Coronary sinus ostium in the lateral right atrium: Implications for the electrophysiologist

Introduction The coronary sinus (CS) is a critical structure that has helped aid electrophysiologists by providing information about epicardial activation along the mitral annulus. Additionally, the advent of biventricular pacing utilizes the CS venous system for lead placement. The CS collects the venous drainage of the atrial and ventricular venous branches and drains into the right atrium (RA). CS anomalies have been recognized, namely atresia of the CS ostium, unroofed CS, connection with the left atrium, and even hypoplasia of the CS. We report 3 cases of CS ostium opening in the midlateral RA. This anomaly may provide new insight into the development of the CS as well as provide an explanation for difficult cannulation of the CS.