An Automated External Defibrillator (AED) is a portable medical device designed to treat Sudden Cardiac Arrest (SCA) by delivering an electrical shock to the heart. When a person collapses from SCA, the heart often enters a chaotic, ineffective rhythm. The AED analyzes this rhythm and delivers corrective electrical therapy if needed. The pads are the direct interface between the AED and the patient, serving as the delivery system for this electrical current.
The Fundamental Role of AED Pads
AED pads function as advanced electrodes that perform multiple roles in the rescue process. Each pad consists of a conductive metallic layer backed by a specialized, water-based adhesive gel called hydrogel. This hydrogel significantly lowers the electrical resistance, or impedance, of the skin, ensuring the electrical current can pass safely and effectively to the heart muscle.
The adhesive backing is important for maintaining firm contact with the patient’s chest, especially since chest compressions must continue before and after a shock. Without reliable adhesion, the electrical pathway would be compromised, risking an ineffective shock or burns. Beyond conducting the shock, the pads continuously act as sensors, relaying the patient’s electrical heart activity back to the AED unit.
The AED analyzes this rhythm to determine if a shockable pattern, such as Ventricular Fibrillation (VF), is present. This sensing capability allows the device to accurately decide whether to advise a shock, ensuring electricity is only delivered when necessary. The entire system—adhesion, low-impedance conduction, and rhythm sensing—must work together to maximize the chance of restoring a normal heart rhythm.
Differentiating Pad Types (Adult vs. Pediatric)
AED pads come in adult and pediatric types to ensure the correct amount of energy is administered based on patient size. Adult pads are intended for individuals over eight years old or weighing more than 55 pounds (25 kg). These pads are larger, which helps spread the electrical current over a greater surface area, effectively lowering the skin impedance.
Pediatric pads are designed for children under eight years old or weighing less than 55 pounds. They are physically smaller to prevent them from touching on a child’s chest, which would otherwise create a short circuit. The most significant difference is that pediatric pads contain special circuitry or a key that reduces the energy output from the AED.
While an adult shock can range from 150 to 360 joules, pediatric pads reduce this energy to a safer level, often around 50 joules. This reduction is necessary because a full adult dose could harm a child’s developing heart. Using adult pads on a child risks delivering an excessive shock, while using pediatric pads on an adult delivers an inadequate and ineffective dose of energy.
Crucial Steps for Pad Placement
Effective defibrillation requires the correct positioning of AED pads to ensure the electrical current passes directly through the heart. Before application, the patient’s chest must be bare, dry, and free of excessive hair or moisture, as these factors increase impedance and reduce shock effectiveness. If possible, a quick shave or wipe-down should be performed to maximize pad contact.
The standard placement for adults is the anterior-lateral position. One pad is placed high on the patient’s right side, just below the collarbone, and the other pad is placed on the lower left side of the rib cage, slightly below the armpit. This diagonal placement is designed to create a direct pathway for the electrical current to cross the heart muscle.
An alternative method, often used for children or when the standard path is obstructed, is the anterior-posterior position. This involves placing one pad in the center of the chest and the other on the patient’s back, usually between the shoulder blades. This front-to-back configuration also ensures the heart lies directly in the path of the electrical current.
Maintaining Pad Readiness and Shelf Life
AED pads are consumables and have a limited shelf life, which must be considered when maintaining a functional AED. The typical lifespan ranges from 18 months to five years, depending on the manufacturer and the hydrogel formulation. The primary reason for this expiration is that the water content in the hydrogel layer slowly dries out over time.
As the hydrogel degrades, the pads lose their conductivity and adhesive properties, meaning an expired set may not stick firmly or deliver the electrical current efficiently. Pads must be stored sealed in their original packaging and kept within the manufacturer’s recommended temperature range. This storage prevents degradation from direct sunlight or extreme conditions.
Pads must be replaced immediately after any use, regardless of whether a shock was delivered to the patient. They must also be replaced before their expiration date to ensure the AED remains in a rescue-ready state for any emergency. Routine checks of the expiration date are a necessary part of AED maintenance.