ICD - implantable cardioverter defibrillator

Implantable Cardioverter Defibrillator (ICD) is a battery powered device used to correct dangerously fast arrhythmias.

ICD function

An implantable cardioverter defibrillator can pace the heart similarly to a pacemaker when the heart rate drops below a certain level but ICD can in addition also detect dangerous arrhythmias (abnormally fast heart beat) and correct it by sending a series of quick impulses to the heart (antitachycardic pacing) or by a small (cardioversion) or bigger (defibrillation) controlled electric shock. ICD is usually implanted under local or general anesthesia under the left collarbone in the same way as a pacemaker. An ICD generator with battery and circuitry is connected to the heart with leads (special wires) passed through one of large veins (axillary, subclavian or cephalic).

ICD indications

  • Primary prevention - no previous cardiac arrest but presence of one of the conditions with a high risk of cardiac arrest, such as severe heart failure due to previous heart attack or dilated cardiomyopathy, channelopathies, e.g. long QT syndrome (LQTS),¬†Brugada syndrome (BrS), short QT syndrome (SQTS), and catecholaminergic polymorphic ventricular tachycardia (CPVT).
  • Secondary prevention - previous successfully resuscitated cardiac arrest with a high risk of recurrence.

ICD vs pacemaker

As stated above, ICD has all the functionality of a normal pacemaker and can correct a slow heart beat by pacing. On top of that, ICD also detects abnormally fast heart rhythms that can lead to cardiac arrest and terminates them by fast pacing or electric shock. ICD box is somewhat bulkier but the implant technique is essentially identical to a normal pacemaker procedure. ICD functionality is often a part of cardiac resynchronization therapy (CRT), called also biventricular pacemaker - or in this case biventricular ICD. Biventricular pacemaker improves the pumping function of the heart by resynchronizing the cardiac contraction in patients with heart failure and LBBB (left bundle branch block, an ECG abnormality). Patients with heart failure and LBBB are often at risk of ventricular arrhythmias and need therefore biventricular ICD (= CRT-D).


ICD implant is a safe procedure but as every surgical procedure it is not entirely risk free. The complications involve bleeding, infection, pneumothorax (collapsed lung), tamponade (bleeding outside the heart), lead dislodgement and wound discomfort. An ICD may fail to terminate arrhythmias and occasionally the device can misinterpret heart rhythm and give inappropriate and painful shocks. However, in most patients the implant is a straightforward and uncomplicated procedure and the ICD provides a valuable protection against the risk of sudden death.

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