An Automated External Defibrillator (AED) is a portable medical device designed to treat sudden cardiac arrest. Its primary function is to analyze the heart’s electrical activity and, if necessary, deliver a controlled electrical shock (defibrillation) to restore a functional heart rhythm. The device is engineered for use by laypersons, providing immediate intervention before emergency medical services arrive.
The AED’s Analysis Phase
The defibrillation process begins when the electrode pads are correctly placed on the patient’s bare chest and plugged into the device. These pads function as sensors, picking up the heart’s electrical signals and transmitting them to the AED’s internal computer system for interpretation. The device immediately begins analysis using pre-programmed algorithms to classify the heart’s rhythm.
During this period, the AED issues a voice prompt, such as “Analyzing heart rhythm” or “Stand clear.” It is imperative that no one touches the patient while the analysis is taking place, as movement or electrical interference can distort the reading. The device listens for specific electrical patterns indicating a life-threatening malfunction correctable by an electrical shock.
The AED’s analysis serves as a fail-safe mechanism, preventing the delivery of a shock if it is not medically appropriate. This safety function ensures that only two specific, chaotic rhythms will prompt the device to advise or deliver an electrical current. If a shockable rhythm is identified, the AED charges its capacitor and instructs the rescuer to press the shock button or, in fully automatic models, delivers the shock after a countdown.
Shockable Cardiac Rhythms
An AED only delivers a shock when it detects one of two specific electrical conditions: Ventricular Fibrillation (VF) or Pulseless Ventricular Tachycardia (pVT). Both rhythms represent an electrical catastrophe in the ventricles, the heart’s lower chambers responsible for pumping blood. In VF, the heart’s electrical signals become entirely disorganized and chaotic, causing the muscle fibers to merely quiver rather than contract effectively.
This quivering action is electrically active but mechanically useless, stopping the heart from pumping blood and leading to sudden cardiac arrest. Pulseless Ventricular Tachycardia (pVT) occurs when the ventricles beat extremely fast, but the rate is so rapid that the heart cannot fill properly, resulting in no detectable pulse. Although the electrical activity in pVT is more organized than in VF, the outcome is the same: circulation ceases.
The electrical shock delivered by the AED is not intended to restart a stopped heart, but rather to momentarily halt all electrical activity within the heart muscle. This massive depolarization aims to create a brief period of electrical silence, giving the heart’s natural pacemaker (the Sinoatrial or SA node) a chance to re-establish a normal, coordinated rhythm. Defibrillation is the definitive treatment for VF and pVT, and the chances of survival diminish significantly with every minute the shock is delayed.
Non-Shockable Cardiac Rhythms and the Role of CPR
The AED advises “No shock advised” when it detects a heart rhythm that will not respond to defibrillation. The two primary non-shockable rhythms are Asystole and Pulseless Electrical Activity (PEA). Asystole, commonly referred to as “flatline,” represents a complete absence of electrical activity in the heart.
Since there is no chaotic electricity to disrupt or reset, an electrical shock would be futile and potentially damaging. Pulseless Electrical Activity is a condition where the heart shows organized electrical activity on a monitor, but the heart muscle is not contracting strongly enough to produce a pulse or pump blood effectively. In PEA, the mechanical failure of the heart is the issue, not a chaotic electrical signal.
In both Asystole and PEA, the immediate and appropriate treatment is high-quality Cardiopulmonary Resuscitation (CPR). CPR involves chest compressions to manually circulate a small amount of blood to the brain and vital organs, buying time until the underlying causes can be identified and treated by advanced medical professionals. The AED will prompt the rescuer to immediately resume chest compressions and continue this life-sustaining intervention until the next rhythm check.