How to read an ECG in six steps: a method for exam strips

Why a fixed routine

Under exam pressure, people jump to the dramatic feature and miss the obvious one. A routine keeps you honest and takes about 30 seconds once practised. This guide complements the earlier ECG basics post, and it sticks to exam-level interpretation.

This is exam-preparation revision, not clinical guidance. In practice, follow your own organisation’s policies and current guidelines.

Step 1: calibration and patient

Check that the paper speed is 25 millimetres per second and the standard calibration is 10 millimetres per millivolt, unless the strip says otherwise. At 25 mm per second, one small square is 0.04 seconds and one large square is 0.2 seconds. Check the patient details and the clinical stem, because the ECG is read in context: chest pain, palpitations, a known electrolyte problem or a drug history change what you look for.

Step 2: rate

For a regular rhythm, divide 300 by the number of large squares between two R waves. If there are four large squares, the rate is 75 per minute. For an irregular rhythm, count the QRS complexes in a 6-second strip, which is 30 large squares, and multiply by ten. Below 60 per minute is bradycardia and above 100 is tachycardia in an adult.

Step 3: rhythm

Ask whether the rhythm is regular. Is there a P wave before every QRS, and a QRS after every P wave? An irregularly irregular rhythm with no clear P waves suggests atrial fibrillation. A saw-tooth baseline suggests atrial flutter. A regular narrow complex tachycardia at about 150 per minute should make you think of flutter with 2 to 1 block or a supraventricular tachycardia.

Step 4: axis

At exam level, a quick check of the QRS in leads I and aVF is enough. If both are mainly positive, the axis is normal. If lead I is positive and aVF is negative, there is left axis deviation. If lead I is negative and aVF is positive, there is right axis deviation. Axis questions are usually linked to a cause, such as a conduction defect or right heart strain, so know the common associations.

Step 5: intervals

Measure the PR interval, the QRS width and the QT interval.

• PR interval: normally 0.12 to 0.20 seconds, which is three to five small squares. A longer PR interval suggests first-degree heart block.
• QRS width: normally under 0.12 seconds, under three small squares. A wider QRS suggests a bundle branch block or a ventricular origin.
• QT interval: depends on rate, so a corrected value is used. A prolonged QT interval raises the risk of a dangerous rhythm. Exact normal limits differ slightly by source and sex, so learn the figure in your course.

Step 6: waves and ST segments

Finally look at the P waves, Q waves, ST segments and T waves in each lead group. ST elevation in two or more contiguous leads in the right clinical context suggests acute myocardial infarction, and the pattern of leads helps you locate it. ST depression and T wave inversion suggest ischaemia. Tall peaked T waves suggest hyperkalaemia, and a widening QRS in that setting is a danger sign. Flattened T waves and U waves may suggest hypokalaemia. Pathological Q waves suggest an old infarct.

A worked strip

Imagine a strip with an irregularly irregular rhythm, no distinct P waves, narrow QRS complexes and about 14 QRS complexes in a 6-second strip. Step one: standard speed and calibration. Step two: rate is 14 times ten, about 140 per minute. Step three: the rhythm is irregularly irregular without P waves, which suggests atrial fibrillation. Step four: the axis is normal. Step five: you cannot measure a PR interval without P waves, and the QRS is narrow. Step six: no ST elevation. The answer is atrial fibrillation with a fast ventricular response, and the stem will ask for the next step, which usually depends on whether the patient is stable.

The routine leads you to the pattern before you jump to the management.

Heart blocks at a glance

First-degree block shows a PR interval longer than 0.20 seconds with every P wave followed by a QRS. Second-degree block type one, also called Wenckebach, shows a PR interval that lengthens until a beat is dropped. Second-degree type two shows a fixed PR interval with occasional dropped QRS complexes and is more concerning. Third-degree, or complete, block shows no relation between the P waves and the QRS complexes, with a slow escape rhythm. Learn each by describing what you see, not by recalling its name alone.

Electrolytes on the strip

Hyperkalaemia moves from tall peaked T waves to a flattened P wave, a widened QRS and eventually a sine-wave pattern. Hypokalaemia may show flat T waves, ST depression and prominent U waves. Hypercalcaemia may shorten the QT interval and hypocalcaemia may lengthen it. Exam questions link these to clinical stems such as renal failure or diuretic use, so learn the pairs. The fluids and electrolytes revision guide is a good companion.

Dangerous rhythms to recognise at once

A few patterns demand immediate action in an exam stem. Ventricular tachycardia appears as a fast, wide-complex rhythm, and if the patient has no pulse it is treated as a cardiac arrest rhythm. Ventricular fibrillation is a chaotic, irregular baseline with no distinct complexes. Asystole shows a flat line, and you check leads and connections to confirm it. Pulseless electrical activity shows an organised rhythm on the monitor without a pulse. The key lesson is that the strip alone does not decide management. Check the patient first, because a disconnected lead can mimic a flat line.

ST elevation: reading the pattern of leads

In general terms, ST elevation in the inferior leads (II, III and aVF) points to the inferior wall, in V1 to V4 to the anterior wall, and in I, aVL, V5 and V6 to the lateral wall. Reciprocal ST depression can appear in opposite leads. Exam questions may give a list of leads and ask which territory is involved, and then which artery is usually responsible. Learn the territories as a short table and rehearse it with cards. In practice, treatment decisions follow local protocols, and the exam question will give you the setting.

The question is usually about the next step

Most ECG questions are not asking you to name a rhythm for its own sake. They ask what you do next. Check the patient first: are they conscious, breathing and perfused? An unstable patient needs emergency care before a long interpretation, and a stable patient with a worrying pattern needs escalation and monitoring rather than a guess.

How to build reading speed

Collect a bank of 30 strips covering the common patterns. Every day, pick five at random, say the six steps aloud and check against the answer. Time yourself: aim for thirty seconds per strip. Add the ones you miss to your cards. In two weeks you will be quicker, and you will have a feel for the patterns that give you trouble. Combine this with exam-style questions that put the strip in a clinical stem.

How to drill it

Practise ten strips a week and say the six steps aloud. Keep a card per pattern in your flashcards, with the image if possible. Use the question bank for strips linked to a stem, and add the patterns you miss to your error log. The DHA exam for nurses and doctors pages show how cardiology sits in the exam.

Two hooks for the ECG

These two small hooks help with common ECG tasks.

• For the rate of a regular rhythm, count the large boxes between two R waves and say 300, 150, 100, 75, 60, 50. It does not work well in an irregular rhythm such as atrial fibrillation.
• WiLLiaM MaRRoW: in left bundle branch block the QRS looks like a W in V1 and an M in V6, and in right bundle branch block it looks like an M in V1 and a W in V6. Shapes vary, and you still need a QRS of 120 ms or more.

Frequently asked questions