title: Restoring heat stress-associated reduction in middle cerebral artery velocity does not reduce fatigue in the heat.
abstract: Heat-induced hyperventilation may reduce PaCO2 and thereby cerebral perfusion and oxygenation and in turn exercise performance. To test this hypothesis, eight volunteers completed three incremental exercise tests to exhaustion: (a) 18 °C ambient temperature (CON); (b) 38 °C (HEAT); and (c) 38 °C with addition of CO2 to inspiration to prevent the hyperventilation-induced reduction in PaCO2 (HEAT + CO2 ). In HEAT and HEAT + CO2 , rectal temperature was elevated prior to the exercise tests by means of hot water submersion and was higher (P < 0.05) than in CON. Compared with CON, ventilation was elevated (P < 0.01), and hence, PaCO2 reduced in HEAT. This caused a reduction (P < 0.05) in mean cerebral artery velocity (MCAvmean ) from 68.6 ± 15.5 to 53.9 ± 10.0 cm/s, which was completely restored in HEAT + CO2 (68.8 ± 5.8 cm/s). Cerebral oxygenation followed a similar pattern. V ˙ O 2   m a x was 4.6 ± 0.1 L/min in CON and decreased (P < 0.05) to 4.1 ± 0.2 L/min in HEAT and remained reduced in HEAT + CO2 (4.1 ± 0.2 L/min). Despite normalization of MCAvmean and cerebral oxygenation in HEAT + CO2 , this did not improve exercise performance, and thus, the reduced MCAvmean in HEAT does not seem to limit exercise performance.
authors.affiliation: Zurich Center for Integrative Human Physiology (ZIHP), Zurich, Switzerland; Institute of Physiology, University of Zurich, Zurich, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Keiser
authors.forename: D
authors.initials: D
authors.lastname: Flück
authors.forename: A
authors.initials: A
authors.lastname: Stravs
authors.forename: F
authors.initials: F
authors.lastname: Hüppin
authors.forename: C
authors.initials: C
authors.lastname: Lundby
mesh_terms: D000328:Adult; D054874:Athletic Performance / Q000502:physiology; D001783:Blood Flow Velocity; D002245:Carbon Dioxide / Q000627:therapeutic use*; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D005221:Fatigue / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control*; D018882:Heat Stress Disorders / Q000209:etiology / Q000503:physiopathology*; D006358:Hot Temperature / Q000009:adverse effects*; D006801:Humans; D006985:Hyperventilation / Q000209:etiology / Q000503:physiopathology / Q000628:therapy*; D008297:Male; D020768:Middle Cerebral Artery / Q000503:physiopathology*; D010101:Oxygen Consumption; D016037:Single-Blind Method; D016896:Treatment Outcome
chemical_list: D002245:Carbon Dioxide
keywords: Brain blood flow; PaCO2; cerebral oxygenation; exercise performance