A device, implanted in the body, that detects potentially fatal abnormal heart rhythms and delivers an electric shock to the heart to restore normal rhythm. The electric impulse may be substantially stronger than the tiny signal used by pacemakers to correct heartbeat. Of the two main components in the name “implantable cardioverter-defibrillator” (commonly abbreviated ICD), a cardioverter is a device that restores normal heart rhythm through an electric shock, and a defibrillator is an apparatus that uses a particularly strong electric shock to counteract the type of abnormal rhythm known as fibrillation. Today’s ICDs are commonly implanted under the collarbone. Early models were placed in the abdomen and were used only for defibrillation.
The heart has four chambers — two atria at the top and two ventricles at the bottom — which contract to pump blood. The atria send blood to the ventricles; the right ventricle pumps blood to the lungs, and the left ventricle to the rest of the body. The contractions of the heart’s chambers and the opening and closing of the heart’s valves must be precisely synchronized for the heart to pump blood efficiently.
Since the ventricles are responsible for pumping blood out of the heart, arrhythmias, or irregularities in rhythm, affecting them can be particularly serious. Such arrhythmias include ventricular tachycardia (abnormally rapid rhythm of the ventricles), ventricular fibrillation (chaotic quivering of the ventricle muscle), and bradycardia (abnormally slow heart rhythm). These arrhythmias can cause sudden cardiac arrest (abrupt loss of heart function), which can be fatal.
ICDs have been used to treat life-threatening ventricular arrhythmias since 1980. A study called the Multicenter Automatic Defibrillator Implantation Trial (or MADIT), the results of which were first published in The New England Journal of Medicine in 1996, showed that implantation of an ICD improved survival in people with coronary heart disease who had reduced left ventricular function and ventricular tachycardia. This study helped make U.S. Vice President Dick Cheney a candidate for the ICD that he received in June 2001.
In March 2002, the results of a landmark second study, called MADIT II, were also published in The New England Journal of Medicine. It compared use of ICDs with standard drug therapy in 1,232 heart attack survivors whose left ventricle was impaired. To be eligible for the study, subjects had to have a left ventricular ejection fraction (the percentage of the total blood in the chamber that is pumped out with each heart beat) of under 30%. (A healthy heart has an average ejection fraction of 50% or 55% to 75%.) In contrast to the previous study, MADIT II did not require that subjects undergo invasive electrophysiological testing, involving running a catheter from the groin to the heart, to certify the presence of a dangerous cardiac arrhythmia. When the study participants were checked some 20 months (on average) later, the people with ICDs were found to have a dramatically reduced — 31% lower — risk of death than people treated with standard drug therapy.
Until recently, only 300,000 people a year in the United States were eligible to receive ICDs under guidelines established by the U.S. Food and Drug Administration (FDA). However, based on these results, the FDA in mid-2002 approved the use of ICDs in people with a prior heart attack and depressed ventricular function, regardless of whether they had undergone invasive electrophysiological testing to look for arrhythmias. This means that about three to four million people in the United States might be candidates for ICDs. Currently ICDs cost about $20,000 or more, but this price is expected to drop significantly as more people use them.