ECG in LBBB

QRS duration	≥0.12 sec
Intrinsicoid deflection	>0.05 sec in lead V6
Common pattern	Loss of q wave and wide slurred R in I, aVL, and V6 rS or QS in V1 ST depression and T wave inversion in leftward leads; ST elevation and upright T waves in right precordial leads
Common pattern	These changes reflect relentlessly leftward vectorial forces. V1 morphology depends on initial leftward or rarely rightward forces. ST and T vectors are usually 180º discordant to the QRS vector.
Variant pattern	Terminal S wave in V6 but not usually in I.
	Mean electrical axis superior and to the left (about -55º).
	ST depression and T wave inversion in leftward leads; ST elevation and upright T waves in right precordial leads
	These changes reflect superior and posterior displacement of loop with a terminal, often rightward, conduction delay. ST and T vectors are discordant to the QRS vector.

ECG_criteria_for_LBBB.htm

Major electrocardiographic criteria for left bundle branch block.

Graphic 81484 Version 1.0