Editing Third-degree atrioventricular block

{{Infobox medical condition (new)
| name            = Third-degree atrioventricular block
| synonyms        = Complete heart block
| image           = CompleteHeartBlock.jpg
| caption         = 12-lead ECG showing complete heart block
| pronounce       =
| field           = [[Cardiology]]
| symptoms        = [[Dizziness]], [[Syncope (medicine)|Fainting]], [[Breathlessness|Shortness of breath]], [[Sudden cardiac death]]
| complications   =
| onset           =
| duration        =
| types           =
| causes          = Fibrosis in [[cardiac conduction system]], [[myocardial infarction]], post-cardiac surgery, medication, [[vagal tone]], electrolyte disturbances
| risks           =
| diagnosis       = [[Electrocardiography|Electrocardiogram]]
| differential    =
| prevention      =
| treatment       = [[Electronic pacemaker|Pacemaker]]
| medication      =
| prognosis       =
| frequency       =
| deaths          =
|alt=}}

'''Third-degree atrioventricular block''' (AV block) is a medical condition in which the electrical impulse generated in the [[sinoatrial node]] (SA node) in the [[atrium (heart)|atrium]] of the [[heart]] can not propagate to the [[ventricle (heart)|ventricles]].<ref name="urlLesson VI - ECG Conduction Abnormalities">{{cite web |url=http://ecg.utah.edu/lesson/6#Complete |title=ECG Conduction Abnormalities |access-date=2009-01-07}}</ref>

Because the impulse is blocked, an accessory pacemaker in the lower chambers will typically activate the ventricles.  This is known as an ''[[escape rhythm]]''. Since this accessory pacemaker also activates independently of the impulse generated at the SA node, two independent rhythms can be noted on the [[electrocardiogram]] (ECG).
* The [[P wave (electrocardiography)|P waves]] with a regular P-to-P interval (in other words, a [[sinus rhythm]]) represent the first rhythm.
* The [[QRS complex]]es with a regular R-to-R interval represent the second rhythm. The [[PR interval]] will be variable, as the hallmark of complete heart block is the lack of any apparent relationship between P waves and QRS complexes.

==Presentation==
People with third-degree AV block typically experience severe [[bradycardia]] (an abnormally low measured heart rate), [[hypotension]], and at times, [[hemodynamic]] instability.<ref>{{cite web|title=Heart Block|url=http://www.nhs.uk/Conditions/Heart-block/Pages/Introduction.aspx|website=NHS Choices|publisher=National Health Service (UK)|access-date=25 August 2015}}</ref>

==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}}

==Diagnosis==
Diagnosis is largely focussed on analysis of the patients 12-lead ECG. A patient with a third-degree AV block will likely have p-waves not corresponding to QRS complexes along with bradycardia.

==Treatment==
[[Atropine]] is often used as a first line treatment of a third-degree heart block in the presence of a narrow QRS which indicates a nodal block, but, may have little to no effect in an infra-nodal block.<ref>{{Cite book|title=Tintinalli's emergency medicine : a comprehensive study guide|others=Tintinalli, Judith E.; Stapczynski, J. Stephan; Ma, O. John; Yealy, Donald M.; Meckler, Garth D.; Cline, David|isbn=9780071794763|edition=  8th |location=New York|page=123|oclc=915775025|date = 2015}}</ref> Atropine works by reducing [[Vagus nerve|vagal]] stimulation through the AV node but will not be effective in those who have had a previous heart transplant.<ref>{{Cite |date=2018-07-05|title=Third-Degree Atrioventricular Block (Complete Heart Block) Treatment & Management: Approach Considerations, Initial Management Considerations, Atropine and Transcutaneous/Transvenous Pacing|url=https://emedicine.medscape.com/article/162007-treatment#d8}}</ref> Other drugs may be utilized such as [[Epinephrine (medication)|epinephrine]] or [[dopamine]] which have positive [[chronotropic]] effects and may increase the heart rate.<ref name=":1">{{Cite book |title=Oxford textbook of critical care |others=Webb, Andrew Roy;  Angus, Derek C.; Finfer, Simon; Gattinoni, Luciano; Singer, Mervyn |isbn=9780199600830 |edition=2nd |location=Oxford |pages=730–735 |oclc=954059445 |date=2016}}</ref> Treatment in emergency situations can involve electrical [[transcutaneous pacing]] in those who are acutely hemodynamically unstable and can be used regardless of the persons level of consciousness.<ref name=":0">{{Cite book|title=Rosen's emergency medicine : concepts and clinical practice|others=Walls, Ron M.; Hockberger, Robert S.;  Gausche-Hill, Marianne|isbn=9780323390163|edition=9th |location=Philadelphia|pages=959|oclc=989157341|date=2017}}</ref> Sedative agents such as a [[benzodiazepine]] or [[opiate]] may be used in conjunction with transcutaneous pacing to reduce the pain caused by the intervention.<ref name=":1" /><ref name=":0" />

In cases of suspected [[Beta blocker|beta-blocker]] overdose, the heart-block may be treated with pharmacological agents to reverse the underlying cause with the use of [[glucagon]]. In the case of a [[calcium channel blocker]] overdose treated with [[calcium chloride]] and [[digitalis]] toxicity may be treated with the [[digoxin immune Fab]].<ref name="BMJ15">{{cite web|title=Atrioventricular block, Treatment Options|url=http://bestpractice.bmj.com/best-practice/monograph/728/treatment/details.html|website=BMJ Best Practice|publisher=British Medical Journal| access-date=25 August 2015}}</ref>

