title: Dehydration accelerates reductions in cerebral blood flow during prolonged exercise in the heat without compromising brain metabolism.
abstract: Dehydration hastens the decline in cerebral blood flow (CBF) during incremental exercise, whereas the cerebral metabolic rate for O2 (CMRO2 ) is preserved. It remains unknown whether CMRO2 is also maintained during prolonged exercise in the heat and whether an eventual decline in CBF is coupled to fatigue. Two studies were undertaken. In study 1, 10 male cyclists cycled in the heat for ∼2 h with (control) and without fluid replacement (dehydration) while internal and external carotid artery blood flow and core and blood temperature were obtained. Arterial and internal jugular venous blood samples were assessed with dehydration to evaluate CMRO2 . In study 2, in 8 male subjects, middle cerebral artery blood velocity was measured during prolonged exercise to exhaustion in both dehydrated and euhydrated states. After a rise at the onset of exercise, internal carotid artery flow declined to baseline with progressive dehydration (P < 0.05). However, cerebral metabolism remained stable through enhanced O2 and glucose extraction (P < 0.05). External carotid artery flow increased for 1 h but declined before exhaustion. Fluid ingestion maintained cerebral and extracranial perfusion throughout nonfatiguing exercise. During exhaustive exercise, however, euhydration delayed but did not prevent the decline in cerebral perfusion. In conclusion, during prolonged exercise in the heat, dehydration accelerates the decline in CBF without affecting CMRO2 and also restricts extracranial perfusion. Thus, fatigue is related to a reduction in CBF and extracranial perfusion rather than CMRO2 .
authors.affiliation: Centre for Sports Medicine and Human Performance, Brunel University London, Uxbridge, United Kingdom; and.
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Trangmar
authors.affiliation: Centre for Sports Medicine and Human Performance, Brunel University London, Uxbridge, United Kingdom; and.
authors.forename: Scott T
authors.initials: ST
authors.lastname: Chiesa
authors.affiliation: Centre for Sports Medicine and Human Performance, Brunel University London, Uxbridge, United Kingdom; and.
authors.forename: Iñaki
authors.initials: I
authors.lastname: Llodio
authors.affiliation: Centre for Sports Medicine and Human Performance, Brunel University London, Uxbridge, United Kingdom; and.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Garcia
authors.affiliation: Centre for Sports Medicine and Human Performance, Brunel University London, Uxbridge, United Kingdom; and.
authors.forename: Kameljit K
authors.initials: KK
authors.lastname: Kalsi
authors.affiliation: Centre for Sports Medicine and Human Performance, Brunel University London, Uxbridge, United Kingdom; and Department of Anaesthesia, The Copenhagen Muscle Research Centre, Rigshospitalet, University of Copenhagen, Denmark.
authors.forename: Niels H
authors.initials: NH
authors.lastname: Secher
authors.affiliation: Centre for Sports Medicine and Human Performance, Brunel University London, Uxbridge, United Kingdom; and j.gonzalez-alonso@brunel.ac.uk.
authors.forename: José
authors.initials: J
authors.lastname: González-Alonso
mesh_terms: D000328:Adult; D001642:Bicycling / Q000502:physiology; D001786:Blood Glucose / Q000378:metabolism; D001921:Brain / Q000098:blood supply* / Q000378:metabolism; D002339:Carotid Arteries / Q000000981:diagnostic imaging / Q000502:physiology*; D002342:Carotid Artery, External / Q000000981:diagnostic imaging / Q000502:physiology; D002343:Carotid Artery, Internal / Q000000981:diagnostic imaging / Q000502:physiology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D003681:Dehydration / Q000378:metabolism / Q000503:physiopathology*; D015444:Exercise / Q000502:physiology*; D006358:Hot Temperature; D006801:Humans; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology*; D014463:Ultrasonography
chemical_list: D001786:Blood Glucose
keywords: cerebral blood flow; dehydration; extracranial blood flow; prolonged exercise