Although classified as a sodium channel blocker, quinidine also blocks K+ channels. Most antiarrhythmic agents have such multiple actions.

Sodium channel blockade results in an increased threshold and decreased automaticity.

Potassium channel blockade results in AP prolongation.

Quindine is used to maintain normal sinus rhythm in patients who have experienced atrial flutter or fibrillation. Also, it is used to prevent ventricular tachycardia or fibrillation.

Quinidine administration results in vagal inhibition (anti-muscarinic) and alpha-adrenergic receptor blockade. The antimuscarinic effect of quinidine facilitates transmission of atrial impulses to the ventricles. In atrial fibrillation such action can increases ventricular rate. To protect against this effect digitalis (digoxin/digitoxin) may be administered because digitalis inhibits transmission through the AV node.

Adverse effects include cinchonism (headaches and tinnitus), diarrhea.

Quinidine is also associated with torsades de pointes, a ventricular arrhythmias associated with marked QT prolongation. This potentially serious arrhythmia occurs in 2% - 8% if patients, even if they have a therapeutic or subtherapeutic quinidine blood level.