Mobitz I (Wenckebach)
Also calledMobitz type 1 · Wenckebach · second-degree AV block type I · Mobitz 1 · second-degree AVB type I
Definition
A form of second-degree AV block in which the PR interval progressively lengthens over successive beats until a single P wave fails to conduct, producing a dropped QRS and a group-beating pattern. In the classic form the PR increment decreases beat to beat, so the RR intervals shorten before the dropped beat, and the pause that contains the blocked P wave is shorter than two P-P intervals. With a narrow QRS the block is most likely in the AV node, although it is occasionally below it.
Clinical noteIn longer cycles (beyond 3:2 conduction), atypical Wenckebach periods that depart from the classic pattern are more common than classic ones, so the beats before the pause can mislead; a run of nearly constant PR intervals can even mimic Mobitz II. A more dependable sign is that the PR interval of the first conducted beat after the blocked P wave is shorter than the ones before the block, the shortest of the cycle. Group beating alone does not identify type I, because dropped QRS complexes produce group beating in Mobitz II as well.
Sources
- Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation. 2019;140(8):e382-e482. doi.org/10.1161/CIR.0000000000000628
- Denes P, Levy L, Pick A, Rosen KM. The incidence of typical and atypical A-V Wenckebach periodicity. Am Heart J. 1975;89(1):26-31. doi.org/10.1016/0002-8703(75)90005-8
- Barold SS, Herweg B. Mobitz type II second-degree atrioventricular block: a commonly overdiagnosed and misinterpreted arrhythmia. Front Cardiovasc Med. 2024;11:1450705. doi.org/10.3389/fcvm.2024.1450705
- Samesima N, God EG, Kruse JCL, et al. Brazilian Society of Cardiology guidelines on the analysis and issuance of electrocardiographic reports - 2022. Arq Bras Cardiol. 2022;119(4):638-680. doi.org/10.36660/abc.20220623
Last reviewed 2 October 2026. Every definition in the EKG Dictionary cites the guideline, consensus statement or original paper it comes from.