.12 seconds (3 small squares) wide and less than 2.5 mm tall. Look for absent, abnormal or extra P waves.
Step 5: PR interval
Normal is 0.12–0.20 seconds (3–5 small squares). A long PR interval suggests heart block; a short one can suggest a pre-excitation pathway.
Step 6: QRS complex
A normal QRS is narrower than 0.12 seconds (3 small squares). A broad QRS may mean a bundle branch block or a ventricular rhythm. Also look for pathological Q waves and for very tall or small complexes.
Step 7: ST segment and T wave
The ST segment should be level with the baseline. ST elevation or depression in a group of related leads, with the right clinical story, may indicate myocardial ischaemia or infarction and needs urgent senior review. Check whether T waves are upright where they should be, or inverted, flattened or peaked.
Step 8: QT interval
Measure from the start of the QRS to the end of the T wave and correct it for heart rate (QTc). A prolonged QTc can be caused by medicines and electrolyte disturbances and increases the risk of dangerous arrhythmias.
Putting it together
Finish with a short report: rate, rhythm, axis, intervals and any abnormalities, then link them to the clinical picture. For example: "Sinus rhythm, rate 78 per minute, normal axis, normal intervals, no ST or T wave changes."
ECG findings must always be interpreted together with the patient's symptoms and examination. If in doubt, ask a senior colleague. For a fuller beginner's guide, see our note ECG Basics Simplified.
Related notes
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Disclaimer
For education and exam preparation only. This is not medical advice and must not be used to diagnose or treat patients. Always follow current clinical guidelines and the advice of qualified senior colleagues. Spotted a mistake? Please tell us.