What is a Monophasic Defibrillator?

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  • Written By: Mary Lou Derksen
  • Edited By: Angela B.
  • Last Modified Date: 29 September 2019
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Sometimes a person’s heartbeat, which is controlled by electrical impulses, becomes irregular and very rapid. Sometimes this irregularity, called ventricular fibrillation, may cause the heart to stop entirely. A heart may also be stopped by the surgeon during open-heart surgery. Doctors use a defibrillator to restart a heart or to try to restore a regular heartbeat via an electric shock to the heart. A monophasic defibrillator is the type commonly used in many hospitals; monophasic means its electricity goes only one direction — to create the shock.

Defibrillators may be external or they may be implanted. An external defibrillator has four primary parts: a monitor, a capacitor, and either two paddles or two sticky defibrillation pads. The capacitor delivers the electricity, which travels from one paddle or pad through the heart to the other paddle or pad. The monitor allows the physician to observe what is happening with the heart rhythm.

An implanted defibrillator is not the same as a pacemaker, though a pacemaker can be part of an implanted defibrillator. The pacemaker controls the heartbeat so it is not too fast or too slow, while the defibrillator portion of the device shocks the heart to help it restore a normal pace when something happens outside of the pacemaker's range of control. An implanted monophasic defibrillator originally did not include a pacemaker.


A monophasic defibrillator must use a fair amount of electricity to give the heart enough electricity to resume normal function; that electricity can cause skin burns, so doctors try to use the smallest possible effective amount. The American Heart Association (AHA) recommends beginning with 360 joules of electrical energy, although other guidelines start as low as 50. As an example of how much electricity is used in defibrillation, some people say it takes 360 joules to generate enough power to start the engine in a diesel truck.

A different defibrillator type is a biphasic defibrillator. It also gives a single electric shock to the heart, but, as in alternating current electricity, half of the shock goes one direction, and the second half goes the other direction. In other words, it goes from one paddle, through the heart to the second paddle, and then back again.

Studies have indicated that a smaller number of joules using a biphasic defibrillator will work as well as a larger amount from a monophasic defibrillator. Using less power means the battery lasts longer, so biphasic shocks are used in implanted defibrillators. Lasting beneficial effects of using a monophasic defibrillator or a biphasic defibrillator are the same.


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