Defibrillation is a medical procedure that delivers a controlled electrical shock to the heart, serving as a life-saving intervention for sudden cardiac arrest. This immediate action is necessary when the heart’s electrical system malfunctions, causing it to stop pumping blood effectively. Sudden cardiac arrest is caused by an electrical problem, unlike a heart attack, which is typically a circulation problem. The application of this electrical energy is often the only way to restore a normal, life-sustaining rhythm, making it a time-sensitive procedure.
The Specific Rhythms That Require Defibrillation
Defibrillation is not a universal treatment for all forms of cardiac arrest; it is specifically designed to treat only those caused by chaotic electrical activity. The two primary rhythms that require this therapy are Ventricular Fibrillation (VF) and Pulseless Ventricular Tachycardia (pVT). Both of these conditions are considered lethal rhythms because they prevent the heart from circulating blood to the rest of the body, leading to immediate circulatory collapse.
In Ventricular Fibrillation (VF), the electrical impulses in the lower chambers of the heart become disorganized and rapid, often exceeding 300 beats per minute. Instead of contracting forcefully, the heart muscle merely quivers uselessly. This chaotic excitation means no blood is pumped to the brain or other vital organs, leading to rapid loss of consciousness.
Pulseless Ventricular Tachycardia is another shockable rhythm where the heart is beating extremely fast, usually over 150 beats per minute, but the rhythm is too rapid to generate a palpable pulse or effective blood flow. Both VF and pVT are electrical problems where the heart tissue still possesses electrical activity, albeit disorganized activity. If a person’s heart has completely stopped electrical activity—a flat line known as asystole—defibrillation is ineffective, as there is no chaotic rhythm to interrupt.
How The Electrical Current Resets The Heart
The purpose of the electrical shock is not to restart a stopped heart, but rather to momentarily stop the chaotic electrical activity completely. The energy delivered by the defibrillator passes through the chest and across the heart muscle, causing a mass depolarization of the cells. This simultaneous depolarization effectively “silences” the multiple, disorganized electrical signals that perpetuate the fibrillation.
This brief cessation of electrical activity is essentially a reboot for the heart’s internal system. By eliminating the chaotic signals, the heart’s natural pacemaker can regain control. The Sinoatrial (SA) node, the heart’s primary pacemaker, is then given the opportunity to re-establish a normal, coordinated sinus rhythm.
The high-energy current must be precisely calibrated to achieve this effect without causing undue damage to the heart tissue. Modern defibrillators utilize a biphasic waveform, which delivers the current in two directions, requiring less energy to successfully terminate the arrhythmia than older monophasic models. When successful, this procedure restores a rhythm that allows the heart to contract properly, enabling it to circulate oxygenated blood throughout the body.
Defibrillation’s Critical Role in the Chain of Survival
Defibrillation is an integral component of the Chain of Survival, which outlines the steps necessary to maximize survival from cardiac arrest. The speed of intervention is paramount. While early Cardiopulmonary Resuscitation (CPR) helps maintain blood flow until defibrillation can occur, only the electrical shock can correct the underlying rhythm problem of VF or pVT.
The chance of survival from a shockable rhythm decreases by approximately 7% to 10% with every minute that passes without defibrillation. This time-dependency makes rapid access to a defibrillator a matter of life and death. For witnessed cardiac arrests, delivering a shock immediately is often prioritized even over other resuscitation interventions.
The widespread availability of Automated External Defibrillators (AEDs) has made this time-sensitive procedure accessible to the public. These portable devices are designed for use by laypeople and utilize voice prompts and visual instructions to guide the user. By enabling rapid intervention in public spaces, AEDs ensure that early defibrillation can be completed long before emergency medical services arrive.