A 53 year-old woman with a history of palpitations since her teenage years presents to the emergency room with an episode of sustained palpitations that have lasted over one hour.

• Supraventricular tachycardia, likely atrioventricular reentrant tachycardia (AVRT), rate 194/min

Comment: Negative P-waves (labeled ‘P’) in the inferior leads are likely retrograde and the timing of the relationship between the P-wave and QRS is most compatible with AVRT as the cause of this arrhythmia.