Differential diagnosis of tachycardia with a typical left bundle branch block morphology

Table 2 Differential diagnosis of tachycardia with a typical left bundle branch block QRS morphology

Arrhythmia	ECG and clinical features
SVT with fixed left bundle branch block	LBBB present on baseline ECG
	QRS during tachycardia usually an identical match
SVT with functional LBBB aberrancy	Most often due to orthodromic AVRT
	At rapid rates, QRS alternans may be present
Atriofascicular antidromic tachycardia	Preexcitation may be minimal or absent during sinus rhythm
	Late QRS transition, leftward axis common
	Frequently coexists with other accessory pathways or AV nodal reentry
SVT with bystander atriofascicular accessory pathway	Accessory pathway does not participate in reentrant circuit of orthodromic AVRT, AVNRT, or atrial tachycardias (including atrial fibrillation and flutter)
Bundle branch reentrant ventricular tachycardia	Associated with acquired structural heart disease (cardiomyopathy, valvular disease)
	Prolonged PR interval and nonspecific IVCD often present during sinus rhythm

SVT: Supraventricular tachycardia; AVRT: Atrioventricular reentrant tachycardia; IVCD: Intraventricular conduction defect; AVNRT: Atrioventricular nodal reentrant tachycardia.