Second degree AV block classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mohammed Salih, M.D., Cafer Zorkun, M.D., Ph.D. [2], Raviteja Guddeti, M.B.B.S. [3]


There are two distinct types of second degree AV block, called type 1 and type 2. The distinction is made between them because type 1 second degree heart block is considered a more benign entity than type 2 second degree heart block. The distinction between Mobitz I and II can be made only when the ratio of atrial to ventricular conduction is not 2:1, because in 2:1 conduction every other beat is conducted to the ventricle and there is no opportunity to observe the PR prolongation that defines type I and II second degree AV block.


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[1][2].
  • 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 almost always 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 infranodal block or infrahisian block are often applied to this disorder, they are not synonymous with it.
  • Infranodal block and infra-Hisian block are terms which refer to the anatomic location of the block, whereas
  • Mobitz II refers to an electrocardiographic pattern associated with block at these levels[3].

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 nonconducted beats - this confirms 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[6].

Another type of classification used to classify second degree AV block is 2:1 block and high grade block (not third degree AV block). In 2:1 block every other atrial impulse is conducted down the ventricle. Higher grade blocks (eg., 3:1) unlike third degree AV block conduct few beats down the ventricle.


  1. Mangi MA, Jones WM, Napier L. PMID 29493981. Missing or empty |title= (help)
  2. Kashou AH, Goyal A, Nguyen T, Chhabra L. PMID 29083636. Missing or empty |title= (help)
  3. 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.
  4. 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.
  5. 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.
  6. 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.