Third-degree AV block can be treated more permanently with the use of a dual-chamber [[artificial pacemaker]].<ref>{{cite journal|last1=Dretzke|first1=J.|title=Compared to single-chamber ventricular pacemakers, dual-chamber pacemakers may reduce the incidence of complications in people with sick sinus syndrome and atrioventricular block|url=http://www.cochrane.org/CD003710/VASC_compared-to-single-chamber-ventricular-pacemakers-dual-chamber-pacemakers-may-reduce-the-incidence-of-complications-in-people-with-sick-sinus-syndrome-and-atrioventricular-block|journal=The Cochrane Database of Systematic Reviews|year=2004 |volume=2004 |issue=2 |pages=CD003710 |doi=10.1002/14651858.CD003710.pub2 |pmid=15106214 |pmc=8095057 |access-date=25 August 2015|display-authors=etal}}</ref>  This type of device typically listens for a pulse from the SA node via lead in the right atrium and sends a pulse via a lead to the right ventricle at an appropriate delay, driving both the right and left ventricles.  Pacemakers in this role are usually programmed to enforce a minimum heart rate and to record instances of [[atrial flutter]] and [[atrial fibrillation]], two common secondary conditions that can accompany third-degree AV block.  Since pacemaker correction of the third-degree block requires full-time pacing of the ventricles, a potential side effect is [[pacemaker syndrome]], and may necessitate the use of a [[Artificial cardiac pacemaker#Biventricular pacing|biventricular pacemaker]], which has an additional 3rd lead placed in a vein in the left ventricle, providing more coordinated pacing of both ventricles.{{cn|date=February 2021}}

The 2005 Joint European Resuscitation and Resuscitation Council (UK) guidelines<ref>{{cite web|title=Peri-arrest arrhythmias|url=https://www.resus.org.uk/EasySiteWeb/GatewayLink.aspx?alId=808|website=Resuscitation guidelines|publisher=Resuscitation Council UK|access-date=25 August 2015}}</ref> state that atropine is the first-line treatment especially if there were any adverse signs, namely: 1) heart rate < 40 bpm, 2) systolic blood pressure < 100&nbsp;mm Hg, 3) signs of heart failure, and 4) ventricular arrhythmias requiring suppression. If these fail to respond to atropine or there is a potential risk of asystole, transvenous pacing is indicated. The risk factors for asystole include 1) previous asystole, 2) complete heart block with wide complexes, and 3) ventricular pause for > 3 seconds. [[Second-degree AV block#Type 2 (Mobitz II/Hay)|Mobitz Type 2 AV block]] is another indication for pacing.

As with other forms of heart block, [[secondary prevention]] may also include medicines to control blood pressure and atrial fibrillation, as well as lifestyle and dietary changes to reduce risk factors associated with [[myocardial infarction|heart attack]] and [[stroke]].

Early treatment of atrioventricular blockade is based on the presence and severity of symptoms and signs associated with ventricular escape rhythm. Hemodynamically unstable patients require immediate medication and in most cases temporary pacing to increase heart rate and cardiac output.

Once the patient is hemodynamically stable, a potentially reversible cause should be evaluated and treated. If no reversible cause is identified, a permanent pacemaker is inserted.{{Citation needed|date=December 2019}} Most stable patients have persistent bradycardia-related symptoms and require identification and treatment of any reversible cause or permanent implantable pacemaker.

Reversible causes of complete AV block should be ruled out before the insertion of a permanent pacemaker, such as drugs that slow heart rate and which induce hyperkalemia.

Complete atrioventricular block in acute myocardial infarction should be treated with temporary pacing and revascularization.<ref name="auto">{{cite web |url= https://www.lecturio.com/concepts/atrioventricular-block/ |title=Atrioventricular Block |website=The Lecturio Medical Concept Library |access-date=3 July 2021}}</ref>
{{Citation needed|date=December 2019}}

Complete atrioventricular block caused by hyperkalemia should be treated to lower serum potassium levels and patients with hypothyroidism should also receive thyroid hormone.<ref name="auto"/>

If there is no reversible cause, the clear treatment of complete atrioventricular block is mostly permanent pacemaker placement.{{Citation needed|date=December 2019}}

==Prognosis==
The prognosis of patients with complete heart block is generally poor without therapy. Patients with 1st and 2nd-degree heart block are usually asymptomatic.<ref>{{Cite journal|journal=Acta Med Scand|year=1976|volume=200|issue=6|pages=457–63|title=Prognosis of patients with complete heart block or arrhythmic syncope who were not treated with artificial pacemakers. A long-term follow-up study of 101 patients.|vauthors=Edhag O, Swahn A |pmid=1015354|doi=10.1111/j.0954-6820.1976.tb08264.x}}</ref>

==See also==
* [[Cardiac pacemaker]]
* [[Electrical conduction system of the heart]]
* [[Electrocardiogram]] (ECG)
* [[Atrioventricular block]]
* [[First-degree AV block]]
* [[Second-degree AV block]]

==References==
{{reflist}}

== External links ==
{{Medical resources
|  DiseasesDB     = 10477
|  ICD10          = {{ICD10|I|44|2|i|30}}
|  ICD9           = {{ICD9|426.0}}
|  ICDO           =
|  OMIM           =
|  MedlinePlus    =
|  eMedicineSubj  = article
|  eMedicineTopic = 162007
|  MeshID         =
}}

{{Circulatory system pathology}}

[[Category:Cardiac arrhythmia]]
[[Category:Medical emergencies]]