QUINIDINE (SUBGROUP 1A)
Quinidine
has actions similar to those of procainamide: it slows the upstroke of the
action potential, slows conduction, and prolongs
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.
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