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

Acute head-out water immersion (HOWI) in hot water increases cerebral blood flow (CBF) until core temperature rises by ~1.0°C. The increase in CBF may promote increases in arterial shear rate that could enhance cerebrovascular reactivity to CO 2 (CVR CO2 ) as a result of elevated nitric oxide (NO) bioavailability. An increase in NO bioavailability improves peripheral vasodilator function, and therefore, a similar response may occur in the cerebral vasculature. This likely only aids in cerebral vasodilation during hypercapnia, and not cerebral vasoconstriction during hypocapnia. However, it is unclear if a single session of hot HOWI alters cerebral artery shear rate and CVR CO2 to hyper- and hypocapnia. Purpose: We tested the hypotheses that during a single session of hot HOWI (39°C), cerebral artery shear rate and hypercapnic CVR CO2 would be greater and hypocapnic CVR CO2 would be lower compared to temperate HOWI (35°C). Methods: Eighteen healthy participants (age: 25±5 y, 6 women) completed two experimental visits. Middle and posterior cerebral artery blood velocities (transcranial Doppler; MCAv and PCAv) were continuously recorded. Blood velocities and diameters of the right internal carotid artery (ICA) and vertebral artery (VA) were obtained via dual-mode Doppler ultrasound to calculate shear rate. Hypocapnic and hypercapnic CVR CO2 were assessed in the MCA and PCA during self-paced hyperventilation (until PETCO 2 decreased by 10 mmHg from baseline) and during 30 s of 7% CO 2 inhalation, respectively. Measures were completed before immersion (PRE) and after a +1.0°C increase in core temperature during hot HOWI and time-matched during temperate HOWI. Data are reported as mean ± SD. Results: There were no differences between conditions at PRE. MCAv (55±9 vs. 64±12 cm/s; P=0.01) and PCAv (29±5 vs. 39±7 cm/s; p<0.01) were lower in hot vs. temperate HOWI at +1.0°C. ICA shear rate was greater in hot vs. temperate HOWI at +1.0°C (247±51 vs. 180±43 s −1 ; p<0.01) but VA shear rate did not differ between conditions (P=0.78). There were no differences between conditions for hypercapnic CVR CO2 (MCA: P =0.48, PCA: P=0.43) or hypocapnic CVR CO2 (MCA: P=0.18). A condition effect was observed for PCA hypocapnic CVR CO2 (p<0.01), but multiple comparisons did not reveal where differences occurred (all P≥0.10). Conclusion: Hot head out water immersion elicits similar changes in hemodynamics and CVR CO2 in intracranial arteries (i.e., MCA and PCA) as temperate head out water immersion. Hot head out water immersion produces a greater shear rate in the ICA compared to temperate water, which may be beneficial for cerebrovascular health if done recurrently. This study was partially supported by the Mark Diamond Research Fund at the University at Buffalo and the Offce of Naval Research Director of Research Early Career Grant (N00014-17-1-2878). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.