Hypohydration Exacerbates Decreases in Cerebral Blood Flow Velocity During Orthostatic Stress

Abstract

PURPOSE: 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. METHODS: Eight subjects completed four orthostatic challenges (temperate conditions) twice before (PRE-EU) and after passive heat stress (∼3h @ 45°C, 50% rh, 1m/s air speed) with (POST-EU) or without (POST-HYPO) fluid replacement of sweat losses (−3% body mass loss). Measurements included CBFV, blood pressure (BP), heart rate (HR), end-tidal CO2(ETCO2), rectal (Trec) and skin (Tsk) temperatures, included being seated (20 min) followed by standing (60 sec) then re-sitting (10 min) with metronomic breathing (15 breaths/ min). RESULTS: Two PRE-EU results were similar. Standing when POST-HYPO resulted in greater CBFV reductions (P<0.05), greater HR increases (P<0.05), but similar BP decreases than when standing during PRE-EU and POST-EUH. POST-HYPO Trec was higher (P<0.05), but Tskand ETCO2were similar to POST-EU and PRE-EU. CONCLUSIONS: These data demonstrate that despite similar BP responses, hypohydration causes a greater CBFV reduction when standing which provides a mechanism for increased risk of orthostatic hypotension.