Implantable cardioverter-defibrillator
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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.
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, overdrive 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.
Unlike external defibrillators, ICDs do not typically cause electrical burns.