Quinidine: an update on therapeutics, pharmacokinetics and serum concentration monitoring

After 70 years of regular use, quinidine remains one of the most useful drugs in the management of both supraventricular and ventricular arrhythmias. Quinidine sulfate, from conventional dosage forms, is absorbed rapidly and reaches peak levels in 1 hour. Sustained-release quinidine sulfate and gluconate are absorbed more slowly and, thus, may provide a more satisfactory response with less fluctuation in the peak and trough at quinidine concentrations. The half-life and absorption characteristics of sustained-release quinidine suggest that an 8- to 12-hour dosage regimen will produce smooth, sustained quinidine levels. Whereas quinidine and digitalis have been used concomitantly for years, dramatic elevations in blood digoxin levels have been observed. When quinidine is added to the regimen of a patient taking digitalis, these increased digoxin levels may be responsible for some of the adverse effects previously attributed to quinidine administration and can be reduced by adjustment of the digoxin dosage. Drug monitoring of serum quinidine concentrations as well as peak levels is essential to assess patient compliance and to determine that therapeutic levels are maintained. This is especially important because of the drug's relatively narrow therapeutic index and the changes in clearance that can result from drug interactions and renal and hepatic disturbances. A personal computer program is available to carry out pharmacokinetic calculations and assist in determining appropriate doses for individual patients.