Implantable Cardiac Defibrillator (cont.)
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How are ICDs designed?
An ICD consists of one or more leads (conducting wires insulated with silicone or polyurethane) and a defibrillator unit. The defibrillator unit is a small titanium case containing a microchip computer, a capacitor, and a battery.
The leads carry electrical signals between the heart and the defibrillator unit. One end of a lead is placed on the inner wall of the heart while the other end is attached to the defibrillator unit. The leads help the defibrillator unit monitor the natural heart rhythm. The leads also deliver electrical shock(s) from the defibrillator unit to the heart when tachycardias occur.
The microchip computer runs the defibrillator, monitors the natural heart rhythm, instructs the capacitor to send electrical shock(s) when tachycardias occur, determines the strength of the shock(s) sent, and also keeps a record of the heart rhythms as well as the shock(s) sent by the defibrillator.
Modern ICDs have programmable features that allow the doctor to change the cutoff heart rate for activating the defibrillator. Tachycardias with rates higher than the cutoff heart rate activate the firing of shocks by the defibrillator. The doctor can also adjust the strength (amount of energy delivered) of each shock, and the number of shocks delivered with each tachycardia episode.
Most defibrillators now have built-in pacemakers as well. The newer defibrillators can have very sophisticated pacing devices equipped with the ability to pace both the atrium and the ventricle (dual chamber pacers). Some defibrillators have rapid pacing capabilities. Rapid pacing can sometimes convert a tachycardia to normal rhythm without administering electric shock(s).
The electric pulses and shocks delivered by the ICDs are of such low energy that they do not harm the patient or family members in physical contact with the patient.
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