An Automated External Defibrillator, or AED, is a portable medical device used when a person has sudden cardiac arrest. It checks the heart's electrical rhythm and can deliver a controlled electric shock if the rhythm is shockable. AEDs matter because rapid defibrillation can greatly improve the chance of survival.
They are designed so trained rescuers and ordinary bystanders can use them with voice prompts and visual instructions.
The AED uses adhesive electrode pads to sense electrical signals from the heart through the chest. Its internal computer analyzes the rhythm and decides whether a shock is advised, most often for ventricular fibrillation or pulseless ventricular tachycardia. If a shock is needed, the device charges a capacitor and releases stored electrical energy across the chest to help reset chaotic heart activity.
CPR, calling emergency services, and following AED prompts all work together to support the person until advanced care arrives.
Understanding Medical Technology: Automated External Defibrillators
A defibrillator does not restart a stopped heart in the way films often show. Its main job is to interrupt certain dangerously disorganized electrical patterns. In ventricular fibrillation, many heart muscle cells receive mixed signals and twitch without making a useful heartbeat.
A shock sends current through a large part of the heart at once. This can stop the chaotic activity for a moment. The heart’s own natural pacing system may then regain control and produce an organized rhythm.
This is why a shock is useful only for particular rhythms. A straight line on a monitor usually cannot be fixed with a shock.
The pads are important because they form the path for electrical current. One pad is normally placed high on the right side of the chest. The other goes lower on the left side, below the armpit area.
This arrangement directs current across the heart. Good contact matters. A wet chest should be dried quickly, since water can affect pad adhesion and create an unwanted path for current.
Very hairy skin can prevent a pad from sticking well. Some AED kits include a razor for this reason.
Medication patches should be removed and the skin wiped before a pad is placed there. Students learning first aid should remember that the device gives clear prompts, so there is no need to guess the next action.
The amount of current reaching the heart is affected by the resistance of the chest. Skin, body tissue, air gaps under pads, and poor pad contact all add resistance. The AED produces a carefully shaped electrical pulse rather than simply applying electricity for a long time.
Many modern devices use a biphasic waveform. In this type of pulse, current flows in one direction and then reverses briefly. This can be effective with less energy than older designs.
The AED measures conditions through the pads and adjusts its output within safe design limits. The capacitor inside stores energy before the shock, then releases it in a very short time.
CPR has a different purpose from the shock. Chest compressions move a small amount of blood to the brain and heart when the person cannot pump blood normally. This buys time, but it does not correct ventricular fibrillation by itself.
Rescuers should minimize pauses in compressions. They stop only when the AED tells them to stand clear for rhythm analysis or shock delivery. During analysis, movement can create electrical noise that confuses the reading.
During the shock, nobody should touch the person, the bed, or connected equipment. Loudly saying clear helps the team check this safety step.
AEDs appear in schools, sports centres, airports, stations, offices, and shopping areas because cardiac arrest can happen far from a hospital. They may have adult pads, child pads, or a child setting. For young children, the device instructions matter especially because pad placement and energy settings can differ.
An AED can say that no shock is advised even when the person is unresponsive. That does not mean care stops.
Emergency services, continued CPR, and repeated rhythm checks remain essential. The key learning point is to act calmly, follow each prompt exactly, and avoid delays caused by fear of doing something imperfectly.
Key Facts
- Sudden cardiac arrest is an electrical problem where the heart stops pumping blood effectively.
- AED stands for Automated External Defibrillator.
- Energy delivered by many adult AED shocks is about 120 J to 200 J, depending on the device.
- Electrical energy stored in a capacitor is E = 1/2 C V^2.
- Current through the chest depends on resistance by Ohm's law: I = V/R.
- Use the AED as soon as it arrives, keep doing CPR when prompted, and make sure nobody touches the person during analysis or shock.
Vocabulary
- Automated External Defibrillator
- A portable device that analyzes heart rhythm and can deliver an electric shock to treat certain life threatening rhythms.
- Defibrillation
- Defibrillation is the delivery of an electric shock intended to stop chaotic heart electrical activity so a normal rhythm can return.
- Electrode pads
- Electrode pads are sticky conductive pads placed on the chest to detect heart signals and deliver the shock.
- Ventricular fibrillation
- Ventricular fibrillation is a dangerous rhythm where the lower chambers of the heart quiver instead of pumping blood.
- Capacitor
- A capacitor is an electrical component that stores charge and energy before releasing it quickly during an AED shock.
Common Mistakes to Avoid
- Touching the person during rhythm analysis or shock delivery is wrong because movement can confuse the AED and a shock can pass through someone in contact with the patient.
- Placing pads too close together is wrong because the current may not pass through enough of the heart to be effective.
- Stopping CPR for a long time while setting up the AED is wrong because blood flow to the brain and heart falls quickly during interruptions.
- Using an AED only after a professional arrives is wrong because AEDs are built for bystanders and earlier shocks are linked to better survival.
Practice Questions
- 1 An AED delivers 150 J of energy in a shock. If the shock lasts 0.010 s, what is the average power delivered during the shock?
- 2 A capacitor inside an AED has a capacitance of 160 microfarads and is charged to 1500 V. Using E = 1/2 C V^2, how much energy is stored in the capacitor?
- 3 Explain why an AED tells rescuers not to touch the person while it analyzes the rhythm and again before it delivers a shock.