Atrial Appendage Closure Devise (IMAGE) Yale University Caption The Watchman device prevents blood clots from forming in the left atrial appendage -- tissue that projects off one of the upper chambers of the heart. Credit Pat Lynch/ YaleNews Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.