title: Cerebrovascular responsiveness to steady-state changes in end-tidal CO2 during passive heat stress.
abstract: This study tested the hypothesis that passive heat stress alters cerebrovascular responsiveness to steady-state changes in end-tidal CO(2) (Pet(CO(2))). Nine healthy subjects (4 men and 5 women), each dressed in a water-perfused suit, underwent normoxic hypocapnic hyperventilation (decrease Pet(CO(2)) approximately 20 Torr) and normoxic hypercapnic (increase in Pet(CO(2)) approximately 9 Torr) challenges under normothermic and passive heat stress conditions. The slope of the relationship between calculated cerebrovascular conductance (CBVC; middle cerebral artery blood velocity/mean arterial blood pressure) and Pet(CO(2)) was used to evaluate cerebrovascular CO(2) responsiveness. Passive heat stress increased core temperature (1.1 +/- 0.2 degrees C, P < 0.001) and reduced middle cerebral artery blood velocity by 8 +/- 8 cm/s (P = 0.01), reduced CBVC by 0.09 +/- 0.09 CBVC units (P = 0.02), and decreased Pet(CO(2)) by 3 +/- 4 Torr (P = 0.07), while mean arterial blood pressure was well maintained (P = 0.36). The slope of the CBVC-Pet(CO(2)) relationship to the hypocapnic challenge was not different between normothermia and heat stress conditions (0.009 +/- 0.006 vs. 0.009 +/- 0.004 CBVC units/Torr, P = 0.63). Similarly, in response to the hypercapnic challenge, the slope of the CBVC-Pet(CO(2)) relationship was not different between normothermia and heat stress conditions (0.028 +/- 0.020 vs. 0.023 +/- 0.008 CBVC units/Torr, P = 0.31). These results indicate that cerebrovascular CO(2) responsiveness, to the prescribed steady-state changes in Pet(CO(2)), is unchanged during passive heat stress.
authors.affiliation: Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, 7232 Greenville Ave., Dallas, TX 75231, USA.
authors.forename: David A
authors.initials: DA
authors.lastname: Low
authors.forename: Jonathan E
authors.initials: JE
authors.lastname: Wingo
authors.forename: David M
authors.initials: DM
authors.lastname: Keller
authors.forename: Scott L
authors.initials: SL
authors.lastname: Davis
authors.forename: Rong
authors.initials: R
authors.lastname: Zhang
authors.forename: Craig G
authors.initials: CG
authors.lastname: Crandall
mesh_terms: D000328:Adult; D001831:Body Temperature / Q000502:physiology; D001833:Body Temperature Regulation / Q000502:physiology; D002245:Carbon Dioxide / Q000097:blood / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D003627:Data Interpretation, Statistical; D005260:Female; D018882:Heat Stress Disorders / Q000378:metabolism / Q000503:physiopathology*; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology; D016857:Hypocapnia / Q000503:physiopathology; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology; D011652:Pulmonary Circulation / Q000502:physiology; D015656:Respiratory Mechanics / Q000502:physiology; D013686:Telemetry
chemical_list: D002245:Carbon Dioxide