"It used to be easy," says Dr. Helene Emsellem, director of the neurophysiology laboratory at George Washington University Medical Center. "You were either dead or alive. We checked the heart, and if it wasn't beating, you were dead.

"Life isn't so simple anymore."

Neither is death.

With the advent of specialized trauma teams, artificial respirators and intensive care units, medicine can temporarily keep "alive" some patients who have lost all brain function and who, without artificial life support, would have no breath, pulse or circulation.

That, along with growing demand for transplant organs, has forced society to broaden its definition of death to include the irreversible loss of all brain function -- brain death.

"What is it that makes us distinctive as human beings?" says Larry McCullough, senior research scholar at the Kennedy Institute for Ethics at Georgetown University. "It's the intellectual capacity, the life of the mind. If that's the case, then the most important organ is the brain, not the heart or the lungs.

"Biomedically, the heart is just a pump and not a very smart one at that."

The brain, which controls even involuntary functions such as breathing, is the organ most vulnerable to loss of oxygen. It suffers permanent damage if its blood supply is interrupted for more than a few minutes.

Forty-three states -- including Maryland and Virginia -- and the District have recognized brain death through laws or court decisions. The concept is recognized by common law in all states. Brain death laws have the backing of the American Medical Association, the American Bar Association and a presidential commission on ethical problems in medicine.

As the presidential commission said in its 1981 report, "the importance customarily accorded to a person's beating heart in differentiating the living from the dead is challenged when a 'dead' person's heart can beat in the chest of a 'living' person whose own heart has not merely stopped but has been removed from his or her body."

Brain death is not euthanasia, or mercy killing. It is a medical diagnosis, made by doctors in accordance with strict medical requirements, which are refinements of the landmark Harvard Criteria, established in 1968. Legislatures and courts, while adopting the concept of brain death, have left its determination up to the medical profession.

Brain death criteria vary slightly from hospital to hospital, but they always include total lack of movement or breathing, total unawareness and unresponsiveness to stimuli, and total absence of reflexes. To ensure that the brain damage is irreversible, two possible causes must be ruled out: hypothermia, or abnormally low body temperature, and drug overdose.

The pronouncement of death must be made by a doctor who is not a member of the transplant medical team, to prevent a potential conflict of interest.

While the concept of brain death is almost universally accepted by the medical community, it is still disconcerting to some laypeople in a society that invests the heart with such symbolic importance and traditionally has equated death with the ceasing of the heartbeat.

Brain-dead patients who are kept breathing by a mechanical respirator sometimes do not look much different from other patients in the hospital's intensive care unit. Their skin color is usually good, and they may appear to be merely asleep.

"You can see the chest rising and falling," says McCullough. "You can feel the pulse. They're still doing a lot of the things we think of as being alive."

In the latest highly publicized case, 26-year-old Pelle Lindbergh, all-star goalie for the Philadelphia Flyers hockey team, was pronounced dead last week in a New Jersey hospital after suffering massive and irreversible injuries to his brain and spinal cord in a car accident. In keeping with the wishes of his family, Lindbergh was kept "alive" on a respirator until his heart, liver, kidneys, pancreas, eyes and portions of his skin were removed for possible transplantation.

Although Lindbergh was officially pronounced dead last Tuesday, more than 48 hours after he was brought to the emergency room, "clinically speaking, he was dead when he got here," says Dr. Louis Gallo, general surgeon at Kennedy Memorial Hospital in Stratford, N.J. Doctors estimate that his brain went without oxygen at least 15 minutes, before rescuers could pry his body from the wreckage of the car.

"You cannot say someone is brain dead simply on the basis of one clinical examination," Gallo says. "We did three or four over the first 24 hours."

Lindbergh showed no response to any stimulus: pinprick to the skin, light in the eyes, hot or cold water injected into the eardrums. His pupils remained fixed and dilated, his eyes wouldn't move when his head was turned, and he could not breathe without a respirator.

Lindbergh was kept breathing on a respirator until his father could travel from Sweden to see him one last time. By then, his fiance and his mother, who was already in this country, had met with the doctors and agreed to authorize donation of his organs for transplantation.

Doctors emphasize the difference between Lindbergh's case and those involving patients who are hopelessly comatose but retain a fraction of brain function -- such as the late Karen Ann Quinlan. Under present law, patients such as Quinlan cannot be declared brain-dead even though they have no hope of regaining consciousness, because their brain damage is not complete. They pose much more complicated ethical dilemmas.

But in cases like that of Pelle Lindbergh, there is little or no controversy.

"Brain death is a settled issue in almost everybody's mind," says Dr. Russell H. Patterson Jr., chairman of neurosurgery at New York Hospital-Cornell Medical Center in New York and a member of the AMA's Council on Ethical and Judicial Affairs. "Coma is a totally different issue."

Unlike a comatose patient, who may survive indefinitely on a respirator, a brain-dead person cannot sustain a heartbeat for more than a few days, even when an artificial respirator is used.

"No technology in the world will allow a brain-dead person to survive under any conditions for more than a week or two," says Patterson.

Nor does brain death replace the traditional heart-and-lungs concept of death, says Dr. Stanley Cohan, associate professor of neurology at Georgetown University Medical Center. "But we as a society have made the decision that when the brain is no longer functioning and can never regain function -- when the brain is destroyed -- the most essential feature of humanness is lost, and it is therefore acceptable to say that the person is dead."

Perhaps nothing is more abhorrent to the doctors, nurses and grieving relatives who face such agonizing choices than the popular but misleading phrase, "pulling the plug."

"It's such a crude, gross statement," says Cohan. "It misses the sense. It's like talking about the silver chalice as if it's a hockey puck.

"No one pulls a plug. You remove a ventilator from a person's airway and turn off the machine. We do that all the time -- but it's people who are already dead."

Transplantation of human organs -- beginning with the first successful kidney transplant in the late 1950s and now including the liver, pancreas and heart -- gave urgency to the issue of brain death. These organs cannot survive more than a few hours after circulation stops, so the ideal donor is an otherwise healthy brain-dead patient whose heart and lungs are kept going until the organs can be surgically "harvested," or removed.

The decision to allow the harvesting of organs for transplant is usually made by the patient's family. To broach such a subject at a time of overwhelming grief for the family requires exquisite sensitivity on the part of the doctors.

The patient may have carried a donor card, or otherwise expressed a wish to donate organs after death. But even in such cases, doctors ask permission from the relatives if possible.

"The card is supposed to be a legally binding document," says Arthur Caplan, associate director of the Hastings Center in Hastings-on-Hudson, N.Y., "but in practice the family has a kind of veto power."

Most grieving families, once they realize there is no hope for the patient, find some solace in knowing that their relative's organs might offer the gift of life to others, says surgeon Gallo, who was involved in the discussions with Lindbergh's family last week.

In the 20 or 25 cases I've dealt with personally," says Gallo, "none of the families has ever said no."