A 72-year-old woman with chronic hypertension and diabetes mellitus presents with progressive dyspnea and fatigue over 2 weeks. Vital signs: BP 142/88 mmHg, HR 48 bpm, RR 22/min. Physical examination reveals an irregular pulse with variable intensity of the first heart sound. ECG shows PR intervals that progressively lengthen until a QRS complex is dropped, after which the PR interval resets and the cycle repeats. Chest X-ray shows mild pulmonary edema. What is the most likely diagnosis?

Explanation

## Diagnosis: Second-Degree AV Block, Mobitz Type I (Wenckebach Phenomenon) ### ECG Pattern Recognition **Key Point:** The hallmark of Mobitz I is **progressive PR prolongation followed by a dropped QRS complex**, then reset of the PR interval. The ECG pattern described: 1. PR interval lengthens with each beat 2. Eventually, a P wave is not followed by a QRS (dropped beat) 3. After the dropped beat, the PR interval resets to its shortest value 4. The cycle repeats ### Comparison of AV Blocks | Feature | Mobitz I (Wenckebach) | Mobitz II | Third-Degree | |---------|----------------------|-----------|---------------| | **PR interval** | Progressively lengthens | Fixed; suddenly dropped | No relationship | | **QRS width** | Normal (< 120 ms) | Wide (≥ 120 ms) | Wide | | **Site of block** | AV node (above His) | Below AV node (infra-nodal) | Complete dissociation | | **Dropped beats** | Occasional | Frequent; 2:1 or worse | Every P wave blocked | | **Prognosis** | Benign; rarely progresses | Ominous; high risk of CHB | Depends on escape rate | | **Symptoms** | Often asymptomatic | Syncope, hypotension | Syncope, shock | | **Treatment** | Observation ± atropine | Pacemaker | Pacemaker | ### Pathophysiology **High-Yield:** Mobitz I occurs at the level of the **AV node** where conduction velocity is slowest and refractoriness is longest. Progressive ischemia or increased vagal tone causes incremental slowing of AV nodal conduction until a P wave fails to conduct. The AV node then recovers, and the cycle repeats. ### Clinical Presentation in This Case - **Symptomatic bradycardia** (HR 48 bpm) with dyspnea and fatigue - **Variable first heart sound** — reflects variable PR intervals (shorter PR = louder S1; longer PR = softer S1) - **Mild pulmonary edema** — consequence of low cardiac output - **Chronic hypertension + diabetes** — risk factors for conduction disease ### Mechanism: Why AV Node? ```mermaid flowchart TD A[Sinus impulse reaches AV node]:::outcome --> B[AV nodal conduction slows incrementally]:::action B --> C[PR interval lengthens with each beat]:::action C --> D{Refractoriness exceeded?}:::decision D -->|No| E[QRS conducts]:::outcome D -->|Yes| F[P wave blocked, QRS dropped]:::outcome F --> G[AV node recovers]:::action G --> H[Next sinus impulse conducts with short PR]:::outcome H --> B ``` **Clinical Pearl:** The variable intensity of the first heart sound is a classic bedside clue. When PR is short, atrial contraction occurs just before ventricular contraction, maximizing ventricular filling and producing a loud S1. When PR is long, atrial and ventricular contractions overlap, reducing LV filling and producing a softer S1. ### Management **Mnemonic: WENCKEBACH** — **W**hy worry? **E**ventually benign, **N**o progression to CHB, **C**onduction recovers, **K**eep observing, **B**radycardia? Try atropine, **A**void pacemaker, **C**hronic monitoring, **H**ypertension/diabetes = risk. 1. **Observation** — most patients are asymptomatic or minimally symptomatic 2. **Atropine 0.6 mg IV** — if symptomatic bradycardia (as in this case) 3. **Pacemaker** — rarely needed; reserved for refractory symptomatic bradycardia 4. **Avoid digoxin and beta-blockers** — may worsen AV block ### Prognosis **High-Yield:** Mobitz I has an **excellent prognosis**. It rarely progresses to higher-degree block and does not require permanent pacing in most cases. Patients can be managed conservatively with monitoring.