A defibrillator is a medical device designed to treat sudden cardiac arrest, typically caused by ventricular fibrillation. This condition involves the heart’s electrical activity becoming chaotic and disorganized, preventing it from effectively pumping blood. The device delivers a controlled, high-energy electrical shock across the chest. This shock temporarily halts the heart’s erratic electrical activity, allowing the heart’s natural pacemaker to reset and restore a functional, synchronized rhythm.
Understanding Energy Output: Joules Versus Volts
The clinically relevant measurement for defibrillation is the energy delivered, expressed in Joules, not the voltage. Voltage is the electrical “pressure” that pushes the charge through the body, while Joules represent the total energy transferred. High voltage is necessary to overcome the body’s natural resistance, known as transthoracic impedance.
The internal voltage required to generate the shock is extremely high, often ranging between 1,000 and 5,000 volts. For adult patients, the therapeutic energy output typically ranges from 120 Joules to 360 Joules, depending on the technology used. For instance, a modern Automated External Defibrillator (AED) may store a charge of around 2,000 volts to deliver a shock of 150 Joules. Joules determine the clinical effectiveness of the shock, quantifying the electrical work done on the heart muscle.
The Difference Between Monophasic and Biphasic Shocks
The energy requirement is influenced by the technology used, specifically the waveform (monophasic or biphasic). Older, monophasic defibrillators send the electrical current in a single direction through the heart. Due to this single-direction flow, monophasic devices generally require a higher energy level, often standardized at 360 Joules, for successful defibrillation.
Modern biphasic defibrillators are the standard and are significantly more energy-efficient and effective. This technology reverses the direction of the electrical current flow partway through the shock, delivering energy in two distinct phases. This bi-directional flow allows for successful rhythm restoration with less energy, typically requiring only 120 to 200 Joules for the first shock. The lower energy requirement minimizes the risk of damage to the heart muscle and surrounding tissue.
How the Electrical Current Restarts the Heart
The electrical charge delivered by the defibrillator aims to achieve global myocardial depolarization. In ventricular fibrillation, the heart’s muscle cells fire randomly and chaotically, resulting in a quivering that cannot pump blood effectively. The high-energy shock acts as a system-wide reset, causing virtually all heart muscle cells to depolarize simultaneously.
This instantaneous depolarization temporarily stops all electrical activity in the heart. This creates a brief period of electrical silence, allowing the heart’s natural pacemaker, the sinoatrial (SA) node, to take over. The SA node is responsible for setting the normal, synchronized rhythm. By eliminating the chaotic activity, the shock gives the SA node the opportunity to re-establish a regular, organized heartbeat.
Device Variations and Standardized Settings
Defibrillators are categorized into manual defibrillators and Automated External Defibrillators (AEDs), which dictates how energy output is managed. Manual defibrillators, found in hospitals and ambulances, require a trained professional to interpret the rhythm and manually select the energy level in Joules. The operator chooses the appropriate dose, which may be escalated if initial shocks are unsuccessful.
AEDs, designed for public use, are programmed with pre-set energy levels, removing the need for manual selection. For adults, AEDs automatically deliver a standardized shock, usually between 150 to 200 Joules. For pediatric patients, specialized pads or a separate key are used to attenuate the energy, delivering a much lower dose, often around 50 Joules, safe for a child’s smaller heart.