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
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.