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This Article Full Text Full Text (PDF) All Versions of this Article: 104/4/976    most recent 01040.2007v2 01040.2007v1 Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Low, D. A. Articles by Crandall, C. G. PubMed PubMed Citation Articles by Low, D. A. Articles by Crandall, C. G.

Cerebrovascular responsiveness to steady-state changes in end-tidal CO₂ during passive heat stress

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 (PET_CO2). Nine healthy subjects (4 men and 5 women), each dressed in a water-perfused suit, underwent normoxic hypocapnic hyperventilation (decrease PET_CO2 20 Torr) and normoxic hypercapnic (increase in PET_CO2 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_CO2 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 PET_CO2 by 3 ± 4 Torr (P = 0.07), while mean arterial blood pressure was well maintained (P = 0.36). The slope of the CBVC-PET_CO2 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_CO2 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 PET_CO2, is unchanged during passive heat stress.

brain blood flow; hyperthermia; hypocapnia; hypercapnia