A defibrillator is a specialized medical device engineered to deliver a controlled, high-energy electrical shock to a person experiencing a life-threatening cardiac rhythm disturbance. Its primary function is not to restart a heart that has flatlined, but rather to interrupt chaotic electrical activity within the heart muscle. The goal of this sudden electrical pulse is to allow the heart’s own intrinsic electrical system to reset and re-establish a coordinated, effective beating pattern. This technology is foundational to emergency medicine and is the definitive treatment for several forms of sudden cardiac arrest.
The Cardiac Rhythms Requiring Intervention
The operating principle of a defibrillator is necessary only when the heart’s electrical system has become dangerously disorganized, specifically in two shockable rhythms. The most common is Ventricular Fibrillation (VF), which is characterized by completely chaotic, disorganized electrical activity in the heart’s lower chambers, the ventricles. Instead of contracting in a coordinated pump, the ventricular muscle fibers merely twitch or “quiver,” preventing any effective blood flow to the body.
The other condition treated by defibrillation is Pulseless Ventricular Tachycardia (Pulseless VT). In this state, the heart’s electrical activity is still rapid and somewhat organized, but the rate is so fast—often exceeding 150 beats per minute—that the ventricles cannot fill properly with blood between contractions. This extreme speed results in a mechanically ineffective contraction, meaning there is no detectable pulse and no functional circulation. Both VF and Pulseless VT represent a failure of the heart to pump blood, requiring the immediate intervention of a defibrillator.
Generating and Storing the Electrical Charge
The ability of a defibrillator to deliver a massive electrical pulse quickly relies on a specialized internal component called a capacitor. This electronic device stores a significant amount of electrical energy, which is measured in units called Joules (J). The device’s internal circuitry, which includes transformers, takes the low-voltage power from the battery and converts it into the high-voltage charge needed for defibrillation.
During the charging phase, the capacitor rapidly accumulates this electrical potential energy, holding it until the moment of discharge. The amount of energy stored can be adjusted by the operator, though Automated External Defibrillators (AEDs) typically have a fixed energy level. For adult patients, modern defibrillators using advanced waveforms often deliver an energy range between 120 and 200 Joules.
How the Shock Resets the Heart
Mechanism of Depolarization
The electrical shock delivered by the defibrillator achieves mass depolarization of the myocardium, simultaneously activating nearly all heart muscle cells with the powerful current. The shock is not meant to “jump-start” the heart, but rather to instantaneously stop all the chaotic, disorganized electrical activity in the ventricles.
This momentary cessation of activity, a temporary state of electrical silence or asystole, is the mechanism for resetting the heart. By silencing all the competing electrical impulses, the defibrillation provides a window for the heart’s natural pacemaker, the sinoatrial (SA) node, to regain control. If the SA node is viable, it can then initiate a new, coordinated electrical impulse that restores a normal, organized rhythm called sinus rhythm.
Biphasic Waveform Technology
Modern defibrillators predominantly utilize a Biphasic waveform, an advancement over Monophasic technology. A Monophasic shock sends the electrical current in only one direction through the heart. In contrast, a Biphasic shock sends the current in one direction and then quickly reverses its polarity, sending the current back in the opposite direction.
This bidirectional current flow is far more effective at terminating chaotic rhythms and requires a lower energy dose compared to the older systems. The Biphasic approach improves the success rate of the defibrillation while reducing the risk of damage to the heart muscle. By adapting to the body’s impedance, Biphasic devices ensure the appropriate energy is delivered to the heart tissue.