This study will evaluate thrombogenicity in patients undergoing left atrial appendage closure using the WATCHMAN device. Parameters of thrombosis and platelet function, as well as other bio-markers, will be measure before, during, and after WATCHMAN implantation at several follow up visits.

Inclusion Criteria:

• Subject indicated and scheduled for WATCHMAN device implantation at IHVI (Inova Heart and Vascular Institute)
• Subject may be of either sex and of any race, and must be >18 years of age.
• Subject must be willing and able to give appropriate informed consent.
• The subject is able to read and has signed and dated the informed consent document including authorization permitting release of personal health information approved by the investigator's Institutional Review Board (IRB).

Exclusion Criteria:

• Subjects with contraindications for WATCHMAN device implantation

Intracardiac thrombus is visualized by echocardiographic imaging, An atrial septal defect repair or closure device or a patent foramen ovale repair or closure device is present, The LAA (Left atrial appendage) anatomy will not accommodate a device, Any of the customary contraindications for other percutaneous catheterization procedures (e.g., patient size too small to accommodate TEE - transesophageal echocardiography probe or required catheters) or conditions (e.g., active infection, bleeding disorder) are present, There are contraindications to the use of warfarin, aspirin, or clopidogrel, The patient has a known hypersensitivity to any portion of the device material or the individual components such that the use of the WATCHMAN Device is contraindicated

• Ligated or oversewn left atrium
• Concurrent participation in any investigational study.
• Subject likely to not be available to complete all protocol-required study visits or procedures, to the best of the subject's and investigator's knowledge
• History or evidence of any other clinically significant disorder, condition, or disease other than those outlined above that, in the opinion of the investigator, may compromise the ability of the subject to give written informed consent, would pose a risk to subject safety or interfere with the study evaluation, procedures, or completion.

Subjects >18 years of age scheduled for left atrial appendage closure procedure with the WATCHMAN device.

1 year

Major adverse clinical outcomes monitored include: all-cause death, embolism (systemic or pulmonary), myocardial infarction, TIA, ischemic or hemorrhagic stroke, device related thrombosis, and major bleeding using the Academic Research Consortium (BARC) definition including BARC type 2, 3 or 5. DRT was defined as the presence of a persistent echogenic mobile mass attached to the WATCHMAN device confirmed by transesophageal echocardiography.