A 6-year-old, 20 kg female with a past medical history of asthma presents to the emergency department with complaints of chest pain, dizziness, and one episode of witnessed syncope while playing soccer. There is a strong palpable pulse with a heart rate of 140 bpm. Blood pressure is 65/35 mmHg, and oxygen saturation is 93% on room air. An EKG is obtained and shows the rhythm below. Which of the following interventions is the MOST appropriate next course of action?