Defibrillation and Electrical Therapy

Defibrillation is the delivery of a controlled electric shock to the heart to terminate a shockable arrest rhythm — ventricular fibrillation or pulseless ventricular tachycardia — so that an organized, perfusing rhythm can resume. Together with related electrical therapies, it is one of the most time-critical interventions in resuscitation, and the spread of automated external defibrillators has extended it beyond the hospital to public settings.

Definition

Defibrillation is the application of an unsynchronized electric shock across the heart to depolarize a critical mass of myocardium and terminate a shockable arrest rhythm, allowing the heart's intrinsic pacemaker activity to re-establish an organized rhythm.

Scope

This topic covers the principle of defibrillation, its role within the resuscitation sequence, the distinction from synchronized cardioversion, and the place of automated external and public-access defibrillation. It is descriptive and educational and does not provide device-specific operating instructions, energy settings, or a substitute for certified training.

Key concepts

• Shockable rhythms as the indication for defibrillation
• Critical-mass depolarization to terminate fibrillation
• Time to first shock and the chain of survival
• Automated external defibrillators (AEDs)
• Public-access defibrillation
• Synchronized cardioversion versus unsynchronized defibrillation
• Minimizing peri-shock pauses in compressions

Mechanisms

A defibrillation shock passes current through the heart and simultaneously depolarizes a large proportion of the myocardium, interrupting the disorganized re-entrant electrical activity of ventricular fibrillation or pulseless ventricular tachycardia. With the chaotic activity terminated, the heart's intrinsic pacemaker tissue can resume an organized rhythm capable of producing a pulse. Because the likelihood of successful defibrillation declines as arrest continues, and because pausing compressions reduces perfusion, guidelines emphasize early shock delivery and minimizing the pauses around it. Synchronized cardioversion, by contrast, times the shock to the cardiac cycle and is used for organized but unstable rhythms rather than for fibrillation.

Clinical relevance

Early defibrillation is a defining link in the chain of survival for shockable cardiac arrest, and automated external defibrillators allow trained and lay responders to deliver it before advanced help arrives. This entry describes the principle for reference; device operation, energy selection, and timing follow current guidelines, device prompts, and formal training rather than this summary.

Epidemiology

Survival from shockable out-of-hospital cardiac arrest falls with each minute of delay to defibrillation, and programs placing automated external defibrillators in public locations have been studied as a way to shorten that interval and improve survival when arrests are witnessed.

Evidence & guidelines

The indications and place of defibrillation in the resuscitation sequence are set out in the advanced life support guidelines of the European Resuscitation Council and the American Heart Association. Trials of public-access defibrillation and of the timing of rhythm analysis relative to compressions have informed how and when shocks are delivered.

History

External defibrillation was developed in the mid-twentieth century and progressively integrated with chest compressions into a unified resuscitation sequence. The later development of automated external defibrillators, which analyze the rhythm and advise a shock, extended defibrillation from hospitals and ambulances to public and lay-responder settings.

Debates

Should a period of CPR precede defibrillation, and how should peri-shock pauses be handled?

The benefit of delivering a period of chest compressions before the first rhythm analysis and shock, versus analyzing and shocking as early as possible, has been tested without a clear overall survival difference, leaving the emphasis on minimizing interruptions around the shock as the practical focus.

Seminal works

• soar-2021
• panchal-2020
• stiell-2011

Frequently asked questions

Which rhythms can be treated with defibrillation?

Defibrillation treats the shockable rhythms — ventricular fibrillation and pulseless ventricular tachycardia — and is not effective for asystole or pulseless electrical activity.

How does defibrillation differ from synchronized cardioversion?

Defibrillation delivers an unsynchronized shock to terminate chaotic fibrillation during arrest, whereas synchronized cardioversion times the shock to the cardiac cycle and is used for organized but unstable rhythms.