Late today Barney B. Clark, the world's first recipient of a permanent artificial heart, successfully underwent a second operation for "minor" complications stemming from an air leak out of his lungs, University of Utah officials reported.

"What started as a day of hope"--when Clark got a look at the snow-covered mountains outside his hospital window--"ended as a day of hope" when the air leak was repaired, said spokesman John Dwan in a 10 p.m. briefing here (midnight EST).

He said that Clark was "doing well" after waking up from the anesthesia and that surgeon William DeVries considered it a "minor incident . . . . He doesn't believe he's been set back at all."

The one-hour surgery was described as "non-emergency in nature" to prevent the development of more serious hazards.

Dr. Chase Peterson, university vice president for health sciences, reported that Clark's doctors were "relieved" to find that the air leakage had come from tiny ruptures on the lungs themselves that resulted from a combination of old lung injuries exacerbated by the artificial heart implantation surgery and the use of a respirator in recent days.

It was the first complication since Clark's historic operation ended at dawn Thursday.

The 61-year-old retired dentist had made remarkable progress since he was given a mechanical heart -- so much so that doctors were surprised at the absence of problems. He was taken off the critical list earlier today and hospital officials said after the second surgery that he was still in "serious" but not critical condition.

When Clark entered the operating room at about 4:45 p.m. local time (6:45 EST), doctors were uncertain about the cause of the "troublesome" leakage of "bubbles of air in the chest wall," technically known as "subcutaneous emphysema," said Peterson.

"It in itself is not terribly harmful," he said, but "if it continues long enough . . . it could be serious." He emphasized the air was apparently trapped in a pocket inside the chest wall tissue itself and "not into the lungs, heart or cardiovascular system."

The air leakage, which caused puffiness in his left chest area, turned out to be bubbling out of several little tears in the lungs. DeVries, who implanted the heart, repaired them with small surgical staples, said Peterson.

Other small ruptures were expected to heal on their own in a few days, he said, and another chest drainage tube was inserted to draw off excess air.

Peterson said that problem was not related to the artificial heart itself, which "never missed a beat." He said Clark had had a long problem with minor emphysema which made his lungs stiff and susceptible to such tears.

Doctors originally suspected that the air was coming from leaks in the plastic tubes that travel from the mechanical heart down through the chest cavity and out to the external, compressed-air power source, said Peterson. They enter Clark's body just above the waist on the left side.

If this had been the case, doctors would have repaired the problem by cutting through the chest wall to search for the leak and patch it up, much as a leak is found and repaired in a bicycle tire, said Peterson.

But there were also concerns before the second operation that something more serious might be causing the problem, said Peterson. There was a possibility, one that doctors considered less likely, that the problem was inside the chest cavity or even, in the worst case, within the heart itself.

This did not turn out to be the case, he said. The worried family, who earlier in the day had met with the press for the first time since the heart implantation, went "from the depths to the heights very quickly" when they learned that the problem had been fixed.

"He's as good as he was before," maintained Peterson.

The announcement of a second operation came only a few hours after the university medical center said Clark had been taken off the critical list. At today's first briefing, a spokesman said Clark offered to buy nurses a round of Cokes and jokingly complained that they didn't know how to brush his teeth, so he would have to do it himself.

Clark's wife, Una Loy, at an emotional meeting with reporters earlier today, told of one of the first times she saw her husband after the 7 1/2-hour surgery to replace his diseased heart with a mechanical device.

"I kept telling him how much I loved him and how happy I was he was still with us," she said in a whispering voice. Clark was on a respirator then and unable to talk. He nodded his head and Mrs. Clark asked if he was trying to tell her he loved her, too.

"I said, 'I'm so thankful. I thought because you have an artificial heart you might not still love us.' "

The Clarks' son, Stephen, a Seattle ear, nose and throat surgeon, said that although his father appeared tired and a bit confused Friday night, he woke up "this morning very much more like the dad we know."

He described the family, which includes a daughter, Karen, and a second son, Gary, as "more hopeful now than we were a couple of days ago."

When surgery began late Wednesday night, doctors said Clark was not expected to survive unless emergency action was taken.

Stephen Clark described his father as having "a strong will to live." But the young Clark cautioned, as do doctors here, that there are still "many, many things that could potentially be a problem."

Earlier in the day, Dr. Peterson reported that Clark's vital signs were in the normal range and the functioning of his mechanical heart "continues to be splendid."

"He told me he feels fine and has no sickness and pain," said Peterson, a physician who has been giving regular updates on Clark's condition. "He's seriously ill but he's doing awfully well."

Peterson said that Clark's condition was upgraded "one word on the scale" because he continued to make medical progress.

The medical staff was attempting earlier to orient Clark, who has been under 24-hour observation and the constant glare of bright hospital light, to the outside world again, Peterson said.

They opened the curtain this morning so he could "look out on the beautiful mountains" and "get a sense of night and day," he said.

The nurses, Peterson said, continued to be amused by Clark, who overheard that they were going out for Cokes and said, "I'll buy."

Since tubes were removed from Clark's throat Friday, he had been talking so much with family and staff that his doctor limited visitors.

The spokesman said that surgeon William DeVries was so encouraged by Clark's progress that he left the hospital Friday night to spend the evening with his own family. The doctor had spent the previous night in Clark's room.