But scientists have never tried anything like this in humans, and the unconscious patients will not be able to consent to the procedure. Indeed, the medical center has been providing free bracelets to be worn by skittish citizens here who do not want to participate should they somehow wind up in the E.R.

“This is ‘Star Wars’ stuff,” said Dr. Thomas M. Scalea, a trauma specialist at the University of Maryland. “If you told people we would be doing this a few years ago, they’d tell you to stop smoking whatever you’re smoking, because you’ve clearly lost your mind.”

Submerged in a frozen lake or stowed away in the wheel well of a jumbo jet at 38,000 feet, people can survive for hours with little or no oxygen if their bodies are kept cold. In the 1960s, surgeons in Siberia began putting babies in snow banks before heart surgery to improve their chances of survival.

Patients are routinely cooled before surgical procedures that involve stopping the heart. But so-called therapeutic hypothermia has never been tried in patients in which a penetrative wound has already occurred, and until now doctors have never tried to replace a patient’s blood entirely with cold saltwater.

In their trial, funded by the Department of Defense, doctors at the University of Pittsburgh Medical Center will be performing the procedure only on patients who arrive at the E.R. with “catastrophic penetrating trauma” and who have lost so much blood that they have gone into cardiac arrest.