Normal myocardial contractile state in the presence of quinidine

Since quinidine is one of the few agents available to treat and prevent ventricular arrhythmias in ambulatory patients, its hemodynamic effects have been reevaluated. When given in therapeutic doses to anesthetized mongrel dogs, quinidine significantly reduced heart rate, aortic pressure and flow, but it did not significantly change the first derivative of the left ventricular pressure curve (left ventricular dp/dt) in nine dogs. A subsequent group of dogs was studied after vagotomy and practolol administration to block cardiac reflexes. This group showed significant reductions in heart rate, aortic pressure and left ventricular dp/dt, with the latter returning to predrug control values when preload, afterload and heart rate were maintained constant. These studies suggest that quinidine does not directly affect myocardial contractility when given in therapeutic doses. Furthermore, the reduction in heart rate in these animals provides support for a direct depressant effect of quinidine on the sinus node. The adverse effects of quinidine on cardiac function previously reported may be due to the use of toxic doses or are secondary to quinidine peripheral circulatory effects, rather than due to a direct reduction in cardiac contractile state.