Similar in structure to a pacemaker, the ICD consists of three parts:

The generator is a small box, usually about 2 inches wide and approximately 3 ounces in weight. Some generators are even smaller, measuring 1 inch in diameter and weighing about half an ounce. They are battery-powered and most use lithium batteries that need to be replaced every four to seven years. The generator is responsible for generating the electric shock.

Attached to the generator are leads, or wires, generally made of platinum with an insulating coating of either silicone or polyurethane. The leads carry the electric shock from the generator.

At the tip of each lead is a tiny device called an electrode that delivers the necessary electrical shock to the heart. The electric shock is created by the generator, carried by the leads and delivered by the electrodes to the heart. The decision of where to put the leads depends on the needs of the patient.

The heart has four chambers. The two lower chambers, or ventricles, perform most of the pumping action. The right ventricle pumps oxygen-poor blood that has returned from the body into the lungs to get more oxygen. The left ventricle takes that oxygen-rich blood and pumps it back out to the body. Though they are pumping the blood to different areas, the ventricles need to be synchronized (pumping at the same time) for blood to circulate properly.

The two upper chambers of the heart are called the atria (or atrium if referring to only one). They also pump blood, but with less force than the ventricles. Blood returning from the body to the heart enters the right atrium, which pumps that blood into the right ventricle and out to the lungs where it picks up a new supply of oxygen. Similarly, the left atrium pumps oxygen-rich blood returning from the lungs into the left ventricle, which in turn pumps the oxygen-rich blood through the aorta and out to the rest of the body. In order for the heart to function properly, the atria must not only be synchronized with each other, but with the ventricles as well.

The lead(s) of an ICD can be placed on one or two of the heart chambers in a variety of combinations depending on the needs of the patient. There are three types of ICDs:

• Single chamber ICD. A lead is attached in the right ventricle. If necessary, a low-energy shock can be delivered to the ventricle to help it contract normally.
  
  
• Dual chamber ICD. In this type of device, one lead is attached in the right atrium and one is attached to the right ventricle. If needed, a low-energy shock is delivered first to the atrium and then to the ventricle. This action helps the heart beast in a normal sequence and rhythm.
  
  
• Biventricular ICD. The leads from the generator are attached in the right atrium, right ventricle and left ventricle. By using this placement, the stimuli helps the heart beat in a more balanced way. This device is used specifically for individuals with heart failure.