This 23-year-old man is referred because of frequent, brief episodes of rapid palpitations over the past year. The following arrythmia recurred while he was in the waiting room of the EP clinic:

• Supraventricular tachycardia, 205/min
• Probably AVRT or AVNRT
• Rightward QRS axis

The arrhythmia did not respond to vagal manoevers and IV adenosine. An hour later, he reported that his symptoms had subsided spontaneously:

• Sinus rhythm, 65/min
• Normal ECG

No delta waves were present. The diagnosis on discharge was “Supraventricular tachycardia: AVNRT or AVRT”. As the arhythmia was frequent and prolonged, causing significant discomfort, the patient was readmitted and EP studies were carried out. A bypass tract was found on the tricuspid annulus, very close to the bundle of His. AVRT was easily induced, and was terminated by ventricular pacing. Cryoablation was successfully performed, and AVRT could no longer be induced.

This is an example of AVRT due to a concealed bypass tract, that conducted only retrogradely, from the ventricle to the atrium. b