Diagnostic accuracy of thermal, hydration, and heart rate assessments in discriminating positive acute kidney injury risk following physical work in the heat

Occupational heat stress increases the risk of acute kidney injury (AKI). This study presents a secondary analysis to generate novel hypotheses for future studies by investigating the diagnostic accuracy of thermal, hydration, and heart rate assessments in discriminating positive AKI risk following physical work in the heat in unacclimatized individuals. Unacclimatized participants (n = 13, 3 women, age: ∼23 years) completed four trials involving 2 h of exercise in a 39.7 ± 0.6 °C, 32 ± 3% relative humidity environment that differed by experimental manipulation of hyperthermia (i.e., cooling intervention) and dehydration (i.e., water drinking). Diagnostic accuracy was assessed via receiver operating characteristic curve analysis. Positive AKI risk was identified when the product of concentrations insulin-like growth factor binding protein 7 and tissue inhibitor of metalloproteinase-2 [IGFBP7∙TIMP-2] exceeded 0.3 (ng∙mL-1)2∙1000-1. Peak absolute core temperature had the acceptable discriminatory ability (AUC = 0.71, p = 0.009), but a relatively large variance (AUC 95% CI: 0.57-0.86). Mean body temperature, urine specific gravity, urine osmolality, peak heart rate, and the peak percent of both maximum heart rate and heart rate reserve had poor discrimination (AUC = 0.66-0.69, p ≤ 0.051). Mean skin temperature, percent change in body mass and plasma volume, and serum sodium and osmolality had no discrimination (p ≥ 0.072). A peak increase in mean skin temperature of >4.7 °C had a positive likelihood ratio of 11.0 which suggests clinical significance. These data suggest that the absolute value of peak core temperature and the increase in mean skin temperature may be valuable to pursue in future studies as a biomarker for AKI risk in unacclimatized workers.

Renal vascular control during normothermia and passive heat stress does not differ between healthy younger men and women

Men are likely at greater risk for heat-induced acute kidney injury compared with women, possibly due to differences in vascular control. We tested the hypothesis that the renal vasoconstrictor and vasodilator responses will be greater in younger women compared with men during passive heat stress. Twenty-five healthy adults [12 women (early follicular phase) and 13 men] completed two experimental visits, heat stress or normothermic time-control, assigned in a block-randomized crossover design. During heat stress, participants wore a water-perfused suit perfused with 50°C water. Core temperature was increased by ∼0.8°C in the first hour before commencing a 2-min cold pressor test (CPT). Core temperature remained clamped and at 1-h post-CPT, subjects ingested a whey protein shake (1.2 g of protein/kg body wt), and measurements were taken pre-, 75 min, and 150 min post-protein. Beat-to-beat blood pressure (Penaz method) was measured and segmental artery vascular resistance (VR, Doppler ultrasound) was calculated as segmental artery blood velocity ÷ mean arterial pressure. CPT-induced increases in segmental artery VR did not differ between trials (trial effect: P = 0.142) nor between men (heat stress: 1.5 ± 1.0 mmHg/cm/s, normothermia: 1.4 ± 1.0 mmHg/cm/s) and women (heat stress: 1.4 ± 1.2 mmHg/cm/s, normothermia: 2.1 ± 1.1 mmHg/cm/s) (group effect: P = 0.429). Reductions in segmental artery VR following oral protein loading did not differ between trials (trial effect: P = 0.080) nor between men (heat stress: -0.6 ± 0.8 mmHg/cm/s, normothermia: -0.6 ± 0.6 mmHg/cm/s) and women (heat stress: -0.5 ± 0.5 mmHg/cm/s, normothermia: -1.1 ± 0.6 mmHg/cm/s) (group effect: P = 0.204). Renal vasoconstrictor responses to the cold pressor test and vasodilator responses following an oral protein load during heat stress or normothermia do not differ between younger men and younger women in the early follicular phase of the menstrual cycle.NEW & NOTEWORTHY The mechanisms underlying greater heat-induced acute kidney injury risk in men versus women remain unknown. This study examined renal vascular control, including both vasodilatory (oral protein load) and vasoconstrictor (cold presser test) responses, during normothermia and heat stress and compared these responses between men and women. The results indicated that in both conditions neither renal vasodilatory nor vasoconstrictor responses differ between younger men and younger women.

