Hot water immersion increases internal carotid artery shear rate but does not alter intracranial vascular reactivity to carbon dioxide

Acute hot water immersion (HWI) increases cerebral blood flow, which may increase arterial shear rate and benefit cerebrovascular function. However, it is unclear whether HWI alters cerebral artery shear rate and cerebrovascular reactivity to carbon dioxide (CVR_CO2). We tested the hypotheses that HWI (39 °C) increases extracranial artery shear rate and intracranial artery hypercapnic CVR_CO2, but reduces hypocapnic CVR_CO2 compared to temperate water immersion (TWI; 35 °C). Eighteen healthy adults completed two experimental visits. Middle and posterior cerebral artery blood velocities (transcranial Doppler; MCAv and PCAv) were continuously recorded. Right internal carotid artery (ICA) and vertebral artery (VA) shear rate were obtained via Doppler ultrasound. Hypocapnic and hypercapnic CVR_CO2 were assessed in the middle cerebral artery (MCA) and posterior cerebral artery (PCA) during self-paced hyperventilation and during 30 s of 7% CO₂ inhalation. Measures were completed pre-immersion (PRE) and at 1.0 °C increase in core temperature during HWI and time-matched during TWI. Data are reported as mean ± SD. There were no differences between conditions at PRE. MCAv (64 ± 12 vs. 55 ± 9 cm/s; <i>P</i> = 0.01) and PCAv (39 ± 7 vs. 29 ± 5 cm/s; <i>P</i> < 0.01) were greater in TWI versus HWI at the 1.0 °C time point. ICA shear rate was greater in HWI versus TWI at 1.0 °C (247 ± 51 vs. 180 ± 43 s^-1; <i>P</i> < 0.01) but VA shear did not differ. Hypocapnic and hypercapnic CVR_CO2 in the MCA and PCA did not differ between conditions. Compared to TWI, MCAv and PCAv are lower in HWI, but HWI augments shear rate in the ICA, which may be beneficial for cerebrovascular health if done recurrently.