BackgroundAlthough the electrcardiographic and electrophysiological properties of ventricular arrhythmias (VA) from the vicinity of lateral tricuspid annulus (TA) have been reported in previous studies, their precise site of origin have not been addressed. ObjectiveTo describe the precise origin of lateral TA-VA and the relevant anatomy. MethodsConsecutive patients with idiopathic lateral TA-VAs were reviewed and analyzed. Three-dimensional mapping system combined with intracardiac echocardiography (ICE) was used for anatomical reconstruction, mapping, and ablation. ResultsDuring the study period, 63 patients with lateral TA-VAs were included. Under ICE view, a prominent enfoldment structure was observed under the valve along lateral TA. The muscular bundle was documented in all patients (100%) within the sub-valvular enfoldment with an average number and diameter of 4±2 and 4.10±0.73mm, respectively. Initial ablation was attempted via the antegrade approach in 15 patients but succeeded in none. To reach the ventricular side of TA, the catheter needed to enter ventricular chamber and retroflexed toward the atrial side with a reverse curve. The earliest activation site was found at the valvular end of muscular bundles in 51 of the 63 (80.9%) patients with a local activation time of -26.78±4.63ms. The VAs were eliminated after an average of 4±2 seconds of ablations. ConclusionThe ventricular adjacent to the lateral TA exhibits a sub-valvular enfoldment-like structure, which is rich in muscular bundles and serves as the origin of TA-VAs in most patients. To reach the origins, a reverse technique is required.