A broad complex tachycardia with conflicting information from pacing manoeuvres.

A patient with a biventricular implantable cardioverter defibrillator for heart failure had a recurrent broad complex tachycardia and underwent electrophysiologic testing. The tachycardia was induced only with ventricular pacing. There was a 1:1 atrioventricular relationship with simultaneous atrial and ventricular activation. However, atrial pacing during tachycardia suggested atrial dissociation from the circuit. The findings, potential mechanisms, and treatment are discussed.