ECG puzzle: Persistent narrow-QRS tachycardia in a 62-year-old woman.

We report a 62-year-old woman with a long history of palpitations who presented with fatigue and acute dyspnea. ECG demonstrated a regular narrow-QRS tachycardia with long RP' intervals and 1:1 atrioventricular conduction. Episodes were recurrent despite amiodarone and beta-blockers, and transthoracic echocardiography revealed severe biventricular dysfunction which improved after rhythm control. We discuss the ECG features, mechanisms, and diagnostic clues of long RP' supraventricular tachycardia in adults.