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Defibrillation is the application of Direct Current (DC) shocks of varying strength to the upper chest of a patient in ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT). This is an attempt to stop the abnormal fibrillating heart momentarily in order to allow the natural pacemaker of the heart take over and restore the heart to normal function.

Most Ambulance Services in the UK either use defibrillators with stick-on pads or paddles through which the shock is delivered. The obvious advantage of the stick-on type pads are that it leaves your hands free to do other important jobs during the resuscitation. Most defibrillators now are simple to use and can easily be operated with the minimum of training. Units are becoming lighter and smaller allowing the immediate care professional to reach a casualty faster.

During the shock
Sometimes the patients arms will jump outwards and upwards. This can obviously be dangerous. Patients arms can land against a hot fireplace for example. Stop this from happening by tucking the arms under the buttocks before you deliver the shock. Or if the patient has a belt on, discreetly tuck their hands into the belt. 

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The difference between cardioversion and defibrillation is pretty simple– the secret lies in timing the shock correctly. If the electricity arrives during the vulnerable period of the T-wave, the  R-on-T thing may occur, an ectopic beat, (or a jolt of external electricity in either case a stimulating electrical signal) landing in the conduction system during the vulnerable period can trigger VF.

The defibrillator cardioverter has the ability to track the QRSs, and to stick a visible marker on each one. This lets the machine figure out when it’s safe to deliver the jolt.

The two main cardioversion situations are:

  • Decompensated, rapid atrial fibrillation- This means that the ventricular response rate to the a-fib is so fast that the chambers can’t fill with blood between the beats. No filling, no blood pressure. Generally if a patient is in rapid a-fib and still has a blood pressure, medics will often try medications first: verapamil, beta-blockers, etc. 

  • VT-with-a-pressure as opposed to VT- with- no- pressure (what we call a pulseless VT).

Monophasic and Biphasic
“Monophasic” means that the current delivered by the machine travels in only one direction between the paddles. This has been the standard way of doing things for many years, but is now seen by many as out of date, and being replaced with a newer method, called “biphasic” defibrillation.

"Biphasic" means that the current initially moves towards the positive paddle, then reverses direction and heads the other way. The difference for us is that biphasic shocks seem to be just as effective as the monophasic ones, but at lower power levels. This is a good thing for a couple of reasons: first, less power applied means less trauma to the patient. Second, less power required means longer battery life, and apparently all implanted defibrillators now use biphasic shocks for this reason – they can also be made smaller.


All information in this section is for guidance and advice only and is assumed to be correct at the time of publishing. Check with your local training department before carrying any of the procedures in this section. The Paramedic Resource Centre cannot be held responsible for any error or omission in any page on this site.

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