Implantable cardioverter defibrillators
An implantable cardioverter-defibrillator (ICD) is a device that is implanted under the skin of patients that are at risk of sudden death due to ventricular fibrillation. The purpose of these devices is to provide defibrillation if the heart enters a potentially lethal rhythm. Implantable cardioverter defibrillators are intended to treat the very serious rhythm problems that
can arise in the lower portion of the heart (the ventricles), not disturbances in other areas. In the near future, ICDs will be available which can be used for patients with these other rhythm problems.
The process of implantation of an ICD is similar to implantation of a pacemaker. Similar to pacemakers, these devices typically include a wire that runs through the right chambers of the heart, usually ending in the apex of the right ventricle.
ICDs constantly monitor the rate and rhythm of the heart and can deliver therapies when the heart rate goes over a set number. All ICDs are programmed to deliver an electrical shock when the ventricles of the heart go faster than the set rate. More modern devices can distinguish between ventricular fibrillation and ventricular tachycardia (VT), and may try to pace the heart faster than its intrinsic rate in the case of VT, to try to break the tachycardia before it progresses to ventricular fibrillation. This is known as fast-pacing or anti-tachycardia pacing (ATP). ATP is only effective if the underlying rhythm is ventricular tachycardia, and is never effective if the rhythm is ventricular fibrillation.
Many modern ICDs use a combination of various methods to determine if a fast rhythm is normal, ventricular tachycardia, or ventricular fibrillation. Rate discrimination evaluates the rate of the lower chambers of the heart (the ventricles) and compares it to the rate in the upper chambers of the heart (the atria). If the rate in the atria is faster than or equal to the rate in the ventricles, then the rhythm is most likely not ventricular in origin, and is usually more benign. If this is the case, the ICD does not provide any therapy.
Rhythm discrimination will see how regular a ventricular tachycardia is. Generally, ventricular tachycardia is fairly regular. If the rhythm is irregular, it is usually due to conduction of an irregular rhythm that originates in the atria, such as atrial fibrillation. Morphology discrimination checks the morphology of every ventricular beat and compares it to what the ICD believes is a normally conducted ventricular impulse for the patient. This normal ventricular impulse is often an average of a multiple of beats of the patient taken in the recent past.
Implantable cardioverter defibrillators are powered by batteries. The variety used has been chosen because of its long service period and because of the very predictable way in which its power is gradually used. Naturally, an ICD which delivers a great many shocks will wear out sooner than one which seldom needs to provide that therapy. Nonetheless, most ICDs last five years and often more. Checking the battery is, of course, an important part of any evaluation of the device, and doctors are careful to do this as a portion of their routine follow-up for patients with ICDs. A normal clinic visit to the electrophysiologist or cardiologist for such a patient usually includes interrogation of the device. This means that we use a piece of equipment called a programmer to "ask" the ICD to show its battery voltage, its settings, any rhythm disturbances it has detected, and any therapy it has delivered since the last visit. Long before the battery is ready to expire, and long before the performance of the ICD changes in any way, an interrogation like this will show the doctor that the time is approaching to consider a replacement ICD. At that point a brief hospital stay is arranged - typically no more than one or two days - to allow the doctor to remove the old ICD and replace it with a fresh one. Replacement procedures are often simpler than original implantations, and so take the doctor and patient even less time.