Arrhythmias or dysrhythmias are abnormal rhythms of the heart. Arrhythmias cause symptoms such as palpitations or dizziness and can cause the heart to pump less effectively. Treatment may include medications, a pacemaker, an implantable defibrillator or surgery.

The heart's natural pacemaker develops an abnormal rate or rhythm, the normal conduction pathway is interrupted or another part of the heart tries to take over as the pacemaker (under some conditions, almost all heart tissue is capable of starting a heartbeat and thus acting like the pacemaker).

You can find further information here: The American Heart Association Website