Varies by problem. Preventing problems from very low heart beat such as heart failure, passing out and in extreme cases cardiac arrest. Also symptoms such as fatigue and shortness of breath from low heart beat may improve. Sometimes it is used when medications treating fast heart beats also causes very slow heart beat. A special resynchronization pacemaker with three leads may be used to treat heart failure.
Prevent syncope. Pacemakers prevent the heart rate to go below certain programmed parameter (usually 60 beats per minute). The most common reasons are sick sinus syndrome and advanced atrioventricular ventricular blockade. Pacemaker prevent the heart to stop in severe bradycardia (caused by medicines or disease) causing syncope with loss of consciousness and injuries related to them.
Pacemaker. It does not let your heart go below certain set rates.