Can you explain how the heart beats? when is pacemaker necessary?

Electrical Impulses : The heart is a muscle that will contract or beat when an electrical impulse is sent to it by the controlling sinus or av node located within the heart. The impulse is transmitted to the heart muscle by the conduction system. If there is a "failure" of the impulse originating node or a "short circuit" in the conduction system, the heart rate slows down and a pacemaker is needed to correct this.

SA node; slow heart: The sino-atrial (sa) node near the top of your right atrium is the natural pacemaker of the heart. A pacemaker is necessary if the native heart beat is so slow that you cannot maintain a normal blood pressure. Generally, a pulse of 60 or greater is needed to maintain a normal blood pressure, although some athletes can have lower heart rates.