Activation of cardiac parasympathetic and sympathetic activity occurs at different skin temperatures during face cooling

Sufficiently cold-water temperatures (<7°C) are needed to elicit the sympathetic response to the cold pressor test using the hand. However, it is not known if stimulating the trigeminal nerve via face cooling, which increases both sympathetic and cardiac parasympathetic activity, also has a threshold temperature. We tested the hypothesis that peak autonomic activation during a progressive face cooling challenge would be achieved when the stimulus temperature is ≤7°C. Twelve healthy participants (age: 25 ± 3 yr, four women) completed our study. Six pliable bags, each containing water or an ice slurry (34°C, 28°C, 21°C, 14°C, 7°C, and 0°C) were applied sequentially to participants' forehead, eyes, and cheeks for 5 min each. Mean arterial pressure (photoplethysmography; index of sympathetic activity) and heart rhythm (3-lead ECG) were averaged in 1-min increments at the end of baseline and throughout each temperature condition. Heart rate variability in the time [(root mean square of successive differences (RMSSD)] and frequency [high-frequency (HF) power] domains was used to estimate cardiac parasympathetic activity. Data are presented as the increase from baseline ± SD. Mean arterial pressure only increased from baseline in the 7°C (13.1 ± 10.3 mmHg; P = 0.018) and 0°C (25.2 ± 7.8 mmHg; P < 0.001) conditions. Only the 0°C condition increased RMSSD (160.6 ± 208.9 ms; P = 0.009) and HF power (11,450 ± 14,555 ms2; P = 0.014) from baseline. Our data indicate that peak increases in sympathetic activity during face cooling are initiated at a higher forehead skin temperature than peak increases in cardiac parasympathetic activity.

Hypohydration attenuates increases in creatinine clearance to oral protein loading and the renal hemodynamic response to exercise pressor reflex

Insufficient hydration is prevalent among free living adults. This study investigated whether hypohydration alters 1) renal functional reserve, 2) the renal hemodynamic response to the exercise pressor reflex, and 3) urine-concentrating ability during oral protein loading. In a block-randomized crossover design, 22 healthy young adults (11 females and 11 males) underwent 24-h fluid deprivation (Hypohydrated) or 24-h normal fluid consumption (Euhydrated). Renal functional reserve was assessed by oral protein loading. Renal hemodynamics during the exercise pressor reflex were assessed via Doppler ultrasound. Urine-concentrating ability was assessed via free water clearance. Creatinine clearance did not differ at 150 min postprotein consumption between conditions [Hypohydrated: 246 mL/min, 95% confidence interval (CI): 212-280; Euhydrated: 231 mL/min, 95% CI: 196-265, P = 0.2691] despite an elevated baseline in Hypohydrated (261 mL/min, 95% CI: 218-303 vs. 143 mL/min, 95% CI: 118-168, P < 0.0001). Renal artery vascular resistance was not different at baseline (P = 0.9290), but increases were attenuated in Hypohydrated versus Euhydrated at the end of handgrip (0.5 mmHg/cm/s, 95% CI: 0.4-0.7 vs. 0.8 mmHg/cm/s 95% CI: 0.6-1.1, P = 0.0203) and end occlusion (0.2 mmHg/cm/s, 95% CI: 0.1-0.3 vs. 0.4 mmHg/cm/s 95% CI: 0.3-0.6, P = 0.0127). There were no differences between conditions in free water clearance at 150 min postprotein (P = 0.3489). These data indicate that hypohydration 1) engages renal functional reserve and attenuates the ability to further increase creatinine clearance, 2) attenuates increases in renal artery vascular resistance to the exercise pressor reflex, and 3) does not further enhance nor impair urine-concentrating ability during oral protein loading.NEW & NOTEWORTHY Insufficient hydration is prevalent among free living adults. This study found that hypohydration induced by 24-h fluid deprivation engaged renal functional reserve and that oral protein loading did not further increase creatinine clearance. Hypohydration also attenuated the ability to increase renal vascular resistance during the exercise pressor reflex. In addition, hypohydration neither enhanced nor impaired urine-concentrating ability during oral protein loading. These data support the importance of mitigating hypohydration in free living adults.

