A 56-year-old male presents with dizziness and palpitations for one week, with occasional syncope. Past medical history includes hypertension (HTN) and coronary artery disease (CAD). Blood pressure (BP) is 105/60 mmHg, heart rate is 34 beats/min with irregular rhythm. Electrocardiogram (ECG) shows a PR interval of 0.22s with occasional dropped QRS complexes after P waves. The most effective treatment is: