How Automated External Defibrillators (AEDs) Work

Ventricular rhythm

The left ventricle is an organized hollow muscle that contracts, squeezing in on itself like closing a fist on a balloon, and it drives the blood round the body. Every pulse beat felt at the wrist represents a contraction of the left ventricle.

Imagine trying to squeeze the balloon by opening and closing your fist so rapidly no real pressure is exerted – not much would come out of the balloon – that’s the equivalent of ventricular tachycardia when the muscle fibres of the left ventricle contract so rapidly there is no time for the chamber either to fill or to empty. Then twitch your fingers on the surface of the balloon, no pressure, just twitch, and even less comes out, that’s the equivalent of fibrillation.

Ventricular tachycardia, an excessively rapid heart beat, occurs when the heart muscle has been insulted in some way, most commonly by a coronary vessel blockage, but possibly by an electric shock or numerous other less common possibilities. The heart’s contraction is grossly inefficient, and soon excessively rapid coordinated contraction is followed by rapid uncoordinated twitching – ventricular fibrillation. The former is known in the profession as V tach (pronounced veetak), the latter as V fib (pronounced veefib). Not long after that there are a few twitches seen on the cardiogram, signalling death – agonal rhythm, and then flat line, no electrical activity at all, and the patient’s heart has stopped.

What the defibrillator can do is defibrillate. Totally logical, yet not understood. When the paramedic is told by the machine, “No shock,” the machine’s software has interpreted the cardiogram as showing no evidence of V tac or V fib, probably there is asystole, the technical term for what they call on the TV shows, “flat line.” If the paramedics (or bystander who might be you) gets to the patient during the time of V tach or V fib, a shock will be recommended by the activated machine. What that shock might do is stop the heart altogether, which on the face of it is a bit frightening, but the expectation is the heart will then start again, spontaneously and with a normal rhythm. But if the heart has stopped altogether, “flat lined,” the AED will most definitely not start it again, and just in case you think if it worked for Frankenstein, why not me? the machine won’t let you. “No shock” means it won’t fire no matter how hard you press that button.

Using the AED

Principle and practice are simple. An electric contact pad is placed on the front of the chest, another on the left side, exact placement is not critical, but contact is. If the person has an inch thick fur on his chest, contact will be inadequate.

Directions for use are written on most machines, or at least will accompany them. After the machine is switched on, and the pads are in place, the software will read the cardiogram the machine produces, and will instruct whether to “Shock” or “No shock,” according to whether there is or is not a state of Vtach or Vfib.

From the operator’s point of view, there must be no other person in contact with the subject to be shocked, and apart from the responsibility of doing the shocking, the operator has the further responsibility of ensuring the subject is “clear” of contacts.


Most countries have what are known as Good Samaritan laws, and provided the operator of the AED is not using it in some consciously malicious manner, there is minimal likelihood of legal repercussion. On the other hand, failing to assist a person in need might bring criticism if not legal action.

The kit

AEDs are proprietary, but in general, apart from the machine which consists of a battery operated electrical generator with cables and pads, there are shears to cut clothing and razor to shave excessive hair. A gel may be provided to improve contact between the pad and the skin.