Drugs and the QTc interval

Many commonly used drugs can prolong the QT_c interval, especially if used in combination with other substances which affect their metabolism. Prolongation of the QT_c interval can cause life-threatening polymorphic ventricular tachycardia also known as torsade de pointes. Women and certain susceptible people are more prone to prolongation of the QT_c interval. This predisposition could be congenital or due to conditions such as hypokalaemia, hypomagnesaemia, renal failure or cardiac failure. Susceptible patients need an electrocardiogram before and after starting drugs that can prolong the QT_c interval. If a drug prolongs the QT_c interval beyond normal limits, the benefits of continuing the drug should be weighed against the relatively rare risk of potentially life-threatening arrhythmias.