This 43-year-old man has a history of paroxysmal narrow QRS tachycardia for over 20 years. The episodes were at first infrequent and terminated spontaneously, however, during the past 2 years they had been occurring every 2-3 months requiring injection of Adenosine:

• Supraventricular tachycardia, 192/min
• Consider Atrioventricular Re-entrant Tachycardia (AVRT), atrial tachycardia or least likely Atrioventricular nodal Re-entrant tachycardia (the interval between the QRS and the retrograde P would be unusually long for AVNRT)

The arrhythmia subsided after IV injection of Adenosine:

• Sinus rhythm, 80/min
• Normal ECG

Comments: The presence of a concealed bypass tract causing episodes of AVRT was strongly suspected. Because of the frequent and prolonged episodes of arrhythmia EP studies were carried out. The presence of a concealed anteroseptal bypass was demonstrated and AVRT was induced. The bypass tract was successfully ablated and AVRT could no longer be induced.