Thermal effects of dorsal head immersion in cold water on nonshivering humans.

Personal floatation devices maintain either a semirecumbent flotation posture with the head and upper chest out of the water or a horizontal flotation posture with the dorsal head and whole body immersed. The contribution of dorsal head and upper chest immersion to core cooling in cold water was isolated when the confounding effect of shivering heat production was inhibited with meperidine (Demerol, 2.5 mg/kg). Six male volunteers were immersed four times for up to 60 min, or until esophageal temperature = 34 degrees C. An insulated hoodless dry suit or two different personal floatation devices were used to create four conditions: 1) body insulated, head out; 2) body insulated, dorsal head immersed; 3) body exposed, head (and upper chest) out; and 4) body exposed, dorsal head (and upper chest) immersed. When the body was insulated, dorsal head immersion did not affect core cooling rate (1.1 degrees C/h) compared with head-out conditions (0.7 degrees C/h). When the body was exposed, however, the rate of core cooling increased by 40% from 3.6 degrees C/h with the head out to 5.0 degrees C/h with the dorsal head and upper chest immersed (P < 0.01). Heat loss from the dorsal head and upper chest was approximately proportional to the extra surface area that was immersed (approximately 10%). The exaggerated core cooling during dorsal head immersion (40% increase) may result from the extra heat loss affecting a smaller thermal core due to intense thermal stimulation of the body and head and resultant peripheral vasoconstriction. Dorsal head and upper chest immersion in cold water increases the rate of core cooling and decreases potential survival time.

Life jacket design affects dorsal head and chest exposure, core cooling, and cognition in 10 degrees C water.

PURPOSE: Personal floatation devices (PFDs) differ in whether they maintain the head out of the water or allow the dorsum of the head to be immersed. Partial head submersion may hasten systemic cooling, incapacitation, and death in cold water. METHODS: Six healthy male volunteers (mean age = 26.8 yr; height = 184 cm; weight = 81 kg; body fat = 20%) were immersed in 10 degrees C water for 65 min, or until core temperature = 34 degrees C, under three conditions: PFD#1 maintained the head and upper chest out of the water; PFD#2 allowed the dorsal head and whole body to be immersed; and an insulated drysuit (control) allowed the dorsal head to be immersed. Mental performance tests included: logic reasoning test; Stroop word-color test; digit symbol coding; backward digit span; and paced auditory serial addition test (PASAT). RESULTS: Core cooling was significantly faster for PFD#2 (2.8 +/- 1.6 degrees C x h(-1)) than for PFD#1 (1.5 +/- 0.7 degrees C x h(-1)) or for the drysuit (0.4 +/- 0.2 degrees C x h(-1)). Although no statistically significant effects on cognitive performance were noted for the individual PFDs and drysuit, when analyzed as a group, four of the tests of cognitive performance (Stroop word-color, digit symbol coding, backward digit span, and PASAT) showed significant correlations between decreasing core temperature to 34 degrees C and diminished cognitive performance. CONCLUSIONS: Performance in more complicated mental tasks was adversely affected as core temperature decreased to 34 degrees C. The PFD that kept the head and upper chest out of the water preserved body heat and mental performance better than the PFD that produced horizontal flotation.