QUINIDINE (SUBGROUP 1A)
Quinidine has actions similar to those of procainamide: it slows the upstroke of the action potential, slows conduction, and prolongs the QRS duration of the ECG, by blockade of sodium channels.
The drug also prolongs the action potential duration by blockade of several potassium channels. Its toxic cardiac effects include excessive QT-interval prolongation and induction of torsades de pointes arrhythmia. Toxic concentrations of quinidine also produce excessive sodium channel blockade with slowed conduction throughout the heart.
Adverse GI effects of diarrhea, nausea, and vomiting are observed in one third to one half of patients. A syndrome of headache, dizziness, and tinnitus (cinchonism) is observed at toxic drug concentra-tions. Idiosyncratic or immunologic reactions, including thrombo-cytopenia, hepatitis, angioneurotic edema, and fever, are observed rarely.
Quinidine is readily absorbed from the GI tract and eliminated by hepatic metabolism. It is rarely used because of cardiac and extra-cardiac adverse effects and the availability of better-tolerated anti-arrhythmic drugs.