This 38-year-old male computer analyst is presenting at the Emergency Department complaining of palpitation and feeling lightheaded. He had experienced 3 similar episodes in the past 5 years for which he was given Adenosine IV with prompt relief. His health had otherwise been excellent. He exercised regularly without discomfort:

• Narrow QRS tachycardia, 214/min
• Probably Atrioventricular Re-entrant Tachycardia (AVRT)
• Less probably AVNRT

Comment: Notice the negative P wave in II, AVF, and the positive P in aVR and V1, that are inscribed in the ST segment, clearly separate from the QRS.  This helps to distinguish AVRT from AVNRT. In AVRT the activation goes from the ventricle up to the atrium through the bypass tract, so that the QRS and P appear separate in the majority of cases. In AVNRT the activation from the AV node travels simultaneously up to the atrium and down to the ventricle, so there is partial or total overlap of the QRS and P in the majority of case. (notice: in the majority of the cases, not all cases! In some patients with AVNRT, particularly in older subjects, the P wave may appear separate from the QRS)

Vagal stimulation is unsuccessful. The patient receives IV Adenosine and converts to sinus rhythm:

• Sinus rhythm, 85/min
• Wolff-Parkinson-White
• Left accessory pathway
• The presence of Wolff-Parkinson- White confirms the diagnosis of AV Re-entrant Tachycardia in ECG 1

Comment: The presence of a q wave (negative delta wave) in lead I identifies a left free wall accessory pathway. As the delta wave is positive in aVF, the pathway is Left lateral or left anterolateral.

The patient consents to undergo EP studies and ablation of the accessory pathway. A left lateral bypass tract is identified and successfully ablated. Following is the post-ablation ECG:

• Sinus rhythm
• ST elevation, with the features of “early repolarization”, a normal variant
• The tracing is otherwise normal