Edwards Lifesciences put enrollment into a program using its FORTIS transcatheter valve replacement therapy on hold, announcing that it wants to investigate reports of valve thrombus.

The device, which is investigational only, was implanted in more than 20 patients with symptomatic mitral regurgitation who were either compassionate cases or enrolled in a high-risk registry. The FORTIS valve incorporates treated bovine pericardial tissue on a cloth-covered, self-expanding frame, a design that is intended to minimize paravalvular leak.

The heart team at St. Thomas' Hospital in London performed the first three cases in February and March of 2014. In addition, a patient treated in Bern, Switzerland, received the device. The researchers shared their experiences about those four cases at EuroPCR in 2014, where they reported that three patients died between four and 76 days after the procedure.

Edwards is working with researchers to investigate the thrombus issue further. “While it is still very early in the program, and we anticipated that optimizing this therapy would be challenging, we are continuing to learn which patients may benefit,” the company announced in a news release.