Automatic internal cardiac defibrillators are more commonly known as implantable cardioverter defibrillators, or ICDs. This life-saving tool can prevent sudden cardiac death in patients at high risk for the development of fatal heart rhythms.
Definition
An ICD consists of a computer, power generator and lead wires that are surgically implanted into the chest wall and attach to one or more of the four heart chambers. The lead wires transmit an electrical shock any time the heart incurs a life-threatening, irregular rhythm, known as an arrhythmia. The Mayo Clinic identifies the two main arrhythmias that ICDs correct as ventricular tachycardia and ventricular fibrillation. These arrhythmias reduce the amount of oxygen delivered to the brain, heart muscle and other tissues of the body, leading to sudden cardiac arrest. It is the job of the ICD to recognize these potentially fatal rhythms and to deliver an electrical shock that will jolt the heart back into its normal rhythm.
Function
The computer chip within an ICD monitors the heart's rhythm. If the heart begins to beat erratically, the ICD's computer will prompt its pulse generator to deliver a low-voltage electrical stimulus through the connecting wires in an attempt to correct the rhythm. If the arrhythmia persists, the ICD will deliver a much stronger, high-energy electrical shock. These stronger bursts can be painful to the patient.
According to the National Heart Blood and Lung Institute, ICDs can function as a single-chamber or dual-chamber stimulator, depending on the number of lead wires connecting the heart to the ICD unit. An important characteristic of ICDs is that they can record the various rhythms detected by a patient's ICD. The physician can then retrieve this information at a later time to observe the ICD's activity and to make adjustments to the ICD's settings, such as when to deliver light shocks versus strong shocks.
Who Needs an ICD
ICDs are recommended for patients with persistent ventricular arrythmias, such as ventricular tachycardia or fibrillation. Patients at high risk for sudden cardiac death or those with a heart muscle that has been weakened by previous heart attacks are also good candidates for ICD therapy. While the largest population of individuals receiving ICDs are adults, children and adolescents that have an inherited, or congenital, heart disease that causes a defect in the structure of the heart muscle may qualify for ICD implantation.
ICDs and Pacemakers
ICDs are commonly mistaken for pacemakers, however, they are not the same thing. Guidelines for the use of ICDs published by the American College of Cardiology explain that pacemakers are used to pace the heart when it beats at a rate too slow to deliver adequate oxygen to the tissues of the body. An ICD, on the other hand, is used to treat potentially fatal, irregular rhythms. Unlike the ICD that can deliver high-energy shocks to override dangerous rhythms, the pacemaker can only emit a low-energy electrical stimulus. If a patient's normal rhythm is too slow and he has recurrent ventricular arrhythmias, he may be a candidate for a combination pacemaker-ICD, which will perform the functions of both devices.
Risk
It is not uncommon for an ICD to deliver an unnecessary shock. This tends to occur in exercising individuals whose heart begins to beat rapidly, or in those whose ICD wires have been damaged. Because children tend to be more active than adults, unnecessary shocks are more common in younger patients.
As with all surgeries, there are risks during the implantation of an ICD. It is possible for the implant site to become infected or to bleed. There is also a risk of damage to the heart, nervous system or lungs. The American College of Cardiology promotes a risk stratification model to determine eligibility for ICD surgery. This stratification weighs the risk of surgery against the benefit of ICD implantation. Generally, but not always, the benefit of therapy outweighs the potential risks involved with receiving the device.
References
- Mayo Clinic: Implantable Cardioverter Defibrillators (ICDs)
- National Heart Lung and Blood Institute: Implantable Cardioverter Defibrillators
- "Journal of Invasive Cardiology"; ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities; Stephen C Vlay, M.D.; 2009
