Physical principles of the defibrillator

Abstract

A defibrillator is a device used to treat ventricular arrhythmias. Defibrillators have various components, including a power source, variable transformer, rectifier, capacitor, switches and paddles. A rectifier, in the capacitor charging circuit, converts AC voltage into DC voltage. DC energy is used rather than AC because it is more effective, causes less myocardial damage and is less arrhythmogenic. A capacitor is the most important part of the defibrillator. A capacitor is formed by a pair of conductors (metal plates) separated by an insulator (a layer of air). A capacitor stores electrical energy in the form of electrical charge. In the capacitor, the quantity of electrical charge stored for a given charge potential is determined by the surface area of the plates, the thickness of the insulating layer and the ability of the capacitor to store charge (permitivity). During the discharge of a capacitor, delivered energy falls exponentially and some of the energy available is lost in circuit resistance, inductor and paddles. The defibrillator has an inductor in its output circuit. The inductor gives optimum shape and duration to the delivered current. In a biphasic (newer type) defibrillator, the effective energy required for defibrillation is less, requiring smaller capacitors. An automated implantable cardioverter-defibrillator (AICD) is a very small defibrillator implanted in the body like an implantable pacemaker. © 2005 Elsevier Ltd. All rights reserved.