How does a defibrillator work?


In the latest in our series of explainers, Jake Port looks at a life saving piece of technology.


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No TV medical drama show is complete without a doctor at some point shouting, “Clear!” as they apply a defibrillator on a dying patient. More often than not, with a massive convulsion, the patient is shocked with hundreds of joules of electricity and miraculously brought back to life.

In the real world, defibrillators are a little less dramatic but they do save countless lives.

How do they work?

As the name suggests, the defibrillator is a device that stops fibrillation – the condition where the heart starts to beat erratically, usually during cardiac arrest.

It does this by generating a powerful electrical current – about 300 joules of electrical energy – which is passed through the heart. This has the effect of stopping the uncontrolled trembling and resetting the beating to normal.

There is a common misconception – thanks to those medical dramas – that a defibrillator is used when the heart stops beating entirely, or flat lines.

Unfortunately, this exaggerates the capabilities of the device.

Rather than restart the heart, the device works to reset the natural pacemaker in our bodies to return the heart to normal function and rhythm.

If the heart has stopped, a defibrillator will do little to restart it so other techniques such as CPR are applied.

It is even possible to have an automatic defibrillator placed inside the body for patients who have any disorder of the heart, known as arrhythmia. These devices, about the size of a pocket watch, administer the appropriate electrical shock if they sense the heart going into fibrillation.

They differ from a pacemaker, however, as they do not assist the heart in maintaining a pace. Rather they deliver an electrical shock when they sense that the heart is approaching a dangerous heartbeat.

The shock produced by the defibrillator is generated via a built-in battery, which releases a massive pulse of energy. This electrical energy is directed down two wires, each ending at a pad, known as a paddle.

With a defibrillator in a hospital, a doctor applies a conductive gel to maximise the flow of electricity to the patient. The paddles have insulated, plastic handles to prevent the user being shocked along with the patient.

An automatic defibrillator at an airport. These compact devices can also be found in shopping centres, school and other public places. – dlewis33/Getty Images

Automatic defibrillators, such as those you might see at a shopping centre, use an adhesive pad, which can be quickly and easily applied by anyone and omit the need to hold the pads during use.

They can also electronically guide a user through the defibrillation process without the need for a medical professional.

Where the pads are placed is crucial.

There are two correct positions – the first above and to the left, and below and to the right of the heart, the second with the paddles placed in front and behind the heart.

These positions ensure that the electrical current moves around and through the heart.

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Jake Port contributes to the Cosmos explainer series.
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