Background: The human sinoatrial node (SAN) pacemaker is a complex structure located at the right atrium (RA)-superior vena cava (SVC) junction. Anatomically, the SAN is tilted such that its superior third (head) is sub-epicardial, while its inferior third (tail) is sub-endocardial. Objective: This study aims to (1) perform in-vivo endocardial and epicardial electroanatomical mapping (EAM) of human SAN in inappropriate sinus node tachycardia (IST) and (2) correlate electrical findings with anatomical observations from thoracoscopy during hybrid SAN sparing IST ablation. Methods: All consecutive patients with 1) Diagnosis of symptomatic IST, refractory or intolerant to drugs and 2) Endocardial and epicardial mapping of SAN during hybrid ablation were included. Local activation time (LAT) was defined using steepest -dV/dT on unipolar electrogram (EGM). Exit zone (SAN-EZ) was defined as the earliest activation site on endocardial and epicardial maps. Endo-epicardial delay (EED) was the time difference between the first endo-epicardial activations. Endocardial–epicardial asynchrony (EEA) was defined as EED≥15 ms. Bipolar EGM morphology and SVC sleeves extension were analyzed. Reversed polarity at SAN-EZ site was defined as a rapid simultaneous deflection in the opposite direction of the initial part of the bipolar EGMs occurring before the onset of the surface P-wave. Results: A total of 56 patients were included. The SAN-EZ area was 1.4 cm2 ± 0.6 and was located in the superior anterior region of the RA in 42 (75\%) of patients and in the mid RA in 14 (25\%) of patients. The earliest activation occurred on epicardial SAN-EZ in all patients. The LAT of the epicardial vs endocardial SAN-EZ was -30.8 ms vs -12.4 ms, p