Hot head-out water immersion does not acutely alter dynamic cerebral autoregulation or cerebrovascular reactivity to hypercapnia

ABSTRACT

Recurring hot head-out water immersion (HOWI) enhances peripheral vascular function and cerebral blood velocity during non-immersion conditions. However, it is unknown if an acute bout of hot HOWI alters cerebrovascular function. Using two experimental studies, we tested the hypotheses that dynamic cerebral autoregulation (dCA) and cerebrovascular reactivity (CVR) are improved during an acute bout of hot (HOT; 39 °C) vs. thermoneutral (TN; 35 °C) HOWI. Eighteen healthy participants (eight females) completed the dCA study, and 14 participants (6 females) completed the CVR study. Both studies consisted of two randomized (TNdCA vs. HOTdCA; TNCVR vs. HOTCVR) 45minute HOWI visits. Middle cerebral artery blood velocity (MCAvmean) was continuously recorded. dCA was assessed using a respiratory impedance device and analyzed via transfer gain and phase in the low-frequency band. CVR was assessed using stepped hypercapnia. Assessments were completed PRE and 30 minutes into HOWI. Values are reported as a change (Δ) from PRE (mean ± SD). There were no differences at PRE for either study. ΔMCAvmean was greater in TNdCA (TNdCA: 4 ± 4 vs. HOTdCA: -3 ± 5 cm/s; P < 0.01) and TNCVR (TNCVR: 5 ± 4 vs. HOTCVR: -1 ± 6 cm/s; P < 0.01) during HOWI. ΔGain was greater in HOTdCA during HOWI (TNdCA: -0.09 ± 0.15 vs. HOTdCA: 0.10 ± 0.17 cm/s/mmHg; P = 0.04). ΔPhase (P > 0.84) and ΔCVR (P > 0.94) were not different between conditions. These data indicate that hot and thermoneutral water immersion do not acutely alter cerebrovascular function in healthy, young adults.

KEYWORDS:

• Cerebral blood flow
• common carotid artery
• blood pressure
• carbon dioxide
• cerebrovascular function
• passive heat stress

Abbreviations

CBF = cerebral blood flow

CCA = common carotid artery

cm H₂O = centimeters of water

CO₂ = carbon dioxide

CVR = cerebrovascular reactivity to hypercapnia

dCA = dynamic cerebral autoregulation

HF = high frequency

HOWI = head-out water immersion

LF = low frequency

MAP = mean arterial pressure

MCA = middle cerebral artery

MCAv_mean = mean middle cerebral artery blood velocity

mm Hg = millimeters of mercury

NO = nitric oxide

PETCO₂ = end-tidal carbon dioxide tension

USG = urine-specific gravity

VLF = very low frequency

Acknowledgments

We thank the participants for their time and commitment to complete the study.

Disclosure statement

No conflicts of interest, financial or otherwise, are declared by the authors.

Additional information

Funding

This study was supported by the Office of Naval Research Director of Research Early Career Grant (No. N00014-17-1-2665).