Implantable Cardioverter-Defibrillator (ICD)

Defibrillators are devices that deliver an electric shock to the heart to terminate an abnormal rhythm and allow the normal rhythm to resume. Defibrillators are famous from television and movies as the devices used to shock cardiac arrest victims in emergency rooms. These external defibrillators remain in common use for the termination of fast rhythms in emergency situations. Prior to 1980, the electrical parts (and the high energy required to shock the heart from the outside of the body) caused defibrillators to be large, bulky devices that occupied toolbox-sized spaces and required frequent recharging from electrical outlets. Advances in technology have allowed engineers and designers to miniaturize the components of a defibrillator into a device approximately the size of a small deck of playing cards. These small devices can be attached to the heart by plastic coated metal wire and the system can function as a self-contained automatic defibrillator. Such a system is commonly referred to as an Implantable Cardiac Defibrillator or ICD. The metal encased electronics is referred to as the ICD generator and the plastic coated metal wire inside the veins and attached to the heart is the ICD lead. The ICD when implanted creates a small but noticeable bulge under the skin of the left chest as shown in this patient.

Because of their small size and the fact that the leads can be inserted via a vein in the shoulder region, most ICDs are placed in skin of the chest wall just below the collarbone. This vein can be reached through the skin in a procedure requiring only local anesthesia.

Two categories of leads exist. The sensing lead or leads are cables that carry the heart rate information back to the heart and, in most modern defibrillators, can carry low energy pacing impulses back to the heart. The defibrillation lead or leads carry the high- energy shock from the generator to the heart when the generator decides that a shock is necessary. In early ICD models, the leads were placed on the exterior of the heart in a surgical procedure involving opening the chest or tunneling through the diaphragm. Such procedures required general anesthesia and involved a long recovery period and frequent complications. These types of leads may still be used in cases of complex heart disease where the tricuspid valve has been replaced with an artificial valve or where operational external leads are already present. Currently the ICD leads used are combination leads which function as both sensing leads and shocking leads in one composite lead.

The ICD generator is the component that houses the battery and the "brain" of the system and is most often placed in a "pocket" formed under the skin and fat on the left chest and above the muscle of the chest as shown in this patient. The metal exterior of the generator often serves as one of the defibrillation leads. The electronics in the generator include circuitry for delivering shocks or other therapies as well as circuitry devoted to memory. The device must make all these decisions quickly and automatically. Finally, the device must possess a generous and reliable power source to allow all this to take place Whenever the ICD delivers a therapy, it records the date and time of the occurrence, a summary of the therapy it delivers, and a recording of the electrical signals from the heart that allows the physician to determine with greater certainty what actually happened. This information may allow for different therapeutic approaches to be tried. Once implanted, ICDs obviously contain a wealth of information and therapeutic options. These options are accessible by communicating through the skin with the ICD using a programmer. Opening the skin is not necessary for most adjustments.

The original ICDs were limited in their approach to the treatment of tachycardias. If a tachycardia was sensed that met or exceeded the heart rate for which the ICD was programmed to pay attention, the ICD delivered a shock. Most patients perceive a shock as a hard blow or "kick" to the chest if they are conscious at the time the shock is delivered. Although the shock is short in duration and results in no damage to the body, it is generally regarded as uncomfortable and even frightening. In later ICD models, other therapeutic options became available that may also be effective but less traumatic than a shock. The ICD can be programmed to recheck the rhythm before delivering a shock and "abort" (not deliver) that shock if the tachycardia stops spontaneously before the shock is ready to be delivered. Antitachycardia pacing (ATP) is a therapy where the pacemaker function of the ICD delivers a series of paced beats at a rate exceeding the tachycardia rate. All current ICDs can also function as basic traditional bradycardia pacemakers in case the sensed heart rate falls below a certain level. Some models of pacemakers have more sophisticated bradycardia pacing capabilities in addition to defibrillation capabilities. These additional features include the ability to increase the pacing rate with activity, rate responsive, ability to pace both the atrium and ventricular in a synchronize fashion like in a dual chamber pacer, as well as all the sophisticated pacing features currently able in most pacemaker implanted for bradycardias.

ICDs arose from the need for better treatments for individuals with life-threatening arrhythmias. As noted elsewhere, the recurrence rate of sudden death in individuals without ICDs who were treated with medications, coronary artery bypass, or angioplasty is up to 30-40% annually. Several clinical studies have shown the remarkable effectiveness of ICDs in preventing sudden death in individuals at risk. One such study released recently is the Antiarrhythmics Versus Implantable Defibrillators (AVID) study sponsored by the U.S. National Institutes of Health. In this study, 1,106 patients with ventricular tachycardia or sudden cardiac death were randomized to receive either medications (amiodarone or sotalol) or an ICD for prevention of death. After one year, there were 39% fewer deaths on the ICD group. After two years, there were 27% fewer deaths among ICD recipients. After three years, the study was halted prematurely because of the clear benefit of having an ICD. Another contemporary trial, the MADIT study, demonstrated a 54% reduction in death among patients with ventricular tachycardia when treated with an ICD rather than antiarrhythmic drugs alone.

Current medical guidelines for ICDs suggest their use in: Patients who have survived at least one episode of cardiac arrest due to a ventricular tachyarrhythmia; Patients who have recurrent, poorly tolerated ventricular tachycardia; and Patients who have been demonstrated to be at risk for sudden cardiac death by having structural heart disease with poor left ventricular function and ventricular tachycardia provoked at electrophysiologic study.

Despite the remarkable effectiveness of ICDs in the prevention of sudden cardiac death in individuals who have ventricular tachycardia or a history of sudden death, the vast majority of victims of sudden cardiac death die without being resuscitated.

It is clear from many studies that individuals with poor left ventricular function are at increased risk of sudden cardiac death; what is not known is whether routine ICD implantation in this population will improve survival. The National Institutes of Health is currently sponsoring the SCD-HEFT (Sudden Cardiac Death-HEart Failure Trial) study to examine the benefit of routine ICD implantation in people with congestive heart failure. Enrollment in this study is ongoing at the Texas Arrhythmia Institute.