Sick Sinus Syndrome — Brady-Tachy Pattern

A 76-year-old woman presents with recurrent syncope, exertional fatigue, and palpitations over 6 months. Holter monitoring reveals sinus bradycardia (36–42 bpm) with sinus pauses up to 4.2 seconds, chronotropic incompetence on exertion (peak HR 78 bpm), and paroxysmal atrial fibrillation (HR 145–165 bpm) followed by 3–5 second pauses after conversion to sinus rhythm. Echocardiography shows preserved left ventricular function. The ECG pattern marked **A** in the diagram is consistent with sick sinus syndrome. Which of the following is the most appropriate definitive management for this patient's symptomatic bradyarrhythmia?