Guess #1: From Blair Foreman, MD, FACC: The tachycardia in the 60-year-old female is AV reciprocating tachycardia utilizing a left-sided pathway in the retrograde fashion. This is demonstrated by the slower cycle length of the tachycardia during left bundle branch abberancy (law of Coumel demonstrating slowing of the tachycardia during bundle branch aberrancy of the ipsilateral side to the bypass tract). In the lab, one can place PVCs during tachycardia when the His is refractory (activated antegrade) and advancing the atrium. Verdict: CORRECT! Guess #2: From Chandra Vasaiwala: CMT (circus movement tachycardia) with and without LBBB aberration Initial half of the ECG in V1 and V6 shows slower heart rate with LBBB (longer circuit) means left side accessory pathway. Later half of the ECG faster heart rate without LBBB (SHORTER CIRCUIT) Treatment: Ablation as the patient has frequent episodes with symptoms Diagnostic approach: 1) 12-lead ECG (for narrow QRS tachycardia) 2) Jugular (frog sign) 3) carotid massage 4) adenosine 5) pharmaceutical therapy Verdict: CORRECT!