The surgeon who treated one of two survivors found Thursday on icy Mount Hood said the eight trapped climbers may have been too cold and confused to understand searchers' helicopters and shouts a day earlier.

Confusion is one symptom of hypothermia, which begins when core body temperature drops below about 95 degrees Fahrenheit. Doctors said Giles Lewis Thompson and Brinton Clark, both 15, owed their lives to their good physical condition and the protective bodies of six companions around them.

Thompson was found on top of Clark in the middle of the huddle in a snow cave they dug Monday in a sudden whiteout. Rescuers said the two were disoriented but awake and groaning.

Exposed to prolonged cold, the body shivers to produce heat and burns fat for extra energy from glucose, said Dr. William Keilson, a hypothermia expert at the Maine Medical Center in Portland, Maine. Blood is shunted from the extremities to keep the brain and heart warm.

The victim is very uncomfortable until body temperature reaches about 86 degrees, when shivering stops and the sensation becomes pleasant. "You may lose pain [nerve] fibers," he said. "It may be a near-death experience."

At this stage, victims may become confused and even undress or lie down in the snow, he said. Dr. Duane Bietz, who treated Clark at Emanuel Hospital in Portland, Ore., speculated that when rescuers searched Wednesday near their cave, the climbers already were too confused to respond. It may also explain why three of the climbers, found dead Wednesday, apparently had left the cave.

The climbers were from Oregon Episcopal School in Portland. Keilson suggested that Molly Schula, 17, who hiked out for help Tuesday with the group guide, may have held up better than the others at the outset because women generally have more body fat than men and withstand cold better.

Below 77 degrees, victims may have no pulse or blood pressure, Keilson said, and no discernible brain activity. He said they appear dead but may sometimes be revived.

"Nobody is dead until they're warm and dead," Keilson said.

Thompson and Clark, both 15, were rushed Thursday night from helicopters to operating rooms, where cardiac surgeons connected them to bypass machines to warm their blood and gently thaw their bodies from the inside out.

Dr. William Long, who operated on Clark at Emanuel Hospital, said Clark arrived at the emergency room disoriented but awake, with a body temperature of 72 degrees Fahrenheit, the highest of any of the eight climbers in the cave. Thompson, awake and groaning, was the next warmest at 68 degrees, according to Barbara Hood, assistant administrator at Providence Medical Center.

The other six had temperatures of 59 degrees or lower. Attempts to revive them were unsuccessful.

Bietz, a thoracic surgeon at Providence, said internal warming is preferable for hypothermia victims because "when you warm the outside, all you do is send cold blood to the heart from the surface," which may stimulate deadly disturbances in heart rhythm.

Bietz said Thompson's surgery began 37 minutes after he arrived, breathing six times a minute and his heart beating 40 times a minute. A four-surgeon team opened his chest and inserted intravenous lines into his aorta and right atrium, an upper chamber of the heart, to connect his circulation to the bypass pump.

"You can put in blood at 41, 42 degrees Centigrade [106 to 108 Fahrenheit]. We were able to warm him . . . to 37 degrees [98.6 Fahrenheit] at about 1 degree every five minutes," Bietz said.

Thompson's heart began to fibrillate dangerously -- beat arrhythmically -- but electric shock restored normal rhythm. Thompson was also connected to a kidney dialysis machine to keep his blood potassium level from rising and interfering with the heart function.

After the three-hour operation, Thompson was kept on a respirator in the cardiac care unit, Bietz said. Doctors operated again yesterday morning, opening the skin in several areas on his lower legs to decompress swollen tissues.

"He's stable in an unstable fashion . . . . We're seeing gradual improvement every few hours," Bietz said yesterday afternoon, putting Thompson's chances of survival at "75-25, maybe 80-20."

Last night, Thompson experienced bleeding caused by tissue damage from extreme cold. A hospital spokeswoman said most of the bleeding was in his arms and legs where the incisions had been made earlier. After receiving a coagulant and blood, he was listed in stable condition again.

A spokeswoman at Emanuel Hospital said Clark was on a respirator, with stable vital signs. Long said her chances of survival were better than 50 percent.