Effects of quinidine versus procainamide on the QT interval.

Eighteen patients were given quinidine and procainamide separately to evaluate whether prolongation of the QT interval by type Ia antiarrhythmic agents is a drug-specific phenomenon. Doses were titrated to achieve standard trough therapeutic levels of quinidine (2 to 5 micrograms/ml) and procainamide (4 to 12 micrograms/ml). In 16 of the 18 patients, the increase in corrected QT interval (QTc) was greater with quinidine than with procainamide, averaging 78 +/- 10 ms (+/- standard error of the mean) with quinidine and 39 +/- 7 ms with procainamide (p less than 0.001). The greater degree of QTc prolongation with quinidine than with procainamide was not due to differences in sinus cycle length, QRS duration, serum potassium level or concomitant drug therapy. Differences in relative drug level did not appear to account for the greater effect of quinidine. Thus, at frequently used plasma levels, quinidine prolongs QTc to a greater degree than does procainamide. This effect does not appear to be due to the comparison of "nonequivalent" drug levels.