Cardiovascular and mood responses to an acute bout of cold water immersion

Cold water immersion (CWI) may provide benefits for physical and mental health. Our purpose was to investigate the effects of an acute bout of CWI on vascular shear stress and affect (positive and negative). Sixteen healthy adults (age: 23 ± 4 y; (9 self-reported men and 7 self-reported women) completed one 15-min bout of CWI (10 °C). Self-reported affect (positive and negative) was assessed at pre-CWI (Pre), 30-min post-immersion, and 180-min post-immersion in all participants. Brachial artery diameter and blood velocity were measured (Doppler ultrasound) at Pre, after 1-min and 15-min of CWI, and 30-min post-immersion (n = 8). Total, antegrade, and retrograde shear stress, oscillatory shear index (OSI), and forearm vascular conductance (FVC) were calculated. Venous blood samples were collected at Pre, after 1-min and 15-min of CWI, 30-min post-immersion, and 180-min post-immersion (n = 8) to quantify serum β-endorphins and cortisol. Data were analyzed using a one-way ANOVA with Fisher's least significance difference and compared to Pre. Positive affect did not change (ANOVA p = 0.450) but negative affect was lower at 180-min post-immersion (p < 0.001). FVC was reduced at 15-min of CWI and 30-min post-immersion (p < 0.020). Total and antegrade shear and OSI were reduced at 30-min post-immersion (p < 0.040) but there were no differences in retrograde shear (ANOVA p = 0.134). β-endorphins did not change throughout the trial (ANOVA p = 0.321). Cortisol was lower at 180-min post-immersion (p = 0.014). An acute bout of CWI minimally affects shear stress patterns but may benefit mental health by reducing negative feelings and cortisol levels.

Acute Kidney Injury Biomarkers and Hydration Assessments Following Prolonged Mild Hypohydration in Healthy Young Adults

The high prevalence of inadequate hydration (e.g., hypohydration, underhydration) is concerning given that extreme heat increases excess hospitalizations for fluid/electrolyte disorders and acute kidney injury (AKI). Inadequate hydration may also be related to renal and cardiometabolic disease development. This study tested the hypothesis that prolonged mild hypohydration increases the urinary AKI biomarker [IGFBP7∙TIMP-2] (product of insulin-like growth factor binding-protein 7 and tissue inhibitor of metalloproteinase-2) compared to euhydration. Additionally, we determined the diagnostic accuracy and optimal cut-offs of hydration assessments for discriminating positive AKI risk ([IGFBP∙TIMP-2] >0.3 ng∙ml^-1)²∙1000^-1). In a block-randomized crossover design, twenty-two healthy young adults (11 females, 11 males) completed 24 h of fluid deprivation (Hypohydrated) or 24 h normal fluid consumption (Euhydrated) separated by ≥72 h. Urinary [IGFBP7∙TIMP-2] and other AKI biomarkers were measured following the 24-h protocols. Diagnostic accuracy was assessed via receiver operating characteristic curve analysis. Urinary [IGFBP7∙TIMP-2] [1.9 (95% CI: 1.0-2.8) vs. 0.2 (95% CI: 0.1-0.3) (ng∙ml^-1)²∙1000^-1, P=0.0011] was markedly increased in Hypohydrated vs. Euhydrated. Urine osmolality [AUC: 0.91, P<0.0001] and urine specific gravity [AUC: 0.89, P<0.0001] had the highest overall performance for discriminating positive AKI risk. Optimal cut-offs with positive likelihood ratio of 11.8 for both urine osmolality and specific gravity were 952 mOsm/kg and 1.025 a.u. In conclusion, prolonged mild hypohydration increased urinary [IGFBP7∙TIMP-2] in males and females. Urinary [IGFBP7∙TIMP-2] corrected to urine concentration was elevated in males only. Urine osmolality and urine specific gravity may have clinical utility for discriminating positive AKI risk following prolonged mild hypohydration.

Acute beetroot juice consumption does not alter cerebral autoregulation or cardiovagal baroreflex sensitivity during lower-body negative pressure in healthy adults

Introduction

Beetroot juice (BRJ) improves peripheral endothelial function and vascular compliance, likely due to increased nitric oxide bioavailability. It is unknown if BRJ alters cerebrovascular function and cardiovagal baroreflex control in healthy individuals.

Purpose

We tested the hypotheses that BRJ consumption improves cerebral autoregulation (CA) and cardiovagal baroreflex sensitivity (cBRS) during lower-body negative pressure (LBNP).

