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Quinidine is the oldest primary antiarrhythmic drug in clinical practice. It is used to control atrial fibrillation and flutter and to suppress ventricular arrhythmias. Quinidine is usually administered orally and absorbed in the small intestine. About 80% of the quinidine in plasma is protein bound. The liver extensively metabolizes quinidine and only 20% of parent drug is excreted unchanged in the urine.

The most common side effects are diarrhea, headache, palpitations, rash and tremor. The major toxic effects are ventricular arrhythmias, hypotension, vomiting, diarrhea, and tinnitus. Hepatic disorders, renal failure and cardiac failure elevate plasma levels. Hyperkalemia increases the effects of quinidine. A synergistic interaction exists between quinidine and digoxin.

Measurement of serum quinidine level is useful to confirm suspected toxicity, adjust the current dose, and assess patient compliance. The circulating half-life is 4 to 9 hours. Steady state levels are reached two days after oral dosing. The recommended sampling time for a trough level is 4 to 6 hours after the last dose.

Therapeutic range (trough level) is 2.0-5.0 ug/mL. Levels >6.0 ug/mL are considered critical values.

Specimen requirement is one plain red top tube. SST tubes are not acceptable.

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