Denton Cooley is a gambler, a tall and handsome Texan who slices open human hearts to the sound of country music. He is a skilled surgeon with among the best hands in American medicine, a man of enormous ego who built the Texas Heart Institute in Houston through the sheer force of personality. He moves in the world of high-stakes medicine, and sometimes that gets him in trouble.

Now is one of those times.

On July 23, Cooley performed a triple bypass operation on a 36-year-old man name Willebrordus A. Meuffels. A short time later, Meuffels suffered a massive heart attack. Attendants rushed him back to the operating room, massaging his heart as they rolled him along the halls of Houston's St. Luke's Episcopal Hospital.

Cooley, believing his patient faced certain death, implanted an artificial heart. For 54 hours, the plastic heart, powered by a large electrical console nearby, kept pumping until Cooley could find a human heart donor that would allow him to perform a heart transplant on Meuffels. That operation occurred on July 26.

At 8 a.m. on Aug. 2, Meuffels died, his body overwhelmed by a massive bacterial invation. Almost immediately thereafter came the questions, including some from the federal government.

No one challenges Cooley's medical judgment, made under fire in the operating room, that his patient would have died without the artificial heart. "If he had not implanted the device, they'd have pulled up the sheet and walked away," said Gregg C. Waddill, associate administrator and councel at St. Luke's.

But there are many questions about the artificial heart that was used and the way Cooley had prepared the medical world for its use. How had the heart been tested? Should Cooley have cleared its use with the Food and Drug Administration in Washington? Was the operation worth the risk? Will it affect future development and use of artificial hearts?

The FDA, suggesting that Cooley violated federal regulations, has informed him that he must receive its approval before using another artificial heart, and the agency wants a full report within 30 days on the details of the operation last month, although FDA officials made clear they are not seeking a confrontation with the prestigious Cooley.

Hospital officials say that Cooley violated no federal regulations and that the details of the heart's testing have been documented to the FDA.

The FDA disputes that claim. "We don't have any information," said Wayne Pines, associate commissioner for public affairs. "To the best of my knowledge, the hospital has never submitted anything to us in writing on that particular artificial heart."

The publicity surrounding the incident, and the fact that the patient died so soon after the operation, are reasons why Cooley's latest gamble may prove costly.

"It's the sort of thing you can get away with once," said Dr. Phillip Oyer of Stanford University Medical Center. "Now he'll have to go through all the hoops."

The incident in Texas is the latest chapter in the checkered history of heart transplant operations, which began in glory and great anticipation in 1967 when Christiaan Barnard performed the first on a South African grocer.

Almost immediately, the rest of the medical world embraced the transplant as the solution to many serious heart ailments, and almost as quickly soured on the operation because so many patients died so soon of post-operative problems, the most troublesome being infection and rejection of the new heart.

At the Texas Medical Center in Houston, Cooley and his former mentor and now rival, Dr. Michael DeBakey, were among the surgeons who gave up on the procedure. DeBakey performed 12 operations over an 18-month period, then stopped in early 1970. "He just discontinued the program because he said the results did not justify the effort that went into them," said Gayle McNutt, a spokesman for the Baylor College of Medicine in Houston.

The heart transplant program at St. Luke's Hospital began on May 3, 1968, and ended in September, 1969. "Most of the patients died within a few weeks or a few months," said Hazel Haby of St. Luke's. Until last month, Cooley had performed only one transplant since 1969, on Valentine's Day in 1978. His patient lived only two weeks after the operation.

In 1977, heart transplants were in such disfavor that the American College of Surgeons was forced to discontinue its register of operations. At that time, 346 operations had been performed worldwide on 338 patients. Only 77 were still alive.

Throughout those years, however, a team of surgeons and researchers at Stanford University, under the direction of Dr. Norman E. Shumway, continued to make steady progress, especially in solving the post-operative complications of transplants. Shumway, who is contrast to Cooley and DeBakey maintained a low public profile, gradually increased the chances of long-term survival and rebuilt the stature of transplants in the eyes of the public and of many other surgeons.

"His experiments preceded by 10 years the first historic transplant," said Spyros Andreopoulos, a Stanford spokesman. "By the time human transplants began, Shumway had refined surgical techniques, had studied the performance of the heart and had methods and techniques for treating infection. This accounts for fact that he has succeeded when others failed."

Shumway's success in fighting infection is especially significant. It resulted both from close monitoring of his patients and from the development of techniques for early sampling of transplanted tissues to see if infection is becoming a problem.

Another thing that has made Shumway successful is his selectivity. About 400 patients are referred to Stanford each year, and their records are reviewed by a team of doctors and researchers for certain criteria. The criteria include being under 55, being in good health except for the heart, and having a form of heart disease fro which there is no treatment other than transplants and which is likely to lead to death within six months without a transplant.

Of the 400 cases referred, about 60 patients are invited to Stanford for further evaluation, and of those about 40 are selected. The medical center performs 25 to 30 heart transplants a year. The other patients usually died before a donor can be found.

Shumway and his team have performed 281 transplants since 1968, with survival rates of about 65 percent after one year, 40 percent after five. William Van Buuren of Mill Valley, Calif., has lived 11 years.

"Improved results and refined techniques have kindled a resurgence of interest in cardiac transplantation and six to 10 more cardiac centers in the United States are endeavoring to utilize this form of therapy," wrote Dr. Richard R. Lower and two other physicians in a journal of the American Heart Association. Lower, a former colleague of Shumway's, heads the transplant program at the Medical College of Virginia.

Artificial hearts overcome a problem that has plagued transplant programs, the lack of donors. Such hearts have been uncer development for years, but to date they have not been cleared for use, which is why Cooley's operation last month raised so many eyebrows.

The July operation was Cooley's second attempt to use an artificial heart. In 1969, he implanted one in Haskell Karp and then went on television with Karp's wife to appeal for a donor of a human heart. The artificial heart worked for 65 hours before the transplant operation, but Karp died 36 hours after the transplant. Cooley was sued for malpractice, and the episode led to a split between Cooley and his mentor DeBakey, setting back the development of artificial hearts.

Until Cooley surprised the medical world with his latest operation, most attention was focused on the University of Utah, where a team has developed an artificial heart, tested it extensively on calves and applied to the FDA for permission to use it. The FDA has asked for more information.

The Utah team intends to use it not as a stopgap but as a permanent replacement for a patient's damaged heart. Right now the artifical heart requires a large power source, but within a few years researchers at Utah hope to develop a portable power source, and eventually the power source will be implanted in the body along with the artificial heart.

Whether Cooley's use of the artificial heart was a success or failure is debated within the medical profession. Cooley, who would not be interviewed for this article, has said the plastic heart performed perfectly. Other physicians, asking not to be quoted, say the success of an operation like that is determined by whether the patient lives. In any case, the operation has put Denton Cooley back in the public eye, an arena he is used to.