Barney B. Clark, the world's first recipient of a permanent artificial heart, has already lived longer than his doctors and family dared dream possible.

On the night of Dec. 1, he was almost dead even before the historic 7 1/2-hour operation at the University of Utah Medical Center began.

In the next 10 days, Clark had an initial burst of quick recovery, followed by a series of complications of varying severity.

It is too soon to predict what Clark's long-term outlook might be. But, interviews with top staff members here suggest, the highly experimental procedure of removing most of his debilitated heart and replacing it with a man-made one is already providing some valuable scientific insights:

* More than a decade of painstaking experiments with calves and sheep helped make the human implantation technically possible, but the jump from healthy animals to a very ill human still proved to be an "enormous one."

* The mechanical heart has "worked superbly" and appears to have been a boon to Clark's recovery, helping to treat the troublesome complications. But there is still concern that the new heart could be overpowering to a body that was long adjusted to a diseased one.

* Experimentally, the absence of a human heart for the first time provides some intriguing possibilities for better understanding the effects of drugs on the cardiovascular system. But experimentation will be limited until Clark's condition becomes more stable.

* There initially appeared to be little concern by Clark and his family about the difficulties of adjusting to the new heart and its cumbersome external power system. But the long-term psychological impact, even for a family as strong as this one, is still troublesome.

* Despite a seizure last week that numbed Clark's brain, there is no evidence that Clark has suffered mental impairment, although this will remain worrisome until he is fully alert. But should damage occur now or in the future, what would be the ethics of keeping such a patient alive with a mechanical heart?

"This pioneering experiment has yielded data which has assumed a significance in its own right," said Dr. Chase Peterson, the university vice president for health sciences.

Peterson acknowledges that intellectually he had thought all along that the artificial heart would work in a human, but "looking back, I was amazed it really did work." On the morning the heart was successfully implanted, Peterson said, "we all had to pinch ourselves and say the predictions were really true."

The evening before, when the emergency surgery began, doctors were uncertain whether their years of preparation would prove fruitless. The 61-year-old retired dentist's heart was beating irregularly, a potentially deadly condition called ventricular tachycardia that could have led to cardiac arrest before the surgery began.

"The patient was too ill," worried Dr. Donald Olsen, the veterinarian who has been implanting artificial hearts in animals here for over a decade. "All my calves and sheep have been perfectly healthy," he said.

But as Clark entered the operating room at 10:15 p.m., "he was very blue," even down to his toenails, because of a lack of oxygen being pumped to his body. His abdomen was also severely swollen, another sign of severe heart failure, Olsen said.

Because of his fluttering heart and the long-term impact of congestive heart disease, Clark was "even sicker than most heart surgery patients," said Dr. Nathan Pace, the anesthesiologist who faced the sensitive task of getting Clark to sleep without setting off heart failure. He did not dare use even the normal pre-operative drugs.

Instead, Pace chose a potent narcotic called Fentanyl, which is 100 times more potent than morphine and acts quickly to numb the brain with little effect on the heart. Once asleep, Clark needed help breathing, so a drug called metocurine was injected to paralyze his muscles and allow insertion of the respirator tubes. This relative of the lethal poison curare again posed the least risk of affecting his heart, he said.

By midnight, Clark was successfully placed on a heart-lung bypass machine that would take over the work of pumping blood throughout the long operation and the more than 20 staff members in the operating room breathed sighs of relief. Six minutes later they found out how lucky they were.

Suddenly Clark's own heart "did stop once and for all," said Peterson and several others present. Fortunately by then, it did not matter. But if the cardiac arrest had occurred 10 minutes earlier he likely would have died, the doctors said.

Shortly afterward, the lower two-thirds of the heart were removed. It was so fragile that it literally "tore open," recalled Robert Jarvik, designer of the man-made heart. "We all gulped because there was no turning back," Peterson said.

Surgeon William DeVries then dropped the artificial heart into the cavity to see if it fit and over the next few hours the laborious process of attaching it to the remnants of the old heart took place.

The most troubling complication of the early morning hours was the discovery that the left side of the new heart was not working properly. There was some discussion between the artificial heart experts and DeVries as to what was wrong; some speculated that heart tissue was causing a valve to stick. Finally, he "reached for the backup heart," Olsen said.

Shortly after 4 a.m. the heart bypass machine was turned off and the mechanical heart was pumping on its own. By 7 a.m. Clark was wheeled into the specially designed intensive care unit.

