Torsades de pointes manifests as prolongation of the QTc interval on the electrocardiogram (ECG).
Class IA drugs (quinidine and disopyramide) and class III drugs (amiodarone) prolong the QTc interval by potassium channel blockade providing the setting for torsades de pointes.
Drug-induced torsades de pointes is often associated with bradycardia because the QTc interval is longer at slower heart rates.
Incessant ventricular tachycardia may be precipitated by drugs that slow conduction of cardiac impulses (class IA and class IC drugs) sufficiently to create a continuous ventricular tachycardia circuit (reentry). Incessant ventricular tachycardia is more likely to occur with high doses of class IC drugs and in patients with a prior history of sustained ventricular tachycardia and poor left ventricular function.
Wide complex ventricular rhythm is usually associated with class IC drugs in the setting of structural heart disease.