With global warming, much attention has been paid to the upper limits of human adaptability. However, the time to reach a generally-accepted core temperature criterion (40.2°C) associated with heat-related illness above (uncompensable heat stress) and just below (compensable heat stress) the upper limits for heat balance remains unclear. Forty-eight (22 men/26 women; 23±4 y) subjects were exposed to progressive heat stress in an environmental chamber during minimal activity (MinAct, 159±34W) and light ambulation (LightAmb, 260±55W) in warm-humid (WH; ~35°C, >60% RH) and hot-dry (HD; 43-48°C, <25% RH) environments until heat stress became uncompensable. For each condition, we compared heat storage (S) and the change in gastrointestinal temperature (∆Tgi) over time during compensable and uncompensable heat stress. Using the slopes of the Tgi response, we estimated the time to reach Tgi=40.2°C. Finally, we examined whether individual characteristics or seasonality were associated with the rate of increase in Tgi. During compensable heat stress, S was higher in HD than in WH environments (p<0.05) resulting in a greater but more variable ∆Tgi (p≥0.06) for both metabolic rates. There were no differences among conditions during uncompensable heat stress (all p>0.05). There was no influence of sex, aerobic fitness, or seasonality, but a larger body size was associated with a greater ∆Tgi during LightAmb in WH (p=0.003). Sustained light activity without intervention in uncompensable thermal environments may result in a Tc of 40.2°C (from a 37°C baseline) in 3-7 hours even in young adults vs. several days under compensable heat stress.
The magnitude of blood pressure (BP) and muscle sympathetic nerve activity (MSNA) responses to laboratory stressors is commonly used to compare neurocardiovascular responsiveness between groups and conditions. However, no studies have rigorously examined the reproducibility of BP and MSNA responsiveness. Here, we assess the within-visit reproducibility of BP (finger photoplethysmography) and MSNA (microneurography) responses to isometric handgrip (HG) and post-exercise ischemia (PEI) in young healthy adults (n=30). In a subset (n=21), we also examined the between-visit reproducibility of responsiveness to HG, PEI, and the cold pressor test (CPT). Intraclass correlation coefficients (ICC) were used as a primary reproducibility measure (e.g., ICC >0.75 is considered very good). Within a visit, the increase in MAP during HG (ICC=0.85 [0.69-0.93]; p<0.001) and PEI (ICC=0.85 [0.69-0.93]; p<0.001) demonstrated very good reproducibility. Further, the between-visit reproducibility of the pressor response to HG (ICC=0.85 [0.62-0.94]; p<0.001), PEI (ICC=0.84 [CI=0.58-0.94]; p<0.001), and the CPT (ICC=0.89 [0.72-0.95] p<0.001) were also very good. However, there was greater variability in both the within- (HG: ICC=0.58 [-0.22-0.85], p=0.001; PEI: ICC=0.33 [-0.24-0.69], p=0.042) and between-visit reproducibility of MSNA responsiveness (HG: ICC=0.87 [0.53-0.96], p=0.001; PEI: ICC=0.24 [-0.62-0.78], p=0.27; CPT: ICC=0.77 [0.29-0.93], p=0.007). The magnitude of the BP response to several standard laboratory stimuli was very good, whereas the variability of the MSNA response to these perturbations was generally less consistent, particularly during PEI. These data provide novel insight for both study design and data interpretation when comparing neurocardiovascular responsiveness between different conditions, groups, or studies, as well as before and after interventions/treatments.
The PSU HEAT protocol has been used to determine critical environmental limits, i.e., those combinations of ambient temperature and humidity above which heat stress becomes uncompensable and core temperature rises continuously. However, no studies have rigorously investigated the reliability and validity of this experimental protocol. Here, we assessed the (1) between-visit reliability and (2) validity of the paradigm. Twelve subjects (5M/7W; 25±4 yr) completed a progressive heat stress protocol during which they walked on a treadmill (2.2 mph, 3% gradient) in a controllable environmental chamber. After an equilibration period, either dry-bulb temperature (Tdb) was increased every 5 min while ambient water vapor pressure (Pa) was held constant (Tcrit experiments) or Pa was increased every 5 min while Tdb was held constant (Pcrit experiments) until an upward inflection in gastrointestinal temperature (Tgi) was observed. For reliability experiments, 11 subjects repeated the same protocol on a different day. For validity experiments, 10 subjects performed a Tcrit experiment at their previously determined Pcrit or vice versa. The between-visit reliability (intraclass correlation coefficient, ICC) for critical environmental limits was 0.98. Similarly, there was excellent agreement between original and validity trials for Tcrit (ICC = 0.95) and Pcrit (ICC = 0.96). Further, the wet-bulb temperature at the Tgi inflection point was not different during reliability (p = 0.78) or validity (p = 0.32) trials compared to original trials. These findings support the reliability and validity of this experimental paradigm for the determination of critical environmental limits for maintenance of human heat balance.
In a series of studies, our laboratory has utilized microneurography coupled with laser Doppler flowmetry to examine sympathetic neural control of the cutaneous microvasculature during thermoregulatory stimuli. However, these techniques have limited clinical utility. Spectral analysis of the low frequency periodic oscillations in laser Doppler flowmetry‐derived red blood cell flux (i.e., skin flowmotion) is a noninvasive technique that has been used to quantify cutaneous microvascular control mechanisms (e.g., endothelial, neurogenic, myogenic). To date, conclusions regarding the mechanistic regulation of cutaneous microvascular function derived from spectral analysis have been primarily limited to basal conditions. Thus, the aim here was to examine the validity of spectral analysis to quantify sympathetic neural control of skin blood flow during a dynamic physiological stimulus (local skin heating to 42°C). We hypothesized that pharmacological blockade of sensory nerves would reduce the spectral power of the neurogenic frequency band during local heating. Eleven young healthy adults (7 women; 22±3 yrs, 25±2 kg/m2) participated. Laser Doppler flux was measured continuously during baseline (33°C) and a standardized local heating protocol (42°C) at a non‐treated control site (con) and a site treated with topical lidocaine (lido; 2.5% prilocaine+2.5% lidocaine) to pharmacologically block sensory nerves. Topical lido was applied for a minimum of 45 minutes prior to testing. Cutaneous vascular conductance was calculated and expressed as a percentage of maximum (local heat of 43°C). A raw Fourier transform was used to calculate spectral power around the 0.0095–0.02 Hz, 0.02–0.05 Hz, and 0.05–0.15 Hz frequency intervals, considered to correspond to endothelial, neurogenic, and myogenic activity, respectively. The initial peak of the local heating response was reduced by lido, (con: 62±2% vs. lido: 25±5%, p<0.01), validating adequate sensory nerve blockade. However, there was no difference between sites during baseline (con: 12±1% vs. lido: 9±2%, p=0.07) or the local heating‐induced plateau (con: 95±3% vs. lido: 87±7%, p=0.20). There was no effect of lido on spectral power of any of the frequency intervals at baseline or during local heating. Total spectral power (i.e., the sum of the spectral power for all frequency bands) was significantly related to conductance in both sites at baseline (con: R2=0.69, p<0.01; lido: R2=0.72, p<0.01) and during local heating (con: R2=0.22, p<0.02; lido: R2=0.71, p<0.01). In conclusion, the physiological increase in cutaneous vascular conductance during local heating was mirrored by an increase in total spectral power. Although blockade of sensory nerves reduced the increase in conductance during the initial peak of the local heating response, it did not affect the neurogenic frequency interval derived from spectral analysis. These preliminary data suggest that skin flowmotion may not be a valid approach to evaluate neural control of cutaneous microvascular function during dynamic condit...
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