Which ECG finding best distinguishes second-degree AV block type I (Wenckebach) from second-degree AV block type II (Mobitz II)?

Explanation

## Distinguishing Features of Second-Degree AV Blocks ### Second-Degree AV Block Type I (Wenckebach) **Key Point:** Progressive lengthening of the PR interval occurs with each successive beat until a P wave is not followed by a QRS complex (dropped beat), then the cycle repeats. - PR interval gradually increases: 0.16 → 0.18 → 0.20 → dropped QRS - The increment of PR prolongation decreases with each beat (decremental conduction) - RR interval progressively shortens before the dropped beat - Usually benign; often seen in athletes and inferior MI ### Second-Degree AV Block Type II (Mobitz II) **Key Point:** The PR interval remains constant, and a QRS complex is suddenly dropped without preceding PR prolongation. - PR interval is fixed and normal or prolonged, but does NOT change - Sudden loss of AV conduction without warning - More dangerous — indicates infranodal block (His bundle or below) - Often progresses to complete heart block; may require pacing ### Comparison Table | Feature | Type I (Wenckebach) | Type II (Mobitz II) | |---------|---------------------|---------------------| | **PR interval pattern** | Progressive lengthening | Constant | | **Dropped beat preceded by** | PR prolongation | No change | | **Site of block** | AV node (nodal) | His bundle or infra-His | | **Clinical significance** | Usually benign | Dangerous; high risk of CHB | | **Pacing indication** | Rarely needed | Often required | | **Associated with** | Inferior MI, athletes | Anterior MI, degeneration | **High-Yield:** The *progressive PR prolongation* is the hallmark of Wenckebach and the single best discriminator from Mobitz II. **Clinical Pearl:** In Wenckebach, the RR interval containing the dropped beat is shorter than the RR interval before it — a subtle but diagnostic finding on careful measurement. **Mnemonic:** **"Wenckebach = Warming up then Dropping"** — PR gets longer and longer until the beat drops. **"Mobitz II = Sudden Surprise"** — no warning, just a missing QRS. [cite:Harrison 21e Ch 226]