Influence of the mode of heating on cerebral blood flow, non‐invasive intracranial pressure and thermal tolerance in humans

Key points The human brain is particularly vulnerable to heat stress; this manifests as impaired cognition, orthostatic tolerance, work capacity and eventually, death. brain's limitation in the often ascribed inadequate cerebral blood flow (CBF), but elevated intracranial pressure commonly observed mammalian models of stroke can on its own cause functional impairment. CBF response incremental strain was dependent mode heating, decreasing by 30% when exposed passively hot, humid air (sauna), while remaining unchanged or increasing with passive hot-water immersion (spa) exercising a hot environment. Non-invasive estimates (nICP) were increased universally 18% at volitional thermal tolerance across all modes stress, therefore may play contributing role eliciting tolerance. sauna, more so than spa exercise, poses greater challenge under mild severe heating due lower similarly nICP. cognitive function, capacity, This vulnerability (CBF); however, (ICP) also stroke. We investigated changes three fundamentally different assessed whether per se ICP. Fourteen fit participants (seven female) heated 40°C core temperature (Tc; oesophageal) via (spa), exposure cycling exercise CO2 inhalation prevent heat-induced hypocapnia. measured duplex ultrasound each 0.5°C increment Tc ICP estimated non-invasively from optic nerve sheath diameter At decreased sauna (P < 0.001), ≥ 0.140). 17% end-tidal clamped eupnoeic 0.001). On contrary, nICP maximum achieved preventing hypocapnia during did not improve 0.146). Therefore, regulation dramatically depending dose whereas responses are not. equivalent strain.