Trauma patients arriving at an emergency room here after sustaining a gunshot or knife wound may find themselves enrolled in a startling medical experiment. Surgeons will drain their blood and replace it with freezing saltwater. Without heartbeat and brain activity, the patients will be clinically dead. And then the surgeons will try to save their lives. Researchers at the University of Pittsburgh Medical Center have begun a clinical trial that pushes the boundaries of conventional surgery — and, some say, medical ethics. By inducing hypothermia and slowing metabolism in dying patients, doctors hope to buy valuable time in which to mend the victims' wounds.