A 60 year old woman is admitted to the cardiology ward because of recurrent palpitations.  On the day after admission, she was having her morning ECG and the initiation of her tachycardia was captured on the ECG (shown below):

• Sinus rhythm 75/min
• Premature ventricular complexes initiate supraventricular tachycardia
• Possible inferior myocardial infarction (Q-waves in III and aVF)

Comment: The supraventricular tachycardia has a long RP interval (QRS to P interval is longer than P to QRS interval).  Retrograde P-waves during tachycardia are noted with “P” below. The combination of this feature and the fact that the tachycardia was initiated by a PVC is highly suggestive of an atypical AVNRT or AVRT with a decrementally conducting accessory pathway as the mechanism of tachycardia.  She was taken to the EP lab and found to have a decrementally conducting septal accessory pathway which was successfully ablated.