---
title: Electrocardiogram (ECG)
description: What a 12-lead ECG records, what its waves and intervals mean, when clinical guidelines call for one and what it cannot show.
type: diagnostic
aliases:
  - EKG
  - electrocardiography
  - 12-lead ECG
  - cardiogram
sources:
  - tier: reference
    org: statpearls
    title: Electrocardiogram
    year: 2023
    url: https://www.ncbi.nlm.nih.gov/books/NBK549803/
  - tier: review
    title: 'Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part I: The Electrocardiogram and Its Technology'
    year: 2007
    doi: 10.1161/CIRCULATIONAHA.106.180200
    url: https://doi.org/10.1161/CIRCULATIONAHA.106.180200
  - tier: guideline
    org: esc
    title: 2024 ESC Guidelines for the management of chronic coronary syndromes
    year: 2024
    doi: 10.1093/eurheartj/ehae177
    url: https://doi.org/10.1093/eurheartj/ehae177
  - tier: guideline
    org: esc
    title: 2024 ESC Guidelines for the management of atrial fibrillation developed in collaboration with the European Association for Cardio-Thoracic Surgery (EACTS)
    year: 2024
    doi: 10.1093/eurheartj/ehae176
    url: https://doi.org/10.1093/eurheartj/ehae176
  - tier: guideline
    org: nice
    title: Transient loss of consciousness ('blackouts') in over 16s (CG109)
    year: 2023
    url: https://www.nice.org.uk/guidance/cg109
sources_checked: 2026-10-11
review: null
---

An electrocardiogram (ECG) is a recording of the heart's electrical activity, taken from the surface of the body without entering it.<Cite n={1} /> The test is safe and painless, with no major risks or complications.<Cite n={1} /> A standard resting ECG is recorded in 12 leads, six limb leads and six chest leads, using ten electrodes placed on the body.<Cite n={1} />

## What it shows

<EcgMonitor preset="normal">

A synthetic teaching model of a normal ECG, not a recording from a real person: the rhythm is regular, and every QRS complex is preceded by a P wave.<Cite n={1} /> The P wave is atrial depolarisation, the QRS complex is ventricular depolarisation, and the T wave is ventricular repolarisation.<Cite n={1} /> Do not compare your own ECG with it.

</EcgMonitor>

<Patient>

Every heartbeat is set off by an electrical impulse; normally it starts in the <Structure id="sa-node">sinus node</Structure> in the <Structure id="right-atrium">right atrium</Structure>.<Cite n={1} /> How the impulse travels through the heart is explained in the article on the [conduction system of the heart](../../anatomy/conduction-system/).

On an ECG this path shows up as a repeating pattern of waves:

- **the P wave**: the atria being electrically activated (depolarisation);<Cite n={1} />
- **the QRS complex**: the ventricles being activated;<Cite n={1} />
- **the T wave**: the ventricles recovering (repolarisation).<Cite n={1} />

The distances between the waves are called intervals. The PR interval is the time from the start of atrial activation to the start of ventricular activation.<Cite n={1} /> The QT interval covers ventricular activation and recovery from start to finish.<Cite n={1} /> From an ECG a doctor can work out the heart rate and rhythm, and can see conduction problems, signs of thickened ventricular walls, damage to the heart muscle and a previous heart attack.<Cite n={1} />

</Patient>

<Student>

The P wave represents atrial depolarisation; as atrial depolarisation is initiated by the SA node in the right atrium, the right atrium depolarises first.<Cite n={1} /> The QRS complex represents ventricular depolarisation and the T wave ventricular repolarisation.<Cite n={1} /> The ST segment depicts the end of ventricular depolarisation and the beginning of ventricular repolarisation; normally it is isoelectric and lies at the same level as the PR segment.<Cite n={1} />

The PR interval runs from the beginning of atrial depolarisation to the start of ventricular depolarisation.<Cite n={1} /> According to StatPearls, the average PR interval is 120–200 ms (3–5 small squares), and the QRS duration is under 120 ms, usually 60–100 ms.<Cite n={1} /> The QT interval runs from the start of depolarisation to the end of repolarisation of the ventricles and has an inverse relation to heart rate.<Cite n={1} /> StatPearls gives a normal heart rate of 60–99 beats per minute; in a regular rhythm a P wave precedes every QRS complex.<Cite n={1} />

The standard paper speed is 25 mm/s.<Cite n={1} /> V1 is placed in the fourth intercostal space at the right sternal border, V4 in the fifth intercostal space in the midclavicular line.<Cite n={2} />

</Student>

<HeartModel highlight={['sa-node', 'right-atrium', 'left-atrium', 'right-ventricle', 'left-ventricle']} />

## When it is done

<Patient>

The main reason for an ECG is symptoms: palpitations, dizziness, chest pain, fainting.<Cite n={1} /> If such symptoms come on suddenly and are severe, do not wait for a test: see the section ["When to call an ambulance"](#when-to-call-an-ambulance).

