A 32-year-old woman complains of intermittent palpitations, often feeling like her heart is 'racing'. These episodes start and stop abruptly and can last from minutes to hours. She sometimes feels lightheaded during episodes. Her resting ECG is normal sinus rhythm. She wears a 24-hour Holter monitor which captures an episode showing a regular, narrow complex tachycardia at 180 bpm with no visible P waves. What is the most likely diagnosis?