Methods

Thirteen healthy adults (age: 26 ± 4 years; 5 women) performed oscillatory (O-LBNP) and static LBNP (S-LBNP) before (PRE) and 3 h after consuming 500 mL of BRJ (POST). Participants inhaled 3% CO2 (21% O2, 76% N2) during a 5 min baseline and throughout LBNP to attenuate reductions in end-tidal CO2 tension (PETCO2). O-LBNP was conducted at ∼0.02 Hz for six cycles (-70 mmHg), followed by a 3-min recovery before S-LBNP (-40 mmHg) for 7 min. Beat-to-beat middle cerebral artery blood velocity (MCAv) (transcranial Doppler) and blood pressure were continuously recorded. CA was assessed using transfer function analysis to calculate coherence, gain, and phase in the very-low-frequency (VLF; 0.020-0.070 Hz) and low-frequency bands (LF; 0.07-0.20 Hz). cBRS was calculated using the sequence method. Comparisons between POST vs. PRE are reported as mean ± SD.

Results

During O-LBNP, coherence VLF was greater at POST (0.55 ± 0.06 vs. 0.46 ± 0.08; P < 0.01), but phase VLF (P = 0.17) and gain VLF (P = 0.69) were not different. Coherence LF and phase LF were not different, but gain LF was lower at POST (1.03 ± 0.20 vs. 1.12 ± 0.30 cm/s/mmHg; P = 0.05). During S-LBNP, CA was not different in the VLF or LF bands (all P > 0.10). Up-cBRS and Down-cBRS were not different during both LBNP protocols.

Conclusion

These preliminary data indicate that CA and cBRS during LBNP in healthy, young adults is largely unaffected by an acute bolus of BRJ.

Glomerular filtration rate reserve is reduced during mild passive heat stress in healthy young adults

We tested the hypothesis that, compared with normothermia, the increase in glomerular filtration rate (GFR) after an oral protein load (defined as the GFR reserve) is attenuated during moderate passive heat stress in young healthy adults. Sixteen participants (5 women; 26 ± 2 yr) completed two experimental visits, heat stress or a normothermic time-control, assigned in a block-randomized crossover design. During the heat stress trial, core temperature was increased by 0.6°C in the first hour before commencing a 2-min cold pressor test (CPT) to assess renal vasoconstrictor responses. One-hour post-CPT, subjects ingested a whey protein shake (1.2 g of protein/kg body wt), and measurements were taken pre-, 75, and 150 min postprotein. Segmental artery vascular resistance was calculated as the quotient of Doppler ultrasound-derived segmental artery blood velocity and mean arterial pressure and provided an estimate of renal vascular tone. GFR was estimated from creatinine clearance. The increase in segmental artery vascular resistance during the CPT was attenuated during heat stress (end CPT: 5.6 ± 0.9 vs. 4.7 ± 1.1 mmHg/cm/s, P = 0.024). However, the reduction in segmental artery vascular resistance in response to an oral protein load did not differ between heat stress (at 150 min: 1.9 ± 0.4 mmHg/cm/s) and normothermia (at 150 min: 1.8 ± 0.5 mmHg/cm/s; P = 0.979). The peak increase in creatinine clearance postprotein, independent of time, was attenuated during heat stress (+26 ± 19 vs. +16 ± 20 mL/min, P = 0.013, n = 13). GFR reserve is diminished by mild passive heat stress. Moreover, renal vasoconstrictor responses are attenuated by mild passive heat stress, but renal vasodilator responses are maintained.

Kidney injury risk during prolonged exposure to current and projected wet bulb temperatures occurring during extreme heat events in healthy young men

Wet bulb temperatures (Twet) during extreme heat events are commonly 31°C. Recent predictions indicate that Twet will approach or exceed 34°C. Epidemiological data indicate that exposure to extreme heat events increases kidney injury risk. We tested the hypothesis that kidney injury risk is elevated to a greater extent during prolonged exposure to Twet = 34°C compared with Twet = 31°C. Fifteen healthy men rested for 8 h in Twet = 31 (0)°C and Twet = 34 (0)°C. Insulin-like growth factor-binding protein 7 (IGFBP7), tissue inhibitor of metalloproteinase 2 (TIMP-2), and thioredoxin 1 (TRX-1) were measured from urine samples. The primary outcome was the product of IGFBP7 and TIMP-2 ([IGFBP7·TIMP-2]), which provided an index of kidney injury risk. Plasma interleukin-17a (IL-17a) was also measured. Data are presented at preexposure and after 8 h of exposure and as mean (SD) change from preexposure. The increase in [IGFBP7·TIMP-2] was markedly greater at 8 h in the 34°C [+26.9 (27.1) (ng/mL)2/1,000) compared with the 31°C [+6.2 (6.5) (ng/mL)2/1,000] trial (P < 0.01). Urine TRX-1, a marker of renal oxidative stress, was higher at 8 h in the 34°C [+77.6 (47.5) ng/min] compared with the 31°C [+16.2 (25.1) ng/min] trial (P < 0.01). Plasma IL-17a, an inflammatory marker, was elevated at 8 h in the 34°C [+199.3 (90.0) fg/dL; P < 0.01] compared with the 31°C [+9.0 (95.7) fg/dL] trial. Kidney injury risk is exacerbated during prolonged resting exposures to Twet experienced during future extreme heat events (34°C) compared with that experienced currently (31°C), likely because of oxidative stress and inflammatory processes.NEW AND NOTEWORTHY We have demonstrated that kidney injury risk is increased when men are exposed over an 8-h period to a wet bulb temperature of 31°C and exacerbated at a wet bulb temperature of 34°C. Importantly, these heat stress conditions parallel those that are encountered during current (31°C) and future (34°C) extreme heat events. The kidney injury biomarker analyses indicate both the proximal and distal tubules as the locations of potential renal injury and that the injury is likely due to oxidative stress and inflammation.

