A 21-year-old man was brought to the emergency department due to dizziness and palpitations that started abruptly an hour ago. He has had similar episodes that provoked fatigue or strong emotions. The patient can usually stop the episodes by squatting and taking a deep breath. However, this time, these actions didn't work. His blood pressure is 65/40 mmHg, and his pulse is 240/min. He is diaphoretic, and his extremities are cold. An ECG is shown. Which of the following is the best next step in the management of this patient?