Editing First-degree atrioventricular block

{{Infobox medical condition (new)
| name            = First-degree AV block
| synonyms        = First degree heart block, PR prolongation
| image           = FirstAVBlock.jpg
| caption         = An ECG showing a first degree AV block of greater than 300 ms
| pronounce       =
| field           = [[Cardiology]]
| symptoms        = Asymptomatic
| complications   = Progression to [[Second-degree atrioventricular block|second]] or [[Third-degree AV block|third degree AV block]]
| onset           =
| duration        =
| types           =
| causes          = Fibrosis in AV node, medication, [[vagal tone]], electrolyte disturbances
| risks           =
| diagnosis       = [[Electrocardiography|Electrocardiogram]]
| differential    =
| prevention      =
| treatment       = Avoidance of AV-nodal-blocking medication
| medication      =
| prognosis       =
| frequency       =
| deaths          =
}}
'''First-degree atrioventricular block''' (AV block) is a disease of the [[electrical conduction system of the heart]] in which electrical impulses conduct from the cardiac [[atrium (heart)|atria]] to the [[ventricle (heart)|ventricles]] through the [[atrioventricular node]] (AV node) more slowly than normal. First degree AV block does not generally cause any symptoms, but may progress to more severe forms of heart block such as [[Second-degree atrioventricular block|second]]- and [[third-degree atrioventricular block]]. It is diagnosed using an [[Electrocardiography|electrocardiogram]], and is defined as a [[PR interval]] greater than 200 milliseconds.<ref name="urlLesson VI - ECG Conduction Abnormalities2">{{cite web|url=http://library.med.utah.edu/kw/ecg/ecg_outline/Lesson6/index.html#First|title=Lesson VI - ECG Conduction Abnormalities|access-date=2009-01-07}}</ref> First degree AV block affects 0.65-1.1% of the population with 0.13 new cases per 1000 persons each year.

== Causes ==

The most common causes of first-degree heart block are AV nodal disease, enhanced [[vagal tone]] (for example in athletes), [[myocarditis]], acute [[myocardial infarction]] (especially acute inferior MI), [[electrolyte disturbances]] and medication.  The medications that most commonly cause first-degree heart block are those that increase the refractory time of the AV node, thereby slowing AV conduction. These include [[calcium channel blockers]], [[beta-blockers]], [[cardiac glycosides]], and anything that increases cholinergic activity such as [[cholinesterase inhibitor]]s.<ref>{{cite journal |url= https://www.ncbi.nlm.nih.gov/books/NBK448164/| title= First Degree Heart Block | website= National Center for Biotechnology Information, U.S. National Library of Medicine | year= 2023 | pmid= 28846254 |access-date= 3 July 2021 | last1= Oldroyd | first1= S. H. | last2= Quintanilla Rodriguez | first2= B. S. | last3= Makaryus | first3= A. N. }}</ref>

== Diagnosis ==

In normal individuals, the AV node slows the conduction of electrical impulses through the heart.  This is manifest on a surface [[Electrocardiography|electrocardiogram]] (ECG) as the PR interval.  The normal PR interval is from 120 [[milliseconds|ms]] to 200 ms in length.  This is measured from the initial deflection of the P wave to the beginning of the [[QRS complex]].<ref>{{cite journal |url= https://www.ncbi.nlm.nih.gov/books/NBK448164/| title= First Degree Heart Block | website= National Center for Biotechnology Information, U.S. National Library of Medicine | year= 2023 | pmid= 28846254 |access-date= 3 July 2021 | last1= Oldroyd | first1= S. H. | last2= Quintanilla Rodriguez | first2= B. S. | last3= Makaryus | first3= A. N. }}</ref>

In first-degree heart block, the AV node conducts the electrical activity more slowly.  This is seen as a PR interval greater than 200 ms in length on the surface ECG.  It is usually an incidental finding on a routine ECG.<ref>{{cite journal |url= https://www.ncbi.nlm.nih.gov/books/NBK448164/| title= First Degree Heart Block | website= National Center for Biotechnology Information, U.S. National Library of Medicine | year= 2023 | pmid= 28846254 |access-date= 3 July 2021 | last1= Oldroyd | first1= S. H. | last2= Quintanilla Rodriguez | first2= B. S. | last3= Makaryus | first3= A. N. }}</ref>

First-degree heart block does not require any particular investigations except for electrolyte and drug screens, especially if an overdose is suspected.<ref>{{cite journal |url= https://www.ncbi.nlm.nih.gov/books/NBK448164/| title= First Degree Heart Block | website= National Center for Biotechnology Information, U.S. National Library of Medicine | year= 2023 | pmid= 28846254 |access-date= 3 July 2021 | last1= Oldroyd | first1= S. H. | last2= Quintanilla Rodriguez | first2= B. S. | last3= Makaryus | first3= A. N. }}</ref>

In comparison to [[second-degree atrioventricular block]], in first-degree block there is an absence of non-conduction or "dropped beats."

