Can you guess what is occuring in this electrocardiogram? In this monthly section, a difficult EP image is provided for our readers to identify. Send us your responses — we will publish the answer in next month’s issue!

Description:
A 60-year-old female presented to the Emergency Department with a tachycardia (see 12-lead ECG obtained during tachycardia). She complained of being light-headed, breathless, and was slightly diaphoretic. Upon questioning, for many years she has felt the same way when she would get in an argument with someone. This is her first visit to the Emergency Department.

Questions:
1. What is the exact diagnosis of this tachycardia: description, mechanism?

2. What diagnostic maneuver or treatment could be initiated to determine the diagnosis?