Barney B. Clark, the remarkable artificial-heart patient who has survived three operations in less than two weeks, improved slightly today after half of his heart was replaced because of a broken valve, University of Utah officials said.

"Dr. Clark has had a good day of recovery. His mental state continues to improve," said Dr. Chase Peterson, university vice president for health services.

Even the mild bacterial pneumonia that had developed in his left lung was responding to antibiotics and had become "somewhat better," Peterson said.

Clark, 61, a retired dentist, remained in critical condition in the university medical center's intensive-care ward, however, and Peterson cautioned that unexpected setbacks were still possible for the world's first permanent artificial-heart recipient. Clark underwent a three-hour operation Tuesday to replace the left side of the plastic and metal heart installed Dec. 2.

The heart had developed a crack in the brace of the artificial mitral valve. Clark's blood pressure had plummeted at 9:45 a.m. Tuesday because the broken valve suddenly allowed blood to regurgitate into the atrium of the heart.

The spot of pneumonia that had developed Monday on the lower part of Clark's left lung improved after doctors administered three or four different antibiotics, Peterson said.

Clark is breathing through a hole cut in his throat and cannot get air to his vocal cords to speak, but he communicated with his wife all morning by mouthing words, asking for water and other things, Peterson said. This showed he was regaining alertness steadily after suffering a massive seizure Dec. 7 that appeared to numb his nervous system.

The seizure, followed by smaller seizures in his left leg, has been blamed on a chemical imbalance in his blood, which Peterson said had been corrected. "We have ruled out significant, major brain damage already," Peterson said. "Whether there is subtle brain damage, I don't know if anyone is going to be able to tell that for a long time."

Peterson said Clark's wife, Una Loy, is "a real tough nut." Despite distress caused by Clark's sudden surgery Tuesday, she had "the best sleep of the whole experience--six hours straight." Peterson said he did not know how long Clark slept.

When the artificial heart was tried out in animals, Peterson said, the mitral valve had failed once in every 100 to 200 experiments.

Dr. Robert K. Jarvik, who designed the artificial heart and attended Peterson's media briefing today, declined to identify the valve's manufacturer.

He said it was the same kind of valve used in more common heart-repair surgery. "I don't want to draw attention to the valve because the forces it sees in an artificial heart are very different from the forces it sees in a natural heart," Jarvik said.

Asked to sum up Clark's condition, Peterson said, "He's worse off than he was before because of the third episode of anesthesia and surgery. He's worse off because of the appearance of the pneumonia. But he is better off because he has had 14 days of good cardiac output."

Clark's heart had become so weak before it was removed two weeks ago that his skin had acquired a bluish tinge, but the artificial heart, run by two air hoses that connect his torso to 375 pounds of equipment, has performed so well that he has returned to a healthy pink.

Peterson said Clark's blood pressure today was 130 over 80, almost normal; his temperature was between 100 and 101 degrees and his cardiac output was seven liters of blood a minute, considered healthy.