Journal of Applied Physiology
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J Appl Physiol 104: 976-981, 2008. First published January 24, 2008; doi:10.1152/japplphysiol.01040.2007
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Cerebrovascular responsiveness to steady-state changes in end-tidal CO2 during passive heat stress

David A. Low,1 Jonathan E. Wingo,1 David M. Keller,1,3 Scott L. Davis,1,2 Rong Zhang,1,2 and Craig G. Crandall1,2

1Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, 2Department of Internal Medicine; University of Texas Southwestern Medical Center at Dallas, Dallas, and 3Department of Kinesiology, University of Texas at Arlington, Arlington, Texas

Submitted 28 September 2007 ; accepted in final form 17 January 2008

This study tested the hypothesis that passive heat stress alters cerebrovascular responsiveness to steady-state changes in end-tidal CO2 (PETCO2). Nine healthy subjects (4 men and 5 women), each dressed in a water-perfused suit, underwent normoxic hypocapnic hyperventilation (decrease PETCO2 ~20 Torr) and normoxic hypercapnic (increase in PETCO2 ~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 PETCO2 was used to evaluate cerebrovascular CO2 responsiveness. Passive heat stress increased core temperature (1.1 ± 0.2°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 PETCO2 by 3 ± 4 Torr (P = 0.07), while mean arterial blood pressure was well maintained (P = 0.36). The slope of the CBVC-PETCO2 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-PETCO2 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 CO2 responsiveness, to the prescribed steady-state changes in PETCO2, is unchanged during passive heat stress.

brain blood flow; hyperthermia; hypocapnia; hypercapnia



Address for reprint requests and other correspondence: C. G. Crandall, Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, 7232 Greenville Ave., Dallas, TX 75231 (e-mail: craigcrandall{at}texashealth.org)




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