Defibrillation and Automated External Defibrillation (AED)

Defibrillation is the delivery of a controlled electric shock across the heart to terminate a lethal, disorganized rhythm — ventricular fibrillation or pulseless ventricular tachycardia — and allow a coordinated, perfusing rhythm to resume. The automated external defibrillator (AED) is a portable device that analyzes the cardiac rhythm and guides a lay or professional rescuer to deliver a shock only when one is indicated.

Definition

Defibrillation is the application of a brief, high-energy electric current to the heart to stop ventricular fibrillation or pulseless ventricular tachycardia; an automated external defibrillator is a portable device that detects these shockable rhythms and prompts delivery of a shock.

Scope

This entry covers the principle of electrical defibrillation, the role of early defibrillation in shockable cardiac arrest, the function of automated external defibrillators, and public-access defibrillation programs. It is descriptive and does not provide device operation instructions or energy settings.

Key concepts

• Shockable rhythms (ventricular fibrillation, pulseless ventricular tachycardia)
• Early defibrillation and time-to-shock
• Automated external defibrillator (AED) rhythm analysis
• Public-access defibrillation
• Integration of defibrillation with chest compressions
• Chain of survival

Mechanisms

In ventricular fibrillation the heart's electrical activity is chaotic and produces no effective pumping. A defibrillation shock depolarizes a critical mass of myocardium simultaneously, briefly halting all electrical activity so that the heart's intrinsic pacemaker can resume an organized, perfusing rhythm. The probability of success falls quickly as the duration of fibrillation lengthens, which is why time-to-shock is critical and why chest compressions are continued around shocks to sustain perfusion. An automated external defibrillator records and analyzes the rhythm and advises a shock only when a shockable pattern is detected, allowing rescuers without rhythm-interpretation skills to defibrillate safely.

Clinical relevance

Early defibrillation is one of the central links in the chain of survival for cardiac arrest and the main reason public-access AED programs exist. This entry describes the principle and evidence for reference; when and how to defibrillate is governed by device prompts, current guidelines, and training rather than by this summary.

Epidemiology

A substantial proportion of out-of-hospital cardiac arrests initially present with a shockable rhythm, and for these patients survival is strongly tied to how quickly a shock is delivered. A randomized trial of public-access defibrillation found that training and equipping lay responders with AEDs increased survival after out-of-hospital cardiac arrest in public settings, supporting the spread of AED programs.

History

External electrical defibrillation was established in the mid-twentieth century and became, alongside chest compressions, a core component of modern resuscitation. The later development of compact, automated external defibrillators made rhythm analysis and shock delivery feasible for lay rescuers, enabling public-access defibrillation programs that are now reflected in international resuscitation guidelines.

Debates

How best to expand and locate public-access AEDs

While early defibrillation clearly improves survival for shockable arrests, the optimal placement, density, and dispatch integration of public-access AEDs — and how to ensure they are actually used in time — remain active questions of resuscitation systems research.

Seminal works

• pad-trial-2004
• soar-2021
• panchal-2020

Frequently asked questions

What does a defibrillation shock actually do?

It momentarily depolarizes most of the heart muscle at once, stopping the chaotic electrical activity of ventricular fibrillation so the heart's own pacemaker can restart an organized, perfusing rhythm.

Can a defibrillator restart a heart that has completely stopped?

No. Defibrillation treats specific disorganized rhythms (ventricular fibrillation and pulseless ventricular tachycardia); an automated external defibrillator analyzes the rhythm and will not advise a shock when none is appropriate, such as in asystole.