LOUISVILLE, Kentucky (Reuters) – The first recipient of the self-contained AbioCor mechanical heart has suffered a severe stroke 3 months after his surgery and is partially paralyzed, his surgeons confirmed on Wednesday.

Fifty-nine-year-old Robert Tools experienced an embolic stroke to the left side of his brain on Sunday at Jewish Hospital in Louisville, doctors said.

"We don't know the prognosis at this time, it could take several weeks. We are optimistic that he will make a recovery, but he's had a severe stroke," surgeon Laman Gray said.

While the artificial heart implanted in Tools' chest on July 2 meant he would be susceptible to blood clots, as with any artificial heart valve, doctors were unable to deliver the necessary doses of anticoagulants because of Tools' tendency to bleed internally.

"The artificial heart was never intended to be used without anticoagulants," Dr. Gray said. "What is really remarkable here is we had the device working for 2 months without anticoagulants."

Tools went without proper doses of the blood-thinning drugs for at least 80% of the time for the first 2 months after his surgery, and more than half the time in recent weeks, his doctors said. At least two procedures were performed to relieve gastrointestinal tract bleeding in the days and weeks following Tools' surgery.

Otherwise, Tools made remarkable progress prior to the stroke. Doctors became optimistic that he might even be able to go home.

"This is still an experiment–we don't know all the in's and out's," Dr. Gray said at a news conference. "His condition is probably a little better than a person with a [real] heart because we don't have to worry about the heart itself," Dr. Gray said, adding he was optimistic about Tools' chances of recovery.

For now, Tools is conscious but back on a ventilator, and his right side was paralyzed except for some slight movement in his right leg.

Neurologist Lynn Simon said the extent of stroke damage to Tools' brain was not known, but his ability to speak may be impaired because the stroke occurred in the left hemisphere.

A few weeks earlier, Tools suffered a "small episode" where he had difficulty talking that may have been a precursor to the stroke, doctors said.

Tools was the first of five patients to receive the AbioCor mechanical heart built by Danvers, Massachusetts-based Abiomed Inc.