Pharmacokinetic studies of quinidine in patients with arrhythmias.

The absorption and disposition of quinidine were measured in nine patients following single oral and intravenous dosing. A new specific chromatographic method was used to measure the drug in plasma and urine. After intravenous administration, the plasma half-life (t1/2beta) was 7.8+/-0.7 h, the volume of distribution (Vd) was 3.0+/-0.5 liters/kg, and the total body clearance was 4.8+/-0.8 ml/min/kg. After oral administration, 87+/-7% (mean+/-SEM) was available to the systemic circulation. Quinidine was removed primarily by hepatic metabolism, with the renal clearance averaging only 1.0+/-0.2 ml/min/kg. Mean quinidine concentrations were estimated in 42 patients on chronic therapy by averaging blood levels during a dosing interval. In patients without heart failure, these corresponded well to mean drug levels predicted from the pharmacokinetic parameters measured after a single intravenous dose, but in patients with heart failure, the values for mean quinidine concentrations were higher than predicted. This suggests that impaired elimination of the drug or a decreased volume of its distribution, or both, may develop in heart failure.