## Distinguishing Wenckebach (Type I) from Mobitz II (Type II) ### Key Pathophysiology **Wenckebach (Type I):** Progressive conduction delay through the AV node until one impulse fails to conduct. The PR interval lengthens with each beat until a P wave is not followed by a QRS. **Mobitz II:** Sudden, unexpected failure of AV conduction without prior PR prolongation. The PR interval remains constant in conducted beats. ### Comparison Table | Feature | Wenckebach (Type I) | Mobitz II (Type II) | |---------|-------------------|-------------------| | **PR interval pattern** | Progressive lengthening → dropped beat | Constant, then sudden drop | | **Dropped beat pattern** | Predictable (cyclic) | Unpredictable | | **Site of block** | AV node (usually) | His bundle or below | | **Escape rhythm** | Junctional, rate 40–60 | Ventricular, rate <40 | | **Prognosis** | Benign, rarely progresses | Ominous, may progress to complete block | | **Treatment** | Observation; atropine if symptomatic | Pacemaker required | **Key Point:** Progressive PR prolongation before the dropped beat is the **hallmark of Wenckebach**. This stepwise delay is absent in Mobitz II, which shows constant PR intervals with sudden block. ### Clinical Pearl **High-Yield:** In acute MI (especially inferior), Wenckebach is common and usually self-limited (AV node ischemia). Mobitz II in acute MI signals extensive conduction system disease and requires urgent pacing. **Mnemonic:** **"Wenck = Worsening PR"** — Wenckebach shows progressive PR lengthening; Mobitz II shows fixed PR then drop. ### Why This Matters The PR interval pattern is the single most reliable ECG discriminator. Wenckebach's progressive prolongation is visible on a single rhythm strip; Mobitz II's constant PR with sudden block is equally distinctive.
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.