An Automated External Defibrillator (AED) is a highly accessible medical device designed to treat sudden cardiac arrest (SCA). This life-threatening condition occurs when an electrical malfunction causes the heart to stop beating effectively. Since the probability of survival decreases significantly every minute, the AED quickly analyzes the heart rhythm and delivers a necessary electrical intervention. Its ease of use and widespread placement allow bystanders to provide immediate, life-saving care.
The Electrical Chaos of Ventricular Fibrillation
Ventricular fibrillation (VF) is the most common cause of sudden cardiac arrest. Normally, the heart’s lower chambers, the ventricles, contract in a strong, synchronized motion to pump blood throughout the body, dictated by a precise electrical impulse pathway. During VF, this electrical activity becomes completely disorganized, turning into a rapid, chaotic mess of signals. Instead of contracting, the ventricular muscle fibers merely twitch or quiver, meaning the heart cannot pump any blood. This leads to immediate loss of consciousness and collapse, resulting in death within minutes without prompt treatment.
How the AED Recognizes the VF Pattern
Once the electrode pads are applied, the AED initiates a hands-off period to analyze the electrical rhythm. The device uses an internal algorithm that processes signals received through the pads, which act as external electrocardiogram (ECG) sensors. This process filters out noise and movement artifacts.
The algorithm performs analysis specifically looking for the distinct characteristics of a shockable rhythm like ventricular fibrillation. VF is identified by its high-frequency, low-amplitude, and irregular electrical pattern on the ECG signal. It lacks the organized peaks and valleys (QRS complexes) that define a normal heartbeat.
The AED contrasts this disorganized activity with non-shockable rhythms, such as a near-flat line (asystole) or a slow, organized rhythm. The device must correctly identify VF (high sensitivity) and avoid unnecessary shocks (high specificity). Only when the algorithm identifies the chaotic electrical signature of VF does the AED advise the user to deliver a shock.
The Critical Steps After VF Detection
Once VF is confirmed, the device immediately announces “SHOCK ADVISED” and begins charging its internal capacitor to accumulate the necessary electrical energy. Modern AEDs typically deliver a biphasic shock, using a lower energy dose (often 120 to 200 Joules) compared to older devices.
The user is instructed to ensure safety by verbally announcing a clear warning, such as “Clear!” or “Stand Clear,” and confirming no one is touching the patient. Once the patient is isolated, the rescuer pushes the shock button to deliver the controlled electrical current across the chest. This electrical discharge is intended to momentarily stop all electrical activity in the heart muscle, essentially resetting the system.
The goal of this reset is to allow the heart’s natural pacemaker (the sinoatrial node) to regain control and re-establish a synchronized, effective rhythm. Immediately following the shock, the AED prompts the user to resume cardiopulmonary resuscitation (CPR) without checking for a pulse. The AED continues to monitor and guide the user through subsequent cycles of CPR and re-analysis until emergency medical services arrive or the patient stabilizes.