A 30-year-old female presents with palpitations and lightheadedness. Her ECG shows a regular narrow complex tachycardia at a rate of 180 bpm. Carotid sinus massage is attempted but fails to terminate the rhythm. Intravenous access is obtained. According to Resuscitation Council UK guidelines (referenced in NICE CG139), what is the most appropriate next pharmacological step for stable narrow complex tachycardia?