title: Active and passive heat stress similarly compromise tolerance to a simulated hemorrhagic challenge.
abstract: Passive heat stress increases core and skin temperatures and reduces tolerance to simulated hemorrhage (lower body negative pressure; LBNP). We tested whether exercise-induced heat stress reduces LBNP tolerance to a greater extent relative to passive heat stress, when skin and core temperatures are similar. Eight participants (6 males, 32 ± 7 yr, 176 ± 8 cm, 77.0 ± 9.8 kg) underwent LBNP to presyncope on three separate and randomized occasions: 1) passive heat stress, 2) exercise in a hot environment (40°C) where skin temperature was moderate (36°C, active 36), and 3) exercise in a hot environment (40°C) where skin temperature was matched relative to that achieved during passive heat stress (∼38°C, active 38). LBNP tolerance was quantified using the cumulative stress index (CSI). Before LBNP, increases in core temperature from baseline were not different between trials (1.18 ± 0.20°C; P > 0.05). Also before LBNP, mean skin temperature was similar between passive heat stress (38.2 ± 0.5°C) and active 38 (38.2 ± 0.8°C; P = 0.90) trials, whereas it was reduced in the active 36 trial (36.6 ± 0.5°C; P ≤ 0.05 compared with passive heat stress and active 38). LBNP tolerance was not different between passive heat stress and active 38 trials (383 ± 223 and 322 ± 178 CSI, respectively; P = 0.12), but both were similarly reduced relative to active 36 (516 ± 147 CSI, both P ≤ 0.05). LBNP tolerance is not different between heat stresses induced either passively or by exercise in a hot environment when skin temperatures are similarly elevated. However, LBNP tolerance is influenced by the magnitude of the elevation in skin temperature following exercise induced heat stress.
authors: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas; School of Health Sciences, Cardiff Metropolitan University, Cardiff, United Kingdom;
authors: J
authors: J
authors: Pearson
authors: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas; Center for Global Health Research, Umea University, Umea, Sweden; and.
authors: R A I
authors: RA
authors: Lucas
authors: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas;
authors: Z J
authors: ZJ
authors: Schlader
authors: China Institute of Sport Science, Beijing, China.
authors: J
authors: J
authors: Zhao
authors: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas;
authors: D
authors: D
authors: Gagnon
authors: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and University of Texas Southwestern Medical Center, Dallas, Texas; craigcrandall@texashealth.org.
authors: C G
authors: CG
authors: Crandall
mesh_terms: D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D015444:Exercise / Q000502:physiology*; D005260:Female; D018882:Heat Stress Disorders / Q000503:physiopathology*; D006358:Hot Temperature / Q000009:adverse effects; D006801:Humans; D008150:Lower Body Negative Pressure / Q000379:methods; D008297:Male; D012881:Skin Temperature / Q000502:physiology*; D013575:Syncope / Q000503:physiopathology
keywords: exercise; heat stress; orthostatic tolerance