What are the classic ECG features of left bundle branch block (LBBB)?

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Multiple Choice

What are the classic ECG features of left bundle branch block (LBBB)?

Left bundle branch block shows a delay in the left ventricle’s activation, which broadens the entire QRS complex. The hallmark is a QRS duration greater than 120 milliseconds, reflecting the slowed intraventricular conduction. In the lateral leads—I, aVL, and V5–V6—you typically see broad, often notched R waves. The ST segments and T waves are discordant with the QRS pattern, meaning they go in the opposite direction to the main QRS deflection (for example, a predominantly positive QRS in those leads is accompanied by ST depression and T wave inversion). This combination—wide QRS with broad/slurred lateral f waves and discordant ST-T changes—best fits classic LBBB.

Left bundle branch block shows a delay in the left ventricle’s activation, which broadens the entire QRS complex. The hallmark is a QRS duration greater than 120 milliseconds, reflecting the slowed intraventricular conduction. In the lateral leads—I, aVL, and V5–V6—you typically see broad, often notched R waves. The ST segments and T waves are discordant with the QRS pattern, meaning they go in the opposite direction to the main QRS deflection (for example, a predominantly positive QRS in those leads is accompanied by ST depression and T wave inversion). This combination—wide QRS with broad/slurred lateral f waves and discordant ST-T changes—best fits classic LBBB.

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