Cardiac resynchronization An ICD is a tiny, battery-operated device for monitoring the rhythm of your heart and detecting irregular heartbeats. It's attached to wires (called leads) which are introduced into the heart through a vein (transvenous). ICDs are very useful in preventing sudden death in patients with known, sustained ventricular tachycardia or fibrillation. 

An ICD is made of the following parts: 

• The pulse generator is the size of a large pocket watch. It contains a battery and electrical circuits that read the electrical activity of your heart.
• The electrodes are wires, called leads, which reach your heart through veins and connect your heart to the rest of the device. Your ICD may have 1 to 3 electrodes.
• Most ICDs have a built-in pacemaker. Your heart may need pacing if it is beating too slowly or too fast, or if you have had a shock from the ICD.
• A subcutaneous ICD is a type of ICD that is placed under the skin. Instead of being implanted in the heart, the lead of this device is implanted in the tissue to the left of the breastbone. This type of ICD is unable to function as a pacemaker.

Reference: Implantable cardioverter-defibrillator: MedlinePlus Medical Encyclopedia. (n.d.). Retrieved from https://medlineplus.gov/ency/article/007370.htm