Cool Head-out Water Immersion Augments Cerebral Artery Blood Velocity In Healthy Young Adults

Abstract

Adequate cerebral blood flow (CBF) is critical for brain function. Thermoneutral head-out water immersion (HOWI) may be a useful modality to increase CBF but it is unclear whether a colder water temperature influences the cerebrovascular responses. PURPOSE: We tested the hypothesis that middle cerebral artery blood velocity (MCAv) and cerebrovascular reactivity to hypercapnia (CVRCO2) would be higher during an acute bout of thermoneutral (TN; 35 °C) vs. cool (COOL; 25 °C) HOWI. METHODS: Ten healthy participants (age: 23 ± 3 y; BMI: 23 ± 3 kg/m2; 4 females) completed two randomized 45-minute HOWI visits. Core temperature (Tcore) was recorded every five minutes via an ingested telemetry pill. Right mean MCAv, end-tidal CO2 tension (PETCO2), and mean arterial pressure (MAP) were continuously recorded. CVRCO2 was assessed using a fixed inspired stepped hypercapnia protocol before (PRE), at 30 minutes of HOWI (HOWI), and 45 minutes after cessation of HOWI (POST). CVRCO2 was calculated as the slope of the linear regression line between MCAv and PETCO2. Values are reported as a change from PRE (mean ± SD). RESULTS: There were no differences between conditions at PRE (all P ≥ 0.09). During HOWI, the change in Tcore, MCAv, PETCO2, MAP, CVRCO2 were not different between TN and COOL (all P ≥ 0.22). Tcore did not change during TN (-0.10 ± 0.14 °C; P = 0.55) or COOL (-0.00 ± 0.22 °C; P > 0.99). MCAv did not increase during TN (4 ± 5 cm/s; P = 0.06) but increased during COOL (7 ± 6 cm/s; P < 0.01). PETCO2 did not change during TN (1 ± 2 mmHg; P = 0.20) or COOL (1 ± 2 mmHg; P = 0.16). MAP increased during TN (11 ± 7 mmHg; P < 0.01) and COOL (16 ± 4 mmHg; P < 0.01). At POST, the change in Tcore, MCAv, and PETCO2 was greater in COOL vs. TN (all P < 0.01). Tcore did not change in TN (P > 0.99), but decreased in COOL (P = 0.04) at POST. MCAv did not change in TN (1 ± 7 cm/s; P > 0.99) but increased in COOL (9 ± 5 cm/s; P < 0.01) at POST. PETCO2 did not change in TN (-1 ± 2 mmHg; P = 0.42) or COOL (1 ± 1 mmHg; P = 0.06) at POST. MAP did not increase in TN (5 ± 8 mmHg; P = 0.06) but increased in COOL (11 ± 10 mmHg; P < 0.01) at POST. CVRCO2 did not change over time (P ≥ 0.11) or between conditions (P ≥ 0.32). CONCLUSION: Cool water augmented MCAv during immersion and MCAv was greater following COOL HOWI vs. TN at 45 minutes post. However, neither condition altered CVRCO2 in healthy young adults. Supported by ONR N00014-17-1-2878