An Automated External Defibrillator (AED) is a portable device designed to deliver an electrical shock to individuals experiencing sudden cardiac arrest. This life-saving intervention aims to interrupt chaotic electrical activity in the heart, specifically ventricular fibrillation or pulseless ventricular tachycardia, allowing the heart’s natural pacemaker to potentially resume a normal, effective rhythm. The device guides users through the process with voice prompts and visual cues, making it accessible even to those without extensive medical training.
Understanding Electrical Energy: Joules
The electrical energy delivered by an AED is quantified in units called joules (J). A joule represents a unit of energy, signifying the electrical energy transferred. This measurement is distinct from voltage, which indicates electrical potential difference, as joules directly reflect the energy delivered to the patient’s heart.
Standard Energy Delivery Levels
AEDs are calibrated to deliver specific amounts of electrical energy, varying by patient age and device technology. For adults, most modern biphasic AEDs typically deliver a fixed shock ranging from 120 to 200 joules. Some biphasic devices may escalate energy for subsequent shocks, potentially reaching up to 300 or 360 joules if the initial shock is unsuccessful. Older monophasic AEDs, less common now, generally deliver a higher fixed energy of 360 joules for adults.
For pediatric patients, energy levels are significantly lower due to their smaller body size and increased sensitivity. Many AEDs use specialized pediatric pads or a child mode to reduce delivered energy, typically between 50 to 70 joules. Guidelines often recommend an initial dose of 2 joules per kilogram of body weight, escalating to 4 joules per kilogram for subsequent shocks.
Factors Determining Energy Output
The energy output of an AED is influenced by several factors. A primary consideration is the patient’s size, specifically distinguishing between adult and pediatric patients, which necessitates different energy levels and sometimes specialized electrode pads. Pediatric pads are designed to reduce the energy delivered, typically for children weighing less than 25 kilograms or under eight years old.
Beyond patient size, the body’s electrical resistance, known as transthoracic impedance, plays a role. Modern AEDs often incorporate impedance sensing technology to measure this resistance, which can vary widely among individuals. This allows some advanced devices to adjust the current delivery to ensure the heart receives an optimal dose. Additionally, if an initial shock is unsuccessful, many AEDs are programmed to deliver escalating energy levels for subsequent shocks.
The Body’s Response to Defibrillation
When an AED delivers an electrical shock, the immediate physiological response is a momentary, widespread depolarization of the heart muscle cells. The primary purpose of this controlled electrical discharge is to briefly halt all disorganized electrical activity in the heart, particularly in cases of ventricular fibrillation. This allows the heart’s natural electrical system, specifically the sinoatrial node, to potentially reset and regain a coordinated, normal rhythm.
The intense electrical stimulation also causes a brief, involuntary contraction of the chest muscles. AEDs incorporate several safety features to protect both the patient and the rescuer. These devices provide clear voice prompts and visual instructions, guiding the user through each step and advising when it is safe to deliver a shock. Many AEDs require the rescuer to press a shock button, ensuring no one is in contact with the patient during the electrical discharge. The amount of electricity delivered is carefully regulated to maximize therapeutic effectiveness while minimizing potential harm to the heart tissue.