Doctors at Humana Heart Institute International tonight operated on William J. Schroeder to correct a "major complication" that arose after the implant of a mechanical heart in his chest earlier in the day.

Dr. Allan M. Lansing, head of the heart program, said at a 10:45 p.m. briefing that he believed the team was successful in stopping excessive bleeding. It resulted, he said, from a needle hole in a suture made in the aortic artery when the artificial heart was being sewn into place.

He said that the world's second recipient of a permanent mechanical heart tonight remained in "critical but stable condition," but cautioned that doctors would watch through the night to make sure that more bleeding did not occur.

"I don't think it's a setback. It is a major complication," Lansing said. He acknowledged that the stress of undergoing a second operation and the blood loss itself could affect the recovery of Schroeder, 52, of Jasper, Ind. "Any time you're run over twice by the same auto, you're going to have twice as much damage."

The bleeding began, but did not appear to be a major problem, as the highly experimental artificial heart implant was being completed, about 2:30 p.m., after nearly seven hours of surgery.

Lansing said that it apparently stemmed from scarring resulting from a coronary bypass operation Schroeder underwent in March, 1983. After the implant, doctors thought it could be controlled with blood tranfusions, but in the succeeding hours the blood loss increased.

"The blood loss was significant," said Lansing, who estimated that before and during the second operation Schroeder lost, and had replaced, 50 percent more blood than is in his entire blood supply.

He said the bleeding, which involved a "welling up" of blood behind the aorta, was controlled by applying pressure and treating the hole with thrombin, a blood-clotting agent. It did not require additional sutures.

The second operation began at about 8:30 p.m. and lasted for about two hours.

Lansing said that in the hours between the two operations, Schroeder had been "wide awake" for a brief period before doctors sedated him again. In the late afternoon, about 4:40 p.m., he awoke with a start and tried to sit up in bed, reacting "rather violently," Lansing said, to a breathing tube that had been placed in his throat.

He was not told of the bleeding problem before being put back to sleep, Lansing said.

Schroeder was awake after the initial surgery long enough to open his eyes and squeeze his surgeons' hands, Lansing reported. "He knows he has survived, but he really feels very little," said Lansing at the late-afternoon news conference.

Schroeder was described then as in critical but stable condition in a special room in the hospital's coronary care unit.

The $15,500 Jarvik-7 heart was implanted by Dr. William C. DeVries, the surgeon who placed the first permanent mechanical heart in dentist Barney B. Clark at the University of Utah two years ago.

Clark went into surgery near death and lived for 112 days with the mechanical heart, dying after several organs failed. The mechanical heart beat to the end.

In contrast to Clark's operation, Lansing said, today's surgery, which was carried out by a 17-member surgical team, was "very tedious" but went relatively smoothly.

"I don't think there were any surprises," Lansing said.

Dr. Robert K. Jarvik, inventor of the mechanical heart, said that during the Clark surgery "I felt certain he would die on the operating table. Today I felt the opposite."

Lansing warned that Schroeder faces the possibility of an unknown array of life-threatening complications, including bleeding, infection, seizures, blood clots or failure of the machine that is now sustaining his life. Schroeder is particularly vulnerable to infection because he has diabetes.

The Clark case generated national debate about the future of the artificial heart, its costs and the quality of life of a person who is permanently attached to a machine.

Since then, inventor Jarvik and his company, Symbion Inc., which manufactures the artificial heart, has acquired rights to a portable machine that will allow patients to move around for several hours a day, escaping the confines of the 323-pound machine that is the chief source of power for the mechanical heart.

Humana Inc., a profitable 91-hospital chain, recently opened a new chapter in the artificial-heart debate, the movement of private industry into testing of the mechanical device.

DeVries, lured to Louisville in July by Humana Inc.'s promise to finance 100 artificial heart implants, said a week ago his "realistic hope" was that a second patient would be able to leave the hospital and even swing a golf club.

At a news conference Nov. 19, Jarvik said, "there definitely is the possibility that the patient could live a year or two." The artificial heart has a life expectancy of three to five years, he said.

