Any irregular and sustained BCT should be assumed to be atrial fibrillation (AF) with either pre-existing bundle branch block or aberrant conduction (Fig 1). It is rare for such a rhythm to be ventricular in origin. Although polymorphic VT is irregular, it is rarely sustained.

AF may occasionally masquerade as polymorphic VT when it is in the presence of pre-excitation (Fig 2). The AF gives rise to an irregular rhythm and the variable conduction down the accessory pathway gives rise to QRS complexes, which do change in morphology due to the presence or absence of delta waves giving a similar appearance to Torsades.

 Fig 1: Atrial fibrillation and left bundle branch block

Fig 2: Pre-excitation and atrial fibrillation

Any BCT that is sustained and irregular is most likely to be AF.