A 72-year-old woman with chronic hypertension and diabetes mellitus type 2 presents with progressive dyspnea and fatigue over 3 weeks. On examination, heart rate is 42 bpm, blood pressure 110/70 mmHg, and she is hemodynamically stable. ECG shows a PR interval of 0.28 seconds with each P wave followed by a QRS complex. Over the next 2 hours, the PR interval progressively lengthens to 0.42 seconds, then suddenly a P wave is not followed by a QRS complex. The next P wave is again followed by a QRS with a PR interval of 0.30 seconds. What is the most likely diagnosis?

Explanation

## Diagnosis: Second-Degree AV Block Type I (Wenckebach/Mobitz I) ### Defining Features of Mobitz I **Key Point:** Mobitz I (Wenckebach) is characterized by **progressive lengthening of the PR interval until a beat is dropped**, then the cycle repeats with a shorter PR interval. ### ECG Pattern Recognition | Feature | Mobitz I | Mobitz II | Third-Degree | |---------|----------|-----------|---------------| | PR interval | Progressively lengthens | Fixed until drop | No relationship | | Dropped beat | After longest PR | Sudden, no warning | Every beat | | PR after drop | Resets (shorter) | Remains fixed | N/A | | QRS width | Usually narrow | Usually wide | Variable | | Escape rate | 40–60 bpm (junctional) | 20–40 bpm (ventricular) | Depends on site | | Location | AV node | Infranodal (His/Purkinje) | Complete block | ### Clinical Timeline in This Case 1. **Initial:** PR 0.28 s (prolonged but conducted) 2. **Progressive:** PR lengthens to 0.42 s 3. **Dropped beat:** One P wave not followed by QRS 4. **Reset:** Next P wave → QRS with PR 0.30 s (shorter than 0.42 s) This **classic Wenckebach pattern** is pathognomonic for Mobitz I. ### Pathophysiology **High-Yield:** Mobitz I occurs at the **AV nodal level** due to: - Increased vagal tone - AV nodal ischemia (usually from RCA disease) - Medications (digoxin, beta-blockers, calcium channel blockers) - Hyperkalemia The AV node has a long refractory period; each impulse takes longer to conduct until one fails to conduct entirely. The node then recovers, and the next impulse conducts with a shorter PR interval. **Mnemonic:** **WENCKEBACH = WENCHES BACK** (PR interval goes up, up, up, then drops; then comes back down) ### Clinical Significance **Clinical Pearl:** Mobitz I is generally **benign** and does not require pacing in most cases because: - The escape rhythm is junctional (rate 40–60 bpm) - Symptoms are usually mild - Progression to complete heart block is rare However, this patient is **symptomatic** (dyspnea, fatigue) and **bradycardic** (42 bpm), suggesting: - Symptomatic bradycardia from the block - Possible need for pacing if symptoms persist ### Management 1. **Identify cause:** Medication review (digoxin, beta-blockers, CCBs), electrolytes, ischemia 2. **Observation:** If asymptomatic, monitor; most resolve spontaneously 3. **Pacing:** Consider if symptomatic bradycardia or signs of hemodynamic compromise 4. **Avoid:** Medications that worsen AV conduction [cite:Harrison 21e Ch 235]