A 70-year-old man with syncope has an ECG showing alternating LBBB and RBBB morphology. Which type of heart block/conduction disease does this represent, and what is the treatment?

• A Second-degree Mobitz I (Wenckebach); vagal manoeuvres and observation
• B First-degree AV block; reassurance and follow-up
• C Accelerated idioventricular rhythm; no treatment required
• D Bilateral bundle branch block (alternating BBB); permanent pacemaker implantation ✓

Explanation

Alternating left and right bundle branch block (bilateral or trifascicular block) is a Class I indication for permanent pacemaker implantation per ACC/AHA/HRS guidelines, as it indicates extensive disease of the His-Purkinje system and high risk of complete heart block and sudden death, particularly in a patient with syncope. This is a more urgent indication than isolated RBBB or bifascicular block.

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

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Written and medically reviewed by the StethoPrep medical team.