## Why "Reassurance and observation; first-degree AV block is benign and requires no treatment" is right The interval marked **B** (PR interval) of 280 ms exceeds the upper normal limit of 200 ms, defining first-degree AV block. According to Guyton & Hall 14e Ch 12 and Harrison 21e Ch 247, first-degree AV block is typically benign, asymptomatic, and requires no treatment. The key clinical pearl is that all atrial impulses still conduct to the ventricles—there is no dropped beat, only delayed conduction. In an asymptomatic patient with no structural heart disease, observation alone is appropriate. ## Why each distractor is wrong - **Immediate pacemaker insertion due to high risk of progression to complete heart block**: This describes Mobitz II (infranodal) second-degree AV block, which shows constant PR interval followed by sudden dropped beats. First-degree block does not progress to complete heart block and does not require pacing. - **Administer intravenous atropine to restore normal AV conduction**: Atropine is used for Mobitz I (Wenckebach) second-degree AV block, which shows progressive PR prolongation followed by a dropped beat. Atropine is not indicated for asymptomatic first-degree block. - **Refer for electrophysiology study and radiofrequency ablation of accessory pathway**: This is the management for Wolff-Parkinson-White (WPW) syndrome, which presents with SHORT PR interval (<120 ms) and a delta wave, not prolonged PR interval. **High-Yield:** First-degree AV block (PR >200 ms) = benign, no treatment; Mobitz I (progressive PR + dropped beat) = AV nodal, atropine-responsive; Mobitz II (constant PR + dropped beat) = infranodal, pacemaker-dependent. [cite: Guyton & Hall 14e Ch 12; Harrison 21e Ch 247]
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