What is an implantable cardioverter defibrillator (ICD)?
An implantable cardioverter defibrillator (ICD) is a device that is implanted in the chest to detect and stop dangerous heart rhythms that can cause sudden cardiac death (SCD). If the heart begins beating dangerously fast or its contractions become disorganized, the ICD delivers an electrical shock to jolt the heart back to a normal rhythm.
ICDs are implanted in women who are at high risk of developing dangerous rhythms in the pumping chambers of the heart. You may be a candidate for ICD implantation if you are at high risk for these abnormal rhythms because of damage to your heart (such as from a previous heart attack or heart failure), or if you have already experienced dangerous heart rhythms. ICDs are implanted in 34,000 American women every year.
How does an ICD work?
There are two parts to an ICD: the generator and the leads. The generator contains a battery and a small computer that constantly monitors and analyzes the heart’s rhythm. It determines when there is a heart rhythm problem (arrhythmia) and is programmed by your doctor to deliver a shock under certain conditions. The generator is implanted underneath the skin, usually on the left side of the chest. It may also be placed elsewhere in the abdomen.
Wires called leads are connected to the generator and run through large veins that lead to the heart, ending in the heart muscle. These leads carry information from the heart muscle to the generator, and electrical pulses or shocks from the generator to the heart muscle. The number and placement of the ICD leads vary (usually from one to three leads in different areas of the heart). Some ICDs are connected to only one of the main pumping chambers of the heart (the right ventricle), others to both main chambers (called bi-ventricular pacing). Your doctor will determine which type of ICD and leads are best for you.
If the ICD detects a heart rhythm problem, it will first try to correct it by pacing the heart, delivering small electrical impulses to try to take over the heart’s rhythm. If this does not work, it will try cardioversion, a series of slightly larger shocks timed when the heart is supposed to beat. If these methods fail, the ICD will deliver one or more large shocks called defibrillation to completely reset the electrical activity of the heart.
Why are women with heart failure at risk for dangerous heart rhythms?
In women with heart failure the heart does not pump as efficiently as it should, either because of damage to the heart muscle from a heart attack or toxins, or because gradual changes over time have caused the heart’s walls to thicken and become stiff. This damage impairs the heart’s mechanical ability to pump blood, but it can also damage the electrical pathways along which the heartbeat travels.
To pump blood effectively, the heart muscle must contract in a coordinated fashion with a regular speed and rhythm. The heart has an electrical system that carries signals telling different areas of the heart muscle when to contract. An arrhythmia is a disturbance in the heart’s normal rhythm. There are many types of arrhythmias, but the most dangerous ones in women and men with heart failure are ventricular tachycardia and ventricular fibrillation. The ventricles are the main pumping chambers of the heart: if they contract too quickly, it is called tachycardia. If the ventricle contracts in a disorganized manner (described by doctors as “quivering” or looking like a “bag of worms”), it is called fibrillation.
Either of these serious heart rhythm problems can cause sudden cardiac death (SCD) if they are not corrected in time. SCD kills as many as 1 in 3 patients with heart failure, although it is 3 to 4 times more common in men than in women.About 17% of women and 34% of men with heart disease die of sudden cardiac death, most of which are caused by ventricular fibrillation.
Patients with heart failure may also lose coordination between the two sides of their heart, preventing the heart from pumping properly. This causes fluid to build up in the lungs and legs, worsening symptoms such as shortness of breath and swelling. In these patients, both sides of the heart need to be electrically paced to beat together. This is called cardiac resynchronization therapy (CRT). Some people need both an ICD and a pacemaker (to provide CRT).