Electrocardiograms (ECGs)

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Spec 7.12(iii)

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Electrocardiograms (ECGs)Spec 7.12(iii)

In short

An electrocardiogram (ECG) records the electrical activity of the heart from electrodes on the skin. The P wave shows atrial depolarisation, the QRS complex ventricular depolarisation and the T wave ventricular repolarisation. Changes in rate, spacing and shape help diagnose abnormal heart rhythms such as tachycardia, bradycardia, ectopic beats and atrial fibrillation.

An electrocardiogram (ECG) is a trace of the electrical changes produced by the heart, recorded by electrodes on the skin. One heartbeat gives a repeating pattern of waves.

Parts of a normal ECG
Feature	What it shows
P wave	Depolarisation of the atria (just before atrial systole)
QRS complex	Depolarisation of the ventricles (just before ventricular systole)
T wave	Repolarisation of the ventricles (as they relax)
R–R interval	Time for one complete heartbeat

heart rate (beats per minute) = 60 ÷ time for one heartbeat (R–R interval, in seconds)

Abnormal rhythms shown on an ECG

Condition	ECG appearance
Tachycardia	Regular pattern, but a rapid heart rate (above 100 beats per minute): short R–R intervals
Bradycardia	Regular pattern, but a slow heart rate (below 60 beats per minute): long R–R intervals
Ectopic heartbeat	An extra or early beat, then a longer gap before the next normal beat
Atrial fibrillation	Irregular, rapid atrial activity: no clear P waves and an irregular R–R interval

Maths skill:

To find heart rate from an ECG, measure the time between two R peaks from the scale given, then divide 60 by it. For example, an R–R interval of 0.8 s gives 75 beats per minute.

A normal ECG on a time grid with the P wave, QRS complex, T wave and R–R interval labelled, above simplified traces of tachycardia, bradycardia, an ectopic beat and atrial fibrillation. — A normal ECG, and what abnormal rhythms look like.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

How does the heart beat without being told by the brain?

Cardiac muscle is myogenic, so it generates its own excitation. The sinoatrial node in the right atrium sets the pace and sends a wave across the atria. The atrioventricular node delays it, then the bundle of His and Purkyne fibres spread it through the ventricles. Nerves only change the rate.

How do you read an ECG?

The P wave is atrial depolarisation, the QRS complex is ventricular depolarisation and the T wave is ventricular repolarisation. The time between two R peaks gives the heart rate: 60 divided by that time in seconds. Irregular spacing or missing P waves can show abnormal rhythms such as atrial fibrillation.

What controls heart rate and breathing rate during exercise?

The medulla oblongata does. Chemoreceptors detect a rise in carbon dioxide and a fall in pH in the blood. The cardiovascular control centre increases heart rate through the sympathetic nerve to the sinoatrial node, and the ventilation centre speeds up and deepens breathing, to deliver oxygen and remove carbon dioxide.

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