 However,the left atrial appendage LAA is the major location of thrombi in patients with AF. In patients with nonrheumatic AF and thrombus in the left atrium, the clot was located in the LAA in 90. And the WATCHMAN Left Atrial Appendage System is a novel device designed to prevent the embolization of thrombi that may form in the LAA. It is hypothesized that it may prevent ischemic stroke and systemic thromboembolism in patients with AF.The PROTECT-Af and PREVAIL trails confirmed this devices efficacy and safety, compared with warfarin, for preventing the combined outcome of stroke,systemic embolism, and cardiovascular death. And the device has been approved by Chinas FDA. 