ECG Pattern of Mobitz Type II Second-Degree AV Block
Mobitz type II second-degree AV block demonstrates a constant PR interval before and after blocked P waves, with sudden non-conducted atrial impulses, and is usually associated with a wide QRS complex. 1
Key Electrocardiographic Features
The defining characteristics of Mobitz type II block include:
- Constant PR intervals: The PR interval remains fixed before and after the blocked P wave—this is the sine qua non of the diagnosis 2, 3
- Sudden dropped QRS complexes: P waves fail to conduct without preceding PR prolongation 4
- Wide QRS complex: Typically ≥120 ms, reflecting infra-nodal (His-Purkinje) disease 2
- Stable sinus rate: Normal sinus rhythm must be present, as vagal surges causing simultaneous sinus slowing can mimic this pattern 3, 5
Critical Distinction from Mobitz Type I
Unlike Mobitz type I (Wenckebach), which shows progressive PR prolongation before the dropped beat and typically has a narrow QRS, Mobitz type II has no visible changes in AV conduction time 4. The European Heart Journal guidelines emphasize that in type II block, "the PR interval is constant before and after the blocked P wave" and "the conduction block is usually in the His-Purkinje system, especially in the case of a wide QRS" 4.
Important Diagnostic Caveats
2:1 AV block cannot be classified as Mobitz type I or II because there are insufficient conducted beats to assess PR interval behavior 1, 5. The QRS width can suggest the level of block, but definitive classification requires observing at least two consecutive conducted P waves.
Common pitfalls to avoid:
- Atypical Wenckebach patterns may superficially resemble Mobitz type II when PR intervals appear constant before block 3, 5
- Concealed His bundle or ventricular extrasystoles can create pseudo-AV block patterns 3, 5
- A shortened PR interval after the blocked P wave effectively excludes true Mobitz type II 5
Anatomic Location and Clinical Significance
The block in Mobitz type II is invariably infra-nodal (His-Purkinje system), particularly when the QRS is wide 2, 4, 2. This anatomic location carries critical prognostic implications: type II block commonly progresses to complete heart block and is associated with a poor prognosis 4. The 2019 ACC/AHA/HRS guidelines note that infra-Hisian block "may progress rapidly and unexpectedly, is associated with a slower and more unpredictable ventricular escape mechanism" 6.
Pacemaker implantation is recommended for Mobitz type II even in asymptomatic patients, particularly when associated with fascicular block, as it represents a class IIa indication 2, 4.