question: What type of arrhythmia is known as bidirectional ventricular tachycardia (BDVT)?
answer: Bidirectional ventricular tachycardia (BVT), which is characterized by an alternating beat-to-beat ECG QRS axis, is a rare but intriguing arrhythmia associated with digitalis toxicity, familial catecholaminergic polymorphic ventricular tachycardia (CPVT), and several other conditions that predispose cardiac myocytes to delayed afterdepolarizations (DADs) and triggered activity. Bidirectional ventricular tachycardia (BVT) is an uncommon type of polymorphic ventricular tachycardia (PVT). Based on similarity of electrocardiographic features, bidirectional ventricular tachycardia has been considered a variant of long QT syndrome. Evidence from human and animal studies attributes BVT to alternating ectopic foci originating from the distal His-Purkinje system in the left and/or right ventricle, respectively. This "ping pong" mechanism of reciprocating bigeminy readily produces the characteristic ECG pattern of BVT and its degeneration to polymorphic VT if additional sites develop bigeminy.