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.