Humana officials say that with government funds for research drying up, private sponsorship of research is the wave of the future. Each mechanical-heart procedure is estimated to cost $100,000 to $250,000. Schroeder was selected as the first Humana patient after the hospital received approval from a local review board and the Food and Drug Administration.

Lansing said the timing of the operation was advanced after Schroeder's serious heart condition worsened. He described the surgery as an "urgent procedure," but not an emergency operation. Although very sick and in danger of dying without the implant, Schroeder was considered to be in better shape at the outset than Clark had been. Clark was near death when the device was implanted in December 1982. Like Clark, Schroeder suffers from the worst form of cardiomyopathy, a chronic disease that weakens the heart muscle and restricts the flow of blood to the body. He could barely walk 20 feet without stopping to rest, and suffered pain and shortness of breath.

Lansing said that before the operation a test showed that the flow of blood from Schroeder's heart was about one-third that of a healthy man.

An estimated 10,000 Americans die each year from cardiomyopathy. A 1982 congressional Office of Technology Assessment study estimated that over 30,000 Americans potentially could be treated with artificial hearts or heart-assist devices.

DeVries and Jarvik say a heart transplant is better than an artificial heart at this stage of development, but Lansing said Schroeder's age and the fact that he has mild diabetes ruled out a transplant because anti-rejection drugs aggravate diabetes. He also has mild kidney failure. The mechanical heart was the only option to imminent death, doctors said.

Schroeder, a heavy smoker for 30 years, was wheeled into the operating room at 7:05 a.m. EST.

DeVries, the only surgeon approved by the FDA to perform the implantation surgery, chose music by Mendelssohn, Vivaldi and the jazz saxophonist Grover Washington as well as baroque lute music for the operation.

"DeVries likes to have music in the operating room," Humana spokesman George Atkins said. "It breaks the tension of the procedure."

DeVries was joined in the second-floor operating room this morning by five doctors, 11 nurses and two photographers. Watching through a window were Jarvik and Dr. Donald Olsen, a pioneer in artificial-heart surgery in animals.

After preparations for surgery, including anesthesia, two lines were implanted in the patient's heart and connected to a heart-lung bypass machine that would keep his vital functions going during the operation.

At about 10 a.m., the lower chambers, or ventricles, of Schroeder's ailing natural heart were removed. The two upper chambers, or atria, were left in place and attached to Dacron cuffs that will help hold the new heart in place.

Similar cuffs were sewn to the aortic and pulmonary arteries that pump blood to the body and lungs. Then the two lower chambers of the mechanical heart were snapped into place.

Drive-line tubes were brought through the left upper abdomen and attached to the external compressor that powers the heart. The present model is slightly smaller than the console that powered Clark's heart.

By 1 p.m. Schroeder's mechanical heart was pumping on its own.

By 2:30 p.m., his chest had been sewn up, and he was taken to the intensive coronary care unit.

Lansing said tonight that it was too early to issue a prognosis, but "we are very satisfied with his course right now.

"We'll be happier at this time tomorrow. But as of this moment, I would say he is just as well off as we could possibly hope he would be. The heart has functioned beautifully. The patient has responded well. His kidneys are functioning. His lung function is quite satisfactiory. The heart function is excellent. And he has awakened . . . . "

Throughout today's surgery, Schroeder's wife, Margaret, and their six children, aged 19 to 31 years, waited with a Roman Catholic priest in a sixth-floor room.

Concerned that the Clark family had sometimes felt isolated and learned from news media what was happening, Humana provided the Schroeder family with frequent updates, Atkins said. CAPTION: Picture 1, William Schroeder with his wife, Margaret, on his way to operating room. Picture 2, Members of the surgical team with artificial heart, shown in foreground. Photos Copyright (c) William Strode; Picture 3, Dr. DrVries is assisted into his surgical gown inside the operating room at the Humana Heart Institute in Louisville. API; Picture 4, Dr. William DeVries holds an artificial heart against chest to show placement. AP