It causes a decrease in refractory period in atrial and ventricular cells of heart muscles. It causes an increase in effective refractory period and decreasing conduction velocity in AV node.
Atrial fibrillation and flutter (while controlling ventricular response rate).
It may cause ventricular beats originating from abnormal place. These beats may cause ventricular tachycardia and fibrillation. (treated with lidocaine or phenytoin)