This 62-year-old woman presents to the Emergency Department with palpitations, dyspnea and weakness. These palpitations have occured once a year since she was 22, with a sudden onset, and have typically resolved with Valsalva. Recently, they had been occurring every few months and not responding to Valsalva, lasting up to 3-4 hours at a time:

• Supraventricular tachycardia:  AV Re-entrant Tachycardia (AVRT) or AV Nodal Re-entrant Tachycardia (AVNRT)

Notice the negative P waves in I, II, III and AVF that are inscribed in the ST segment, clearly separate from the QRS. This suggests AVRT. 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 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 (but not in all cases!). In some patients, particularly in older ones, the conduction to the atria may be slower, and P waves may be separate from the QRS during AVNRT. For that reason, both AVRT and AVNRT should be considered in the differential:

The patient received an injection of IV adenosine, and sinus rhythm was restored:

• Normal sinus rhythm
• Wolff-Parkinson-White, with left-sided bypass pathway