hi and welcome. heart has an intrinsic rate and rhythm to sustain life. heart rate s below 40 and above 140 are usually not normal and are the consequence of many abnormalities of the conduction system of the heart. for example low rates in heart blocks including missed beats and premature beats ( possibly due to hardening of the conduction wires due to age or ischemia/ reduced blood supply to them in case of heart attack)... fast heart rates more likey are due to structural heart diseases ( flutters or fibrillations are these fast rhythm abnormalities) reduced blood supply to heart, or structural changes especially of left ventricle after massive heart attacks or dilatation of heart after certain viral illnesses all predispose to these fatal rapid heart rates. MOSTLY pace makers are inserted to treat heart blocks in which conduction is not anticipated or documented, a device with the size of a small biscuit, under the skin on the left chest below the collar bone with the wires inserted via the subclavian veins to the right side chambers of the heart . it depends on the patient condition as whether to pace single or dual chambers. some times these patients with dilated hearts can also trigger abnormally fast and chaotic rhythm that causes the patient to die in minutes ...thus pacemakers with defibrillating capacity are inserted so when the chaotic rhythm starts , it is detected and a shock to the patient is delivered and the rhythm stops. the pace maker generated rhythm then takes over again with a rate adjusted by the doctor....IT IS IMPLANTED BY A CARDIOLOGIST BASICALLY AND HE IS THE ONE WHO KEEPS ITS FOLLOW UP. thanks