A 68-year-old woman with chronic obstructive pulmonary disease presents with dyspnoea and chest discomfort. Her 12-lead ECG shows a prolonged PR interval (>200 ms) with normal QRS duration. Which of the following is the most common cause of first-degree atrioventricular block in this patient population?

Explanation

## Most Common Cause of First-Degree AV Block **Key Point:** Increased vagal tone (parasympathetic dominance) is the most common physiological cause of first-degree AV block in the general population, particularly in athletes, elderly individuals, and those with high parasympathetic tone. ### Pathophysiology of First-Degree AV Block First-degree AV block (PR interval > 200 ms) results from **delayed conduction through the AV node**. The delay occurs because: 1. Vagal stimulation increases acetylcholine release at the AV node 2. Acetylcholine hyperpolarizes nodal cells and slows depolarization 3. This prolongs the refractory period of the AV node 4. Conduction velocity through the node decreases ### Comparison of Causes of First-Degree AV Block | Cause | Mechanism | Frequency | Clinical Context | |-------|-----------|-----------|------------------| | **Increased vagal tone** | ↑ ACh, ↓ AV nodal conduction | Most common | Athletes, elderly, sleep apnoea | | Acute inferior MI | Ischaemia of AV nodal artery | Common | Acute coronary event | | Lyme disease | Spirochete inflammation of AV node | Rare | Endemic areas (USA, Europe) | | Digoxin toxicity | ↑ Vagal tone + direct AV nodal depression | Uncommon | Therapeutic/toxic levels | **High-Yield:** First-degree AV block is **benign and asymptomatic** when due to vagal tone. It often resolves with exercise or atropine administration. This distinguishes it from pathological causes. ### Clinical Pearl In an elderly patient with COPD presenting with dyspnoea, the most likely explanation for first-degree AV block is **increased parasympathetic tone** from chronic hypoxia, sleep apnoea, or simply age-related vagal dominance. This is a **benign finding** requiring no treatment. **Mnemonic — Causes of First-Degree AV Block (ACID):** - **A**cetylcholine (vagal tone) - **C**oronary ischaemia (inferior MI) - **I**nflammation (Lyme disease, myocarditis, rheumatic fever) - **D**rugs (digoxin, beta-blockers, calcium channel blockers, amiodarone) ### Differential Features ```mermaid flowchart TD A[First-Degree AV Block<br/>PR > 200 ms]:::outcome --> B{Clinical Context?}:::decision B -->|Athlete or elderly| C[Increased vagal tone]:::action B -->|Acute chest pain| D[Inferior MI]:::urgent B -->|Tick exposure<br/>endemic area| E[Lyme disease]:::action B -->|On digoxin| F[Digoxin toxicity]:::urgent C --> G[Benign, no treatment]:::outcome D --> H[Requires reperfusion therapy]:::urgent E --> H F --> H ``` **Warning:** Do NOT assume all first-degree AV blocks are benign. Always correlate with clinical context. Acute inferior MI with first-degree AV block may progress to higher-degree block and requires close monitoring.