Editing Third-degree atrioventricular block (section)

==Cause==
[[Image:Rhythm strip showing third degree heart block.jpg|thumb|upright=1.8|[[Electrocardiography#Leads|Leads I and II]] demonstrating complete AV block. Note that the P waves are not related to the QRS complexes (PP interval and QRS interval both constant), demonstrating that the atria are electrically disconnected from the ventricles. The QRS complexes represent an escape rhythm arising from the ventricle.]]
[[File:Complete A-V block with resulting junctional escape.png|thumb|upright=1.4|[[Atrial tachycardia]] with complete A-V block and resulting junctional escape]]
Many conditions can cause third-degree heart block, but the most common cause is [[coronary ischemia]].  Progressive degeneration of the electrical conduction system of the heart can lead to third-degree heart block.  This may be preceded by [[first-degree AV block]], [[second-degree AV block]], [[bundle branch block]], or [[bifascicular block]]. In addition, acute myocardial infarction may present with third-degree AV block.<ref>{{Cite journal |last1=Knabben |first1=V. |last2=Chhabra |first2=L. |last3=Slane |first3=M. |year=2022 |title=Third-Degree Atrioventricular Block |url=https://www.ncbi.nlm.nih.gov/books/NBK545199/ |pmid=31424783 |access-date=3 July 2021 |website=National Center for Biotechnology Information, U.S. National Library of Medicine}}</ref>

An inferior wall myocardial infarction may cause damage to the AV node, causing third-degree heart block.  In this case, the damage is usually transitory.  Studies have shown that third-degree heart block in the setting of an inferior wall myocardial infarction typically resolves within 2 weeks.<ref>{{cite journal|last1=Sclarovsky|first1=S|last2=Strasberg|first2=B|last3=Hirshberg|first3=A|last4=Arditi|first4=A|last5=Lewin|first5=RF|last6=Agmon|first6=J|title=Advanced early and late atrioventricular block in acute inferior wall myocardial infarction.|journal=American Heart Journal|date=July 1984|volume=108|issue=1|pages=19–24|pmid=6731277|doi=10.1016/0002-8703(84)90539-8}}</ref> The escape rhythm typically originates in the AV junction, producing a narrow complex escape rhythm.{{cn|date=February 2021}}

An anterior wall myocardial infarction may damage the distal conduction system of the heart, causing third-degree heart block. Initially demonstrated by animal studies, this is due to a stark reduction in the Kv β-subunit of the voltage-gated K+ channels in the pacemaker cells of the atrioventricular junction, causing significantly decreased propagation of ions across gap junctions between cardiac cells and thus prolonging the PR interval.<ref> Nikolaidou, T.; Cai, X.J.; Stephenson, R.S.; Yanni, J.; Lowe, T.; Atkinson, A.J.; Jones, C.B.; Sardar, R.; Corno, A.F.;  Dobrzynski, H.; Withers, P.J.; Jarvis, J.C.; Hart, G.; & Boyett, M.R. (2015). "Congestive heart failure leads to prolongation of the PR interval and atrioventricular Junction enlargement and ion channel remodelling in the rabbit." PLOS ONE, 10(10), e0141452. https://doi.org/10.1371/journal.pone.0141452 </ref> This is typically extensive, permanent damage to the conduction system, eliciting a necessity for a permanent pacemaker to be placed. <ref> Atrioventricular block, third degree. (2012). Clinical Veterinary Advisor, 58–61. https://doi.org/10.1016/b978-1-4160-9979-6.00038-6 </ref> The escape rhythm typically originates in the ventricles, producing a wide complex escape rhythm.

Third-degree heart block may also be [[congenital]] and has been linked to the presence of [[lupus erythematosus|lupus]] in the mother.<ref>{{cite journal|last1=Brucato|first1=A.|last2=Previtali|first2=E.|last3=Ramoni|first3=V.|last4=Ghidoni|first4=S.|title=Arrhythmias presenting in neonatal lupus.|journal=Scandinavian Journal of Immunology|date=September 2010|volume=72|issue=3|pages=198–204|pmid=20696016|doi=10.1111/j.1365-3083.2010.02441.x|hdl=2434/635678|url=https://air.unimi.it/bitstream/2434/635678/2/Arrhtmyas%20in%20CHB.pdf|doi-access=free}}</ref> It is thought that maternal [[antibodies]] may cross the [[placenta]] and attack the heart tissue during [[gestation]]. The cause of congenital third-degree heart block in many patients is unknown. Studies suggest that the prevalence of congenital third-degree heart block is between 1 in 15,000 and 1 in 22,000 live births.{{cn|date=August 2022}}

[[Hyperkalemia]] in those with previous cardiac disease<ref>{{Cite journal |vauthors=Sohoni A, Perez B, Singh A |year=2010 |title=Wenckebach Block due to Hyperkalemia: A Case Report |journal=Emerg Med Int |volume=2010 |pages=879751 |doi=10.1155/2010/879751 |pmc=3200192 |pmid=22046534 |doi-access=free}}</ref> and [[Lyme disease]] can also result in third-degree heart block.<ref>{{cite journal|last1=Forrester|first1=JD|last2=Mead|first2=P|title=Third-degree heart block associated with lyme carditis: review of published cases.|journal=Clinical Infectious Diseases|date=October 2014|volume=59|issue=7|pages=996–1000|pmid=24879781|doi=10.1093/cid/ciu411|doi-access=free}}</ref>

=== Hypermagnesemia ===
AV block may be observed in patients with [[hypermagnesemia]] who are receiving excessive intravenous doses of [[Magnesium sulfate (medical use)|magnesium sulfate]].<ref name=":2">{{Cite book |url=https://www.worldcat.org/oclc/1007160054 |title=Advanced perioperative crisis management |date=2017 |others=Matthew D. McEvoy, Cory M. Furse |isbn=978-0-19-022648-0 |location=New York |oclc=1007160054}}</ref>{{Rp|page=281}}