[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical skills](https://durgical.com/en/clinical-skills/) › [The ECG](https://durgical.com/en/clinical-skills/areas/ecg/) › Reading an ECG step by step

# Reading an ECG step by step

- Interpretation

A fixed order for reading a heart tracing, so that the rate, rhythm, conduction and the shape of every wave are checked and nothing is missed.

Updated Nov 13, 2026

## Key points

- Paper at 25 mm/s: small square 40 ms, large square 200 ms; calibration 10 mm = 1 mV.
- Regular rate = 300 ÷ large squares (or 1500 ÷ small squares) between two R waves; irregular rate = QRS complexes in the 10-second strip × 6.
- Normal values: PR 120–200 ms, QRS under 120 ms, QTc under about 450 ms in men and 460 ms in women; axis −30° to +90°.
- aVR is normally negative: an upright P and QRS in aVR with a negative lead I suggests limb-lead reversal (or dextrocardia).
- Always compare with an old ECG and with the patient: a new change matters more than an old one.

On this page

- The basics Key points
- What it is
- Reference values and concepts
- Reading it step by step
- Normal values on the 12-lead ECG (paper at 25 mm/s)
- Patterns and common mistakes
- Next step
- Memory aid
- Elsewhere in DurgiPedia

- Study Test yourself

- Connections Connections

## 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

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

| Measure | Normal | If outside the range, think of |
| --- | --- | --- |
| Rate | 60–100 bpm | Below: sinus bradycardia, AV block; above: sinus or other tachycardia |
| P wave | Under 120 ms, under 2.5 mm in II | Wide and notched: left atrial enlargement; tall: right atrial enlargement |
| PR interval | 120–200 ms | Long: first-degree AV block; short: pre-excitation or junctional rhythm |
| QRS duration | Under 120 ms | 120 ms or more: bundle branch block, pre-excitation, paced or ventricular rhythm, hyperkalaemia |
| QTc | Under about 450 ms (men), 460 ms (women) | Long: drugs, low potassium, magnesium or calcium, congenital long QT |
| Axis | −30° to +90° | Left: left anterior fascicular block, inferior infarct; right: right ventricular strain, left posterior fascicular block |
| T wave | Upright in I, II, V3–V6; inverted in aVR | Inverted: ischaemia, strain, cardiomyopathy, PE; peaked: hyperkalaemia |

## 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.

## Memory aid

Rate, Rhythm, Intervals, Axis, Morphology, Compare (RRIAMC): six stops, every ECG.

## Test yourself

Think of the answer, then tap the question to see it.

## A regular ECG shows 4 large squares between consecutive R waves. What is the rate?

About 75 bpm (300 ÷ 4). With the standard paper speed of 25 mm/s, each large square is 200 ms, so the RR interval is 800 ms.

## Lead I shows an inverted P, QRS and T, and aVR is upright; the chest leads have normal R-wave progression. What is the most likely explanation?

Left and right arm electrodes swapped. Dextrocardia also inverts lead I, but it gives loss of R-wave progression across V1–V6. Re-site the leads and repeat.

## A tracing shows QRS complexes that are mainly positive in lead I and mainly negative in aVF, and positive in lead II. What is the axis?

Normal, between 0° and −30°: lead I positive and aVF negative put it in the left quadrant, but a positive lead II keeps it above −30°. A negative II would mean left axis deviation.

## QT is 440 ms at a heart rate of 60 bpm. What is the QTc by Bazett's formula?

440 ms: at 60 bpm the RR interval is 1 second, and √1 = 1, so the QTc equals the measured QT. That is normal in a man and in a woman.

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## Elsewhere in DurgiPedia

- [ECG basics: waves, intervals and leads](https://durgical.com/en/physiology/ecg-basics-waves-intervals-and-leads/)
- [The cardiac electrical axis](https://durgical.com/en/physiology/the-cardiac-electrical-axis/)
- [The ECG in ischemia and infarction](https://durgical.com/en/physiology/the-ecg-in-ischemia-and-infarction/)
- [The ECG in potassium and calcium disorders](https://durgical.com/en/physiology/the-ecg-in-potassium-and-calcium-disorders/)
- [The cardiac conduction system](https://durgical.com/en/physiology/the-cardiac-conduction-system/)

## Connections

## Related 10

- [Recording a 12-lead ECG](https://durgical.com/en/clinical-skills/recording-a-12-lead-ecg/)
- [Sinus rhythm, sinus tachycardia, bradycardia and arrhythmia](https://durgical.com/en/clinical-skills/sinus-rhythm-sinus-tachycardia-bradycardia-and-arrhythmia/)
- [AV blocks: first, second (Mobitz I and II) and third degree](https://durgical.com/en/clinical-skills/av-blocks-first-second-and-third-degree/)
- [Right and left bundle branch block and fascicular blocks](https://durgical.com/en/clinical-skills/right-and-left-bundle-branch-block-and-fascicular-blocks/)
- [Atrial fibrillation and atrial flutter](https://durgical.com/en/clinical-skills/atrial-fibrillation-and-atrial-flutter/)
- [Narrow-complex tachycardia (SVT)](https://durgical.com/en/clinical-skills/narrow-complex-tachycardia/)
- [Broad-complex tachycardia, VF and the arrest rhythms](https://durgical.com/en/clinical-skills/broad-complex-tachycardia-vf-and-the-arrest-rhythms/)
- [STEMI and its territories, and STEMI mimics](https://durgical.com/en/clinical-skills/stemi-and-its-territories-and-stemi-mimics/)
- [The long QT interval](https://durgical.com/en/clinical-skills/the-long-qt-interval/)
- [Left and right ventricular hypertrophy and atrial enlargement](https://durgical.com/en/clinical-skills/left-and-right-ventricular-hypertrophy-and-atrial-enlargement/)

## Related conditions 5

- [Myocardial Infarction (Heart Attack)](https://durgical.com/en/conditions/myocardial-infarction/)
- [Atrioventricular (heart) block](https://durgical.com/en/conditions/atrioventricular-block/)
- [Atrial fibrillation](https://durgical.com/en/conditions/atrial-fibrillation/)
- [Acute pericarditis](https://durgical.com/en/conditions/acute-pericarditis/)
- [Hyperkalaemia](https://durgical.com/en/conditions/hyperkalaemia/)

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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](https://www.ncbi.nlm.nih.gov/books/NBK549803/) (StatPearls (NCBI Bookshelf), 2023); [ECG T Wave](https://www.ncbi.nlm.nih.gov/books/NBK538264/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-skills/reading-an-ecg-step-by-step/