## ECG Characteristics of Mobitz Type I (Wenckebach) **Key Point:** Mobitz type I is characterized by **progressive PR prolongation** until a P wave fails to conduct (dropped QRS), after which the PR interval resets and the cycle repeats. ### Mechanism 1. Each successive impulse encounters increasing AV nodal refractoriness 2. Conduction time lengthens progressively (PR interval increases) 3. Eventually, one P wave finds the AV node completely refractory and is blocked 4. The AV node then recovers fully, and the cycle restarts with a normal PR interval ### ECG Pattern - **Hallmark:** "Footprints in the sand" — PR intervals get progressively longer - The PR interval of the beat **after** the dropped beat is shortest - The RR interval containing the dropped beat is less than twice the preceding RR interval ### Clinical Significance - **Location:** AV node (above the bundle of His) - **Prognosis:** Generally benign; rarely progresses to complete heart block - **Symptoms:** Usually asymptomatic; no pacing required unless symptomatic **High-Yield:** Mobitz I = **progressive PR** → dropped beat → reset. This is the **classic teaching point** for NEET PG exams. **Clinical Pearl:** Mobitz I is often seen in athletes, acute MI (inferior wall), and digitalis toxicity. It does NOT require pacing in asymptomatic patients.
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