Thermal and Mountain Medicine Division, U.S. Army Research Institute of Environmental Medicine Natick United States
Cold water immersion presents one of the most challenging environmental conditions, due to the risk of hypothermia. The environmental stress becomes greater with colder water temperature, greater immersion depth, and longer duration of exposure. Understanding these influences improves mission planning and risk mitigation. The Army Technical Bulletin Medical 508 TB MED 508, Prevention and Management of Cold-Weather Injuries, provides guidelines for risk management. These guidelines are used by the Army Ranger School during the 6th Army Ranger Training Battalion 6RTB Florida or swamp Phase, where waterborne movements are a key component of the environment under which leadership skills are evaluated. In winter months when air and water temperatures fall, the waterborne movements are modified to reduce risk while achieving mission objectives. The table in TB MED 508 that recommends immersion duration limits based on water temperature and depth was developed using a mathematical model. The model was developed from principles of human thermoregulation and biophysics, and validated against laboratory data. During Ranger School, chronic exertional fatigue, sleep loss, and negative energy balance result in blunted thermoregulatory responses to cold, including delayed vasoconstriction and shivering, and greater heat loss due to reduced subcutaneous body fat and decreased tissue insulation. This increases the risk of hypothermia relative to a typical research study population. To develop the immersion table for Ranger School students, a lower body fat composition was used in describing the individual 12 percent, compared to 15 percent, and a higher target core temperature Tc of 35.5 degrees C was used, compared to a typical limit of 35.0 degrees C. The model was validated against Ranger School students undergoing a cold air exposure however, it was never validated against students during cold-water immersion.