Relation of ventricular premature beat suppression to serum quinidine concentraton determined by a new and specific assay.

Fourteen patients who were receiving quinidine in doses of 800 to 1800 mg. per day for ventricular arrhythmias underwent Holter monitoring during a steady state dosing interval at a mean of four days after the initiation of quinidine therapy. Serum quinidine concentration, determined by a specific high performance liquid chromatography method, was measured hourly during the dosing interval. Ventricular premature beat (VPB) frequency during quinidine therapy was compared to the baseline VPB frequency. A reduction in VPB frequency of at least 90% was required to substantiate the presence of a therapeutic response to quinidine. In 12 of the 14 patients a therapeutic response to quinidine was present at serum levels ranging from 0.72 to 5.92 micrograms/ml. There was no group correlation between serum quinidine concentration and VPB frequency, but there was a tendency in individual patients for VPB frequency to decrease as serum quinidine level increased. Quinidine toxicity was not observed in these 14 patients. Because of the wide variation in response to quinidine, a serum quinidine concentration that is within the therapeutic range is not necessarily the optimal serum quinidine concentration for an individual patient. The clinician may therefore consider increasing the dose if there is no evidence of quinidine toxicity and the ventricular rhythm disturbance is not adequately controlled.