How an AED works on the heart

When a heart goes into an uncoordinated electrical activity called fibrillation, the heart twitches ineffectively, can't pump blood and appears to have stopped beating. This condition is called sudden cardiac arrest (SCA) and often accompanies severe heart attacks.

The Automated External Defibrillator (AED) is a small, portable device that administers an electric shock to the heart. A built-in computer assesses the victim's heart rhythm, judges whether defibrillation is needed, and then administers an appropriate level of shock. Audible and visual prompts guide the user through the process. A AED unit will stun the heart, disrupting the electrical chaos, and allow the normal electrical sequencing of the heart and pumping action to resume.

The AED is designed to be used by a layperson "first responder" - a person with AED training. No medical training is needed to use a AED. Here is a summary of how a AED unit works:

1. The first prompts tells the user to connect the electrodes to the AED and to stick the adhesive electrode pad(s) to the victim's bare chest. The electrodes send the heart's electrical rhythm data (ECG) to the AED unit.

2. The AED then analyzes (and sometimes displays) the ECG heart rhythm. It checks characteristics such as frequency, shape, slope, amplitude and heart rate. Based on these characteristics, the AED determines whether or not a shock is needed.

3. If a shock is needed, the AED will prompt the user to either press a button to deliver the shock, or to stand back to deliver the shock automatically. The AED starts building up the requires electrical charge and then delivers an electrical shock to the victim.

4. The AED next re-analyzes the heart rhythm to determine if another shock is needed. If so, another shock is administered.

5. If a "shockable" heart rhythm is not detected, the AED will prompt the user to check the victim for a pulse and to perform CPR.

* In all cases, CPR should be administered - using an AED for resuscitation is not enough.*