Brain oxygen consumption is maintained during prolonged exercise in the heat despite reductions in cerebral blood flow

Steven Trangmar, Scott T Chiesa, Kameljit Kalsi, Niels H. Secher, Jose Gonzalez-Alonso

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review


Aim: Exercise in the heat with concomitant dehydration induces a significant cardiovascular strain on the human body. Blood flow to the brain (CBF) and extra-cranial tissues including the skin may also be compromised, potentially challenging brain oxygen uptake (brain VO2) and local heat dissipation. Whether dehydration reduces regional blood flow across the head and reduces brain VO2 during strenuous exercise in the heat remains unknown. Methods: We assessed CBF and extra-cranial blood flow in the internal, external and common carotid arteries (ICA, ECA and CCA) using Doppler ultrasonography in ten cyclists (VO2PEAK: 59 ± 2 ml/kg/min), who performed two hours of prolonged cycling exercise in a warm environment (182 ± 6 W; 35ºC), without fluids, to induce moderate dehydration (DEH; 3.1 ± 0.3% body mass loss). Blood samples were obtained from the brachial artery and left internal jugular vein for a-vO2 differences and calculation of brain VO2 using the Fick principle. Participants returned one week later to repeat the protocol whilst hydration was maintained with regular ingestions of a carbohydrate/electrolyte drink (Gatorade®). Results: In the dehydration trial, CBF was elevated after 30 min of strenuous exercise in the heat (+13% from rest to 30 min; P < 0.05), before declining to baseline values at 120 min (P < 0.001). Extra-cranial blood flow increased from rest to 60 min before declining prior to exhaustion; overall an indication of a reduced total blood flow to the head. Reductions in CBF were accompanied by increases (P < 0.05) in a-vO2 diff, resulting in a stable brain VO2. Fluid ingestion sufficient to prevent dehydration maintained CBF (≥ 25% increase vs. rest; P < 0.05) and extra-cranial blood flow throughout exercise. Conclusions: The present findings demonstrate that progressive dehydration during prolonged exercise in the heat results in a distinct circulatory strain on the brain. However, brain VO2 appears not to be compromised due to compensatory increases in cerebral oxygen extraction. Regular fluid ingestion precludes the reductions in blood flow to regional vascular beds in proximity to the brain.
Original languageEnglish
Title of host publicationProceedings of Training and Competing in the Heat, Aspetar, Doha, Qatar
Publication statusPublished - Mar 2014

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