Quinidine plasma concentration and exertional arrhythmia.

Quinidine gluconate was used to treat arrhythmia induced with maximal exercise testing. Twenty-nine subjects who had previously developed frequent premature contractions on testing were selected for further study. After a control maximal exercise test, quinidine (10 mg. per kilogram) in solution was given orally in a single dose, and two hours later the same test was repeated. Recurrence of premature contractions was completely prevented in five of the 19 subjects tested; suppression was better than the mean value in three others, and in 11 subjects it was below the mean value. The plasma concentration at two hours was 1.68 plus or minus 0.31 ug per milliliter, which is a therapeutic level. Raising the dose to 15 mg. per kilogram eliminated the premautre contractions in six subjects whose response to 10 mg. per kilogram had not been complete, but not in two others. Lowering the dose to 5mg. per kilogram lowered the plasma level to below the therapeutic level. No differences between the responses to the drug of the otherwise healthy subject and those with symptomatic heart disease could be found. Compared with the responses to the control tests, there were small but significant changes in the second test in heart rates, blood pressure, and duration of exercise. Aerobic working capacity estimated by Vo2max was unchanged. Mild toxic effects manifested by malaise and diarrhea were a common finding with both 10mg per kilogram and 15mg per kilogram of quinidine, but not with 5mg per kilogram.