Using an experimental model of extreme heat exposure that incorporates brief periods of light physical activity to simulate activities of daily living, the extent of thermal strain reported herein more accurately represents what would occur during actual heatwave conditions. Despite matching metabolic heat generation and environmental conditions, we show that older adults have augmented core temperature responses, likely due to age-related reductions in heat dissipating mechanisms.

Profound heat stress can damage the gastrointestinal barrier, leading to microbial translocation from the gut and subsequent systemic inflammation. Despite the greater vulnerability of older people to heat wave-related morbidity and mortality, it is unknown if age modulates gastrointestinal barrier damage and inflammation during heat stress. Therefore, the aim of this study was to determine if aging impacted enterocyte damage and systemic inflammatory responses to a 3-h exposure to very hot and dry (47 °C, 15% humidity) heat with accompanying activities of daily living (intermittent activity at 3 METS). Data from 16 young (age 21 to 39 years) and 16 older (age 65 to 76 years) humans were used to address this aim. In each group, log-transformed plasma concentrations of intestinal fatty acid binding protein (I-FABP_log), interleukin-8 (IL-8_log), and tissue factor (TF_log) were assessed as indices of enterocyte damage, systemic inflammation, and blood coagulation, respectively, before and after the 3-h heat exposure. In the younger cohort, I-FABP_log concentration did not increase from pre to post heat exposure (p = 0.264, d = 0.20), although it was elevated in the older group (p = 0.014, d = 0.67). The magnitude of the increase in I-FABP_log was greater in the older participants (p = 0.084, d = 0.55). Across all participants, there was no correlation between the change in core temperature and the change in IFABP_log. There was no change in IL-8_log in the younger group (p = 0.193, d = 0.23) following heat exposure, but we observed a decrease in IL-8_log in the older group (p = 0.047, d = 0.48). TF_log decreased in the younger group (p = 0.071, d = 0.41), but did not change in the older group (p = 0.193, d = 0.15). Our data indicate that I-FABP_log concentration (an index of enterocyte damage) is increased in older humans during a 3-h extreme heat exposure. Future studies should determine whether this marker reflects increased gastrointestinal barrier permeability in older individuals during heat exposure.

Comparing healthy young and older adults, we assessed left ventricular cardiac function (using echocardiography) during two separate 3-h extreme heat exposures in a very hot and dry or hot humid climate type. Although the augmentation of diastolic function and cardiac output were similar between age groups, older adults showed a greater increase in mitral annular systolic velocity with heat exposure, indicating a stronger reliance on systolic mechanisms to maintain stroke volume.

Herein we present plasma epinephrine and norepinephrine concentrations to 3 h of very hot and dry, as well as hot and humid, heat exposures with accompanying activities of daily living in young and older participants. We found 1) increased plasma concentrations of epinephrine in young and older adults following the hot and humid, but not the very hot and dry exposures and 2) decreased concentrations of norepinephrine in both groups following exposure to both conditions.

Addressing two unique aims, we observed that sublingual sufentanil administration does not impair tolerance or cardiovascular responses to lower-body negative pressure (LBNP)-induced progressive central hypovolemia. Second, despite pain perception being reduced, sublingual sufentanil did not attenuate mean blood pressure responses to a cold pressor test (CPT).

We present novel data on between-visit cerebrovascular and cardiovascular reactivity reproducibility during the CPT. Relative agreement was stronger for BP and mean MCAv reactivity during the second minute of the CPT, suggesting that future work should use protocols >1 min. Cerebrovascular and cardiovascular variables had similar reproducibility; changes in [Formula: see text] and perceived pain significantly predicted CVCi reactivity. Controlling [Formula: see text] could improve reproducibility; future work could consider pain perception while assessing cerebrovascular reactivity to the CPT.