(A) Post-mortem dog heart (no device implanted), showing the exterior view of the left atrial appendage (LAA). (B) Diagrammatic view of the Watchman LAA obliteration method. Metal struts with anchoring hooks secure the device within the body of the appendage cavity. A fabric membrane filter covers the atrial surface of the device, preventing thrombi from escaping into the left atrial chamber. A center hub is used to connect the device to the catheter delivery system. (C) Dog autopsy specimen 28 days after Watchman implant showing a cross-sectional cutaway view of the Watchman device. The fabric membrane is covered with a fine layer of endocardium, and the metal struts are shown holding the device in place. Pectinate muscles internal to the LAA cavity are labeled. (D) Dog autopsy specimen showing a view of the former LAA ostium, now completely obliterated by the endocardium-covered fabric membrane. This view is from the left atrial cavity, where it is clear that thrombi potentially residing the left atrial appendage body could no longer escape into the left atrium and systemic circulation.