Electrocardiogram (ECG)

Also known as: ECG, EKG, P wave, QRS complex, T wave, PR interval, QT interval, 12-lead ECG

The electrocardiogram (ECG) records, from electrodes on the skin, the combined electrical activity of the heart over time. Its waves (P, QRS, T) and intervals (PR, QRS, QT) trace the sequence of atrial and ventricular depolarisation and repolarisation.

Show on body map Cardiac Conduction System & ECG

Overview

PR interval0.12–0.20 s

QRS duration<0.12 s

QTc≲440 ms (men), ≲460 ms (women)

Paper speed25 mm/s (1 small square = 0.04 s)

Normal axis−30° to +90°

Leads. The standard 12-lead ECG has six limb leads and six chest (precordial) leads. The limb leads view the heart in the frontal plane: I, II and III are bipolar leads forming Einthoven's triangle, and aVR, aVL and aVF are augmented unipolar leads. The chest leads, V1–V6, view it in the horizontal plane.

Polarity. A wave of depolarisation travelling toward a lead's positive electrode gives an upward deflection. One travelling away gives a downward deflection. The normal frontal-plane QRS axis lies between −30° and +90°.

Paper conventions. At the standard speed of 25 mm/s, one small square (1 mm) represents 0.04 s and one large square (5 mm) represents 0.20 s. The standard calibration is 10 mm = 1 mV. Heart rate equals 300 divided by the number of large squares between consecutive R waves, or 1500 divided by the number of small squares.

Repolarisation. The T wave normally points the same way as the QRS. This is because the ventricles repolarise in the opposite direction to depolarisation, from epicardium to endocardium, and an opposite-direction wave of opposite charge gives a deflection of the same sign.

Parts & subdivisions

P wave
Atrial depolarisation. Normally less than 0.12 s wide and less than 2.5 mm tall, upright in lead II and inverted in aVR.
PR interval
From the start of the P wave to the start of the QRS, normally 0.12–0.20 s. It consists mainly of the AV nodal delay.
QRS complex
Ventricular depolarisation, normally less than 0.12 s (usually 0.06–0.10 s). Atrial repolarisation is hidden within it.
ST segment
The plateau phase, normally isoelectric (flat). Elevation or depression indicates injury or ischaemia.
T wave
Ventricular repolarisation.
QT interval
From the start of the QRS to the end of the T wave: the total duration of ventricular electrical activity. It shortens as heart rate rises. The rate-corrected QTc (Bazett formula: QT ÷ √RR) is normally up to about 440 ms in men and 460 ms in women. A QTc above 500 ms carries a high risk of arrhythmia.
U wave
A small wave sometimes seen after the T wave. It is prominent in hypokalaemia.

Function

Detects disorders of rhythm and conduction.
Detects ischaemia and infarction and localises the territory affected.
Shows chamber hypertrophy, electrolyte disturbances, drug effects and pericardial disease.

Clinical significance

Localising STEMI

ST elevation in II, III and aVF indicates an inferior infarct (RCA). ST elevation in V1–V4 indicates an anterior or anteroseptal infarct (LAD). ST elevation in I, aVL, V5 and V6 indicates a lateral infarct (circumflex). A posterior infarct shows as ST depression with tall R waves in V1–V3.

Rhythm recognition

Atrial fibrillation gives an irregularly irregular rhythm with no P waves. Atrial flutter gives sawtooth F waves at about 300/min, often with 2:1 block, producing a ventricular rate of about 150/min. Wolff–Parkinson–White syndrome gives a short PR interval and a delta wave. Ventricular tachycardia gives a broad, regular complex tachycardia. Ventricular fibrillation gives chaotic, irregular deflections.

A systematic approach

Read every ECG in the same order: rate, rhythm, axis, P waves, PR interval, QRS, ST segments, T waves and QT. Then compare with a previous ECG and interpret it in the clinical context.

Test yourself

Which ECG interval mainly reflects conduction delay through the AV node?

QT interval
PR interval
QRS duration
ST segment

Answer: PR interval

The PR interval runs from the start of atrial depolarisation to the start of ventricular depolarisation (normally 0.12–0.20 s). Most of it is the deliberate delay in the AV node, about 0.09 s.

Which ECG pattern defines Mobitz type II second-degree AV block?

Progressive PR prolongation until a QRS is dropped
Constant PR interval with sudden non-conducted P waves
Complete dissociation of P waves and QRS complexes
A PR interval above 200 ms with every P wave conducted

Answer: Constant PR interval with sudden non-conducted P waves

In Mobitz II block the PR interval stays constant until a P wave suddenly fails to conduct. The block is usually below the AV node, can progress to complete heart block and generally needs a pacemaker. Progressive PR lengthening describes Mobitz I (Wenckebach) block.

What is the earliest typical ECG sign of hyperkalaemia?

Tall, peaked T waves
Prominent U waves
A short PR interval with a delta wave
Prolonged QT interval

Answer: Tall, peaked T waves

As potassium rises, the T waves first become peaked. Next the P waves flatten and the PR interval lengthens, then the QRS widens, and finally a sine-wave pattern appears before VF or asystole. U waves are a sign of hypokalaemia.