An Automated External Defibrillator (AED) is a portable medical device designed to treat sudden cardiac arrest by delivering an electrical shock to the heart. Cardiac arrest occurs when the heart’s electrical system malfunctions, preventing it from beating effectively. The AED analyzes the heart’s electrical activity and is programmed to advise a shock only when it detects a specific, treatable electrical problem, aiming to restore a normal, effective heart rhythm.
The Role of the AED in Cardiac Arrest
The human heart relies on a precisely timed electrical signal to coordinate the contraction of its four chambers. Cardiac arrest results when this signaling becomes chaotic or ceases, preventing the heart from pumping blood. The AED monitors the heart’s activity via electrode pads and uses built-in algorithms to determine if the electrical pattern can be corrected.
The electrical shock delivered by the AED is not intended to start a heart that has completely stopped, but rather to serve as a “reset button.” This process, called defibrillation, momentarily stuns the heart’s chaotic electrical activity. The goal is to allow the heart’s natural pacemaker to regain control and re-establish an organized, effective rhythm. If the AED determines a shockable rhythm is present, it prompts the rescuer to deliver the electrical therapy.
The Shockable Rhythms
An AED is programmed to recognize and deliver a shock for only two specific heart rhythms: ventricular fibrillation and pulseless ventricular tachycardia. Both conditions represent an electrical failure that causes the heart muscle to stop pumping blood effectively. Immediate defibrillation is the specific treatment for both rhythms.
Ventricular Fibrillation (VF)
The most common shockable rhythm is Ventricular Fibrillation (VF), characterized by disorganized electrical activity in the ventricles. Instead of contracting, the ventricular muscle merely quivers in a chaotic, useless motion. This quivering means no blood is pumped, resulting in cardiac arrest and a complete absence of a pulse. The AED shock interrupts this electrical chaos, allowing a normal rhythm to potentially resume.
Pulseless Ventricular Tachycardia (pVT)
The second shockable rhythm is Pulseless Ventricular Tachycardia (pVT), which involves a very rapid electrical signal originating in the ventricles. In pVT, the heart beats so quickly that the chambers do not have time to properly fill with blood before contracting. This speed prevents effective blood flow, meaning there is no pulse. Because this rapid electrical malfunction leads to circulatory collapse, the AED treats pVT with the same defibrillation protocol used for VF.
Non-Shockable Rhythms and Why the AED Stays Silent
The AED advises “No Shock Advised” when the heart’s electrical problem cannot be fixed by an electrical reset. This includes two main non-shockable rhythms: asystole and pulseless electrical activity. In these scenarios, administering a shock would be ineffective and delay necessary interventions, such as high-quality Cardiopulmonary Resuscitation (CPR).
Asystole
Asystole is commonly known as a flatline, representing the complete absence of measurable electrical activity in the heart. Since there is no chaotic or fast electrical signal to interrupt, an electrical shock would be futile. When an AED detects asystole, it instructs the rescuer to continue chest compressions and follow advanced life support protocols.
Pulseless Electrical Activity (PEA)
Pulseless Electrical Activity (PEA) occurs when the heart’s electrical system appears organized, but the muscle fails to contract forcefully enough to create a pulse. The problem is not an electrical malfunction but rather a mechanical or chemical issue, such as severe blood loss or a drug overdose. Since the AED corrects electrical chaos, a shock cannot fix this underlying mechanical failure. The AED remains silent and directs the rescuer to continue CPR while medical professionals address the non-electrical cause of the arrest.