Fig 1. a. Illustration of the RA showing the usual anatomic description of the SN complex represented as a crescent-shape structure with a “comma” disposition in which its “head” is placed at lateral SVC-RA junction and its “tail” extends toward the posteroinferior aspect of the CT. b . 3-D activation map of a patient who underwent SNM for IST in which the RF set of lesions was extended from the earliest spot at the lateral SVC-RA junction towards the posteroinferior aspect of the CT crossing next to the PN (dark dots). Note the anatomic extrapolation between the SVC and RA.c. 3-D voltage map of a patient who underwent previous SNM for IST at the lateral SVC-RA junction (low voltage zone in green) where earliest activation under isoproterenol was documented. Ablation at this place did not control the HR with the need for an extension to the posteroinferior CT with the same result. d. 3-D Activation map of the same patient performed during redo procedure under isoproterenol showing shifting of the earliest activation to the anterosuperior SVC-RA junction at the AR (in red). RA: right atrium, CT: crista terminalis, AR: arcuate ridge, SNM: Sinus Node Modulation, SVC: Superior Vena Cava, IVC: Inferior Vena Cava, HR: Heart rate. PN: phrenic nerve, IST: inappropriate sinus tachycardia. Reproduced from: Anderson RH, Yen SH, Becker AE. The surgical anatomy of the conduction tissues. Thorax. 1983; 38:408–20.