What is an implantable cardioverter-defibrillator (ICD)?

Updated: Oct 11, 2019
  • Author: Daniel M Beyerbach, MD, PhD; Chief Editor: Jeffrey N Rottman, MD  more...
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An implantable cardioverter-defibrillator (ICD) is a specialized implantable electronic device designed to directly treat a cardiac tachyarrhythmia, whereas a permanent pacemaker is an implanted device that provides electrical stimuli, thereby causing cardiac contraction when intrinsic myocardial electrical activity is inappropriately slow or absent. A pacemaker senses intrinsic cardiac electric potentials, and, if these are too infrequent or absent, transmits impulses to the heart to stimulate myocardial contraction.

All modern ICDs are equipped with a demand pacing system and serve the dual functions of emergency defibrillation and backup pacing. If a patient has a ventricular ICD and the device senses a ventricular rate that exceeds the programmed threshold, the device may be programmed to deliver antitachycardia pacing therapy (ATP) or defibrillation. With antitachycardia pacing, the device delivers a preset number of rapid pulses in succession in an attempt to terminate the ventricular tachycardia. If antitachycardia pacing therapy is ineffective within a prespecified number of trials or a prespecified time period, or if the ventricular rate exceeds a preset rate, the device delivers a high-energy electric shock to reset the heart’s electrical activity.

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