Pharmacokinetic analysis of intravenously and orally administered quinidine in horses

A pharmacokinetic study was made, using 7 healthy adult horses (weighing between 400 and 560 kg) given quinidine gluconate IV and quinidine sulfate orally. The apparent volume of distribution of quinidine base was 3.10 +/- 0.79 L/kg, total body clearance was 5.49 +/- 2.40 ml/minute/kg, and plasma half-life was 6.65 +/- 3.00 hours. The systemic availability of quinidine sulfate after oral administration of a 10 mg/kg dose was 48.5 +/- 20.4%. Oral administrations of quinidine sulfate in doses of 10 mg/kg and 10 g produced peak plasma concentrations of 0.79 microgram/ml at 146 minutes and 1.47 microgram/ml at 131 minutes, respectively. In 4 horses given 4 oral doses of 10 mg of quinidine sulfate/kg, each dose separated by 2 hours, maximal plasma concentrations of quinidine averaged 13% higher than the preceding minimal plasma concentration. The application of these pharmacokinetic determinations to the clinical use of quinidine in the horse is discussed.