Early After-depolarizations and Torsade De Pointes: Implications for the Control of Cardiac Arrhythmias by Prolonging Repolarization

Common clinical features in drug-induced torsade de pointes include hypokalemia and cycle-length prolongation just prior to initiation of the arrhythmia. In canine Purkinje fibres, drugs known to be associated with torsade de pointes, such as quinidine, sotalol or N-acetylprocainamide, consistently produce early after-depolarizations (EADs) and triggered activity at slow drive rates; for quinidine, these abnormalities are exaggerated by low extracellular potassium. Triggered activity can be abolished in vitro in two ways. First, action-potential shortening with abolition of EADs can be accomplished by increasing stimulation rates, beta-stimulation and action-potential shortening antiarrhythmics such as lidocaine. Second, triggered activity can be suppressed, with less prominent effects on EADs, by magnesium, alpha- and/or beta-adrenergic blockade and calcium-channel blockers. The parallels between these in vitro findings and clinical torsade de pointes suggest that EADs and triggered activity play a role in the genesis of the clinical arrhythmia. Further research directed at determining the mechanisms underlying the cellular abnormalities and their propagation to the whole heart should yield information that will increase the safety of antiarrhythmic therapy.