Reading an ECG step by step

What it is

The 12-lead electrocardiogram records the heart's electrical activity from twelve viewpoints: six limb leads (I, II, III, aVR, aVL, aVF) look at the frontal plane and six chest leads (V1–V6) look at the horizontal plane. Each beat shows a P wave (atrial depolarisation), a PR interval (the delay through the atrioventricular node and the His–Purkinje system), a QRS complex (ventricular depolarisation), an ST segment and a T wave (ventricular repolarisation). On standard paper running at 25 mm/s, one small square is 40 ms and one large square is 200 ms; with standard calibration, 10 mm of height equals 1 mV. Reading well is about order, not speed: details first (patient, date, calibration, lead placement), then rate, rhythm, intervals, axis and morphology (P, QRS, ST, T and QT), and finally a one-line conclusion compared with any previous tracing and with the patient in front of you. The same sequence works in an OSCE station, on a ward round and at 3 a.m., and it stops the commonest error: seeing one striking abnormality and missing the second one.

Reference values and concepts

• Time: 1 small square = 40 ms, 1 large square (5 small) = 200 ms, 5 large squares = 1 second at 25 mm/s.
• Voltage: the calibration pulse should be 10 mm tall (1 mV); half-standard (5 mm) makes complexes look small.
• Normal sinus rate 60–100 bpm; a 10-second rhythm strip shows 10–16 complexes in that range.
• P wave: under 120 ms wide and under 2.5 mm tall in lead II; upright in I and II, inverted in aVR.
• PR interval: 120–200 ms; QRS: under 120 ms (110–119 ms is incomplete block or a nonspecific delay).
• QT is measured from QRS onset to the end of T and corrected for rate (QTc, Bazett: QT ÷ √RR in seconds); above 500 ms is high risk.
• Axis: normal −30° to +90°; left deviation below −30°; right deviation above +90°. Quick check: leads I and aVF (or I and II).
• T waves are normally upright in I, II and V3–V6 and inverted in aVR; an isolated inverted T in III or V1 can be normal.

Reading it step by step

1. Check name, date, time and symptoms at recording — an ECG means little without the clinical moment it captures.
2. Confirm calibration (10 mm = 1 mV) and paper speed (25 mm/s) — wrong settings change every interval and voltage.
3. Look for lead errors: lead I negative with aVR positive — limb electrodes may be swapped; repeat before diagnosing.
4. Calculate the rate — 300 ÷ large squares if regular, complexes in 10 s × 6 if irregular.
5. Decide the rhythm: regular or not, a P before every QRS, a QRS after every P — this separates sinus, AF, flutter and blocks.
6. Measure PR, QRS and QT and correct the QT — long PR means AV delay, wide QRS means a block, pre-excitation or ventricular origin.
7. Work out the axis from leads I and aVF (then II) — deviation points to fascicular block, hypertrophy or a ventricular rhythm.
8. Inspect P, QRS, ST and T lead by lead, in territories — Q waves, ST change and T inversion map ischaemia and strain.
9. Compare with any previous ECG — a new block or new ST change is acute until proved otherwise.
10. Write a one-line conclusion — rhythm, rate, axis, intervals and key findings, and what they mean for this patient.

Normal values on the 12-lead ECG (paper at 25 mm/s)

Patterns and common mistakes

• Trusting the machine's printed report: automated reads miss flutter, blocks and subtle ST change and often misread the QT.
• Limb-lead reversal (right and left arm) mimics a lateral infarct or dextrocardia; chest leads placed too high mimic septal Q waves.
• Fixing on the first abnormality (for example, ST elevation) and missing a second one, such as complete heart block or a long QT.
• Missing P waves hidden in the T wave, which hides 2:1 block or atrial flutter at 150 bpm.
• Bazett's formula over-corrects the QT at fast rates and under-corrects at slow rates; measure in the lead with the longest clear QT, usually II or V5.
• Reading T-wave inversion in V1–V3 as ischaemia in a young person (persistent juvenile pattern) or forgetting it can be the only sign of a pulmonary embolism.

Next step

• Any ST elevation, new block or dangerous rhythm: show the ECG to the senior at once and act on the patient, not the paper.
• Repeat the ECG if the leads look swapped, the trace is noisy or symptoms change (serial ECGs catch evolving ischaemia).
• Find the old ECG in the record and write whether each finding is new or old.
• Link the reading to bloods where needed: potassium, magnesium, calcium and troponin.
• Document the conclusion in the notes, with the time, and sign and date the ECG itself.

Test yourself

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Elsewhere in DurgiPedia

• ECG basics: waves, intervals and leads
• The cardiac electrical axis
• The ECG in ischemia and infarction
• The ECG in potassium and calcium disorders
• The cardiac conduction system

Connections

← All clinical skills

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Electrocardiogram (StatPearls (NCBI Bookshelf), 2023); ECG T Wave (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.