second-degree atrioventricular block

Most people with Wenckebach (Type I Mobitz) do not show symptoms. However, those that do usually display one or more of the following:{{cite web |url= https://www.lecturio.com/concepts/atrioventricular-block/ | title= Atrioventricular Block | website= The Lecturio Medical Concept Library |access-date= 3 July 2021}}

• Light-headedness
• Dizziness
• Syncope (fainting)

There are two non-distinct types of second-degree AV block, called Type 1 and Type 2. In both types, a P wave is blocked from initiating a QRS complex; but, in Type 1, there are increasing delays in each cycle before the omission, whereas, in Type 2, there is no such pattern.{{cite web |url=http://library.med.utah.edu/kw/ecg/ecg_outline/Lesson6/index.html#Wenckebach |title=Lesson VI - ECG Conduction Abnormalities |access-date=2009-01-07}}

Type 1 second-degree heart block is considered more benign than type 2 second-degree heart block.http://health.medicscientist.com/wp-content/uploads/2011/04/SeconddegreeAVblock2_thumb.jpg{{Dead link|date=January 2022 |bot=InternetArchiveBot |fix-attempted=yes }} The type 1 does not have structural changes found on histology.

Both types are named after Woldemar Mobitz.{{WhoNamedIt|synd|2824}}{{cite journal | vauthors = Mobitz W | title = Über die unvollständige Störung der Erregungsüberleitung zwischen Vorhof und Kammer des menschlichen Herzens. | journal = Zeitschrift für die Gesamte Experimentelle Medizin | location = Berlin | date = 1924 | volume = 41 | pages = 180–237 | doi = 10.1007/BF02758773 | s2cid = 87410648 }} Type I is also named after Karel Frederik Wenckebach,{{cite journal | vauthors = Wenckebach KF | title = De Analyse van den onregelmatigen Pols. III. Over eenige Vormen van Allorythmie en Bradykardie. | journal = Nederlands Tijdschrift voor Geneeskunde | location = Amsterdam | date = 1898 | volume = 2 | page = 1132 }} and type II is also named after John Hay.{{cite journal | vauthors = Silverman ME, Upshaw CB, Lange HW | title = Woldemar Mobitz and His 1924 classification of second-degree atrioventricular block | journal = Circulation | volume = 110 | issue = 9 | pages = 1162–1167 | date = August 2004 | pmid = 15339865 | doi = 10.1161/01.CIR.0000140669.35049.34 | doi-access = free }}{{cite journal | vauthors = Hay J | year = 1906| title = Bradycardia and cardiac arrhythmia produced by depression of certain of the functions of the heart | url = https://zenodo.org/record/1945247| journal = The Lancet | volume = 1906 | issue = 1| pages = 139–143 | doi=10.1016/s0140-6736(01)44443-6}}

===Type 1 (Mobitz I/Wenckebach)===

Type 1 Second-degree AV block, also known as Mobitz I or Wenckebach periodicity, is almost always a disease of the AV node. Wenckebach published a paper in 1906 on progressively lengthening PR intervals{{cite journal| vauthors = Wenckebach KF |title=Beiträge zur Kenntnis der menschlichen Herztätigkeit [Contributions to the knowledge of human cardiac activity]|journal=Arch Anat Physiol|date=1906|pages=297–354}} that was later classified as Type I in Mobitz's 1924 paper.{{cite journal| vauthors = Mobitz W |title=Über die unvollständige Störung der Erregungsüberleitung zwischen Vorhof und Kammer des menschlichen Herzens [On the partial block of impulse conduction between atrium and ventricle of human hearts]|journal=Z Gesamte Exp Med|date=1924|volume=41|pages=180–237 |doi=10.1007/bf02758773 |s2cid=87410648}} Thus, both "Mobitz type I" and "Wenckebach block" refer to the same pattern and pathophysiology.

In Wenckebach's 1906 paper, his original observations were of increasing delay in contraction of the atria and ventricles that shortened after a brief pause. He later observed this in the electrocardiogram (ECG) after Einthoven's 1901 invention. In modern practice, Mobitz I heart block is characterized by progressive prolongation of the PR interval on consecutive beats followed by a blocked P wave (i.e., a dropped QRS complex). After the dropped QRS complex, the PR interval resets and the cycle repeats. This grouped beating was described as "Luciani periods" after Luigi Luciani's work in 1873.{{cite journal | vauthors = Silverman ME, Upshaw CB, Lange HW | title = Woldemar Mobitz and His 1924 classification of second-degree atrioventricular block | journal = Circulation | volume = 110 | issue = 9 | pages = 1162–1167 | date = August 2004 | pmid = 15339865 | doi = 10.1161/01.CIR.0000140669.35049.34 | doi-access = free }} The result is a lengthening of the R-R intervals as each subsequent P-wave reaches an increasingly refractory AV node until the impulse fails to conduct, which ultimately results in a blocked QRS complex.{{Cite web |title=ECG Learning Center - An introduction to clinical electrocardiography |url=https://ecg.utah.edu/lesson/6 |url-status=live |archive-url=https://web.archive.org/web/20220814034316/https://ecg.utah.edu/lesson/6 |archive-date=2022-08-14 |access-date=2022-08-14 |website=ecg.utah.edu |publisher=University of Utah}} A blocked P-wave may not be present under some circumstances, such as coexisting sinoatrial block.[https://litfl.com/diagnosis-wenckebach-squared/ Diagnosis, Wenckebach Squared?]

