Structural Heart outperformed all the other franchises at Boston Scientific Corp. last year, besting its guidance of $475 million for that business in 2018. One of the drivers behind that growth is its Watchman franchise. The left atrial appendage closure (LAAC) device was first approved by the U.S. FDA to reduce the risk of stroke in patients with non-valvular atrial fibrillation (AF) in March 2015.