Impact of whole-body hot water immersion on cerebral haemodynamics and cerebrovascular reactivity to carbon dioxide

Passive heating may be an alternative or adjunct to exercise to maintain cardiovascular and cerebral function and health. Hot water immersion (HWI) improves peripheral vascular function via shear-mediated mechanisms; however, it remains unclear if acute HWI can improve cerebrovascular function. This study determined the influence of HWI on cerebral haemodynamics and subsequent cerebrovascular reactivity to carbon dioxide (CVRco2) within the internal carotid artery (ICA), an index of cerebrovascular function. Fifteen healthy adults (six female, mean age 28 ± 8 years) completed two experimental trials separated by at least 48 hours. Females were tested in the same menstrual cycle phase for both trials. In the HWI trial, participants were immersed in 39°C water to the sternum for 60 min whilst in the CON trial they rested in 21°C air in the same semi-recumbent posture and duration as in the HWI trial. Thermal and haemodynamic variables were assessed throughout trials. CVRco2 was determined before and 45 min after the 60 min HWI and CON interventions by participants breathing four minutes of a hypercapnic gas (6% CO2, 21% O2, N2 balance) whilst ICA blood velocity and diameter were measured by duplex ultrasound. At 60 min of the interventions core body temperature and heart rate were higher on HWI than CON (CON, 36.9 ± 0.3°C; HWI, 38.1 ± 0.3°C, P < 0.001. CON, 60 ± 10 bpm; HWI, 96 ± 13 bpm, P < 0.001). No differences were observed between the HWI and CON interventions for ICA diameter (P = 0.34), ICA blood velocity (P = 0.97), ICA shear rate (P = 0.75) and ICA blood flow (CON, 311 ± 80 ml·min-1; HWI, 338 ± 92 ml·min-1, P = 0.39). Compared to CON, HWI intervention reduced mean arterial blood pressure (CON, 82 ± 7 mmHg; HWI, 67 ± 8 mmHg, P < 0.001), and increased ICA conductance (CON, 3.9 ± 1.3 ml·min-1·mmHg-1; HWI, 5.2 ± 2.0 ml·min-1·mmHg-1, P = 0.03). A 45 min after the interventions no difference was detected for CVRco2 between the trials (CON, pre: 13.9 ± 9.2 to post: 11.3 ± 6.1 ml·min-1·mmHg-1; HWI, pre: 14.6 ± 7.9 to post: 10.9 ± 5.4 ml·min-1·mmHg-1; Interaction P = 0.65). HWI reduced blood pressure and increased ICA conductance (i.e. autoregulation) to maintain blood flow to the brain. Nevertheless, despite differences in cerebral vascular tone between interventions, HWI did not influence subsequent cerebrovascular function, as assessed by cerebrovascular reactivity to carbon dioxide. These findings support hot water immersion as an intervention to reduce blood pressure, which if repeated regularly may help to maintain cardiovascular and cerebral health.