Significance of the Hypothermia Level for Neurophysiological Reactions of Human Organism to Experimental Whole-Body Cooling

Effects of hypothermia on the character of changes in central hemodynamics, heart rate variability (HRV) and EEG were studied during an experimental whole-body cooling of 30 healthy male subjects at the age of 18–20 years. The subjects were divided into 2 equal groups by the median body temperature in the total sample (34.1 °С) during cooling. The experiment consisted of 3 steps: rest at +20 °С, 10-min exposure to -20 °С in a hypothermal chamber and rewarming at +20 °С. The parameters registered at each step included EEG, blood pressure (BP), heart rate (HR) and temperature in the external auditory meatus. In the cooling chamber, body temperature lowered reliably by 1.4–3.1 °С on the average. In both groups hypothermia reduced the index of regulatory systems strain, increased total HRV power and BP, and decreased HR. During rewarming all these parameters tended to return to baseline values. Subjects with a more distinct hypothermia (33.1 °С) displayed a weakened baroreflex reaction (slight HR reduction) accompanied by a stronger vascular reaction (significant BP rise). High baroreflex stability in the subjects with low-level hypothermia (35 °С) was evidenced by a significant HR decrease in response to the BP rise. Changes in the EEG amplitude and frequency patterns during cooling declared themselves by enhancement of the subcortex regulatory activity (reliable rise in the theta-activity). Also, the alpha-activity was increased in the subjects with high-level hypothermia and decreased in 73 % of the subjects with a low-level hypothermia which points to desynchronization of the main rhythm.