Defibrillator

Defibrillator

Defibrillators are life saving devices for treating life threatening disorders of cardiac rhythm. Defibrillator is a device for delivering high energy direct current electric shocks in a controlled manner to correct abnormalities in cardiac rhythm. The basic circuitry involves generation of a high voltage current which charges a capacitor and stores the energy for rapid delivery through large electrodes to be placed over the chest of the recipient.

The voltage may be as high as 2200 volts in a biphasic defibrillator, and 5000 volts in a monophasic defibrillator [1], but the current is of the order of milliamperes and lasts only a few milliseconds. Most of the current day defibrillators are equipped with ECG monitoring facility as well so that there is no need to connect a separate ECG monitor to identify the cardiac rhythm.

There are controls to select the energy level, which is usually expressed in Joules. Energy levels range from 5 Joules to 360 Joules for a monophasic defibrillator while it ranges from 2 Joules to 200 Joules usually for a biphasic defibrillator. Monophasic defibrillators deliver a unidirectional pulse of energy while the biphasic defibrillator reverses the polarity of the current after the initial phase. Biphasic defibrillation is more effective and requires only lower energy.

Principle of defibrillation

Normal heart has a synchronized activation sequence starting from the sinoatrial node, conducted down to the ventricles through the conduction system, producing a regular electrocardiogram.

In an abnormal rhythm known as ventricular fibrillation, the ventricles stand still due to multiple wavelets of activation known as ventricular fibrillation. Ventricular fibrillation is fatal, unless promptly corrected by direct current shock using a defibrillator. Defibrillator shock simultaneously depolarizes a large portion of the ventricular cells so that the fibrillary activity cannot be sustained. This gives a chance for the sinoatrial node, the regular pacemaker to take over the sequential activation of the heart.

Components of a defibrillator

A defibrillator requires a high energy capacitor for storing the high voltage pulse, circuitry to generate and deliver the pulse, ECG monitor and electrodes (paddles) to deliver the shock to the recipient. A battery backup is essential to work the device in case of power failure.

Advances in defibrillators

Two of the important advances in defibrillator technology are the automated external defibrillators (AED) and implantable cardioverter defibrillator (ICD). AEDs are automated devices which give instructions to the operator regarding the cardiac rhythm and therapy to be delivered. AEDs are commonly being used in public places like airports and railway stations, meant to be used by laypersons or minimally trained personnel.

ICDs are small devices which can be implanted beneath the skin and function automatically for several years. They detect and treat life threatening disorders of cardiac rhythm. Both ICDs and AEDs are becoming very popular these days because of their potential for saving lives.

Reference

1. Joseph L Sullivan. Letter by Sullivan regarding article, “Hands-on defibrillation: an analysis of electrical current flow through rescuers in direct contact with patients during biphasic external defibrillation”. Circulation. 2008 Dec 2;118(23):e712; author reply e713.