A 55-year-old woman with chronic rheumatic heart disease is found to have progressive AV conduction delay on serial ECGs over the past 6 months. Which of the following is NOT a recognized feature or consequence of second-degree AV block (Mobitz I and Mobitz II combined)?

Question

Accepted Answer

B. The site of block in Mobitz I is always infranodal (below the AV node), whereas in Mobitz II it is always nodal. ## Second-Degree AV Blocks: Mobitz I vs. Mobitz II

### Overview
Second-degree AV block is characterized by intermittent failure of AV conduction—some atrial impulses conduct to the ventricles, others do not.

### Comparison Table

| Feature | Mobitz I (Wenckebach) | Mobitz II |
|---------|----------------------|----------|
| **Site of block** | AV node (nodal) | Infranodal (His bundle or below) |
| **PR interval pattern** | Progressive lengthening until a beat is dropped | Constant (fixed) |
| **QRS duration** | Normal (< 120 ms) | Wide (≥ 120 ms) |
| **Ventricular rate** | Usually normal | May be slow |
| **Clinical significance** | Benign; rarely progresses to CHB | Serious; may progress to CHB |
| **Pacing requirement** | Usually NOT required | Often required (prophylactic) |
| **Common causes** | Increased vagal tone, drugs (digoxin), inferior MI | Anterior MI, degenerative disease, fibrosis |

### Why Option 2 (Site of block) is INCORRECT

**Key Point:** The critical error in option 2 is the statement that the site of block in Mobitz I is "always infranodal." This is FALSE.

- **Mobitz I block:** Site is **nodal** (AV node itself)
- **Mobitz II block:** Site is **infranodal** (His bundle, bundle branches, or Purkinje system)

The option reverses this relationship, making it the incorrect statement.

### Mechanism: Why Site Matters

**Mobitz I (Nodal Block):**
1. AV node has decremental conduction properties (slowing with each impulse)
2. PR interval progressively lengthens
3. Eventually, one impulse fails to conduct (dropped beat)
4. Cycle resets; PR interval shortens again
5. Pattern repeats: "group beating"

**Mobitz II (Infranodal Block):**
1. His bundle or bundle branches have fixed refractoriness (no decremental properties)
2. PR interval remains constant
3. Impulses either conduct normally or fail abruptly
4. High risk of sudden progression to CHB

### Clinical Pearl
**High-Yield:** The **QRS width** is a reliable clue to the site of block:
  - Narrow QRS → Mobitz I (nodal)
  - Wide QRS → Mobitz II (infranodal)

### Management

| Mobitz I | Mobitz II |
|----------|----------|
| Observation; treat underlying cause | Prophylactic pacemaker |
| Atropine if symptomatic | Do NOT use atropine (may worsen) |
| Rarely requires pacing | High mortality without pacing |

### Mnemonic
**"WENCKEBACH = WENCHING" (Wenckebach = Mobitz I)**
- **W**idening PR interval
- **E**ventually a beat is **E**xcluded (dropped)
- **N**odal block
- **C**ontraction of PR interval after the dropped beat (cycle resets)
- **K**eeps the patient **K**icking (benign)

Answer

A. Mobitz I block is generally benign and rarely requires pacing, whereas Mobitz II block often requires prophylactic pacing

Answer

C. Mobitz I block typically has a longer PR interval in the conducted beat compared to the PR interval in the first beat of the cycle

Answer

D. Mobitz II block is associated with a wide QRS complex and a risk of sudden progression to complete heart block

A 55-year-old woman with chronic rheumatic heart disease is found to have progressive AV conduction delay on serial ECGs over the past 6 months. Which of the following is NOT a recognized feature or consequence of second-degree AV block (Mobitz I and Mobitz II combined)?

Explanation

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