University of Utah surgeons began emergency surgery tonight to implant an artificial heart, the first approved for experimental use by the Food and Drug Administration, in a dying Seattle dentist.

The surgery had been scheduled Thursday morning, but surgeons decided to proceed more quickly when the patient's condition took a critical turn, medical center officials said.

In a late-evening briefing, university medical center spokesman John Dwan said the operation on the 61-year-old patient was expected to last as long as four hours or more. Doctors had decided that they "really could not wait any longer," Dwan said.

Officials said surgery began about 11:30 p.m. (1:30 a.m. EST).

Dr. Chase Peterson, university vice president for health sciences, said the decision to advance surgery was made because the patient's failing heart began to beat abnormally, a condition called cardiac arrhythmia. "The picture was rather ominous," he said, for further deterioration and possible sudden death.

Dwan said that, after the preparations for surgery had begun, the patient's heart apparently stopped momentarily. Dwan also reemphasized an earlier decision not to disclose the patient's identity until after the operation.

The operation called for implanting a fist-sized, man-made pump, attached to a compressed-air power system that must be wheeled around on a large cart plugged into a wall outlet. It is the first human test of the plastic and metal machine painstakingly tested in animals over the last 15 years.

The experimental operation is a "moment of great human hope" that puts us "on the threshold of something as exciting and thrilling as has ever been accomplished in medicine," Peterson said.

In a news briefing earlier today, hospital staff members and developers of the heart were cautious about prospects for success.

Dr. Willem Kolff, who heads the artificial organs division here, emphasized that "our expectations should not be too high," given the newness of the technology.

He recalled that the first patients to receive the artificial kidney he developed in the 1930s died but that the device is now used extensively around the world.

He said researchers "would learn from it, whether or not it does go well." And he emphasized that it offers a "last chance" for the chosen recipient, a critically ill man who "has no other alternatives . . . . How large that chance is depends to a great extent on luck."

Dwan said the identity of the patient, who has volunteered for the surgery, would be kept secret until after the operation to "protect his privacy."

The man was required to sign an 11-page consent document twice, with a 24-hour "cooling-off" period between signatures. Doctors said he was not a candidate for a human heart transplant because of his age.

The recipient was selected after a worldwide search by Dr. William DeVries, who heads the operating team. He is the only physician in the country approved by the FDA to implant the so-called "Jarvik-7" heart, named for its developer Dr. Robert Jarvik.

Jarvik said today that success in this or subsequent operations would pave the way for "broadspread clinical applications" that could extend the lives of tens of thousands of heart patients.

Heart disease, the nation's No. 1 killer, claims about 650,000 lives annually.

Advocates predict that the artificial heart eventually could satisfy a demand that cannot be met now by transplants of human hearts, which frequently trigger a patient's immune system and cause the body to reject the foreign object maintaining life.

But, mindful of the euphoria that accompanied the original heart transplants, Jarvik added that "there is a lot of work left to be done" before the artificial heart could achieve routine usage. It must prove its reliability and advance to more portable form to give patients greater freedom, he said.

The heart that was to be implanted will completely replace the recipient's heart. But it is a "cumbersome" device, Jarvik said.

It must be attached by tubes to a large, external power system that would restrict the patient to the "home environment."

It has a temporary power source that could allow outside trips for as long as three hours, he said. But a highly portable, battery-powered system is already being developed.

Dwan said that developers of the Jarvik heart had implanted it in more than 100 animals in recent years and that six calves and sheep with the device are alive.

The animal that lived longest after such an implant was a calf that survived nearly 9 months with an earlier version of the heart before developing an infection and dying.

Infection is likely to be a serious risk for the human recipient, should he survive the operation.

The patient was selected only after meeting rigorous FDA criteria specifying that the operation should be conducted only as a last resort for volunteers dying of inoperable degenerative disease of the heart muscle or who would die during heart surgery.

The University of Utah team has proceeded cautiously on its project to avoid the controversy that surrounded two previous attempts by Texas heart surgeon Denton Cooley to implant a different artificial heart temporarily until a human heart was available for transplant. Both patients died after receiving the transplants.

The Utah attempt is different since the artificial heart being used here is intended as a permanent heart substitute and a heart donor is not being sought.