The drama of the operation was followed by two days of elation as the heart "performed extremely well" with "no unexpected events," said Larry Hastings, the technician in charge of the equipment.

The cumbersome heart power system was being monitored by an Apple II computer system called "COMDU," for cardiac output monitor and diagnostic unit. It took minute-by-minute readings of blood output from both sides of the heart.

It was COMDU that told doctors during the operation that the left heart was not working, Olsen said. In the early days after implantation, it once reported that the left side was pumping more blood than the right, leading technicians to check for malfunctioning. They found that the heart "driver" that powers the heart had a slight problem and immediately switched to the backup system, he said.

"I have absolutely unshakable confidence in the future of the artificial heart now," said Dr. Willem Kolff, in assessing performance of the equipment.

The 71-year-old Dutch scientist, who heads the artificial organs program here, earlier pioneered the development of the artificial kidney used in renal dialysis and was instrumental in the 1957 implantation of an artificial heart inside the chest of a dog for the first time in this country.

Kolff reports some "satisfaction and reassurance" in the warm reception he said he received at a government heart meeting in Washington, D.C., last week. Although the National Institutes of Health has long supported the artificial heart work, it has not received top priority and funds have sometimes been difficult to obtain, researchers here said.

In addition to performing well in its own right, the Jarvik-7 artificial heart has proved an aid in treating the complications that have arisen in Clark's recovery.

Anesthesiologist Pace noted that the second surgery on Dec. 4, to repair air leaks in Clark's lungs, was "all the difference in the world" from the treacherous operation three nights earlier. In contrast to the earlier problem of selecting drugs that would not harm the heart, this time "there was nothing I could do that would make it stop," he said. Common anesthetic drugs were administered.

The mechanical heart and its independent control system were later called upon for help in flushing out fluid accumulating in Clark's lungs. And when minor kidney failure that had plagued Clark before surgery worsened, doctors were again able to increase his blood pressure and send more blood through the kidney, Peterson noted.

The absence of a human heart also provides an unusual laboratory for watching the effects of drugs used in his treatment. A drug called nitroprusside was administered to help open up his arteries as part of the effort to improve kidney function. While in a normal patient this drug might also affect the heart, in this case its effect was "purely on the blood vessels," Peterson said.

But there are still unanswered questions about the hidden effect of the artificial heart on a patient whose body had become used to a debilitated organ that was putting out one-seventh as much blood before the implant.

"One concern has been have we improved his function too rapidly? The rest of his body -- his kidneys, his brain, his liver -- have been used to a very low blood flow and now as we increase the blood flow maybe we're overpowering them," said Dr. Jeffrey Anderson, a cardiologist who referred Clark to the University of Utah program. Clark was suffering from cardiomyopathy, a progressive deterioration of the heart muscle for which there is little treatment.

It is this concern that still haunts doctors, since breakage of the blood vessels to the brain could cause a hemorrhage with permanent brain damage. There is also the continuing worry that clots could form on the surface of the artificial heart, which is a foreign surface to the body, and be released into the blood stream and cause a stroke, they said.

These were major concerns last week when Clark suffered a major seizure, but doctors concluded that it was more likely a result of chemical imbalances that would do no permanent harm.

Peterson has reported daily that there is no evidence from brain tests or clinical signs that permanent brain damage has occurred. But he admits that doctors are "still a little concerned about the possibility of small hemorrhages or clots" that might not have shown up on the tests.

Only a restoration to the awake and joking state that Clark was in before the seizure will provide that reassurance, he said. He compared the present waiting period with crossing the desert at night with a compass and a flashlight. "You set a course, but you're still waiting for the sun to come up."

In the meantime, Peterson dismisses theoretical questions from reporters about how the hospital would treat an artificial heart patient, should he suffer permanent brain damage.

His clear daily descriptions of the roller-coaster course of Clark's illness have provided a close look at experimental medicine that is seldom seen by the public.

This is an added benefit, Peterson believes. "It's useful to patients to understand that doctors do not throw thunderbolts down from Olympus, but rather try to understand a situation and develop an approach for treating it."

Full analysis of Clark's case and the mountains of material that are accumulating will take weeks and months. University of Utah doctors said they plan to digest what they have learned first before undertaking a second artificial heart implantation.

"We don't know yet a lot of things about Dr. Clark," Peterson said. "But we're going to stay with it until we darn well known all that there is to know."