A 67-year-old woman presents to the Emergency Department with rapid regular palpitations for the past hour. She had similar episodes a few times a year since her early twenties, usually lasting from 30 minutes to 1-2 hours, and often relieved by the Valsalva maneuver:

• Supraventricular tachycardia, 210/min
• P waves are seen, following closely the QRS complex, negative in the inferior and left precordial leads.
• AV nodal re-entrant tachycardia (AVNRT) or Atrioventricular re-entrant tachycardia (AVRT)

When seen, the P waves are usually hidden by the QRS complex or “attached” to the end of the QRS complex in AVNRT, while they follow the QRS by 40-60 msec in AVRT. This is due to the fact that the activation of the atria and the ventricles occurs almost simultaneously from the AV junction in AVNRT, while it is sequential (atria –> ventricles –> atria –> ventricles –>) in AVRT. In older patients, however,  the P wave may also be separate from the  QRS in AVNRT because of delay in the upward conduction to the atria. Therefore, in this 67 year older patient, the separation of the P from the QRS cannot be used to assist with the diagnosis.

Vagal stimulation was not successful, and so IV adenosine was administered, resulting in rapid restoration of sinus rhythm:

• Sinus tachycardia, 105/min
• Wolff-Parkinson-White
• The supraventricular tachycardia in the first ECG was Atrioventricular Re-entrant Tachycardia (AVRT)

The delta waves are isoelectric in I and negative in aVL, which indicates the presence of a left bypass tract.