A 38-year-old woman with no past history presents to the emergency department with severe palpitations, shortness of breath and light-headedness. ECG shows a narrow complex tachycardia at 190 bpm consistent with AV nodal re-entry tachycardia. Blood pressure is 110/70 mmHg and she has no chest pain. Vagal manoeuvres are unsuccessful. According to standard Australian practice, what is the most appropriate next acute treatment?