On the eve of becoming the second human to receive an artificial heart, William J. Schroeder remained a "good candidate" for the experimental operation although his health was rapidly deteriorating, doctors said.

At the Humana Heart Institute International, where the operation was to begin at 8 a.m. Sunday morning, medical director Allan M. Lansing told reporters that the 52-year-old Schroeder, of Jasper, Ind., was "a fighter" who had "no question" that this was his last hope.

In an operation expected to last five or more hours, Schroeder's diseased and weakened heart was to be replaced with an improved version of the plastic-and-metal Jarvik-7 device that was implanted in Seattle dentist Barney B. Clark two years ago.

Compared with Clark, who survived 112 days with his artificial heart, Lansing said Schroeder had better prospects for long-term survival. "He can't live much longer without it . . . . Yet, I think he's strong enough to survive it," he said.

Schroeder was reported in good spirits as he and his wife, Margaret, and their six children belatedly celebrated Thanksgiving with a dinner of turkey and pumpkin pie. He spent later hours conserving his strength for the operation, Lansing said.

Schroeder's operation is different in several ways from Clark's and will provide a crucial second trial of the mechanical heart in a human. In Clark's case, the heart worked, but the patient suffered an array of physical and mental problems that led physicians and scientists to question the experiment's value.

Changes since the Clark implant include the way candidates are selected and the design of the artificial heart itself, as well as the location and sponsorship of the experiment. This attempt is being directed by Dr. William C. DeVries, who implanted the device into Clark at the University of Utah medical center in Salt Lake City on the night of Dec. 1, 1982.

DeVries recently moved to Louisville and joined the staff at the Humana Heart Institute at Humana Hospital-Audubon, a for-profit hospital run by Humana Inc. Humana's enticements for DeVries to leave Utah included paying for up to 100 artificial heart implants if sufficient progress were made.

When Clark underwent emergency surgery in Utah he was near death, debilitated by a heart condition that had sapped his energy and threatened his life. Based on that experience, surgeon DeVries hoped to make a second attempt on a healthier patient with a better chance of survival.

A review board at Humana Hospital-Audubon and the Food and Drug Administration, which oversees the development of experimental medical devices, agreed that, as with the Clark case, the only candidates for an artificial heart should be patients for whom no further medical or surgical treatment is available.

Like Clark, Schroeder suffers from the most severe form of a serious heart condition called cardiomyopathy. Schroeder first suffered a heart attack in 1982, stopped work in March 1983 and underwent a coronary bypass operation at Humana, the hospital said. He was diagnosed last month as having the most serious form of progressive heart deterioration, and thus became a possible candidate for the mechanical heart.

Lansing tonight disclosed new details about Schroeder's health, noting that he is a diabetic who has been troubled in recent months with tooth and gall bladder infections. He is recuperating from gall bladder surgery on Nov. 17, a day after he was unanimously chosen by Humana's selection committee to receive the heart implant.

Like Clark, Schroeder was for many years a smoker, but Lansing said that he did not appear to have serious lung problems that might interfere with his recuperation. This was a major factor in Clark's repeated medical crises before his death.

Although the mechanical heart is substantially the same as the one Clark received, the Jarvik-7 device Schroeder is set to receive has undergone a significant improvement. A welded valve that failed about two weeks after Clark's operation has been replaced with a one-piece unit made of titanium, said Dr. Robert Jarvik, its inventor. Like Clark's, Schroeder's two-chambered artificial heart will be powered by a 323-pound air compressor connected by a pair of plastic tubes that pass through the skin of the abdomen.

Clark was tethered to the machine, which was the size of a refrigerator, but Schroeder was expected to have the option of using a new, 12-pound system, about the size of a camera bag, that has been approved for use in this case for up to three hours a day, once the patient's condition has stabilized.

"There definitely is the possibility that the patient could live a year or two," Jarvik told reporters at a news conference last week. "We're substantially better-prepared than we were at the University of Utah."

The commercial implications and the handling of the new implant by a for-profit group will be watched with interest by the research community. DeVries has said he was attracted to Humana because of its financial commitment to perform a number of implants and because of frustration with the University of Utah bureaucracy.

Humana's offer to pay for the initial artificial heart implants, as needed, could cost it from $10 million to $25 million, based on estimated costs of $100,000 to $250,000 per operation.

Humana owns and operates 91 hospitals worldwide and has roots in Kentucky. Through marketing, research on needs of patients and physicians, and an emphasis on rewarding efficiency, the chain grew to become the world's third largest with profits last year of $193 million.

DeVries does not work directly for Humana, but is part of a private practice in cardiovascular surgery at the hospital. According to Humana, he is donating his services for the experiments involving the artificial heart.

Based on the experience with Clark, DeVries and his colleagues have produced an expanded informed consent form that Schroeder signed. The seven-page document specifies the "substantial risks involved" and notes that during Clark's life with the artificial heart, he experienced "kidney and lung problems, a pneumothorax air in the lung cavity valve breakage, seizures, bleeding complications and depressions. He remained hospitalized during the entire 112-day period."