Dr. Claude Lenfant was flying home to Washington yesterday, reading a newspaper story about William J. Schroeder, the artifical-heart recipient whose remarkable progress has surprised even his doctors. Reading over his shoulder was a man with heart disease.

A week after the Schroeder implant and two years to the day after Barney B. Clark was wheeled into the operating room to receive the world's first artifical heart, Lenfant's seatmate was excited.

"He thought he had another 20 years to live. He viewed himself as a candidate and said, "I think it's terrific. Maybe I could have one of those,"" said Lenfant, head of the National Heart, Lung and Blood Institute.

Lenfant said he told the man: "Please keep in mind we have only two cases. The first died after 112 miserable days. The second happened just one week ago. . . . It's an experiment. Nobody can conclude at this time that it will be something of value to the American public, even people with severe heart disease."

Although there was initial elation that the surgery two years ago at the University of Utah hospital in Salt Lake City had saved Clark's life, the Seattle dentist struggled through pain, mental confusion and numerous complications before his death on March 23, 1983.

Clark consented to the highly experimental operation, hoping that it might help him, but well aware that he was donating his living body to test a chancy technology.

The experiment with Schroeder, 52, who received an artifical heart Nov. 25, is just beginning. A week ago he was "fading away" from a failing heart. Yesterday, he ate breakfast with his family and listened to country music.

His temperature was reported to be normal, with no signs of infection, and he continued to make "slow but steady progress," said Robert Irvine, spokesman for Humana Hospital-Audubon in Louisville.

"If all goes well, he will remain on the [portable popwer] unit up to the maximum limit" of three hours, while resting in bed, said Irvine. "His condition is very stable and he is stronger today than yesterday."

Despite the focus on Schroeder's current condition, his future is uncertain.

"There's a tendency to forget that he's not a patient getting known therapy but a subject who is receiving a new and little-known and very innovative device," said Arthur Caplan, a medical ethics specialist at New York's Hastings Center.

"I apologize for being enthusiastic. I want you to know how everybody here feels," said Humana heart chief Dr. Allan M. Lansing in a news briefing after Schroeder got out of bed for the first time. But, he added, "I fully expect that sometime in the future, whether it is next week, next month, or next year, that we will have some bad things to say" about Schroeder's progress.

Regardless of this patient's fate, the success of the artificial heart as an effective therapy for serious heart disease will not be known until it has been tested on more patients. But the debate on whether it will be desirable or cost-effective on a larger scale already is underway.

Proponents and critics agree that there has been progress since the Clark case. "This fellow is responding much better," said Lenfant, in part because of improvements in patient selection. Although experimental devices must be tested on individuals who are seriously ill and have no other options, Clark was sicker than Schroeder.

Also, he said, "the device is better," largely because of the option of using a portable power system that will allow Schroeder to move around more freely for up to three hours a day. Clark was thethered by two lines to a 370-pound drive system.

Humana already is screening other applicants, Dr. William C. DeVries, the surgeon who left Utah last summer, frustrated with bureaucracy and lack of money there, has government permission to do five more operations, probably within a year.

A review of Humana's first six patients must be made before seeking further Food and Drug Administration permission to proceed. Also, at some point, said Lansing, "we'll have to be prolonging life one to three years at least. If everybody dies in the first year, no matter how happy, we don't have the right apparatus."

Dr. Robert Jarvik, inventor of the mechanical heart, said that medical centers in Minnesota, Texas, Florida, as well as the Utah group, are training to use his equipment.

The cost of the Clark experiment was estimated to be $250,000. Humana, a profitable company that believes it can make the procedure more efficient, estimates that the initial cases here may cost between $100,000 and $150,000.

But Jarvik said he believes that the cost could come down "quite substantially" in the next few years, perhaps as low as $50,000 for the entire procedure, with roughly half for the equipment and half for the hospital care.

Human-heart transplants have remained largely in the $80,000 to $100,000 range, although costs are coming down.

Jarvik said he hopes to simplify his mechanical heart unit further by powering it with a battery pack that could be worn around the waist.

He estimated the potential market to be 10,000 to 50,000 heart patients each year. The Office of Technology Assessment estimates in a 1982 study that more than 30,000 Americans under age 65 could be candidates yearly.

Critics question both the Jarvik approach and the costs, worrying that it will be either a life-saving device for the rich or a drain on the public pocketbook.

Lenfant has appointed an advisory panel to review artifical heart research. Much of the $200 million the government has spent on artificial-heart research has focused on alternative approaches, particularly electrically powered, left-ventricular-assist devices that would help pump blood out of the left side of the heart. The Jarvik-7 heart replaces both lower heart chambers.

Caplan called the current Jarvik-7 heart the "Model T" of the field, but said it is never too soon to worry about who will pay for it.

For 50,000 artifical heart implants annually, he said the cost would be $6 billion annually at the current estimates cost of $125,000 each. Even at Jarvik's reduced rate of $50,000 each, the cost would come to $2.5 billion annually. Kidney dialysis currently costs taxpayers roughly $2 billion each year.

Caplan said the "technological imperative" will advance the artifical heart regardless of cost, particularly with the backing of private industry. Humana has pledged to pay for the first 100 implants at its Louisville hospital, assuming that there will be sufficient progress.

Replied Lansing, "You can't stop progress."

Humana chairman David Jones readily acknowledges that his company stands to "gain in stature" should the Schroeder operation succeed. But he said that given government-fund cuts in medical research, it is natural for the private sector to begin picking up the tab.