Submitted on November 5, 2008
Revised on February 26, 2009
Accepted on March 5, 2009
Dynamic Cerebral Autoregulation during Passive Heat Stress in Humans
David A. Low1, Jonathan E. Wingo2, David Melvin Keller2, Scott L. Davis3, Jian Cui4, Rong Zhang2, and Craig G. Crandall2*
1 Institute for Exercise and Environmental Medicine
2 Presbyterian Hospital of Dallas
3 University of Texas Southwestern Medical Center at Dallas
4 Penn State University, College of Medicine
* To whom correspondence should be addressed. E-mail: craigcrandall{at}texashealth.org.
This study tested the hypothesis that passive heating impairs cerebral autoregulation. Transfer function analyses of resting arterial blood pressure and middle cerebral artery blood velocity (MCAVmean), as well as MCAVmean and blood pressure responses to rapid deflation of previously inflated thigh cuffs, were examined in 9 healthy subjects under normothermic and passive heat stress (increase core temperature 1.1±0.2 °C, P<0.001) conditions. Passive heating reduced MCAVmean (
8±8 cm•s-1, P=0.01), while blood pressure was maintained (-1±4 mmHg, P=0.36). Coherence was decreased in the very low frequency range during heat stress (0.57±0.13 to 0.26±0.10, P=0.001) but was >0.5 and similar between NT and HS in the low (0.07-0.20 Hz, P=0.40) and high (0.20-0.35 Hz, P=0.12) frequency ranges. Transfer gain was reduced during heat stress in the very low (0.88±0.38 to 0.59±0.19 cm•s-1•mm Hg-1, P=0.02) frequency range but was unaffected in the low and high frequency ranges. The magnitude of the decrease in blood pressure (normothermia: 20±4, heat stress: 19±6 mmHg, P=0.88) and MCAVmean (13±4 to 12±6 cm•s-1, P=0.59) in response to cuff deflation was not affected by the thermal condition. Similarly, the rate of regulation of cerebrovascular conductance (CBVC) after cuff release (0.44 ± 0.22 to 0.38 ± 0.13 CBVC units•s-1, P = 0.16) and the time for MCAVmean to recover to pre-cuff deflation baseline (10.0±7.9 to 8.7±4.9 s, P=0.77) were not affected by heat stress. Counter to the proposed hypothesis, similar rate of regulation responses suggest that heat stress does not impair the ability to control cerebral perfusion after a rapid reduction in perfusion pressure, while reduced transfer function gain and coherence in the very low frequency range during heat stress suggest that dynamic cerebral autoregulation is improved during spontaneous oscillations in blood pressure within this range.
