Whole-body hot water immersion effect on cerebral haemodynamics and subsequent cerebrovascular reactivity to carbon...

To test the hypothesis that hot water immersion (HWI) improves cerebrovascular function via shear-mediated mechanisms, this study determined cerebrovascular reactivity to carbon dioxide ( $\mathrm{CV}{R}_{C{O}_2}$ ) before and after 60 min of 39°C HWI and a 21°C air control (CON) in 15 healthy participants. Thermal and haemodynamic variables were assessed throughout the trials, and $\mathrm{CV}{R}_{C{O}_2}$ was determined by a 4-min inhalation of hypercapnic gas (6% CO₂, 21% O₂, N₂ balance) and the assessment of internal carotid artery (ICA) blood flow by duplex ultrasound before and 45 min after HWI and CON. At 60 min of the interventions, core body temperature (CON, 36.9 ± 0.3°C; HWI, 38.1 ± 0.3°C, P < 0.01), heart rate (P < 0.01) and ICA conductance (P < 0.01) were higher in HWI than CON, while, mean arterial blood pressure was lower (CON, 82 ± 9 mmHg; HWI 65 ± 8 mmHg, P < 0.01). No differences were observed for ICA diameter, ICA blood velocity, ICA shear rate and ICA blood flow between HWI and CON (all P > 0.05). ICA $\mathrm{CV}{R}_{C{O}_2}$ did not change after either CON (pre: 13.9 ± 9.2 to post: 11.3 ± 6.1 mL min^-1 mmHg^-1) or HWI (pre: 14.6 ± 7.9 to post: 10.9 ± 5.4 mL min^-1 mmHg^-1; Interaction P = 0.65). In conclusion, HWI reduced blood pressure and increased ICA conductance (i.e. autoregulation) to maintain blood flow to the brain; however, HWI did not influence subsequent cerebrovascular function, as assessed by $\mathrm{CV}{R}_{C{O}_2}$ .