Cardiovascular baroreceptors mediate susceptibility to hypothermia

The maintenance of excessively high peripheral blood flow through dilated blood vessels during immersion in cold water could explain some individuals' predisposition to hypothermia. We hypothesized that interpersonal differences in vascular reactivity could account for contrasting susceptibility to hypothermia.

Twenty-two highly fit, volume replete subjects undergoing Navy SEAL training were recruited for this study. Vascular reactivity in these trainees was determined in a thermal-neutral environment by measuring changes in forearm blood flow (FBF) while decreasing their BP with the application of lower body negative pressure (LBNP). FBF was also measured during exposure of these subjects to ice cold water. BP, heart rate, stroke volume, and skin temperatures were also recorded.

Changes in FBF induced by a fall in BP correlated with an individual's reduction in FBF caused by ice water immersion (n = 17, r = 0.84, p < 0.001). A subject's decrement in BP induced with LBNP correlated inversely with the fall in skin temperature in response to cold water immersion (n = 19, r = 0.70, p < 0.001). Finally, we found that sodium excretion also correlated with cold-induced decrements in peripheral blood flow (n = 7, r = 0.83, p < 0.05).

It is suggested that contrasting cardiovascular baroreceptor sensitivity and vascular responsiveness contribute to individual differences in susceptibility to hypothermia. Furthermore, the trend toward dietary salt restrictions may not be salutary in the Navy SEAL who must frequently operate in cold ambient environments.