Maj. David DeGroot, Ph.D., went looking for the cold facts, and after years of study, he may have them.
Now the field investigation team leader for the Injury Prevention Program, Army Institute of Public Health, U.S. Army Public Health Command in Aberdeen, Md., DeGroot actually began his quest to discover why the human body cools down faster under certain circumstances when he was a research physiologist in the Thermal and Mountain Medicine Division at the U.S. Army Research Institute of Environmental Medicine at Natick Soldier Systems Center.
DeGroot wanted to learn more about why four Soldiers died from hypothermia in 1995, during the final phase of Ranger School, in hopes of developing medical techniques to help prevent such tragedies. He and his team used data collected from eight Soldier volunteers, who were fitted with microdialysis fibers, muscle temperature probes and skin temperature sensors. They were then put into the 102-degree waters of an immersion tank, followed by a trip to an environmental chamber, where the air temperature was a relatively cool 66 degrees.
“The key part of the study was using microdialysis to ‘pharmoco-dissect’ the mechanisms that control skin blood flow,” DeGroot said. “Human skin is unique in that it is under dual neural control -- there is a vasoconstrictor system, as in all mammals, that functions to reduce blood flow through the vasculature. However, humans also have an active vasodilator system in the skin, which serves to increase blood flow, which as a result increases heat transfer to the environment.”
DeGroot built upon t
he 1999 study by USARIEM’s John Castellani, Ph.D. Castellani was part of a team that conducted the institute’s initial study at Camp Rudder on Eglin Air Force Base, Fla., soon after the deaths. Castellani’s work led to adjustments to the immersion and exposure tables Rangers use to determine what amount of exposure to cold is safe.
Castellani found that exposure to cold air after exercise led to a faster rate of decline in core temperature, higher skin temperature, and higher rate of heat transfer through the skin.
“The follow-up question to John Castellani’s 1999 study was, what mechanism was responsible for the increased skin heat flux and accelerated body core temperature decline that he reported when cold exposure was preceded by active heating, (such as) exercise in an immersion pool?” DeGroot said.
To answer that, DeGroot and his team used several pharmacological agents.
“By comparing the control site between the active and passive heating trials, we could demonstrate the effect of prior exercise on skin blood flow,” DeGroot said. “During the passive heating trial, skin blood flow decreased 40 percent from baseline. However, during the active heating trial, it only decreased 28 percent, which is a statistically significant difference.
“These data indicate that prior exercise leads to higher skin blood flow during subsequent cold exposure. So, why?
Botox, which is typically used for cosmetic purposes, was used at one microdialysis site because it also stops vasodilation of skin blood vessels. Skin blood flow at this site decreased more than 50 percent during the cold exposure, bringing it in line with the passive heating trial.
“Based on the available data, we conclude that active vasodilation over-rode a competing vasoconstrictor signal and resulted in higher skin blood flow,” DeGroot said. “Using these data, researchers might be able to develop pharmacological agents to maintain the vasoconstrictor response to cold and thereby reduce the risk of hypothermia.”
With a better understanding of the human body’s response to cold, researchers are a step closer to helping future Soldiers ward off hypothermia.