A 72-year-old woman with chronic hypertension and diabetes mellitus type 2 presents with recurrent episodes of dizziness over the past 2 weeks. She denies syncope or chest pain. Vital signs: BP 138/82 mmHg, HR 56 bpm, regular. ECG shows PR interval of 280 ms with each P wave followed by a QRS complex. Subsequent ECGs over 3 hours show progressive PR prolongation (320 ms, then 380 ms), followed by a single dropped QRS complex after a P wave, then resumption of 1:1 conduction with PR 240 ms. What is the most likely diagnosis?

Explanation

## ECG Pattern Recognition **Key Point:** Mobitz type I (Wenckebach) AV block is characterized by progressive PR prolongation followed by a non-conducted P wave (dropped QRS), then reset of the cycle with shorter PR interval. ### Diagnostic Criteria for Mobitz Type I | Feature | Present in This Case | Significance | |---------|----------------------|--------------| | Progressive PR lengthening | Yes (280 → 320 → 380 ms) | Hallmark finding | | Dropped QRS after P wave | Yes (one beat) | Confirms block | | PR resets after dropped beat | Yes (returns to 240 ms) | Cycle repeats | | Site of block | AV node | Benign, usually reversible | | Ventricular rate | Regular (despite dropped beat) | Grouped beating pattern | ### Mechanism of Wenckebach Phenomenon ```mermaid flowchart TD A["Atrial impulse arrives at AV node"]:::action B["AV nodal conduction time increases"]:::action C["PR interval progressively lengthens"]:::action D{"Next impulse finds AV node refractory?"}:::decision D -->|Yes| E["P wave not conducted: QRS dropped"]:::outcome D -->|No| F["Conduction resumes with shorter PR"]:::outcome E --> G["Cycle repeats"]:::action F --> G ``` ### Clinical Context **High-Yield:** Mobitz type I block occurs at the **AV node level** (proximal to the bundle of His). The AV node has the longest refractory period and slowest conduction velocity of all cardiac tissues, making it the site of physiologic block. **Clinical Pearl:** Wenckebach is common in: - Acute inferior wall MI (RCA ischemia → AV node ischemia) - Digitalis toxicity - Increased vagal tone (athletes, sleep apnea) - Degenerative conduction disease in elderly In this patient, chronic hypertension and diabetes suggest degenerative AV nodal disease. ### Prognosis and Management - **Asymptomatic Wenckebach**: Observation; no pacing required - **Symptomatic (dizziness, syncope)**: Atropine or temporary pacing - **Progression risk**: Low; rarely progresses to complete heart block - **Contrast with Mobitz II**: Mobitz II is infranodal, carries high risk of sudden complete block, and requires pacemaker