Edwards Lifesciences (NYSE:EW) today called a halt to enrollment in a clinical trial for its Fortis mitral valve replacement after discovering blood clots in some of the 20 patients implanted with the device.

"We observed evidence of valve thrombosis that we believe warrants additional investigation," the Irvine, Calif.-based company said. Fortis is a transcatheter mitral valve implant designed to treat patients with leaky heart valves.

"We are working closely with the trial investigators and heart teams to gather additional information in this early study of transcatheter mitral valve replacement therapy," Edwards said. "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. We remain committed to and are encouraged by the early experience in transcatheter mitral valve replacement therapy."

EW shares dipped 1.4% to $129.87 apiece in mid-morning trading today.

It’s not the 1st hiccup for the Fortis clinical program. Back in February 2014, Edwards said a regulatory snag had delayed the start of the Fortis program. The 1st implantations came a month later, when a surgical team at London’s St. Thomas’ Hospital implanted the 1st 3 patients with the Fortis TMVI device.