In an [[electrophysiology study]], this corresponds to a prolonged A-H interval that shows the time between atrial depolarization and [[His bundle]] depolarization near the AV node.

== Treatment ==

The management includes identifying and correcting electrolyte imbalances and withholding any offending medications.  This condition does not require admission unless there is an associated [[myocardial infarction]].  Even though it usually does not progress to higher forms of heart block, it may require outpatient follow-up and monitoring of the ECG, especially if there is a comorbid [[bundle branch block]].  If there is a need for treatment of an unrelated condition, care should be taken not to introduce any medication that may slow AV conduction.  If this is not feasible, clinicians should be very cautious when introducing any drug that may slow conduction; and regular monitoring of the ECG is indicated.<ref>{{cite journal |url= https://www.ncbi.nlm.nih.gov/books/NBK448164/| title= First Degree Heart Block | website= National Center for Biotechnology Information, U.S. National Library of Medicine | year= 2023 | pmid= 28846254 |access-date= 3 July 2021 | last1= Oldroyd | first1= S. H. | last2= Quintanilla Rodriguez | first2= B. S. | last3= Makaryus | first3= A. N. }}</ref>

== Prognosis ==

Isolated first-degree heart block has no direct clinical consequences. There are no symptoms or signs associated with it. It was originally thought of as having a benign prognosis. In the [[Framingham Heart Study]], however, the presence of a prolonged PR interval or first degree AV block doubled the risk of developing [[atrial fibrillation]], tripled the risk of requiring an [[artificial pacemaker]], and was associated with a small increase in mortality. This risk was proportional to the degree of PR prolongation.<ref>{{cite journal |vauthors=Cheng S, Keyes MJ, Larson MG, McCabe EL, Newton-Cheh C, Levy D, Benjamin EJ, Vasan RS, Wang TJ | title=Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular block | journal=JAMA | year=2009 | volume=301 | issue=24 | pages=2571–2577 | doi=10.1001/jama.2009.888| pmid=19549974 | pmc=2765917 }}</ref>

A subset of individuals with the triad of first-degree heart block, [[right bundle branch block]], and either [[left anterior fascicular block]] or [[left posterior fascicular block]] (known as [[trifascicular block]]) may be at an increased risk of progression to [[complete heart block]].<ref>{{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>

== FAA controversy ==
In October 2022 the [[FAA]] quietly changed the medical policy in their Guide for Aviation Medical Examiners to define a first-degree AV block as a PR interval greater than 300ms. <ref>{{cite web|title=Guide for Aviation Examiners, Archives and Updates
|url=https://www.faa.gov/ame_guide/monthly_guide_updates|website=FAA Guide for Aviation Medical Examiners|publisher=faa.gov|accessdate=31 October 2022|archive-url=https://web.archive.org/web/20221031145709/https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/media/Archives.pdf|archive-date=31 October 2022|url-status=dead}}</ref> This was considered controversial <ref>{{cite web|title=The FAA has very quietly tacitly admitted that the EKGs of pilots are no longer normal
|url=https://stevekirsch.substack.com/p/the-faa-has-very-quietly-tacitly|website=Substack|date=17 January 2023 |publisher=substack.com|accessdate=31 March 2023|archive-url=https://web.archive.org/web/20230331044406/https://stevekirsch.substack.com/p/the-faa-has-very-quietly-tacitly|archive-date=31 March 2023}}</ref> in part because accepted guidelines for the PR interval remain unchanged at 200ms,<ref>{{cite journal|title=First Degree Heart Block
|url=https://www.ncbi.nlm.nih.gov/books/NBK448164/|website=National Library of Medicine|year=2023 |publisher=nih.gov|pmid=28846254 |accessdate=31 March 2023 |last1=Oldroyd |first1=S. H. |last2=Quintanilla Rodriguez |first2=B. S. |last3=Makaryus |first3=A. N. }}</ref> and in part because at the time there was no explanation of why the FAA required the change. The FAA has subsequently provided a statement explaining its reasoning.<ref>{{cite web|title=New FAA cardiac health rules not prompted by COVID-19 shots|website=[[Associated Press]] |date=20 January 2023 |url=https://apnews.com/article/fact-check-faa-pilots-heart-covid-373861551871|accessdate=12 May 2023}}</ref>

==See also==
* [[Atrioventricular block]]
* [[Second-degree atrioventricular block]]
* [[Third-degree atrioventricular block]]

==References==
{{reflist}}
== External links ==
{{Medical resources
|  DiseasesDB     = 10477
|  ICD10          = {{ICD10|I|44|0|i|30}}
|  ICD9           = {{ICD9|426.11}}
|  ICDO           =
|  OMIM           =
|  MedlinePlus    =
|  eMedicineSubj  = emerg
|  eMedicineTopic = 233
|  MeshID         =
}}
{{Circulatory system pathology}}

{{DEFAULTSORT:First-Degree Atrioventricular Block}}
[[Category:Cardiac arrhythmia]]