- **Chest pain.** The European guidelines on chronic coronary syndromes call the resting 12-lead ECG an indispensable part of the first assessment of a person with chest pain.<Cite n={3} /> The condition that often lies behind such pain is described in the article on [coronary heart disease](../../disease/coronary-heart-disease/).
- **Fainting.** The UK NICE guideline on transient loss of consciousness in people over 16 says to record a 12-lead ECG at the initial assessment.<Cite n={5} />
- **Atrial fibrillation.** Before risks are assessed and treatment begins, this arrhythmia has to be documented on an ECG.<Cite n={4} /> A 12-lead ECG is recommended for everyone with [atrial fibrillation](../../disease/atrial-fibrillation/).<Cite n={4} />

An ECG is also used to check how a pacemaker is working, to monitor the effects and side effects of medicines, and to assess the heart before an operation and during anaesthesia.<Cite n={1} />

</Patient>

<Student>

2024 ESC chronic coronary syndromes guidelines: "the resting 12-lead ECG remains an indispensable component of the initial evaluation of a patient with chest pain".<Cite n={3} /> Dynamic ST-segment changes recorded during ongoing angina can be crucial for diagnosing transient myocardial ischaemia.<Cite n={3} />

NICE CG109 (recommendation 1.1.2.2): record a 12-lead ECG using automated interpretation, and treat as a red flag a conduction abnormality (for example, complete right or left bundle branch block or any degree of heart block), evidence of a long or short QT interval, and any ST segment or T wave abnormalities.<Cite n={5} /> NICE recommends urgent referral for cardiovascular assessment of anyone with transient loss of consciousness who also has an ECG abnormality, with the referral reviewed and prioritised by an appropriate specialist within 24 hours.<Cite n={5} />

If a 12-lead ECG with automated interpretation is not available, NICE CG109 requires the ECG of a person with transient loss of consciousness to be reviewed by a healthcare professional trained and competent in identifying abnormalities including long QT (corrected QT more than 450 ms) and short QT (corrected QT less than 350 ms) intervals, ventricular pre-excitation, ventricular hypertrophy and pathological Q waves.<Cite n={5} /> These are NICE criteria for red flags after loss of consciousness, not general QT reference values.

2024 ESC AF guidelines: ECG documentation of the arrhythmia is required before risk stratification and management.<Cite n={4} /> Confirmation can come from a standard 12-lead ECG (a 10 s recording) or, by consensus opinion with limited evidence, 30 s or more on a single-lead or multiple-lead device that provides an ECG; photoplethysmography-based wearables do not qualify.<Cite n={4} /> A 12-lead ECG is warranted in all AF patients to confirm rhythm, determine ventricular rate, and look for signs of structural heart disease, conduction defects or ischaemia.<Cite n={4} /> In AF there are no discernible and regular P waves, and there is no specific pattern to RR intervals in the absence of AV block.<Cite n={4} />

</Student>

## When to call an ambulance

If there is chest pain, someone has lost consciousness or severe palpitations have started, call your local emergency number (112 in the EU, 911 in the US and Canada, 999 in the UK) straight away. Do not wait for an ECG appointment or any other planned test: this is the site's general safety rule, repeated at the bottom of every page. The signs of a heart attack are listed in the article on [coronary heart disease](../../disease/coronary-heart-disease/).

## How it is done

<Patient>

The clinic will tell you how to prepare: follow the instructions of your doctor or clinic. You will be asked to change into an examination gown and take off metal objects such as jewellery and watches.<Cite n={1} /> Chest hair is sometimes shaved so that the electrodes stick well.<Cite n={1} /> The electrodes are attached to the arms, legs and chest.<Cite n={[1, 2]} /> You lie still while the machine records the ECG for about 10 seconds.<Cite n={1} /> The room should be warm so that you do not shiver.<Cite n={1} />

</Patient>

<Student>

The standard ECG is recorded in the supine position; tracings recorded sitting or upright should not be considered equivalent to standard supine ECGs.<Cite n={2} /> On the printout, each column of leads represents successive 2.5-second intervals of a continuous 10-second record.<Cite n={2} /> The limb leads are colour-coded to avoid misplacement.<Cite n={1} />

</Student>

## What the results mean

<Patient>

The doctor looks at whether the waves and intervals are normal and assesses the heart rate and rhythm.<Cite n={1} /> Many machines print an automatic report, but every computer-generated report must be checked by a doctor.<Cite n={2} /> The doctor interprets the ECG together with your symptoms, the examination and other tests. A normal ECG does not mean the heart is definitely healthy: see the section ["Limitations"](#limitations).

</Patient>

<Student>

The 2024 ESC guidelines list indirect signs of chronic coronary syndrome on the resting ECG: signs of previous MI (pathological Q or R waves) and conduction abnormalities (mainly left bundle branch block and impaired atrioventricular conduction); atrial fibrillation is also not uncommonly associated with the syndrome.<Cite n={3} />

</Student>

## Limitations

<Patient>

A normal resting ECG does not rule out heart disease: for example, the ECG is often normal after an angina attack.<Cite n={3} /> Artefacts and distortions in the recording make it harder to read and can lead to a wrong conclusion.<Cite n={1} /> Longer ECG recording is covered in the article on [Holter monitoring](../holter-monitoring/).

</Patient>

<Student>

Errors in ECG analysis can lead to misdiagnosis and delay appropriate treatment.<Cite n={1} /> The duration of AF required for diagnosis on monitoring devices is not clear cut, and asymptomatic episodes make diagnosis more challenging.<Cite n={4} />

</Student>

<Quiz />