Defibrillatoris a treatment for life-threateningcardiac dysrhythmias, specificallyventricular fibrillation(VF) andnon-perfusing ventricular tachycardia(VT).^[1][2]A defibrillator delivers a dose ofelectric current(often called a countershock) to theheart. Thisdepolarizesa large amount of theheart muscle, ending the dysrhythmia. Subsequently, the body’snatural pacemakerin thesinoatrial nodeof the heart is able to re-establishnormal sinus rhythm.^[3]

In contrast to defibrillation, synchronized electricalcardioversionis an electrical shock delivered in synchrony to thecardiac cycle. Although the person may still be critically ill, cardioversion normally aims to end poorly perfusingcardiac dysrhythmias, such assupraventricular tachycardia.^[1][2]

Defibrillators can be external, transvenous, orimplanted(implantable cardioverter-defibrillator), depending on the type of device used or needed.^[4]Some external units, known asautomated external defibrillators(AEDs), automate the diagnosis of treatable rhythms, meaning that lay responders or bystanders are able to use them successfully with little or no training.

Defibrillation is often an important step incardiopulmonary resuscitation(CPR).^[5][6]CPR is an algorithm-based intervention aimed to restore cardiac and pulmonary function.^[5]Defibrillation is indicated only in certain types ofcardiac dysrhythmias, specificallyventricular fibrillation(VF) andpulseless ventricular tachycardia.^[1][2]If the heart has completely stopped, as inasystoleorpulseless electrical activity (PEA), defibrillation is not indicated. Defibrillation is also not indicated if the patient is conscious or has a pulse. Improperly given electrical shocks can cause dangerous dysrhythmias, such asventricular fibrillation.^[1]

Survival rates for out-of-hospital cardiac arrests are poor, often less than 10%.^[7]Outcome for in-hospital cardiac arrests are higher at 20%.^[7]Within the group of people presenting with cardiac arrest, the specific cardiac rhythm can significantly impact survival rates. Compared to people presenting with a non-shockable rhythm (such as asystole or PEA), people with a shockable rhythm (such as VF or pulseless ventricular tachycardia) have improved survival rates, ranging between 21-50%.