The trial of a 24-year-old Baltimore nurse, charged with killing an unconscious hospital patient by disconnecting his respirator, opened yesterday with defense attorneys agruing that the patient was effectively dead hours before his life-supporting machine was unhooked.

The Baltimore criminal court jury will have to determine, defense attorneys said, exactly when death occurs to decide the guilt or innocence of nurse Mary Rose Robaczynski, accused of killing cabdriver Harry Gessner. By arguing that the trial's outcome hinges on the complex question of when death occurs, defense attorneys made the Robaczynski trial the latest forum for the nationwide debate on the fluctuating legal and medical definitions of death.

But, while Robaczynski's attorneys focused on that issue yesterday, the prosecutors' opening statement were devoted to a simple recital of what they say occurred at Maryland fGeneral Hospital in the early morning hours of March 8, 1978.

On that morning, the 48-year-old Gessner, who had suffered a "respiratory arrest" the previous day, was lying unconcious in the hospital's special care unit, where Robaczynski and nurse Anja Hammaja were on duty, Assistant State's Attorney Howard Gersh todl the jurors yesterday.

On the stand later yesterday, Hammaja testified that between 5 and 5:30 a.m. on March 8, in the hallway outside Gessner's room she ran into Robaczynski, who said: "Oh good, he's going now. It's a nice time to go.'"

Rushing into Gessner's room, Hammaja found the tubing that had hooked him to the respirator disconnected, she testified. She also testified she heard no sound from the respirator alarm system that should have set off a piercing noise because of the disconnection.

Hammaja said she ran into the hallway and called Robaczynski's name. At that, according to Hammaja's testimany, Robaczynski tried to quiet her, saying "Shhh..."

Hammaja testified that she told Gessner's doctor that morning about her suspicions that Robaczynski had disconnected the respirator.On March 17 she and another nurse made similar allegations to the nursing supervisor, Hammaja said.

When Robaczynski's supervisor confronted her later that day with the allegation that she had disconnected Gessner's respirator and the machines of other unconscious patients, Robaczynski allegedly acknowledged that she had.

"I only did it to the GORKS," Robaczynski told the supervisor, according to Gersh's opening statement.

"GORKs" according to nurse Hammaja's testimony, was nurse's slang for "God Only Really Knows," and referred to patients who were continually unconscious and had virtually no hope of recovery.

Robaczynski was asked to resign from the hospital on March 17. After a 5-month investigation by the Baltimore city prosecutors, she was indicted on charges of murdering Gessner and three other patients by unhooking their respirators. Separate trials are scheduled in each murder case.

In the defense's opening statement yesterday, attorney Joseph F. Murphy Jr. told the jurors: "You cannot commit the murder of someone who is in fact dead. If Harry Gessner was braindead when the respirator was disconnected, your nverdict must be not guilty."

Murphy argued that Gessner, after his respiratory arrest on March 7, was not immediately treated fand "was not successfully resuscitated" Gessner already was "brain dead" when brought to the special care unit and hooked to the respirator, Murphy said.

"Now you will all be asked to determine," Murphy told the six men and six women on the jury, "when the brain dies."

The question of what constitutes brain death has fcaused debate among doctors fand lawyers for years.

The issue of fbrain death has fbeen raised in previous cour controversies, although in a different context.

In the Karen Ann Quinlan case, the New Jrsey Supreme Court ruled in 1976 that it was legal for doctors to remove Quinlan's life support systems -- although she had not suffered total brain death -- because there was little chance of her ever regaining consciousness.Quinlan, now free of any life-support systems, remains in a coma.

In the Washington, D.C. beating death of 85-year-old Gladys Werlich, one defendant has appealed his murder conviction, arguing that a doctor who disconnected Mrs. Werilch's respirator interrupted the chain of circumstances that could have linked him to her death. The defense contends that she had not suffered brain death.

Under at 1972 Marland law, death may be legally declared when there is an absence of "spontaneous brain function" and no hope of a person's breathing or his heart being able to operate on their own, according to University of Maryland law professor Kenneth Abraham. But the law leaves to doctors using "ordinary standards of medical practice" the determination of preciously what constitiutes the absence of brain function.

But the law heaves to doctors fthe determination of precisely what constitutes the absence of brain function.

The most widely accepted fdefinition of brain death was set out by the Harvard Medical School committee in 1968. The criteria it proposed were that the patient be unresponsive to even the most painful stimuli, that there be no movement or breathing, no reflexes and no brain waves. The brain waves must be lacking for at least 24 hours before the patient will be declared dead.