Quinidine and Verapamil interaction

Proarrhythmic effects Like many other drugs (including all other Class Ia antiarrhythmics), quinidine prolongs the QT c interval, and this can lead to torsades de pointes , a life-threatening ventricular arrhythmia (see OVERDOSAGE ).

This hazard may be decreased if partial atrioventricular block is achieved prior to initiation of quinidine therapy, using conduction-reducing drugs such as digitalis, verapamil, diltiazem, or a β-receptor blocking agent.

Hepatic clearance of quinidine is significantly reduced during coadministration of verapamil , with corresponding increases in serum levels and half-life.

For example, when quinidine and verapamil are coadministered in doses that are each well tolerated as monotherapy, hypotension attributable to additive peripheral α-blockade is sometimes reported.

Interactions with other dihydropyridine calcium channel blockers have not been reported, but these agents (including felodipine, nicardipine, and nimodipine ) are all dependent upon P450 IIIA4 for metabolism, so similar interactions with quinidine should be anticipated.

Until further data are obtained, avoid combined therapy of verapamil and quinidine in patients with hypertrophic cardiomyopathy.

Concomitant administration of quinidine [see Drug Interactions ( 7.10 )] preceded the severe hypotension in 3 of the 8 patients (2 of whom developed pulmonary edema).

Quinidine In a small number of patients with hypertrophic cardiomyopathy, concomitant use of verapamil and quinidine resulted in significant hypotension.

There has been a report of increased quinidine levels during verapamil therapy.

The electrophysiological effects of quinidine and verapamil on AV conduction were studied in 8 patients.