Voluntary Cooling during Exercise Is Augmented in People with Multiple Sclerosis Who Experience Heat Sensitivity

INTRODUCTION

We tested the hypothesis that people with multiple sclerosis (MS) who experience heat sensitivity voluntarily engage in cool-seeking behavior during exercise to a greater extent than healthy controls.

METHODS

In a 27.0°C ± 0.2°C, 41% ± 2% RH environment, seven participants with relapsing-remitting MS who exhibited heat sensitivity and seven healthy controls completed two randomized trials cycling for 40 min (EX) at 3.5 W·kg-1 metabolic heat production, followed by 30 min recovery (REC). In one trial, participants were restricted from engaging in cooling (CON). In the other trial, participants voluntarily pressed a button to receive 2 min of ~2°C water perfusing a top (COOL). Mean skin and core temperatures and mean skin wettedness were recorded continuously. Total time in cooling provided an index of cool-seeking behavior. RPE, total symptom scores (MS only), and subjective fatigue (MS only) were recorded every 10 min.

RESULTS

Core temperature (+0.5°C ± 0.1°C) and skin wettedness (+0.53 ± 0.02 a.u.) increased but were not different between groups or trials at end exercise (P = 0.196) or end recovery (P = 0.342). Mean skin temperature was reduced in COOL compared with CON at end exercise (P ≤ 0.002), with no differences between groups (P ≥ 0.532). MS spent more total time in cooling during EX (MS, 13 ± 3 min; healthy, 7 ± 4 min; P < 0.001) but not REC (MS, 2 ± 1 min; healthy, 0 ± 1 min; P = 0.496). RPE was greater at end exercise in MS (P = 0.001). Total symptom scores increased during exercise (P = 0.005) but was not different between trials (P = 0.321), whereas subjective fatigue was not attenuated in the cooling trial (P = 0.065).

CONCLUSION

Voluntary cooling is augmented in MS but does not consistently mitigate perceptions of heat-related symptoms or subjective fatigue.

Occupational heat exposure and the risk of chronic kidney disease of nontraditional origin in the United States

Occupational heat exposure is linked to the development of kidney injury and disease in individuals who frequently perform physically demanding work in the heat. For instance, in Central America, an epidemic of chronic kidney disease of nontraditional origin (CKDnt) is occurring among manual laborers, whereas potentially related epidemics have emerged in India and Sri Lanka. There is growing concern that workers in the United States suffer with CKDnt, but reports are limited. One of the leading hypotheses is that repetitive kidney injury caused by physical work in the heat can progress to CKDnt. Whether heat stress is the primary causal agent or accelerates existing underlying pathology remains contested. However, the current evidence supports that heat stress induces tubular kidney injury, which is worsened by higher core temperatures, dehydration, longer work durations, muscle damaging exercise, and consumption of beverages containing high levels of fructose. The purpose of this narrative review is to identify occupations that may place US workers at greater risk of kidney injury and CKDnt. Specifically, we reviewed the scientific literature to characterize the demographics, environmental conditions, physiological strain (i.e., core temperature increase, dehydration, heart rate), and work durations in sectors typically experiencing occupational heat exposure, including farming, wildland firefighting, landscaping, and utilities. Overall, the surprisingly limited available evidence characterizing occupational heat exposure in US workers supports the need for future investigations to understand this risk of CKDnt.

