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Understanding AEDs: How They Work and What They Look For in Cardiac Rhythms


In moments of medical emergency, especially cardiac arrest, every second counts. Quick and efficient intervention can often be the difference between life and death. Automated External Defibrillators (AEDs) are remarkable devices designed to provide timely assistance in such critical situations. But how exactly do they operate, and what cardiac rhythms do they target? Let’s delve into the workings of these life-saving machines.

What is an AED?

An AED is a portable electronic device that automatically analyzes a person’s heart rhythm and delivers an electric shock, known as defibrillation, to restore a normal heartbeat. It’s a crucial tool in the chain of survival for sudden cardiac arrest (SCA) victims, as it can be used by laypeople with minimal training.

How Does an AED Work?

  1. Detection: When turned on, an AED guides the user through step-by-step instructions, often via visual or voice prompts. The first step is to attach the electrode pads to the victim’s chest. These pads contain sensors that detect the heart’s electrical activity.
  2. Analysis: Once the pads are in place, the AED begins analyzing the heart rhythm to determine if a shockable rhythm is present. A shockable rhythm is typically either ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT), which are chaotic and ineffective heart rhythms that can lead to cardiac arrest.
  3. Shock Delivery: If the AED detects a shockable rhythm, it will charge up and instruct the user to stand clear of the victim while it delivers a shock through the electrode pads. This shock is intended to reset the heart’s electrical activity, allowing it to resume a normal rhythm.
  4. CPR Integration: In many modern AED models, CPR (Cardiopulmonary Resuscitation) instructions are integrated into the device. After a shock is delivered, the AED prompts the rescuer to perform CPR for a specific duration before re-analyzing the heart rhythm.
  5. Non-shockable Rhythms: If the AED detects a non-shockable rhythm, such as asystole (flatline) or pulseless electrical activity (PEA), it will prompt the rescuer to continue CPR and may offer additional guidance or support until emergency medical services arrive.

Cardiac Rhythms Detected by an AED:

  1. Ventricular Fibrillation (VF): This is a rapid, erratic heart rhythm that prevents the heart from effectively pumping blood. It’s one of the most common rhythms leading to sudden cardiac arrest.
  2. Pulseless Ventricular Tachycardia (VT): VT is a fast, regular rhythm originating in the heart’s ventricles. Like VF, it can lead to cardiac arrest if not treated promptly.
  3. Asystole (Flatline): Asystole refers to the absence of any detectable electrical activity in the heart. While not shockable, CPR and other advanced interventions are necessary for treatment.
  4. Pulseless Electrical Activity (PEA): PEA is a condition in which the heart’s electrical activity is present but doesn’t result in a pulse or effective cardiac output. It requires prompt medical attention but does not typically respond to defibrillation.

Where Can I Receive AED Training?

Chicago CPR Academy offers AED training and hands on practice during all of their offered classes. Whether you sign up for one of our classes at our training facility in Northbrook or have us come to your workplace we will have practice AEDs to use and gain confidence on how to operate them quickly, safely and effectively. Check out what we have to offer! Chicago CPR Academy, Northshore Chicagoland.


Automated External Defibrillators are invaluable tools in the fight against sudden cardiac arrest. Their ability to quickly analyze cardiac rhythms and deliver appropriate shocks can significantly increase the chances of survival for those experiencing a life-threatening heart rhythm. While AEDs are user-friendly and designed for use by bystanders, it’s essential to remember that they are not a substitute for professional medical care. Training in CPR and AED usage can empower individuals to confidently respond to cardiac emergencies and potentially save lives. With wider accessibility and awareness of AEDs, we can create safer communities where prompt action can make all the difference.




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