E.A. Larson1, B.R. Ely1, M.A. Francisco1, V.E. Brunt1,2, S.M. Harris1, J.R. Halliwill, FACSM1, C.T. Minson, FACSM1

1University of Oregon, Eugene, OR; 2University of Colorado Boulder, Boulder, CO

Increased conduit vessel shear stress acutely improves vascular function in a dose-dependent manner and potentiates many of the beneficial vascular adaptations that accompany chronic heat therapy. However, physical and physiologic differences exist between sexes which influence thermoregulation and may alter the adaptive shear stimulus accompanying a single bout of passive heat stress. PURPOSE: To compare the hemodynamic response to acute passive heat exposure (APHE) between sexes. METHODS: 10 women (W) and 11 men (M) completed a one-hour APHE session immersed to the level of the sternum in 40°C water. Rectal temperature (Tre), mean arterial pressure (MAP), and brachial artery hemodynamics (ultrasound) were measured at baseline (seated rest before APHE) and every 15 min throughout APHE. Subject characteristics were compared using unpaired t-tests. A 2 X 5 mixed model ANOVA was used to compare Tre, MAP, and brachial hemodynamics across time and between sexes. RESULTS: Both body mass (W: 62.9 ± 8.1 kg, M: 74.2 ± 5.1 kg, P < 0.01) and body surface area (W: 1.71 ± 0.14 m2 M: 1.93 ± 0.10 m2, P < 0.001) were lower in W compared to M. W had a higher T than M (P < 0.05) at baseline (W: 37.5 ± 0.1°C, M: 37.2 ± 0.1°C) and after 60 min APHE (W: 38.7 ± 0.03°C, M: 38.5 ± 0.1°C). W had a lower MAP than M (P < 0.05) at baseline (W: 85 ± 2 mmHg, M: 88 ± 2 mmHg) and throughout APHE (W: 74 ± 2 mmHg, M: 82 ± 3 mmHg at 60 min APHE). W and M had a similar baseline brachial shear rate (W: 170 ± 30 1/s, M: 105 ± 19 1/s). Brachial shear rate was elevated to a greater extent in W than M during APHE (P < 0.01), reaching 651 ± 49 1/s and 396 ± 24 1/s at 60 min APHE, respectively. The sex difference in brachial shear response to APHE was the result of a greater increase in brachial velocity seen in W (+47 ± 6 cm/s) compared to M (+35 ± 4 cm/s) with APHE (P < 0.1). This elevated brachial velocity allowed for a similar increase in brachial blood flow between sexes with APHE (W: +339 ± 53 mL/min, M: +369 ± 37 mL/min) despite the smaller brachial diameter in W compared to M (P < 0.0001) at baseline (W: 0.31 ± 0.01 cm, M: 0.41 ± 0.01 cm) and throughout APHE (W: 0.37 ± 0.01 cm, M: 0.46 ± 0.01 cm at 60 min APHE). CONCLUSION: These data indicate the presence of a sex difference in the hemodynamic response to a single bout of passive heat stress and may have implications for sex-specific vascular adaption accompanying chronic heat therapy.

Supported by APS Porter Pre-Doctoral Fellowship.

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