Association between left atrial myocardium and coronary sinus muscular sleeve: Insights from left-sided concealed accessory pathway.

Left-sided concealed accessory pathways (LCAPs) are a common cause of paroxysmal supraventricular tachycardia (PSVT) treated with radiofrequency ablation (RFA); however, the inferoposterior myocardial bundle, myocardial sleeve connections between the left atrium and coronary sinus (LA-CS), and the creation of mitral isthmus block (MIB) can complicate the procedure by inducing complex electrophysiological changes. We report a case of a 29-year-old male with an LCAP who developed a functional MIB during RFA, which was characterized by dual-component potentials within the LA-CS myocardial sleeves and dynamic shifts in the retrograde atrial activation sequence. Initial ablation via a retrograde aortic approach at the 3 o'clock position on the mitral annulus modified the activation pattern without eliminating the pathway. Subsequent transseptal remapping localized the effective target at the 1 o'clock position, where ablation successfully produced ventriculoatrial dissociation. The patient had no PSVT recurrence at one-year follow-up. These findings are consistent with the presence of multiple LA-CS electrical connections and clarify how a functional MIB can modulate atrial activation by altering breakthrough sites. Recognizing these electrophysiological patterns and promptly considering a functional MIB in cases of unexpected activation changes are essential for optimizing outcomes in complex LCAP ablations.