Case Presentation
A 26-year-old man presented to emergency department with acute onset palpitations with a history of such episodes since two months. The electrocardiogram revealed a regular short RP narrow QRS tachycardia at the rate of 145 beats per minute. The echocardiogram showed a left ventricular ejection fraction of 0.25 during tachycardia but was normal during sinus rhythm. He underwent an electrophysiology study which revealed eccentric activation in the coronary sinus during right ventricular pacing and during tachycardia (Figure 1) , suggestive of a retrograde conducting concealed left free wall accessory pathway (AP). The mitral annulus was mapped via the transseptal route and the AP was localized to the left anterolateral mitral annulus. Radiofrequency energy was delivered during entrainment of the orthodromic tachycardia by left ventricular (LV) lateral wall pacing(Figure 2 left panel) . A sudden change in QRS morphology was seen during energy (Figure 2 right panel) , coincident with elimination of accessory pathway conduction (Figure 3 left panel) . What is the mechanism?