Defibrillation is a medical procedure that uses an electrical charge to stop life-threatening abnormal heart rhythms, reversing Sudden Cardiac Arrest (SCA). This therapy is the definitive treatment for specific electrical malfunctions of the heart and is the most impactful intervention for survival. The procedure delivers a controlled electrical shock across the chest, aiming to restore a heart that has stopped pumping blood effectively back to a normal, organized rhythm.
The Electrical Crisis: Rhythms Requiring Defibrillation
Defibrillation is specifically required for cardiac arrest caused by two primary electrical disturbances: Ventricular Fibrillation (VF) and pulseless Ventricular Tachycardia (VT).
In ventricular fibrillation, the heart’s electrical signals become chaotic and disorganized, causing the lower chambers of the heart, the ventricles, to merely quiver instead of contract effectively. This chaotic electrical activity prevents the heart from pumping blood to the brain and other organs, resulting in immediate sudden cardiac arrest.
Pulseless ventricular tachycardia is an extremely rapid, regular rhythm originating in the ventricles. The impulses occur so quickly that the heart chambers do not have time to fill with blood before the next contraction, rendering the pumping action ineffective. Both VF and pulseless VT result in a loss of a palpable pulse and require an immediate electrical shock for correction.
Mechanism of Action: How the Shock Resets the Heart
The electrical current delivered during defibrillation is designed to achieve a near-simultaneous depolarization of the myocardial cells. This massive electrical charge overwhelms the heart muscle, causing a momentary cessation of all electrical activity. The purpose is not to “jump-start” the heart but to enforce a temporary silence that interrupts the chaotic electrical circuits causing the arrhythmia.
Following this enforced electrical silence, the heart’s natural pacemaker, the sinoatrial (SA) node, can successfully regain control. By eliminating the competing, erratic electrical activity, defibrillation provides the opportunity for this natural system to resume its function and initiate a coordinated, life-sustaining heartbeat. Current defibrillators often utilize biphasic waveforms, which are more efficient and require lower energy levels to achieve this electrical reset.
The Critical Role in Survival: The Time Factor
Defibrillation’s effectiveness is highly dependent on how quickly it is administered after a cardiac arrest begins. For every minute that defibrillation is delayed in a shockable rhythm, the patient’s chance of survival declines by approximately 7% to 10%. This rapid decline underscores why time is the most important factor in the chain of survival for cardiac arrest.
While Cardiopulmonary Resuscitation (CPR) is an immediate and necessary intervention, it is not a substitute for the electrical shock. CPR works by manually circulating oxygenated blood to the brain and organs, helping to sustain them until the underlying electrical problem can be fixed. Only defibrillation can correct the chaotic electrical rhythm of ventricular fibrillation or pulseless ventricular tachycardia, transforming it into an organized rhythm. CPR buys time, but defibrillation provides the cure for the electrical malfunction.
Accessibility and Technology: Defibrillation Devices
Advancements in technology have broadened the accessibility of defibrillation, extending its reach beyond hospital walls. Automated External Defibrillators (AEDs) are portable devices designed for use by laypeople and first responders in public settings. The AED automatically analyzes the heart rhythm and provides voice prompts, delivering a shock only if a shockable rhythm is detected. The widespread placement of AEDs in public spaces like airports and schools has drastically reduced the time-to-shock.
For individuals at high risk for sudden cardiac arrest, an Implantable Cardioverter-Defibrillator (ICD) offers long-term protection. Surgically placed under the skin, an ICD constantly monitors the heart’s rhythm and automatically delivers a corrective electrical shock internally when a dangerous arrhythmia is detected.