A "very, very sick" Barney B. Clark was rushed into emergency surgery yesterday after a fracture developed in a valve in his artificial heart and half of the mechanical device had to be replaced, University of Utah officials said.

The world's first recipient of a permanent artificial heart survived the more than three-hour operation in extremely critical condition after suffering his most servere setback since the plastic and aluminum device was implanted Dec. 2.

But two hours after the surgery ended, spokesman John Dwan announced that Clark was back in his room, "semiconscious," and responding to doctors' questions.

Still, Dr. Chase Peterson, university vice president for health sciences, described Clark at a late afternoon briefing as a "very, very sick man who has gone through multiple complications." In addition to his heart malfunction, he has developed mild pneumonia for the first time.

Asked about the chances of enduring three operations after almost dying on the operating table before the heart could be implanted, Peterson responded that Clark would face "great difficulty."

"Clearly he has gone through another stress.... He has been listed as critical and is obviously more critical now," Peterson said.

Clark's temperature began to rise late Monday, and doctors discovered that the 61-year-old retired dentist had contracted a "mild pneumonia" in his left lower lung. "He is being given antibiotics and there is no reason to think it can't be attended to," said Peterson.

He noted that Clark has suffered previous complications, including air leaks in the lungs, kidney failure, and a major seizure, and "from each of these he's had a correction."

But yesterday morning, five hours into the 13th day since the heart implant, the Utah medical staff noticed a sudden drop in Clark's blood pressure. With Clark was Dr. Robert Jarvik, inventor of the artificial heart, who happened to be paying a visit, as well as nurse Cheryl Perkins.

Clark's blood pressure plunged from the normal range of 140 over 80 to 80 over 40. The cardiac output of blood from the heart also dropped to a range of as low as 1.8 liters per minute, from the normal range of 7, Peterson reported. Both his blood pressure and his blood flow fell to levels that were about the same as Clark experienced before the historic heart implant was attempted.

The staff checked the 375-pound external power system attached to the artificial heart, but concluded that the heart itself was malfunctioning, Peterson said.

Clark has been only periodically conscious since his seizure last Tuesday, but the blood pressure drop apparently "startled his body and he responded by being more alert," said Peterson. Doctors were able to discuss the need for "exploratory surger" with Clark and his wife, Una Loy, before going ahead.

After blowing a goodbye kiss to his wife, Clark was wheeled into the operating room at 10:45 a.m. Salt Lake City time [12:45 p.m. Washington time].Surgery began about 11:30 a.m., spokesmen said. After receiving general anesthesia Clark was put on the heart-lung bypass machine he had used for several hours during the initial implant operation.

Peterson said Clark remained on the machine for only 46 minutes and it took less than half an hour to replace the faulty left pump or "ventricle." He said that the problem was a break in the "mitral valve," which connects the upper and lower chambers of the left side of the heart. Blood, instead of being pushed out of the lower part of the heart into the arteries of the body, regurgitated back into the left upper heart chamber or atrium.

"It was basically a fracture -- a stress fracture," said heart designer Jarvik, "and why it occurred I cannot tell you." He said that similar problems have occurred in experimental animals.