Effects of heat stress on dynamic cerebral autoregulation during large fluctuations in arterial blood pressure.
Impaired cerebral autoregulation during marked reductions in arterial blood pressure may contribute to heat stress-induced orthostatic intolerance. This study tested the hypothesis that passive heat stress attenuates dynamic cerebral autoregulation during pronounced swings in arterial blood pressure. Mean arterial blood pressure (MAP) and middle cerebral artery blood velocity were continuously recorded for approximately 6 min during normothermia and heat stress (core body temperature = 36.9 +/- 0.1 degrees C and 38.0 +/- 0.1 degrees C, respectively, P < 0.001) in nine healthy individuals. Swings in MAP were induced by 70-mmHg oscillatory lower body negative pressure (OLBNP) during normothermia and at a sufficient lower body negative pressure to cause similar swings in MAP during heat stress. OLBNP was applied at a very low frequency ( approximately 0.03 Hz, i.e., 15 s on-15 s off) and a low frequency ( approximately 0.1 Hz, i.e., 5 s on-5 s off). For each thermal condition, transfer gain, phase, and coherence function were calculated at both frequencies of OLBNP. During very low-frequency OLBNP, transfer function gain was reduced by heat stress (0.55 +/- 0.20 and 0.31 +/- 0.07 cm x s(-1) x mmHg(-1) during normothermia and heat stress, respectively, P = 0.02), which is reflective of improved cerebrovascular autoregulation. During low-frequency OLBNP, transfer function gain was similar between thermal conditions (1.19 +/- 0.53 and 1.01 +/- 0.20 cm x s(-1) x mmHg(-1) during normothermia and heat stress, respectively, P = 0.32). Estimates of phase and coherence were similar between thermal conditions at both frequencies of OLBNP. Contrary to our hypothesis, dynamic cerebral autoregulation during large swings in arterial blood pressure during very low-frequency (i.e., 0.03 Hz) OLBNP is improved during heat stress, but it is unchanged during low-frequency (i.e., 0.1 Hz) OLBNP.
Brothers
R Matthew
RM
Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas, University of Texas Southwestern Medical Center, Dallas, Texas 75231, USA.
Zhang
Rong
R
Wingo
Jonathan E
JE
Hubing
Kimberly A
KA
Crandall
Craig G
CG
D000328:Adult; D000704:Analysis of Variance; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology*; D001831:Body Temperature / Q000502:physiology; D001833:Body Temperature Regulation / Q000502:physiology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006706:Homeostasis / Q000502:physiology*; D006358:Hot Temperature; D006801:Humans; D008150:Lower Body Negative Pressure; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology; D012815:Signal Processing, Computer-Assisted; D013312:Stress, Physiological / Q000502:physiology*; D016683:Supine Position; D013686:Telemetry