Quinidine dosage in children using population estimates.

A total of 36 plasma quinidine concentrations from nine hospitalized pediatric patients with cardiac arrhythmias were examined retrospectively to determine factors significantly affecting quinidine dosing. Each plasma quinidine concentration was obtained after at least 24 h of inpatient therapy. Doses of quinidine base varied from 7.7 to 45.6 mg/kg/day and from 179 to 921 mg/m2/day. Six of the eight children who responded to quinidine achieved normal sinus rhythm with plasma concentrations less than 2.0 micrograms/ml. Stepwise multilinear regression analysis demonstrated that the plasma concentration was significantly affected by the quinidine dose and by the time after the dose that the plasma was obtained, but not by the age of the patient. Patient population estimates were then derived to predict the quinidine dosage necessary to achieve given plasma concentrations. The group of patients receiving digoxin concurrently were predicted to obtain higher plasma quinidine levels on smaller doses and a shorter quinidine elimination half-life compared with those patients not on digoxin. While currently recommended quinidine doses by body weight are frequently insufficient, recommended quinidine doses by body surface area are excessive. Children require larger quinidine doses on a body weight basis and respond to a wide range of plasma quinidine concentration. Patient population estimates provide useful information on dosing of infrequently prescribed drugs in children.