Cold Water Immersion: Simultaneous Assessment of Cerebral Oxygenation, Vascular Function, and Thermoregulatory Responses.

INTRODUCTION: Developing a greater understanding of how the body reacts following cold water immersion (CWI) and its impact on human performance is required for the development of future monitoring technologies and countermeasures to reduce cold-induced impairments. Currently, there are no field-deployable technologies for real-time monitoring of cold-induced neurochemical and metabolic changes, limiting physiological assessment tools for monitoring the impact of cold immersion on cognitive and physical performance. Functional near-infrared spectroscopy (fNIRS) may be an effective technique for monitoring the impact of cold exposures on neurophysiological functions; however, a deeper examination of the fNIRS patterns of responses is needed to progress the interpretability of this technique in austere environments. Therefore, the purpose of this study was to examine the cerebral hemodynamic responses over the prefrontal cortex (PFC) as well as respiratory and common carotid artery (CCA) responses during a 10-minute CWI.

MATERIALS AND METHODS: Twenty-six participants (age = 23.6 ± 4.3 years) completed 2 testing visits, which consisted of a 10-minute thermoneutral water immersion (TWI; 35 °C) or CWI (15 °C). Functional near-infrared spectroscopy-derived oxygenated hemoglobin (O2Hb) was measured over the PFC, as well as respiratory rate, tidal volume, and CCA diameter, were measured at the beginning and end of the immersion period. In addition, skin temperature (Tsk) and thermal perception were measured during each condition. Repeated measures ANOVA's were used to examine the condition and time course of response changes for Tsk, respiratory rate, tidal volume, thermal perception, and CCA diameter. A general linear model analysis was used to examine differences in beta values of the O2Hb between the TWI and CWI conditions. This study was approved by the university's institutional review board (IRB: 2115890).

RESULTS: There was no significant change in O2Hb during the TWI condition (P = .15), however, O2Hb significantly increased during the CWI (P < .01). The CCA diameter did not change during the TWI (P = .84) but increased during the CWI (P < .01). There was a significant decrease in Tsk (P < .01) during CWI and greater thermal perception compared to TWI (P < .01), which showed no significant changes (P = .06-.82). Respiratory rate remained unchanged (P = .59). Tidal volume was significantly greater during the CWI compared to TWI (P < .01).

CONCLUSIONS: Initial exposure to CWI induced a cold shock response with a 43% increase in tidal volume, without a corresponding increase in respiratory rate. This suggested a potential, intuitive, cold-mitigation strategy in cold naïve participants. In addition, the O2Hb responses indicated a short-lived, cold-induced mismatch between cerebral oxygen demand and supply. The increase in cerebral oxygenation reflects a cold-induced increase in cerebral blood flow to prevent localized temperature drops, along with elevated counter-current vascularization and metabolic heat generation.