Editing Cardiac arrest (section)

===Defibrillation===
[[File:Defibrillator-809447 1920.jpg|thumb|upright=1.3|An automated external defibrillator stored in a visible orange mural support]]
Defibrillation is indicated if an shockable rhythm is present; the two shockable rhythms are [[ventricular fibrillation]] and [[ventricular tachycardia]]. These shockable rhythms have a 25-40% likelihood of survival, compared with a significantly lower rate (less than 5%) in non-shockable rhythms.<ref>{{cite journal | vauthors = Woolcott OO, Reinier K, Uy-Evanado A, Nichols GA, Stecker EC, Jui J, Chugh SS | title = Sudden cardiac arrest with shockable rhythm in patients with heart failure | journal = Heart Rhythm | volume = 17 | issue = 10 | pages = 1672–1678 | date = October 2020 | pmid = 32504821 | pmc = 7541513 | doi = 10.1016/j.hrthm.2020.05.038 }}</ref> The non-shockable rhythms include [[asystole]] and pulseless electrical activity.

Ventricular fibrillation involves the [[Ventricle (heart)|ventricles]] of the heart rapidly contracting in an disorganized pattern, and thereby limiting blood flow from the heart. This is due to an uncoordinated electrical activity.<ref>{{Cite web |date=2022-03-24 |title=Arrhythmias - What Is an Arrhythmia? {{!}} NHLBI, NIH |url=https://www.nhlbi.nih.gov/health/arrhythmias |url-status=live |archive-url=https://web.archive.org/web/20240926180105/https://www.nhlbi.nih.gov/health/arrhythmias |archive-date=2024-09-26 |access-date=2024-01-31 |website=www.nhlbi.nih.gov |language=en}}</ref> The electrocardiogram (ECG) shows irregular QRS complexes at a very high rate (>300  beats per minute).<ref>{{cite book | vauthors = Ludhwani D, Goayal A, Jagtap M | chapter = Ventricular Fibrillation |date=2024 | title = StatPearls | chapter-url= http://www.ncbi.nlm.nih.gov/books/NBK537120/ |access-date=2024-01-31 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=30725805 }}</ref> In ventricular tachycardia, the ECG will show a wide complex rhythm at a rate higher than 100 beats per minute.<ref>{{cite book | vauthors = Foth C, Gangwani MK, Ahmed I, Alvey H | chapter = Ventricular Tachycardia |date=2024 | title = StatPearls | chapter-url=http://www.ncbi.nlm.nih.gov/books/NBK532954/ |access-date=2024-01-31 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=30422549 }}</ref> These two rhythm lead to hemodynamic instability and compromise, resulting in poor perfusion to vital organs (including the heart itself).

[[File:Defibrillation Electrode Position.jpg|thumb|Demonstration of electrode pad placement for defibrillation]]
A defibrillator — either implanted or external — delivers an electrical current that results in the entire myocardium simultaneously depolarized thereby stopping the arrhythmia.<ref>{{Cite web | vauthors = Knight BP | date = 23 January 2023 | veditors = Page RL, Dardas TF | title = Basic principles and technique of external electrical cardioversion and defibrillation | work = UpToDate |url=https://www.uptodate.com/contents/basic-principles-and-technique-of-external-electrical-cardioversion-and-defibrillation |access-date=2024-01-18 }}</ref> Defibrillators can deliver energy as monophasic or biphasic waveforms, although biphasic defibrillators are now the most common.<ref>{{cite journal | vauthors = Leng CT, Paradis NA, Calkins H, Berger RD, Lardo AC, Rent KC, Halperin HR | title = Resuscitation after prolonged ventricular fibrillation with use of monophasic and biphasic waveform pulses for external defibrillation | journal = Circulation | volume = 101 | issue = 25 | pages = 2968–2974 | date = June 2000 | pmid = 10869271 | doi = 10.1161/01.CIR.101.25.2968 }}</ref><ref name="Schneider 1780–1787">{{cite journal | vauthors = Schneider T, Martens PR, Paschen H, Kuisma M, Wolcke B, Gliner BE, Russell JK, Weaver WD, Bossaert L, Chamberlain D | display-authors = 6 | title = Multicenter, randomized, controlled trial of 150-J biphasic shocks compared with 200- to 360-J monophasic shocks in the resuscitation of out-of-hospital cardiac arrest victims. Optimized Response to Cardiac Arrest (ORCA) Investigators | journal = Circulation | volume = 102 | issue = 15 | pages = 1780–1787 | date = October 2000 | pmid = 11023932 | doi = 10.1161/01.CIR.102.15.1780 }}</ref> Prior studies suggest that biphasic shock is more likely to produce successful defibrillation after a single shock, however rate of survival is comparable between the methods.<ref name="Schneider 1780–1787"/>

In out-of-hospital arrests (OHA), the defibrillation is made by an [[automated external defibrillator]] (AED), a portable machine that can be used by any user. The AED provides voice instructions that guide the process, automatically checks the person's condition, and applies the appropriate electric shocks. Some defibrillators even provide feedback on the quality of [[cardiopulmonary resuscitation|CPR]] compressions, encouraging the lay rescuer to press the person's chest hard enough to circulate blood.<ref>{{cite web|title=Zoll Automated External Defibrillator (AED) Plus|url=http://www.lifeassisttraining.com/zollaedplus.html|archive-url=https://web.archive.org/web/20110621004342/http://www.lifeassisttraining.com/zollaedplus.html|archive-date=2011-06-21|work=Life Assistance Training}}</ref>

There is increasing use of public access to defibrillators. This typically involves placing AEDs in publicly-accessible places and training staff in these areas on how to use them. This allows defibrillation to occur prior to the arrival of emergency services, which has been shown to increase the chances of survival. People who have cardiac arrests in remote locations have worse outcomes.<ref name="Lyon-2004">{{cite journal | vauthors = Lyon RM, Cobbe SM, Bradley JM, Grubb NR | title = Surviving out of hospital cardiac arrest at home: a postcode lottery? | journal = Emergency Medicine Journal | volume = 21 | issue = 5 | pages = 619–624 | date = September 2004 | pmid = 15333549 | pmc = 1726412 | doi = 10.1136/emj.2003.010363 }}</ref>

Defibrillation cannot be applied to asystole and CPR must be initiated first in this case. A similar concept, [[cardioversion]], utilizes the same defibrillation machine but is used for other rhythms such as [[atrial fibrillation]] and [[supraventricular tachycardia]]. In these rhythms, the machine is "synchronized" to the QRS complex to avoid shocking on the T wave (and inducing VT or VF). Cardioversion can be done electively for rhythm control, or urgently if the rhythm is unstable.