Arrhythmias are abnormal beats
The term "arrhythmia" refers to any change from the normal sequence of electrical impulses, causing abnormal heart rhythms. Arrhythmias may be completely harmless or life-threatening.

Some arrhythmias are so brief (for example, a temporary pause or premature beat) that the overall heart rate or rhythm isn't greatly affected. But if arrhythmias last longer, they may cause the heart rate to be too slow or too fast or the heart rhythm to be erratic – so the heart pumps less effectively.

• A fast heart rate (in adults, more than 100 beats per minute) is called tachycardia.
• A slow heart rate (less than 60 beats per minute) is referred to as bradycardia.

• Normally, the heart's most rapidly firing cells are in the sinus (or sinoatrial or SA) node, making that area a natural pacemaker.
• Under some conditions almost all heart tissue can start an impulse of the type that can generate a heartbeat. Cells in the heart's conduction system can fire automatically and start electrical activity. This activity can interrupt the normal order of the heart's pumping activity.
• Secondary pacemakers elsewhere in the heart provide a "back-up" rhythm when the sinus node doesn't work properly or when impulses are blocked somewhere in the conduction system.

• The heart's natural pacemaker develops an abnormal rate or rhythm.
• The normal conduction pathway is interrupted.
• Another part of the heart takes over as pacemaker.