A 68-year-old man presents with recurrent syncope. ECG shows sinus rhythm at 70 bpm with 3:2 AV conduction. The PR interval remains constant at 0.26 sec in all conducted beats, with sudden unexpected dropped QRS complexes. QRS duration is 150 ms with LBBB morphology. What is the most likely diagnosis?

• A Mobitz type I (Wenckebach) second-degree AV block
• B First-degree AV block with non-conducted premature atrial contractions
• C Mobitz type II second-degree AV block ✓
• D High-grade AV block

Explanation

Mobitz type II is characterized by a constant PR interval with intermittent non-conducted P waves, typically occurring at the His-Purkinje level, hence the wide QRS. Mobitz type I shows progressive PR prolongation before a dropped beat and usually has a narrow QRS. The constant PR with sudden dropped beats and wide QRS is classic for Mobitz II, which carries high risk of progression to complete heart block and often requires permanent pacing.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.