A 62-year-old woman with a history of atrial fibrillation is found to have a prolonged PR interval (220 ms) on ECG. Which is the most common site of atrioventricular block in patients with acquired atrioventricular nodal disease?

Explanation

## Atrioventricular Block — Site of Conduction Delay **Key Point:** The atrioventricular (AV) node is the most common site of conduction delay and block in acquired atrioventricular disease, accounting for approximately 60–70% of all AV blocks. ### Anatomical Sites of AV Block Conduction delay or block can occur at three anatomical levels: | Site | Frequency | Clinical Features | PR Interval | QRS Duration | |---|---|---|---|---| | **AV Node (AV nodal block)** | 60–70% (most common) | Benign, often asymptomatic; responsive to atropine | Prolonged (>200 ms) | Normal (<120 ms) | | Infranodal (His bundle, bundle branches) | 20–30% | More serious; may progress to complete block; atropine-resistant | Prolonged or normal | Widened (>120 ms) | | Atrial muscle | Rare (<5%) | Intra-atrial conduction delay | Prolonged | Normal | | Ventricular muscle | Rare (<5%) | Ventricular conduction abnormality | Normal | Widened | ### Why AV Node is Most Common 1. **Intrinsic properties:** The AV node has the slowest conduction velocity in the cardiac conduction system (0.02–0.05 m/s), making it the physiological site of delay 2. **Vagal innervation:** The AV node is richly innervated by parasympathetic fibers; increased vagal tone (common in elderly, athletes, sleep apnea) causes nodal delay 3. **Drug sensitivity:** AV nodal conduction is slowed by beta-blockers, calcium channel blockers, and digoxin — common medications in elderly patients 4. **Ischemia:** The AV node receives dual blood supply (AV nodal artery from RCA in 80%, from LCx in 20%), but ischemia from RCA occlusion (inferior STEMI) commonly affects the node **High-Yield:** A **prolonged PR interval with normal QRS duration** (as in this case: 220 ms PR, normal QRS) is diagnostic of **AV nodal block** (first-degree or Mobitz I second-degree). If the QRS were widened, it would suggest infranodal block. **Clinical Pearl:** AV nodal block is usually benign and often resolves spontaneously. It is **atropine-responsive** because it is vagally mediated. In contrast, infranodal block is atropine-resistant and may require pacing. **Mnemonic:** **AVN = Atropine-responsive, Vagal, Nodal** — the AV node is the site of vagally-mediated, atropine-sensitive conduction delay in acquired disease.