A defibrillator delivers a controlled electrical shock to the heart. Its purpose is to interrupt abnormal, life-threatening heart rhythms, allowing the heart to resume a normal, effective beat. This intervention is crucial during sudden cardiac arrest, when the heart unexpectedly stops pumping blood effectively.
How Defibrillators Restore Heart Rhythm
When sudden cardiac arrest occurs, the heart’s electrical activity often becomes chaotic, known as ventricular fibrillation (VF) or pulseless ventricular tachycardia (pVT). In these rhythms, the heart’s lower chambers quiver ineffectively, preventing blood circulation. A defibrillator analyzes this disorganized activity and, if a shockable rhythm is detected, delivers a timed electrical impulse.
This electrical impulse momentarily resets the heart muscle cells. This brief interruption allows the heart’s natural pacemaker to re-establish a coordinated and effective rhythm. A defibrillator does not “restart” a heart that has completely flatlined (asystole); rather, it corrects an electrical malfunction.
Factors Affecting Defibrillation Success
Several factors influence defibrillator effectiveness. The time from collapse to defibrillation is a significant factor; survival decreases by 7% to 10% for every minute without intervention. Each minute of delay to the first shock can lower defibrillation success by about 6%.
The type of cardiac arrest rhythm also plays a role; defibrillators are effective only for ventricular fibrillation and pulseless ventricular tachycardia. Rhythms like asystole or pulseless electrical activity do not respond to defibrillation. The quality and duration of cardiopulmonary resuscitation (CPR) performed before defibrillation also impact success. High-quality CPR, with adequate compression rate, depth, and minimal interruptions, helps maintain blood flow and makes the heart more receptive to the shock.
Understanding Survival Rates
Defibrillation success is distinct from overall survival to hospital discharge. For out-of-hospital cardiac arrest (OHCA), the overall survival rate in the United States is often below 10%. When bystander CPR is performed, survival rates are around 9%. If an automated external defibrillator (AED) shock is also delivered, this rate increases to about 38%. With both bystander CPR and AED use, the survival rate can reach 50%.
Public access defibrillation programs show a median survival to hospital discharge of 40%, with ranges from 9.1% to 83.3%. When a bystander uses an AED, patients are more likely to survive to discharge, with rates up to 66.5% compared to 43% for those shocked by emergency medical services. In-hospital cardiac arrest (IHCA) survival rates range from 6% to 20%.
The Critical Role of Timely Intervention
Immediate action is crucial in cardiac arrest. While defibrillators are effective for specific heart rhythms, their success relies on quick deployment alongside high-quality CPR. Every minute without defibrillation reduces the chance of a positive outcome.
Early, effective CPR maintains blood flow to the brain and heart, buying time until a defibrillator can be used. Increased public awareness, widespread CPR training, and greater accessibility of AEDs improve survival rates. These combined efforts create a “chain of survival” that leads to better outcomes for individuals experiencing sudden cardiac arrest.