Second degree AV block classification
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. ; Associate Editor(s)-in-Chief: Sara Zand, M.D. Mohammed Salih, M.D., Cafer Zorkun, M.D., Ph.D. , Raviteja Guddeti, M.B.B.S. 
There are 4 distinct types of second degree AV block. The distinction is made between them because type 1 second-degree heart block is considered a more benign entity than the other types. In mobitz type 1 second degree AV block there is evidence of gradually PR prolongation and dropped beat and grouped beating pattern. In mobitz type 2 AV block there is suddenly dopped beats without evidence of preceding PR prolongation. In atrioventricular block with the pattern of 2:1, there is every other beat without conducting down to the ventricle. In a high-grade AV block, there are two or more consecutive P waves without conducting down to the ventricle. It is important to determine the anatomic site of AV block. In Mobitz type 1 AV block, the site is usually within the AV node, but in Mobitz type II AV block the site is almost always below the AV node. In the presence of wide QRS complex and 2:1 AV conduction it is more likely that the site of AV block is intranodal or infranodal. In some cases, second-degree atrioventricular block must be differentiated from other causes of pauses such as non-conducted premature atrial contractions or atrial tachycardia with block.
|Atrioventricular block||First-degree atrioventricular block||
|Second-degree AV block||
|Third-degree AV block (complete heart block)||
Type 1 (Mobitz I / Wenckebach)
- Type 1 second degree AV block, also known as Mobitz I or Wenckebach periodicity which is a disease of the AV node.
- Mobitz I heart block is characterized by progressive prolongation of the PR interval on the electrocardiogram (EKG) 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.
- 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.
- This is a benign condition for which no specific treatment is needed.
Type 2 (Mobitz II)
- Type 2 second degree AV block, also known as Mobitz II is almost always a disease of the distal conduction system (His-Purkinje System).
- Although the terms intranodal block or infrahisian block are often applied to this disorder, they are not synonymous with it.
- Infranodal block and infra-Hisian block are terms that refer to the anatomic location of the block, whereas,
- Mobitz II refers to an electrocardiographic pattern associated with block at these levels.
- 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.
- 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.
Differentiating Mobitz I from Mobitz II in the Presence of a 2:1 Conduction
Likely EKG findings that help differentiate Mobitz type I from type II in the presence of a 2:1 conduction ratio include:
- Very long PR interval (> 300 msec) or narrow QRS complex - indicates the block is at the level of AV node
- Administration of atropine enhances AV nodal conduction resulting in less frequent non conducted beats - this confirms the type I Mobitz
- Mobitz I is worsened by carotid sinus massage which slows AV nodal conduction, unlike Mobitz II.
- Carotid sinus massage paradoxically eliminates infranodal block by slowing the sinus rate.
- Another type of classification used to classify second-degree AV block is 2:1 AV block and high-grade AV block.
- In 2:1 AV block every other atrial impulse is conducted down the ventricle.
- Higher grade AV blocks (eg., 3:1) unlike third degree AV block conduct few beats down the ventricle.
- ↑ Kusumoto, Fred M.; Schoenfeld, Mark H.; Barrett, Coletta; Edgerton, James R.; Ellenbogen, Kenneth A.; Gold, Michael R.; Goldschlager, Nora F.; Hamilton, Robert M.; Joglar, José A.; Kim, Robert J.; Lee, Richard; Marine, Joseph E.; McLeod, Christopher J.; Oken, Keith R.; Patton, Kristen K.; Pellegrini, Cara N.; Selzman, Kimberly A.; Thompson, Annemarie; Varosy, Paul D. (2019). "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". Circulation. 140 (8). doi:10.1161/CIR.0000000000000628. ISSN 0009-7322.
- ↑ Mangi MA, Jones WM, Napier L. PMID 29493981. Missing or empty
- ↑ Kashou AH, Goyal A, Nguyen T, Chhabra L. PMID 29083636. Missing or empty
- ↑ Li X, Xue Y, Wu H (2018). "A Case of Atrioventricular Block Potentially Associated with Right Coronary Artery Lesion and Ticagrelor Therapy Mediated by the Increasing Adenosine Plasma Concentration". Case Rep Vasc Med. 2018: 9385017. doi:10.1155/2018/9385017. PMC 5933017. PMID 29850368.
- ↑ Fu Md J, Bhatta L (2018). "Lyme carditis: Early occurrence and prolonged recovery". J Electrocardiol. 51 (3): 516–518. doi:10.1016/j.jelectrocard.2017.12.035. PMID 29275956.
- ↑ Tuohy S, Saliba W, Pai M, Tchou P (January 2018). "Catheter ablation as a treatment of atrioventricular block". Heart Rhythm. 15 (1): 90–96. doi:10.1016/j.hrthm.2017.08.015. PMID 28823599.
- ↑ Schernthaner C, Kraus J, Danmayr F, Hammerer M, Schneider J, Hoppe UC, Strohmer B (March 2016). "Short-term pacemaker dependency after transcatheter aortic valve implantation". Wien. Klin. Wochenschr. 128 (5–6): 198–203. doi:10.1007/s00508-015-0906-4. PMID 26745972.