Effects of antiarrhythmics and hypokalemia on the rate adaptation of cardiac repolarization.

OBJECTIVES: In normal conditions, sudden heart rate acceleration provokes a rapid reduction in ventricular action potential duration (APD). The protracted APD rate adaptation favors early afterdepolarizations and precipitates arrhythmia. Nevertheless, it is uncertain as to whether the rate-dependent changes of ventricular repolarization can be adversely modified by arrhythmogenic drugs (quinidine and procainamide) and hypokalemia, in comparison to the agents with safe therapeutic profile, such as lidocaine.

DESIGN: The rate adaptation of QT interval and monophasic APD obtained from the left ventricular (LV) and the right ventricular (RV) epicardium was examined during rapid cardiac pacing applied in isolated, perfused guinea-pig heart preparations.

RESULTS: At baseline, an abrupt increase in cardiac activation rate was associated with a substantial reduction of the QT interval and ventricular APD in the first two cardiac cycles, which was followed by a gradual shortening of repolarization over subsequent pacing intervals. The time constants of the fast (τfast ) and slow (τslow ) components of the APD dynamics determined from a double exponential fit were longer in RV compared to LV chamber. Quinidine, procainamide, and hypokalemia prolonged ventricular repolarization and delayed the rate adaptation of the QT interval and APD in LV and RV, as evidenced by increased τfast and τslow values. In contrast, lidocaine had no effect on the dynamic changes of ventricular repolarization upon heart rate acceleration.

CONCLUSIONS: The rate adaptation of ventricular repolarization is delayed by arrhythmogenic interventions, such as quinidine, procainamide, and hypokalemia, but not changed by lidocaine, a clinically safe antiarrhythmic agent.