Cerebral Autoregulation Is Not Different Between Hot And Thermoneutral Head-Out Water Immersion In Healthy Participants

Abstract

Recurring hot head-out water immersion (HOWI) enhances peripheral vascular function and cerebral blood velocity during non-immersion conditions (i.e., rest, aerobic exercise, and heat stress). However, it is not known if an acute bout of hot HOWI alters cerebrovascular function (i.e., cerebral autoregulation). PURPOSE: We tested the hypothesis that dynamic cerebral autoregulation is improved during and following an acute bout of hot (HOT) vs. thermoneutral (TN) HOWI. METHODS: Seventeen healthy participants (age: 23±2 y, 5 females) completed two randomized trials which consisted of 30 min of HOT (39°C) or TN (35°C) HOWI. Beat-to-beat blood pressure (MAP), middle cerebral artery blood velocity (MCAv), and end-tidal CO2 tension (PETCO2) were recorded continuously. After 5 min of seated rest, participants breathed through a respiratory impedance device for 5 min while maintaining PETCO2 values to assess cerebral autoregulation using Fourier transformation (n = 11). Cerebral autoregulation testing was completed pre, 30 min into HOWI (during), and immediately post HOWI. Values are reported as a change from baseline (α: mean ± SD). RESULTS: MAP, MCAv, PETCO2, gain, and phase were not different between HOT and TN at pre (P > 0.23 for all). αMAP was lower in HOT vs. TN during (-3±6 vs. 9±5 mmHg; P<0.01) and post (0±5 vs. 10±7 mmHg; P<0.01). αMCAv was lower in HOT vs. TN during (-3±5 vs. 4±4 cm/s; P<0.01) and post (-5±7 vs. 0±5 cm/s; P<0.01). αPETCO2 was lower in HOT vs. TN during (1±2 vs. 2±2 mmHg; P<0.01) and post (-3±2 vs. -1±2 mmHg; P<0.01). αGain was not different between HOT and TN during (HOT: -0.03±0.15 vs. TN: -0.08±0.05 cm/s/mmHg; P=0.18) or post (HOT: 0.04±0.12 vs. TN: -0.03±0.07 cm/s/mmHg; P=0.18). αPhase was not different between HOT and TN during (HOT: 2.9±9.0 vs. TN: 1.0±8.8°; P=0.74) or post (HOT: -1.2±12.8 vs. TN: -2.0±9.7°; P=0.74) CONCLUSION: These data indicate that an acute bout of hot water immersion attenuates cerebral blood velocity vs. thermoneutral water immersion. This response is likely due to the differences in arterial blood pressure and/or arterial carbon dioxide between conditions. However, cerebral autoregulation during and following hot water immersion is not different compared to thermoneutral water immersion in healthy participants. Supported by Office of Naval Research Award N00014-17-1-2665