8. QRS Complex - ECG assessment and ECG interpretation made easy
Channel: Dr.G Bhanu Prakash Animated Medical Videos
Date: 2022-05-28

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QRS Complex Morphology

Main features to consider:

Width of the complexes: Narrow versus broad.
Voltage (height) of the complexes.
Spot diagnoses: Specific morphology patterns that are important to recognise.

QRS Width
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Normal QRS width is 70-100 ms (a duration of 110 ms is sometimes observed in healthy subjects). The QRS width is useful in determining the origin of each QRS complex (e.g. sinus, atrial, junctional or ventricular).

Narrow complexes (QRS less than 100 ms) are supraventricular in origin.
Broad complexes (QRS greater than 100 ms) may be either ventricular in origin, or due to aberrant conduction of supraventricular complexes (e.g. due to bundle branch block, hyperkalaemia or sodium-channel blockade).

Narrow QRS Complex Morphology
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Narrow (supraventricular) complexes arise from three main places:

Sino-atrial node (= normal P wave)
Atria (= abnormal P wave / flutter wave / fibrillatory wave)
AV node / junction (= either no P wave or an abnormal P wave with a PR interval less than 120 ms)

Broad QRS Complex Morphology
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Broad/Wide QRS Complexes

A QRS duration greater than 100 ms is abnormal
A QRS duration greater than 120 ms is required for the diagnosis of bundle branch block or ventricular rhythm

Broad complexes may be ventricular in origin or due to aberrant conduction secondary to:

Bundle branch block (RBBB or LBBB)
Hyperkalaemia
Poisoning with sodium-channel blocking agents (e.g. tricyclic antidepressants)
Pre-excitation (i.e. Wolff-Parkinson-White syndrome)
Ventricular pacing
Hypothermia
Intermittent aberrancy (e.g. rate-related aberrancy)

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