A 52-year-old woman from Mumbai presents with palpitations and dyspnea for 3 days. On examination, heart rate is 78/min and regular. Her 12-lead ECG reveals a PR interval of 0.28 seconds, normal QRS duration of 0.09 seconds, and a QT interval of 0.42 seconds. The PR interval is uniformly prolonged across all leads. There is no evidence of ST elevation or T-wave inversion. What is the most likely diagnosis based on the ECG findings?

Explanation

## First-Degree Atrioventricular Block (PR Prolongation) ### Definition and ECG Criteria **Key Point:** First-degree AV block is defined as **PR interval >0.20 seconds** with a 1:1 AV conduction ratio (every P wave is followed by a QRS complex). ### ECG Findings in AV Blocks | Type | PR Interval | P:QRS Ratio | QRS Duration | Clinical Significance | |------|-------------|-------------|--------------|----------------------| | 1° AVB | >0.20 s | 1:1 | Normal | Delayed AV conduction; usually benign | | 2° AVB (Mobitz I) | Progressive ↑ | Variable | Normal | Wenckebach pattern; usually benign | | 2° AVB (Mobitz II) | Normal or ↑ | 2:1 or variable | Often widened | High risk of progression to 3° AVB | | 3° AVB | Variable | No fixed ratio | Normal or widened | Complete dissociation; requires pacing | ### Analysis of This Case 1. **PR interval = 0.28 seconds** → Prolonged (normal: 0.12–0.20 s) 2. **Uniform prolongation** → Same duration across all leads 3. **1:1 AV conduction** → Every P wave is followed by a QRS (implied by regular rhythm and normal rate) 4. **Normal QRS duration** → 0.09 s (no intraventricular conduction delay) 5. **No dropped beats** → Rules out 2° AVB 6. **No AV dissociation** → Rules out 3° AVB **High-Yield:** The **key distinguishing feature** of 1° AVB is that **every atrial impulse is conducted to the ventricles, but conduction is delayed**. This is different from 2° AVB, where some P waves are not followed by QRS complexes. ### Causes of First-Degree AV Block - **Vagal hypertonicity** (athletes, elderly) - **AV nodal disease** (ischemia, fibrosis, infiltration) - **Medications:** beta-blockers, calcium channel blockers, digoxin, amiodarone - **Acute myocarditis or pericarditis** - **Hyperkalemia** - **Hypothyroidism** **Clinical Pearl:** First-degree AV block is usually **asymptomatic and benign**, requiring no specific treatment unless symptomatic bradycardia develops. Monitoring is recommended if caused by acute MI or myocarditis. **Mnemonic — AVB Classification:** **"1° = Delay, 2° = Drop, 3° = Dissociation"** - 1° = PR prolongation (delay in conduction) - 2° = Dropped QRS (some P waves not conducted) - 3° = Complete dissociation (no P-QRS relationship)