CLINICAL INTRODUCTION

An adult with no known cardiovascular disease presented with acute onset palpitations and near-syncope during exercise. On admission, ECG revealed wide QRS tachycardia with left bundle branch block and left superior axis morphology (Figure 1A). Because of haemodynamic instability, the patient was urgently cardioverted to sinus rhythm (Figure 1B). Serum electrolyte and cardiac troponin levels were normal. Bedside echocardiography revealed normal left ventricular size and function. Subsequent coronary angiography did not demonstrate any significant coronary artery stenosis.