Tachycardia culprit near the mitral annulus

Background: Mahaim fibers are accessory pathways (APs) known for their unique slow, exclusively antegrade decremental conduction and absence of retrograde conduction.Typically, Mahaim-type APs are located on the right side, with atrial insertion points at various locations along the tricuspid annulus.However, left-sided Mahaim fibers have been reported occasionally.Case presentation: A 30-year-old man with a structurally normal heart presented with wide complex tachycardia characterized by right bundle branch block (RBBB) morphology and left axis deviation, resembling ventricular tachycardia (VT).However, his young age, structurally normal heart, and the tachycardia's responsiveness to adenosine raised suspicion for antidromic atrioventricular reentrant tachycardia (AVRT).An electrophysiological study confirmed the diagnosis, identifying a left-sided decrementally conducting Mahaim like pathway as the underlying mechanism.While wide QRS tachycardia with RBBB morphology, left axis deviation, and no baseline preexcitation is often mistaken for VT, this case represented a notable exception.Conclusions: We described a rare mechanism of wide-complex tachycardia resulting from antidromic AVRT involving a left-sided Mahaim-like pathway which was successfully abated along the posterior mitral annulus.