Cool Under Pressure – Using Science to Stave Off Hypothermia

By Bob Reinert for USAG-Natick Public Affairs

Seventeen years after four soldiers died from hypothermia during the final phase of Ranger School, researchers at the U.S. Army Research Institute of Environmental Medicine at Natick Soldier Systems Center continue to study how the human body cools down, in hopes of one day developing medical techniques to help prevent such tragedies.

“You can’t design possible countermeasures — pharmacological treatments, perhaps — until you know mechanisms,” said Capt. David DeGroot, Ph.D., a research physiologist in USARIEM’s Thermal and Mountain Medicine Division, who is leading the study. “You’ve got to understand the basic mechanism before you (say), ‘Okay, now how do I target it?’

“This is going to allow us to get further insight with the actual mechanisms so that we can follow it up with, Okay, what could we possibly do in terms of an intervention to mitigate that rate of core temperature drop?”

Dr. John Castellani, serving as an Army captain with USARIEM at the time, was a member of the team that conducted the institute’s initial study at Camp Rudder on Eglin Air Force Base, Fla., soon after the February 1995 deaths. He still works at the institute as a research physiologist.

Castellani said that the original study led to adjustments to the tables Rangers use to determine what amount of exposure to cold is safe.

 “The swamp portion of training takes place at the very tail end of Ranger School, so soldiers have lost a lot of muscle, fat,” Castellani said. “They’re also, during that time frame, purposefully not being fed, so they may have very little food on board, and they’re also sleep deprived a lot as part of that part of the training.”

“So we studied Ranger students who were finishing up Ranger School. We tested them immediately as they came out of the swamp.”

Castellani followed these studies with the Rangers by trying to understand how physically fatigued soldiers are more susceptible to hypothermia.

“What John found was if you exposed people to cold air after they exercised, they cooled off faster than people who were warmed up passively,” DeGroot said. “So there was something about that prior exercise that led to a faster rate of decline in core temperature, higher skin temperature, higher rate of heat transfer through the skin.

“The follow-up question was always, ‘why? What’s controlling that skin temperature? What’s the mechanism responsible for this abnormal response?’”

DeGroot and his team are studying that mechanism with the help of eight soldiers from the Human Research Volunteer Program at NSSC, who are fitted with microdialysis fibers, muscle temperature probes and skin temperature sensors. They are then put into the 102-degree waters of an immersion tank, followed by a trip to an environmental chamber, where the air temperature is a relatively cool 66 degrees.

“Now that doesn’t sound very cold, (but) all he’s wearing is a pair of shorts and a pair of socks, and he’s at rest,” said DeGroot of one volunteer. “A normal response in the cold is that the blood vessels in the skin are going to constrict, and that’s to limit the rate of heat loss from the core out to the environment. What varies is how we warm them up prior to the cold exposure — exercise versus passive.”

The use of microdialysis fibers, implanted under the skin to gather samples, was in its “infancy” when Castellani did his study, DeGroot said. Things have changed since then.

“(We are) using some different techniques that, frankly, we didn’t have a dozen years ago,” DeGroot said. “We didn’t have the technical capability to do this study.

“This is unique. Off the top of my head, I can think of (only) six other labs in the world that use microdialysis to study the control of skin blood flow.”

With a better understanding of the human body’s response to cold, USARIEM researchers likely will be in a better position to help future soldiers ward off hypothermia.

“Everything in science is incremental,” said DeGroot, “just building off of others.”