For a low heart rate they usually implant a pacemaker, not a defibrillator. Pacemakers kick in when the heart rate gets too slow. A defibrillator is used for ventricular fibrillation ( v-fib) to shock the heart out of v-fib. There can be a combination device. When the heart rate gets too slow it is usually from second or third degree heart block. The pacemaker then stimulates the beats needed and is set to do that when the heart rate slows below a set rate. They are very safe and have been used for many years. Sometimes when the heart rate gets too slow the ventricals of the heart will try to take over and can lead to v-fib which would require a response from a defibrillator. As a critical care RN for 50 years, working CCU/ ICU/ER I have witnessed their useage. The question to ask is what is the cause of the slow heart rate. Sometimes medications can slow the heart rate. Ask if his EKG shows any damage from a heart attack. Third degree blocks need a pacemaker.