“…The sinus node is a crescent-shaped cluster of myocytes, approximately 1 to 2 cm long, located in the high right atrium near the junction of the crista terminalis and the superior vena cava . The sinus node is composed of pacemaker (P) cells, which are responsible for impulse formation, and perinodal or transitional cells, which transmit the impulse to the surrounding atrial tissue, resulting in myocardial depolarization . Sinus node dysfunction was first described in 1968 and encompasses a wide array of abnormalities, including persistent sinus bradycardia, chronotropic incompetence, SA exit block, and sinus arrest.…”