Causes
Long QT syndrome can either be inherited as congenital mutations of ion channels carrying the cardiac impulse/action potential or acquired as a result of drugs that block these cardiac ion currents.
Common causes for torsades de pointes include diarrhea, hypomagnesemia and hypokalemia. It is commonly seen in malnourished individuals and chronic alcoholics. Certain combinations of drugs resulting in drug interactions may contribute: decreasing the metabolism of a medication causing QT elongation such as clarithromycin (Biaxin) or haloperidol (Haldol), taken concomitantly with a specific cytochrome P450 inhibitor like fluoxetine (Prozac), cimetidine (Tagamet); foods like grapefruit will result in higher than normal doses of the medication responsible for the QT elongation. Since these specific drugs worsen the elongation of the QT wave in a dose-dependent manner, inhibition of drug metabolism raises the risks of developing a malignant torsades de pointes arrhythmia.
Various other medications are known to cause QT elongation. Examples include amiodarone, methadone, lithium, chloroquine, erythromycin, and phenothiazines. It can also be the side effect of some antiarrhythmic medications such as sotalol, procainamide and quinidine. The gastrokinetic drug cisapride (Propulsid) was withdrawn from the US market in 2000 after such interactions lead to deaths caused by long QT syndrome-induced torsades de pointes.
Read more about this topic: Torsades De Pointes