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1 Human Factors, Centre de Recherches du Service de Sante des Armees, La Tronche, France
2 Thermal and Mountain Medicine, US Army Research Institute of Environmental Medicine, Natick, Massachusetts, United States
* To whom correspondence should be addressed. E-mail: robert.carteriii{at}us.army.mil.
Hypohydration is associated with orthostatic intolerance, however little is known about cerebrovascular mechanisms responsible. This study examined if hypohydration reduces cerebral blood flow velocity (CBFV) in response to an orthostatic challenge. Eight subjects completed four orthostatic challenges (temperate conditions) twice before (PRE-EU and PRE-HYP) and following recovery from passive heat stress (~3h @ 45°C, 50% rh, 1m/s air speed) with (POST-EU) or without (POST-HYP) fluid replacement of sweat losses (-3% body mass loss). Measurements included CBFV, mean arterial pressure (MAP), heart rate (HR), end-tidal CO2 (ETCO2), core (Tre) and skin (Tsk) temperatures. Test sessions included being seated (20 min) followed by standing (60 sec) then re-sitting (60 sec) with metronomic breathing (15 breaths/min). CBFV and MAP responses to standing were similar during PRE-EU and PRE-HYP. Standing POST-HYP exacerbated the magnitude (-28.0 ±1.4% of baseline) and duration (9.0 ± 1.6 sec) of CBFV reductions, and increased cerebrovascular resistance (CVR) compared to POST-EU (-20.0 ± 2.1% and 6.6 ± 0.9 sec). Standing POST-EU also resulted in a reduction in CBFV, and a smaller decrease in CVR compared to PRE-EU. MAP decreases were similar for POST-EU (-18 ± 4 mmHg) and POST-HYP (-21 ± 5 mmHg) from seated to standing. These data demonstrate that despite similar MAP decreases, hypohydration and prior heat stress (despite apparent recovery) produce greater CBFV reduction when standing. These observations suggest that hypohydration and prior heat stress are associated with greater reductions in CBFV with greater CVR which likely contribute to orthostatic intolerance.
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