Abstract
Increases in body temperature from heat stress (i.e., hyperthermia) generally impairs cognitive function across a range of domains and complexities, but the relative contribution from skin versus core temperature changes remains unclear. Hyperthermia also elicits a hyperventilatory response that decreases the partial pressure of end-tidal carbon dioxide (PetCO2) and subsequently cerebral blood flow that may influence cognitive function. We studied the role of skin and core temperature along with PetCO2 on cognitive function across a range of domains. Eleven males completed a randomized, single-blinded protocol consisting of poikilocapnia (POIKI, no PetCO2 control) or isocapnia (ISO, PetCO2 maintained at baseline levels) during passive heating using a water-perfused suit (water temperature ~ 49°C) while middle cerebral artery velocity (MCAv) was measured continuously as an index of cerebral blood flow. Cognitive testing was completed at baseline, neutral core-hot skin (37.0 ± 0.2°C-37.4 ± 0.3°C), hot core-hot skin (38.6 ± 0.3°C-38.7 ± 0.2°C), and hot core-cooled skin (38.5 ± 0.3°C-34.7 ± 0.6°C). The cognitive test battery consisted of a detection task (psychomotor processing), 2-back task (working memory), set-shifting and Groton Maze Learning Task (executive function). At hot core-hot skin, poikilocapnia led to significant (both p < 0.05) decreases in PetCO2 (∆−21%) and MCAv (∆−26%) from baseline, while isocapnia clamped PetCO2 (∆ + 4% from baseline) leading to a significantly (p = 0.023) higher MCAv (∆−18% from baseline) compared to poikilocapnia. There were no significant differences in errors made on any task (all p > 0.05) irrespective of skin temperature or PetCO2 manipulation. We conclude that neither skin temperature nor PetCO2 maintenance significantly alter cognitive function during passive hyperthermia.
Highlights
IntroductionElevations in core temperature (i.e., hyperthermia) increases physiological (e.g., cardiovascular, metabolic), psychological (e.g., thermal discomfort), and neurological (e.g., central processing) strain relative to thermoneutral environments and can lead to impairments in cognitive function (Hancock et al, 2007; Taylor et al, 2016; Schmit et al, 2017)
Elevations in core temperature increases physiological, psychological, and neurological strain relative to thermoneutral environments and can lead to impairments in cognitive function (Hancock et al, 2007; Taylor et al, 2016; Schmit et al, 2017)
There was a time-point effect for Tre (Table 2; p < 0.001, ηp2 = 1.00), where pairwise comparisons determined Tre was significantly higher in the Hot Core–Hot Skin (HC-HS) and Hot Core–Cooled Skin (HC-CS) compared to BASE and Neutral Core–Hot Skin (NC-HS)
Summary
Elevations in core temperature (i.e., hyperthermia) increases physiological (e.g., cardiovascular, metabolic), psychological (e.g., thermal discomfort), and neurological (e.g., central processing) strain relative to thermoneutral environments and can lead to impairments in cognitive function (Hancock et al, 2007; Taylor et al, 2016; Schmit et al, 2017). The magnitude of impairment from thermal stress may be task-dependent, where higherorder cognitive tasks (e.g., executive function, vigilance, working memory) or those requiring motor coordination are more vulnerable to impairment compared to where simple task performance (e.g., psychomotor processing) (Gaoua et al, 2011; Piil et al, 2017). Higher skin temperature with no or just minor elevations in core temperature increased thermal perception and discomfort, along with eliciting more errors (Gaoua et al, 2012) and slower reaction times (Malcolm et al, 2018) on executive function tasks. Based on the potential influence of skin temperature as an independent factor, it is of interest to tease out the relative contribution of skin versus core temperature influences on cognitive function across a range of task domains and complexities
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