An Automated External Defibrillator (AED) is a portable device designed to treat sudden cardiac arrest caused by an electrical malfunction in the heart. This malfunction is most commonly Ventricular Fibrillation (VF), a life-threatening rhythm where the heart’s electrical activity becomes chaotic and disorganized. Instead of pumping blood effectively, the ventricles merely quiver, leading to a rapid loss of pulse and consciousness. When applied, the AED initiates a precise sequence of analysis, warning, and shock delivery to restore a normal heart rhythm. Delivering this life-saving electrical therapy quickly is crucial, as the chance of survival decreases significantly with every minute that passes without defibrillation.
How the AED Recognizes Ventricular Fibrillation
The moment the adhesive electrode pads are correctly placed on the patient’s bare chest, the AED begins to monitor the heart’s electrical activity. Electrical signals travel through the body and are picked up by the pads for the machine’s internal analysis system. The AED employs sophisticated algorithms to analyze the electrical signal waveform, searching for the characteristic signature of VF. This signature is identified by its chaotic, high-frequency, and low-amplitude pattern, indicating continuous, uncoordinated depolarization of the ventricular muscle cells.
The analysis phase is a brief period where the AED must differentiate between a shockable rhythm, such as VF or pulseless Ventricular Tachycardia (VT), and a non-shockable rhythm, like asystole (a flatline) or Pulseless Electrical Activity (PEA). The machine is programmed to ensure a shock is only delivered when a shockable rhythm is present, preventing inappropriate therapy. The voice prompt, “Analyzing heart rhythm, do not touch the patient,” signals the start of this automated diagnostic process, which typically takes only a few seconds.
Preparing for Defibrillation: Warnings and Charging
Once the AED confirms Ventricular Fibrillation, it immediately enters the preparation phase to deliver a therapeutic electrical discharge. The machine announces its finding with a clear voice prompt, typically “Shock Advised” or “Shock Needed.” The AED begins to internally charge its capacitor to the required energy level, often between 120 and 200 Joules for adult patients on modern biphasic defibrillators. This audible charging process prepares the device to deliver the necessary current to reset the heart’s electrical system.
This phase is accompanied by strict safety warnings from the AED, such as “Stand Clear” or “Everyone clear of the patient.” No one must touch the patient during charging to avoid accidental electrocution. The charging period is intentionally fast, minimizing the pre-shock pause—the time chest compressions are stopped before shock delivery—because every second of delay reduces the chances of successful defibrillation. Once the capacitor is fully charged, the AED will issue a final instruction to the user, indicating the shock is ready to be delivered.
Safe Shock Delivery: User Action and Confirmation
For most semi-automatic AEDs, the final step requires direct intervention from the rescuer to deliver the shock. The machine will illuminate the shock button, usually with a flashing light, and prompt the user to “Press the shock button now.” Before pressing, the rescuer must perform a final visual check and verbally confirm that the patient is clear, often using the phrase, “I’m clear, you’re clear, everybody’s clear.” This action guarantees no one is in contact with the patient during delivery.
The brief, controlled electrical current is then delivered across the chest, passing through the heart muscle. The purpose of this discharge is not to restart the heart, but to temporarily stop all chaotic electrical activity by simultaneously depolarizing a large portion of the heart muscle. This momentary electrical silence allows the heart’s natural pacemaker, the sinoatrial node, the opportunity to regain control and initiate a coordinated, life-sustaining rhythm. The physical effect on the patient is a sudden muscle contraction due to the electrical impulse.
Immediate Post-Shock Care and Reassessment
Immediately following the shock delivery, the AED will prompt the user to resume Cardiopulmonary Resuscitation (CPR). The instruction is typically, “Begin CPR,” or “Start chest compressions,” and must be followed without checking for a pulse or rhythm. Chest compressions are needed because the heart muscle remains stunned after the electrical reset, requiring manual circulation of oxygenated blood to the brain and vital organs. The AED is programmed to guide the rescuer through two full minutes of high-quality CPR, providing a metronome-like beat for compressions.
After the two-minute period of continuous compressions, the AED will automatically re-enter the analysis cycle and instruct the rescuer to stand clear once more. The machine analyzes the heart rhythm to determine if the initial shock successfully terminated Ventricular Fibrillation. If the chaotic rhythm persists, the AED will advise and prepare for a second shock. If a normal rhythm is detected, the AED will continue to monitor the patient while the rescuer waits for emergency medical services to arrive.