title: Heat stress reduces cerebral blood velocity and markedly impairs orthostatic tolerance in humans.
abstract: Orthostatic tolerance is reduced in the heat-stressed human. This study tested the following hypotheses: 1) whole body heat stress reduces cerebral blood velocity (CBV) and increases cerebral vascular resistance (CVR); and 2) reductions in CBV and increases in CVR in response to an orthostatic challenge will be greater while subjects are heat stressed. Fifteen subjects were instrumented for measurements of CBV (transcranial ultrasonography), mean arterial blood pressure (MAP), heart rate, and internal temperature. Whole body heating increased both internal temperature (36.4+/-0.1 to 37.3+/-0.1 degrees C) and heart rate (59+/-3 to 90+/-3 beats/min); P<0.001. Whole body heating also reduced CBV (62+/-3 to 53+/-2 cm/s) primarily via an elevation in CVR (1.35+/-0.06 to 1.63+/-0.07 mmHg.cm-1.s; P<0.001. A subset of subjects (n=8) were exposed to lower-body negative pressure (LBNP 10, 20, 30, 40 mmHg) in both normothermic and heat-stressed conditions. During normothermia, LBNP of 30 mmHg (highest level of LBNP achieved by the majority of subjects in both thermal conditions) did not significantly alter CBV, CVR, or MAP. During whole body heating, this LBNP decreased MAP (81+/-2 to 75+/-3 mmHg), decreased CBV (50+/-4 to 39+/-1 cm/s), and increased CVR (1.67+/-0.17 to 1.92+/-0.12 mmHg.cm-1.s); P<0.05. These data indicate that heat stress decreases CBV, and the reduction in CBV for a given orthostatic challenge is greater during heat stress. These outcomes reduce the reserve to buffer further decreases in cerebral perfusion before presyncope. Increases in CVR during whole body heating, coupled with even greater increases in CVR during orthostasis and heat stress, likely contribute to orthostatic intolerance.
authors.affiliation: Department of Pharmacology and Physiology, Drexel University College of Medicine, Philadelphia, PA, and Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, 7232 Greenville Ave., Dallas, TX 75231, USA.
authors.forename: Thad E
authors.initials: TE
authors.lastname: Wilson
authors.forename: Jian
authors.initials: J
authors.lastname: Cui
authors.forename: Rong
authors.initials: R
authors.lastname: Zhang
authors.forename: Craig G
authors.initials: CG
authors.lastname: Crandall
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D001831:Body Temperature / Q000502:physiology; D001921:Brain / Q000098:blood supply* / Q000502:physiology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D004244:Dizziness / Q000503:physiopathology*; D005260:Female; D006339:Heart Rate / Q000502:physiology; D018882:Heat Stress Disorders / Q000503:physiopathology*; D006801:Humans; D008150:Lower Body Negative Pressure; D008297:Male; D012039:Regional Blood Flow / Q000502:physiology; D017585:Ultrasonography, Doppler, Transcranial; D014655:Vascular Resistance / Q000502:physiology