One of the baseline assumptions when determining if an individual has Mobitz I heart block is that the atrial rhythm has to be regular. If the atrial rhythm is not regular, there could be alternative explanations as to why certain P waves do not conduct to the ventricles.{{citation needed|date=February 2021}}

Mobitz I heart block is almost always a benign condition for which no specific treatment is needed for the rhythm itself. It can be seen in myocardial ischemia, propranolol use, digitalis use, rheumatic fever, and chronically in ischemic heart disease and other structural diseases (amyloidosis, mitral valve prolapse, aortic valve disease, and atrial septal defect). In symptomatic cases, intravenous atropine or isoproterenol may transiently improve conduction.{{cite book | vauthors = Lilly LS |title=Pathophysiology of heart disease : a collaborative project of medical students and faculty |date=2007 |publisher=Wolters Kluwer/Lippincott Williams & Wilkins |location=Philadelphia |isbn=978-0-7817-6321-9 | oclc = 938327813 |edition=4th}}

File:Type I A-V block 5-to-4 Wenckebach periods.png with acute inferior infarction complicated by Type I A-V block manifest in the form of 5:4 Wenckebach periods; R-P/P-R reciprocity.]]

File:Sinus rhythm with 3-to-2 and 2-to-1 Type II A-V block.png; RBBB]]

===Type 2 (Mobitz II/Hay)===

Type 2 Second-degree AV block, also known as Mobitz II, is almost always a disease of the distal conduction system (His-Purkinje System).

Mobitz II heart block is characterized on a surface ECG by intermittently non-conducted P waves not preceded by PR prolongation and not followed by PR shortening. There is usually a fixed number of non-conducted P waves for every successfully conducted QRS complex, and this ratio is often specified in describing Mobitz II blocks. For example, Mobitz II block in which there are two P waves for every one QRS complex may be referred to as 2:1 Mobitz II block.{{cite book| vauthors = Dubin D |title=Rapid interpretation of EKG's : ... an interactive course|year=2000|publisher=Cover Publ.|location=Tampa, Fla.|isbn=978-0912912066|edition=6th}}^:181

The medical significance of this type of AV block is that it may progress rapidly to complete heart block, in which no escape rhythm may emerge. In this case, the person may experience a Stokes-Adams attack, cardiac arrest, or sudden cardiac death. The definitive treatment for this form of AV Block is an implanted pacemaker.

The impairment is usually below the AV node.{{cite journal | vauthors = Wogan JM, Lowenstein SR, Gordon GS | title = Second-degree atrioventricular block: Mobitz type II | journal = The Journal of Emergency Medicine | volume = 11 | issue = 1 | pages = 47–54 | year = 1993 | pmid = 8445186 | doi = 10.1016/0736-4679(93)90009-V }} Although the terms infranodal block or infrahisian block are often applied to this disorder, they refer to the anatomic location of the block, whereas Mobitz II refers to an electrocardiographic pattern.

Because type I Mobitz block occurs in regular cycles, there is always a fixed ratio between the number of P waves and the number of QRS complexes per cycle. This ratio is often specified when describing the block. For example, a Mobitz type I block which has 4 P waves and 3 QRS complexes per cycle may be referred to as 4:3 Mobitz Type I block.^:179

Type II Mobitz block also usually occurs with a fixed P:QRS ratio, with a set number of P waves for every successfully elicited QRS.^:179 This ratio is also frequently specified in referring to 3:1, 4:1, 5:1, or higher Mobitz type II block. Higher numbers of P waves for every QRS indicate more severe block^:181 and ratios of 3:1 and above are also referred to as "High Grade" AV Block{{cite journal | vauthors = Kusumoto FM, Schoenfeld MH, Barrett C, Edgerton JR, Ellenbogen KA, Gold MR, Goldschlager NF, Hamilton RM, Joglar JA, Kim RJ, Lee R, Marine JE, McLeod CJ, Oken KR, Patton KK, Pellegrini CN, Selzman KA, Thompson A, Varosy PD | display-authors = 6 | title = 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society | journal = Journal of the American College of Cardiology | volume = 74 | issue = 7 | pages = e51–e156 | date = August 2019 | pmid = 30412709 | doi = 10.1016/j.jacc.2018.10.044 | doi-access = free }}^e60. Of course, because type II Mobitz block is unstable by nature, it is common for the P:QRS ratio in Mobitz type II block to change over time.{{citation needed|date=December 2013}}

The P:QRS ratio is always of the form X:(X − 1) in type I Mobitz block.

In the case of 2:1 block (2 P waves for every QRS complex) it is impossible to differentiate type I from type II Mobitz block-based solely on the P:QRS ratio or on a pattern of lengthening PR intervals.^:182 In this case, a lengthened PR interval with a normal QRS width is most likely indicative of a type I-like pathology, and a normal PR interval with a widened QRS is most likely indicative of a type II-like pathology.^:182

• Electrocardiogram (ECG or EKG)
• SA node
• AV node
• Atrioventricular block
• First-degree AV block
• Third-degree AV block

{{Reflist}}

{{Medical resources

| DiseasesDB = 10477

| ICD10 = {{ICD10|I|44|1|i|30}}

| ICD9 = {{ICD9|426.12}}, {{ICD9|426.13}}

| ICDO =

| OMIM =

| MedlinePlus =

| eMedicineSubj = emerg

| eMedicineTopic = 234

| MeshID =

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{{Circulatory system pathology}}

Category:Cardiac arrhythmia