AMIODARONE AND QUINIDINE
The QT interval prolonging effects of quinidine and amiodarone are increased when they are used together, and torsade de pointes has occurred (see drugs that prolong the QT interval, for a general discussion of QT prolongation). Quinidine levels can be increased by up to 40% by amiodarone. (18)
Concurrent use should generally be avoided due to the QT prolonging effects of the combination. If the two drugs are considered essential, the dosage of quinidine may need to be reduced by 30 to 50%. Levels should be monitored where possible. The QT interval on the ECG should also be monitored and patients observed closely for quinidine-related adverse effects.