Pacemaker: 

A pacemaker is a medical device which uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart.  The primary purpose of the pacemaker is to maintain an adequate heart rate, either because the heart's native pacemaker is not fast enough, or there is a block in the heart's electrical conduction system.

Internal Defibrillator: 

An internal defibrillator, also known as an automatic internal cardiac defibrillator (AICD), is a medical device which can deliver a therapeutic dose of electrical energy or shocks to correct various life threatening arrhythmias.  This allows normal sinus rhythm to be reestablished by the body's natural pacemaker.