The Food and Drug Administration and the University of Utah, concerned about the unexpected need to replace a key valve in Barney B. Clark's artificial heart, will review the first permanent mechanical heart implant carefully before attempting a second one.

"We have mutually agreed" on the need for this to "make sure we have all the information we need," said Glenn Rahmoeller, of FDA's Office of Medical Devices. "Both sides think that's a reasonable thing to do."

The FDA originally had given Dr. William DeVries, the surgeon, permission to implant seven of the mechanical hearts, but the agency and the hospital have decided to review the situation after only the first operation because they are concerned about the malfunctioning mitral valve.

Although the problem had occurred in a few experimental animals, "it occurred much earlier in the patient," said Rahmoeller, who heads the FDA's division of cardiovascular devices. Among the questions for the future, he said, will be whether this valve or others should be used in subsequent implants.

Dr. Chase Peterson, a University of Utah official, said that the university had intended to inform the FDA of its review of the Clark case in any event.

Meanwhile, Clark's heart yesterday continued to perform well after emergency surgery Tuesday to replace the left half, and his condition, although still critical, seems to be improving slowly. He has fought off the mild pneumonia that developed on Monday.

Peterson, when asked about Clark's long-term recovery, offered a cautious response: "It's inappropriate for physicians to be pessimistic or optimistic. We simply have to be realistic."

This includes watching for additional complications. Although Clark's major organs have been functioning better than before the operation to replace his malfunctioning heart, he has been less alert than in the first days after the implantation. "The big question everyone has is, when will he be better, and is his brain all right. We don't know," cautioned Peterson.

Because the artificial heart is highly experimental, it could be implanted only in a dying man who had no alternatives. Clark, who had suffered from heart muscle deterioration for several years, was in such poor health when he underwent the implantation that he was a likely candidate for further complications.

Thus far he has weathered three operations, a major seizure and pneumonia, and hospital staff members, impressed with Clark's stamina and courage, hope he will now have a chance to stabilize and recover less eventfully. But, despite the optimism, there is still a nagging worry, says one top staff member, about further complications and "whether it is worth it for Dr. Clark."

Clark and his wife, Una Loy, were well aware of the risks when he agree to receive the artificial heart, and both agreed to the surgery to repair it. But, according to one informed source, at one point last week even Clark's strongly supportive wife voiced misgivings about how many medical interventions the future might bring.

"It's a day-by-day situation," said another staff member. In contrast to the initial euphoria after the heart implantation, when hospital doctors suggested that Clark, 61, might go home in a few weeks, no one is willing to say now when, or even if, that will occur.

Although Clark now appears to be doing well and the world is cheering for him, the prospect of a long recovery raises difficult questions:

* Was the device tested prematurely in humans?

* Were unrealistic expectations raised by the extensive media coverage of the operation and the inevitable cautious optimism voiced by some university officials?

* In the long run, it is worthwhile to invest in such research, given the competing health-care concerns and the unanswered questions about the quality of life for a person permanently attached by six-foot hoses to a 375-pound machine?

To such questions, Dr. Willem Kolff, 71, the leader of the university's artificial-organs program, responds that he remains "absolutely confident" about the future of the artificial heart.

But, viewing the artificial-heart experiment from a distance, medical philosopher Arthur Caplan of New York's Hastings Center sees the implant in humans as "premature."

Despite the 11-page consent form warning of the possible risks, "it's very hard to do informed consent. These patients are very desperate and are faced with imminent death," Caplan said.

Caplan said he believes animal experiments should have been continued longer, until the artificial heart was truly portable, and that, in any case, "There should be no hurry to do the research if society is not willing to commit the resources to give it out perform the operations . This is just going to add fuel to rising health-care costs if it's successful."