An 80 year old man is referred for intermittent palpitations.  A Holter monitor is ordered and an episode of palpitations that lasted for approximately 30 minutes is recorded.  The onset and termination are shown below:

• Initially Sinus rhythm
• Premature atrial complex (PAC) in top panel is conducted with a very long AV interval with subsequent onset of tachycardia. During tachycardia, P-wave is seen at terminal part of QRS complex
• Bottom panel shows PAC which terminates tachycardia

Comment: The p-wave at the onset of the tachycardia is inferred because the T-wave amplitude and morphology is slightly different from the preceding sinus beats. The onset of a tachycardia with a very long AV interval (conducted down antegrade slow pathway) and p-wave at terminal part of QRS complex (retrograde fast pathway activation) is typical for the onset of AV Nodal Reentrant Tachycardia (AVNRT)