A comparison of plasma concentration arrhythmia response and electrocardiographic changes in patients requiring quinidine therapy

Quinidine is a naturally occurring substance used in the treatment of cardiac arrhythmias. It may be administered orally or parenterally to control both ventricular and supraventricular arrhythmias. Because of its low therapeutic index, it is rational to obtain plasma concentrations of quinidine as a guide to the safest and most effective dose. Unfortunately, past studies that were designed to compare plasma quinidine concentration to arrhythmias control and electrocardiographic (ECG) changes used nonspecific assay techniques. Such methods can neither completely nor consistently separate quinidine from its metabolites and contaminants which may differ in their myocardial and possibly their toxic effects. Our experience has been that arrhythmia control can often be achieved with plasma quinidine concentrations lower than the reported therapeutic ranges. The availability of a high performance liquid chromatography (HPLC) method provides an opportunity to establish a more accurate therapeutic range using a highly specific assay technique. This has not been adequately studied although it is suggested that therapeutic efficacy may be achieved at a concentration 8 9 of 1.0 or 0.7 mcg/ml.