TIL: for mechanical valves, ASA + VKA is better than VKA alone
A systematic review1 showed that adding an antiplatelet agent (e.g. aspirin) to a vitamin K antagonist decreased the risk of both a thromboembolic event or death.
Per the review, "Compared with anticoagulation alone, the addition of an antiplatelet agent reduced the risk of thromboembolic events (odds ratio (OR) 0.43, 95% confidence interval (CI) 0.32 to 0.59; P < 0.00001) and total mortality (OR 0.57, 95% CI 0.42 to 0.78; P = 0.0004)." Of note, major bleeding events were increased when aspirin was added.
This led to the Chest guidelines2 and AHA/ACC guidelines3 to recommend adding low-dose aspirin to a vitamin K antagonist for patients with mechanical valves. For bioprosthetic valves, the guidelines are different.
1. Massel, David R, and Stephen H Little. “Antiplatelet and anticoagulation for patients with prosthetic heart valves.” The Cochrane database of systematic reviews ,7 CD003464. 9 Jul. 2013, doi:10.1002/14651858.CD003464.pub2
2. Whitlock, Richard P et al. “Antithrombotic and thrombolytic therapy for valvular disease: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines.” Chest vol. 141,2 Suppl (2012): e576S-e600S. doi:10.1378/chest.11-2305
3.