A defibrillator or AED is a life-saving machine that delivers and electric shock to the heart which when used promptly, in the case of sudden cardiac arrest, can restart the heart.

When someone has a sudden cardiac arrest the heart commonly goes into a state of ventricular fibrillation (VF), a kind of wobbling muscle spasm rather than a proper heart beat which means that blood is not being pumped around the body. While it is in this state applying an electric shock from a defibrillator can restart the heart and help save that persons life. Performing CPR (cardio pulmonary resuscitation) helps to prolong the period of time that the heart remains in this shockable rhythm until defibrillation is available.

The UK Resuscitation council estimates that there are around 30 000 cardiac arrests in the UK outside of hospital every year. So knowing how to perform CPR and increasing the number of AED's available in public places will directly increase the survival rates of cardiac arrest victims.

The scientific evidence to support early defibrillation is overwhelming; the delay from collapse to delivery of the first shock is the single most important determinant of survival. If defibrillation is delivered promptly, survival rates as high as 75% have been reported. The chances of successful defibrillation decline at a rate of about 10% with each minute of delay.