A 62-year-old man with a 3-day history of anterior wall myocardial infarction presents with syncope. His 12-lead ECG shows a wide QRS complex with intermittent dropped beats. The feature marked **B** in the diagram—sudden non-conducted P waves without progressive PR lengthening on the preceding conducted beats—is observed. Which of the following is the most appropriate immediate management?

Explanation

## Why transcutaneous pacing followed by transvenous pacing as a bridge to permanent pacemaker implantation is right The feature marked **B**—sudden non-conducted P waves WITHOUT progressive PR lengthening—is the hallmark of Mobitz Type II 2nd-degree AV block. This pattern, combined with wide QRS (bundle branch block), indicates an **infra-Hisian block** (block in the His bundle or below). According to Harrison 21e Ch 247, Mobitz Type II is **rarely benign** and carries a **high risk of progression to complete heart block**, particularly in the setting of anterior MI with LAD septal involvement. The patient is **symptomatic (syncope)**, indicating hemodynamic compromise. Management requires **immediate transcutaneous pacing** for stabilization, followed by **transvenous pacing as a bridge to permanent pacemaker implantation**, which is indicated regardless of symptoms in Mobitz Type II due to the infra-Hisian location and high progression risk. ## Why each distractor is wrong - **Observation with serial ECGs; pacemaker only if symptoms persist or progression to 3rd degree block occurs**: This is the approach for **Mobitz Type I (Wenckebach)**, which is often benign and AV nodal in location. Mobitz Type II requires **prophylactic permanent pacing** even in asymptomatic patients because of the high risk of sudden progression to complete block and unreliable ventricular escape rhythms. - **Atropine 0.5 mg IV bolus to enhance AV nodal conduction**: Atropine may be useful in **AV nodal blocks** (Mobitz Type I or 3rd-degree AV nodal block), but Mobitz Type II is an **infra-Hisian block** below the AV node. Atropine will not improve conduction at the His bundle or bundle branch level and delays necessary pacing. - **Temporary transvenous pacing without permanent pacemaker, as the conduction defect may resolve with revascularization**: While revascularization is essential in acute anterior MI, Mobitz Type II with infra-Hisian block has a **high inherent risk of sudden progression to complete block** and does not reliably resolve with revascularization alone. Permanent pacemaker is the standard of care to prevent sudden cardiac death. **High-Yield:** Mobitz Type II = infra-Hisian block + wide QRS + high progression risk = **permanent pacemaker indicated regardless of symptoms**; Mobitz I = AV nodal block + narrow QRS + benign = pacemaker only if symptomatic. [cite: Harrison 21e Ch 247]