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How Does an AED Work on the Heart - The Science Behind Using an AED


Almost everyone is aware that in times of emergency, using an AED can save a life. When someone has a cardiac arrest, using an AED located nearby can be just the thing to save someone’s life. Although AEDs are not mandatory, the more access we have to these machines, the more likely we are able to help those in heart-related emergencies. In these emergencies, an AED can be used to shock a heart back into a normal rhythm. You may know how to use an AED, but do you actually know how an AED works on the heart? Here is the science behind using an AED to save a life!


What Does Using an AED Do?

AED stands for Automatic External Defibrillator and it is a device that can return an erratic heart beat (a heart in fibrillation) back to normal. When someone experiences a cardiac arrest, their heart begins to beat in an unsynchronized rhythm that will render an individual unconscious. It is when someone has a cardiac arrest that they are in need of an AED right away! When using an AED, the machine will monitor the person’s heart rhythm and give you instructions that will assist in returning the heart back to normal. The faster someone gets attention from an AED, the more likely they are to survive!

AED - Using an AEDHow Does the AED Work?

In an emergency, it is crucial to call 911 right away and then immediately ask someone to grab an AED. Once an AED is brought, the pads are placed on the individual’s upper body, and the defibrillator analyzes the person’s heart. If the heart is in fibrillation (which will be the case for a cardiac arrest victim), then the AED will instruct you to administer a shock. Each AED will know, through heart monitoring, if a shock is required. The machine will then build up an electric charge and administer it once the person is clear and the ‘Shock’ button has been pressed. Once an AED has delivered a shock, the machine will once again analyze the heartbeat for any further action.

Another prominent feature of the Philips AED is the voice activated prompts. As the machine analyzes the heart rhythm, it will give you commands based upon the need of the individual. It will give assistance such as:

  • Instruct responders for AED pad application
  • Analyze the individual’s heart
  • Identify when a shock is advised
  • Instruct responders to stand back
  • Identify when a shock is being administered
  • Instruct responders to perform CPR

All of these instructions are simple and straight-forward – using an AED is easy for anyone!


How Do AED Shocks Work?

Once the AED has identified that a shock is needed, it will inform you that the machine is charging. During this period, the machine is actually building up an electromagnetic charge that will be administered as a shock once you press the button. The AED will tell you to stay clear of the patient (like when a doctor yells ‘CLEAR’ on TV and everyone backs up), and then instruct you to press the shock button when clear. Once pressed, the AED will release the shock from the main machine, down the pad wires, and through the pads into the individual – crossing from one pad to another through the heart.

The shock is a precisely measured electric current (typically 150 Joules of Energy for an adult and 50 joules for a child) that is delivered to pads and sent through the heart. These shocks work to ‘de-fibrillate’ the erratic rhythm (fibrillation) of the heart and ‘kick’ it back into a normal beat. The shocks given when using an AED will actually ‘stop’ the heart so it can naturally ‘restart’ normally once again. Just as CPR uses compressions to help push the heart into a normal beat, the AED uses an electrical current kick-start it (much more effectively).

Take a look at this video and see how the heart (beating erratically at first), is given a shock, and then returns to a normal heartbeat. Although this video’s defibrillation is done through direct access to the exposed heart with paddles, an AED machine and pads works similarly.

Note: Once a heart has stopped beating completely, an AED cannot revive it. An AED is only able to ‘de-fibrillate’ a heart in fibrillation (quivering).  This is why it is so important to get an AED right away, so you are able to catch the heart while it is still moving, albeit erratically.


How do AED Pads Work?AED Pads - Using an AED

With each AED is an included set of pads. These pads are easily located on the face of every Philips AED, and are simple to attach. When using an AED, simply remove each pad from the case, peel off the sticker-backs, and apply them to the patient as indicated. Each set of pads will have detailed diagrams of the necessary placement on the individual’s body (one pad on the right-hand side near on top of the chest, the second pad along the ribs of the left-hand side of the body). Once you have stuck these stickers on in the proper locations the pads will act as heart sensors and electric conductors.

Each pad has electrodes (sensors) that will help the machine monitor the heart rhythm. Once they have analyzed the pace of the heart, they send the information back to the machine to determine if a shock is needed. If a shock is required, the electrical current built up in the primary device will send the shock to the pads, and through the heart.

It is for this reason that proper pad placement is essential. The pad locations are placed specifically so the electrical shock travels through the heart between pads.


Now that you know the science behind using an AED, you can apply this knowledge in emergencies. Purchase an AED for your location and be prepared for any heart-related emergency. It could be just the thing to save a life!






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