A 28-year-old woman presents with recurrent episodes of palpitations and light-headedness. An ECG during symptoms shows a regular narrow-complex tachycardia at 180 bpm, with no visible P waves. Vagal manoeuvres are unsuccessful. She is haemodynamically stable with BP 120/75 mmHg and oxygen saturation 99% on room air. What is the most appropriate next step in acute management?