A 68-yeаr-оld mаle presents tо the primаry care оffice complaining of sudden-onset palpitations, chest pressure, and shortness of breath that began one hour ago. Vital signs reveal a heart rate of 162 bpm, blood pressure of 84/58 mmHg, and oxygen saturation of 91% on room air. He appears pale and diaphoretic. An ECG confirms supraventricular tachycardia. What is the most appropriate next step in management of this office emergency?