Class Ia antiarrhythmic drugs—including drugs such as quinidine, procainamide, and disopyramide—affect the cardiac cycle in specific ways and lead to specific ECG changes, as shown on the rhythm strip below. Class Ia antiarrhythmics

• block sodium influx during phase 0, which depresses the rate of depolarization.
• prolong repolarization and the duration of the action potential.
• lengthen the refractory period (in accessory pathways as well).
• decrease contractility.