Editing Cardiac arrest (section)

===Mechanism===
[[File:Ventricular fibrillation.png|thumb|upright=1.3|Ventricular fibrillation]]
The definitive electrical mechanisms of cardiac arrest, which may arise from any of the functional, structural, or physiologic abnormalities mentioned above, are characterized by arrhythmias.<ref name="Saunders-2015" /> [[Ventricular fibrillation]] and pulseless or sustained [[ventricular tachycardia]] are the most commonly recorded arrhythmias preceding cardiac arrest. These are rapid and erratic arrhythmias that alter the circulatory pathway such that adequate blood flow cannot be sustained and is inadequate to meet the body's needs.<ref name="Saunders-2015" />

The mechanism responsible for the majority of sudden cardiac deaths is ventricular fibrillation. Ventricular fibrillation is a tachyarrhythmia characterized by turbulent electrical activity in the ventricular myocardium leading to a heart rate too disorganized and rapid to produce any meaningful cardiac output, thus resulting in insufficient perfusion of the brain and essential organs.<ref>{{Cite web|title=Ventricular tachycardia - Symptoms and causes|url=https://www.mayoclinic.org/diseases-conditions/ventricular-tachycardia/symptoms-causes/syc-20355138|access-date=2021-11-29|website=Mayo Clinic|language=en}}</ref> Some of the electrophysiologic mechanisms underpinning ventricular fibrillations include ectopic automaticity, re-entry, and triggered activity.<ref>{{cite journal |vauthors=Szabó Z, Ujvárosy D, Ötvös T, Sebestyén V, Nánási PP |date=2020-01-29 |title=Handling of Ventricular Fibrillation in the Emergency Setting |journal=Frontiers in Pharmacology |volume=10 |pages=1640 |doi=10.3389/fphar.2019.01640 |pmc=7043313 |pmid=32140103 |doi-access=free}}</ref> However, structural changes in the diseased heart as a result of inherited factors (mutations in ion-channel coding genes, for example) cannot explain the sudden onset of cardiac arrest.<ref>{{cite journal |vauthors=Rubart M, Zipes DP |date=September 2005 |title=Mechanisms of sudden cardiac death |journal=The Journal of Clinical Investigation |volume=115 |issue=9 |pages=2305–2315 |doi=10.1172/JCI26381 |pmc=1193893 |pmid=16138184}}</ref>

In ventricular tachycardia, the heart also beats faster than normal, which may prevent the heart chambers from properly filling with blood.<ref>{{Cite web|title=Ventricular fibrillation - Symptoms and causes|url=https://www.mayoclinic.org/diseases-conditions/ventricular-fibrillation/symptoms-causes/syc-20364523|access-date=2021-11-29|website=Mayo Clinic|language=en}}</ref> Ventricular tachycardia is characterized by an altered QRS complex and a heart rate greater than 100 beats per minute.<ref>{{cite journal | vauthors = AlMahameed ST, Ziv O | title = Ventricular Arrhythmias | journal = The Medical Clinics of North America | volume = 103 | issue = 5 | pages = 881–895 | date = September 2019 | pmid = 31378332 | doi = 10.1016/j.mcna.2019.05.008 | s2cid = 199437558 }}</ref> When V-tach is sustained (lasts for at least 30 seconds), inadequate blood flow to heart tissue can lead to cardiac arrest.<ref>{{cite journal | vauthors = Baldzizhar A, Manuylova E, Marchenko R, Kryvalap Y, Carey MG | title = Ventricular Tachycardias: Characteristics and Management | journal = Critical Care Nursing Clinics of North America | volume = 28 | issue = 3 | pages = 317–329 | date = September 2016 | pmid = 27484660 | doi = 10.1016/j.cnc.2016.04.004 }}</ref>

[[Bradycardia|Bradyarrhythmias]] occur following dissociation of spontaneous electrical conduction and the mechanical function of the heart resulting in [[pulseless electrical activity]] (PEA) or through complete absence of electrical activity of the heart resulting in [[asystole]]. Similar to the result of tachyarrhythmias, these conditions lead to an inability to sustain adequate cardiac output.<ref name="Saunders-2015" />