Elucidating left atrial electrical potential with microelectrode catheter: A case of coronary sinus ostial atresia with small persistent left superior vena cava.

A 51-year-old woman presented with recurring palpitations. Electrocardiography revealed narrow QRS tachycardia with short RP configuration. Computed tomography showed coronary sinus (CS) ostial atresia along with a small persistent left superior vena cava (PLSVC). Electrophysiological study identified the retrograde earliest atrial activation site (EAAS) at the CS ostium without decremental properties, and para-Hisian pacing suggested retrograde atrioventricular nodal conduction. Using a 1.6-Fr microelectrode catheter distally placed in the CS via the PLSVC, EAAS was confirmed within the left atrium, not the CS ostium. Transseptal approach revealed a left lateral accessory pathway, which was successfully eliminated.