Nearly two years ago, Seattle dentist Barney B. Clark's first spoken request after the historic implant of a permanent artificial heart was for a glass of water. Today, William J. Schroeder, who received the world's second man-made heart Sunday, asked for a can of beer.

Recalling Schroeder's early morning request with a smile, a tired but relieved Dr. William C. DeVries, the surgeon responsible for both mechanical-heart implants, said his new patient was "doing very, very well," had been taken off the respirator and was "very alert."

But he cautioned in his first public comments on Schroeder's progress that, with such a highly experimental procedure, "you live on the edge of a possible disaster any time."

Such disasters plagued the recovery of pioneer patient Clark until his death at the University of Utah hospital in Salt Lake City on March 23, 1983, after 112 days with the mechanical device beating in his chest. But his spirit lives on at the Humana Heart Institute International where Schroeder is being treated.

"I personally think Schroeder has one person to thank for his new opportunity to live. That person is Barney Clark . . . . His contribution led to a new hope for life for other people," said Dr. Robert Jarvik, the inventor of the mechanical heart used in both operations.

"We learned a lot of things from Dr. Clark," DeVries agreed. "I was most impressed with how smoothly things went this time."

Comparisons between the maiden operation at the University of Utah and the second artificial-heart implant here are made frequently. And officials and researchers with Humana Inc., the profitable hospital chain that convinced DeVries to switch sites, say that they sought to learn the lessons of the Clark case before launching their program.

"With Clark, the question was could it be done? He showed it could be done. Here the question is how should it be done? We took what Utah had done and reviewed and modified it," George Atkins, head of public affairs for Humana, said in an interview.

Observers of both efforts point to several changes, some of them controversial, that have occurred since the Clark implant. These include differences in the selection of patients, in the mechanical heart itself, in patient management, hospital security, in financial resources and in publicity generated by the hospital.

In general, artificial-heart candidates, to be eligible for consideration, must be suffering from a severe form of heart disease and have no other treatment options. But this time, a required waiting period was modified to allow doctors to select patients who had a better chance of survival.

DeVries noted today that Schroeder has "a better prognosis than Dr. Clark. He was healthier going into it than Dr. Clark was . . . . It's a little better test of the heart."

Schroeder is 52; Clark was 61 at the time of his operation in December 1982. Both had seriously impaired blood flow from weakened hearts, but steroid drug therapy and smoking had seriously damaged many of Clark's organs, particularly his lungs. Because Schroeder also had smoked, doctors carefully checked his lung functions before accepting him as a candidate and found no signs of damage.

Clark was near death when he was rushed into surgery. DeVries said today that Schroeder, although in better overall physical condition, was in greater danger before the surgery from a rapidly failing heart than had been portrayed.

"He was not going to live to survive the weekend . . . . Saturday night I was very worried," DeVries said. But rather than rushing into surgery as with Clark, he was able to wait until the scheduled Sunday morning operation.

Participants later said that surgery went far more smoothly than the Clark procedure, "almost routinely," Jarvik said.

Jarvik, who observed both operations, said that Humana "recognized the need for good coordination and preparation of an implant team."

The hospital-chain company, which appears to have spared little expense, has drawn criticism from those who are concerned about the movement of private enterprise into such research. Humana has pledged to fund 100 artificial-heart implants if sufficient scientific progress is made.

Responding to the criticism, DeVries said today that, in contrast to his experience at Utah, "I've been able to set up this program exactly the way I wanted without any questions."

And, he added, "I was able to select the patient without consideration of whether he can pay the bill." In the case of Clark, whose care cost about $250,000, the state-funded Utah university was worried about how to pay the bill and sought contributions.

DeVries, who is donating his services for the implant, said he was "very impressed with the so-called marriage" of his clinical research and Humana's corporate resources.

The more extensive artificial-heart surgical team of seven has allowed DeVries to get some rest and even have dinner with his wife Monday night. "That's a real change from the University of Utah," he said.

Humana's resources also allowed it to send teams of doctors and nurses to Utah to practice on animals and later conduct elaborate rehearsals here to put everything carefully in place.

There even was consultation with Utah's public relations staff to prepare for the media and the impact of publicity.

Warned that Clark's house had been broken into and photo albums stolen after his operation was announced, Humana's public relations chief, Robert Irvine, noted that hospital officials sat down with Shroeder's family and warned them to have someone stay at their house and "put the photo albums and other things away."

Humana's Atkins said that overall, the implant was approached with the "more focused" team approach used by the company in all its efforts.

There also is a corporate commitment to push ahead as quickly as possible, an approach that concerns some outside critics.

After Clark's death, Utah deliberated the ethics of the mechanical-heart implant, and an institutional review board prevented DeVries from moving ahead as quickly as he wanted. Humana already is interviewing other implant candidates, and DeVries, the only surgeon with approval from the Food and Drug Administration to implant the mechanical-heart pump, said today that he hopes within the next year to complete the five additional implants that have been approved.

But for now, the lanky Utahan is sticking close to his patient.

Today, the hospital staff encouraged Schroeder, who is still considered in critical but stable condition, to move his arms and legs in bed. He continues to receive intravenous feeding, ice chips and Popsicles. He has not received liquids by mouth.

What about that beer? At a medical update this afternoon, Humana heart chief Dr. Allan M. Lansing noted that a Milwaukee radio station had sent a case of beer, and said Schroeder may get one within a few days, "whenever he is up to it."

Lansing said the man with the new mechanical heart today expressed feelings of "gratitude and hope. Gratitude that he had another chance and hope . . . . He made it over the first hurdle. Maybe the odds are on his side."