George Washington University Hospital has suspended a prominent cardiac surgeon pending the outcome of internal and criminal investigations into the death of a 66-year-old patient who was taken off a life support system and injected with a medication sometimes used to stop the heart, according to sources.

The surgeon, Dr. Benjamin L. Aaron, director of the hospital's chest and cardiovascular surgery, removed the bullet lodged one inch from President Reagan's heart after an assassination attempt in March 1981.

The woman patient underwent an unsuccessful bypass operation on March 17 and died March 20. According to sources, the hospital notified the U.S. attorney's office about the case on March 28 after conducting an internal investigation the previous week.

The woman, identified by a police source as Mary Fisk of Delaware, had been removed from a respirator with the permission of her family, sources said. Aaron injected Fisk with potassium chloride after other doctors had pronounced her dead and her heart was no longer pumping, although it was still emitting electrical pulses, according several sources.

The internal hospital investigation apparently was prompted by a nurse who became disturbed when at least one doctor -- not Aaron -- refused to enter the injection on Fisk's medical chart. It could not be determined yesterday whether the family had been informed in advance about the injection.

Potassium chloride stops the heart from beating when administered in large doses and is often used during open heart surgery. In this case, sources said, Aaron and other doctors believed Fisk was dead once life support systems were suspended.

Aaron's suspension followed a review by a panel of five of the hospital's doctors of the circumstances surrounding Fisk's death, according to a source who has read a five-page summary of the panel's findings. Sources said that the nurse who prepared the syringe of potassium chloride was placed on administrative leave briefly, and it was unclear yesterday if she had returned to work.

A D.C. police source said that Fisk was cremated before a request was made for a criminal investigation. The source said that the death report filed by the D.C. medical examiner is based on information supplied by the hospital, and that the cause of death is listed as a failure of the left ventricle.

Aaron, 53, a nationally renowned surgeon credited along with his colleagues with saving the president's life, could not be reached for comment yesterday.

"Dr. Aaron has fully cooperated with the U.S. attorney's office and we are confident that when the review is complete that the matter will be closed," said Robert Muse, Aaron's lawyer. "Dr. Aaron committed no improper act and the statements of every witness to the event demonstrates this. There is no basis for assertion of wrongdoing."

Hospital officials refused to comment last night, noting only that an "internal review . . . is ongoing."

The U.S. attorney's office is conducting the criminal investigation with assistance from the D.C. police department's homicide unit. Law enforcement sources said yesterday that although their investigation was not completed, they did not expect a criminal indictment to result because interviews with hospital doctors and other doctors familiar with the case show they are all in agreement that Fisk was dead at the time of the injection and that it would be impossible to prove otherwise because the body had been either cremated, as the police source said, or embalmed.

Several of Aaron's colleagues described Aaron's three-day efforts with Fisk as "heroic" and expressed dismay that the hospital would take the unprecedented step of asking the U.S. attorney's office to investigate a doctor's actions in the death of a patient whose family and doctors had agreed to remove her from life support systems.

"It's unconscionable," said Dr. Mitchell Mills, the other cardiac surgeon at the hospital who spoke to everyone who was present in the hospital room at the time of the injection. "It is unequivocal that she was already pronounced dead . . . . This patient had had no mechanical activity of the heart for three days."

"He's one of the pillars of the medical community," said another doctor who knows Aaron well and has worked with him. "I'm disturbed by this. He has nothing but the highest standards, morally, ethically and surgically."

Others said the case highlighted the ethical and legal difficulties doctors and hospitals are facing as sophisticated medical treatment can prolong the life of hopelessly ill or comatose patients.

According to some medical sources, even if Fisk had not been injected with the potassium chloride, all her vital signs would have stopped within three minutes.

"If you have spent three days trying to save a patient, there is an element of tension in the room," said one source. "All he [Aaron] did was try to abbreviate this deathwatch that was an event of no consequence because the woman was dead."

In addition, these sources said a number of doctors and Fisk's family had agreed to end Fisk's life support system and fully expected her death. They said the family had only recently been notified about the investigation.