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QRS complex: ventricular depolarisation, normal width, bundle branch blocks and other causes of a wide QRS.

The QRS complex is the tallest and sharpest part of each ECG beat. It reflects the depolarisation of the ventricles, which then contract and pump blood to the lungs and body. A Q wave is the first downward deflection, the R wave the first upward deflection, and the S wave the downward deflection after R; not all three are seen in every lead. Normally the impulse spreads through the fast His–Purkinje system, so the QRS is narrow. When conduction is blocked or the beat starts in the ventricle itself, the QRS becomes wide.

ECGVentriclesBundle branch block

Typical adult≈ 70–100 ms

Upper limit≈ 110 ms

Incomplete block / IVCD110–119 ms

Complete bundle branch block≥ 120 ms

Figure 1. The QRS complex (highlighted) ends at the J point, where the ST segment begins.

Wide QRS complex

Left or right bundle branch block (QRS ≥ 120 ms with typical shapes)
Non-specific intraventricular conduction delay, often in cardiomyopathy and heart failure
Ventricular beats and ventricular tachycardia, ventricular pacing
Pre-excitation (WPW, delta wave)
Hyperkalaemia and sodium-channel-blocking drugs (class I antiarrhythmics, tricyclic antidepressant overdose)

Figure 2. Left: normal narrow QRS. Middle: left bundle branch block (lead V6). Right: right bundle branch block with rsR′ (lead V1). Both blocks: QRS ≥ 120 ms.

Amplitude and shape

Very tall QRS voltages may indicate ventricular hypertrophy (for example from hypertension or aortic stenosis), although they are common in young, slim people. Low voltage occurs with pericardial effusion, obesity, emphysema, hypothyroidism and infiltrative heart disease. Pathological Q waves may indicate an old myocardial infarction.

Seek emergency care

if a wide-QRS tachycardia, new bundle branch block with chest pain, or fainting is found. A new left bundle branch block with symptoms of ischaemia is treated as a possible heart attack.

References

Surawicz B, Childers R, Deal BJ, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances. Circulation. 2009;119(10):e235–e240. doi:10.1161/CIRCULATIONAHA.108.191095
Thygesen K, Alpert JS, Jaffe AS, et al. Fourth universal definition of myocardial infarction (2018). Eur Heart J. 2019;40(3):237–269. doi:10.1093/eurheartj/ehy462

This article is for general information and does not replace a consultation with a doctor. ECG findings must be interpreted by a physician together with symptoms and other tests.

Published on 7 July 2019