Reference:
1. Bun SS, Latcu DG, Wedn AM, Saoudi N. Intrapulmonary vein “echo” beats. HeartRhythm Case Rep . 2018;4:464-465
2. Takasugi N, Kubota T, Okura H. Sinus impulse-triggered firing within the superior vena cava as an unusual cause of atrial bigeminy. J Cardiovasc Electrophysiol . 2019;30:2523-2524