What is an artificial pacemaker? Why it is called a life-saving instrument?

It is an electronic device, which regularly sends a small amount of electrical charge for maintaining the rhythmicity of the heart. It is implanted subcutaneously in the upper thoracic region having a connection with the heart.
The artificial pacemaker is called a life-saving instrument because, it stimulates the heart electrically at regular intervals to maintain its beats. Thus, it replaces the SA node as the originator of the cardiac impulse. It is to be noted that some pacemakers stimulate the heart at a fixed rate, others sense when the natural heart rate falls below a predetermined value and then stimulate the heart.