Acute Beetroot Juice Ingestion Does Not Alter Renal Hemodynamics during Normoxia and Mild Hypercapnia in Healthy Young Adults

Arterial hypercapnia reduces renal perfusion. Beetroot juice (BRJ) increases nitric oxide bioavailability and may improve renal blood flow. We tested the hypothesis that acute consumption of BRJ attenuates both decreases in blood velocity and increases in vascular resistance in the renal and segmental arteries during acute hypercapnia. In fourteen healthy young adults, blood velocity and vascular resistance were measured with Doppler ultrasound in the renal and segmental arteries during five minutes of breathing a carbon dioxide gas mixture (CO₂) before and three hours after consuming 500 mL of BRJ. There was no difference between pre- and post-BRJ consumption in the increase in the partial pressure of end-tidal CO₂ during CO₂ breathing (pre: +4 ± 1 mmHg; post: +4 ± 2 mmHg, p = 0.4281). Segmental artery blood velocity decreased during CO₂ breathing in both pre- (by −1.8 ± 1.9 cm/s, p = 0.0193) and post-BRJ (by −2.1 ± 1.9 cm/s, p = 0.0079), but there were no differences between pre- and post-consumption (p = 0.7633). Segmental artery vascular resistance increased from room air baseline during CO₂ at pre-BRJ consumption (by 0.4 ± 0.4 mmHg/cm/s, p = 0.0153) but not post-BRJ (p = 0.1336), with no differences between pre- and post-consumption (p = 0.7407). These findings indicate that BRJ consumption does not attenuate reductions in renal perfusion during acute mild hypercapnia in healthy young adults.

Sugar-sweetened soft drink consumption acutely decreases spontaneous baroreflex sensitivity and heart rate variability

In healthy humans, fructose-sweetened water consumption increases blood pressure variability (BPV) and decreases spontaneous cardiovagal baroreflex sensitivity (cBRS) and heart rate variability (HRV). However, whether consuming commercially available soft drinks containing high levels of fructose elicits similar responses is unknown. We hypothesized that high-fructose corn syrup (HFCS)-sweetened soft drink consumption increases BPV and decreases cBRS and HRV to a greater extent compared with artificially sweetened (diet) and sucrose-sweetened (sucrose) soft drinks and water. Twelve subjects completed four randomized, double-blinded trials in which they drank 500 mL of water or commercially available soft drinks matched for taste and caffeine content. We continuously measured beat-to-beat blood pressure (photoplethysmography) and R-R interval (ECG) before and 30 min after drink consumption during supine rest for 5 min during spontaneous and paced breathing. BPV was evaluated using standard deviation (SD), average real variability (ARV), and successive variation (SV) methods for systolic and diastolic blood pressure. cBRS was assessed using the sequence method. HRV was evaluated using the root mean square of successive differences (RMSSD) in R-R interval. There were no differences between conditions in the magnitude of change from baseline in SD, ARV, and SV (P ≥ 0.07). There were greater reductions in cBRS during spontaneous breathing in the HFCS (-3 ± 5 ms/mmHg) and sucrose (-3 ± 5 ms/mmHg) trials compared with the water trial (+1 ± 5 ms/mmHg, P < 0.03). During paced breathing, HFCS evoked greater reductions in RMSSD compared with water (-26 ± 34 vs. +2 ± 26 ms, P < 0.01). These findings suggest that sugar-sweetened soft drink consumption alters cBRS and HRV but not BPV.

The requirement for physical effort reduces voluntary cooling behavior during heat exposure in humans

We tested the hypothesis that cool-seeking behavior during heat exposure is attenuated when physical effort is required. Twelve healthy adults (mean(SD), 24(4) years, four women) underwent three experimental trials during two hours of exposure to 41(1) °C, 20(0)% relative humidity in which subjects undertook intermittent exercise alternating between seated rest and cycling exercise at ~4 metabolic equivalents every 15 min. In all trials, subjects wore a water perfused suit top. In the control trial (Control), no water perfused the suit. In the other trials, subjects were freely able to perfuse 2.1(0.2) °C water through the suit. In one cooling trial, subjects received two minutes of cooling by pressing a button (Button). The other cooling trial permitted cooling by engaging in isometric handgrip exercise at 15% of maximal grip strength (Handgrip), with cooling maintained throughout the duration the required force was produced or until two minutes elapsed. In both Button and Handgrip, a one-minute washout proceeded cooling. Core temperature increased over time in all trials (P<0.01) and there were no differences between trials (P = 0.32). Mean skin temperature at the end of heat exposure was lowest in Button [34.2(1.5) °C] compared to Handgrip [35.6(0.8) °C, P = 0.03] and Control [36.9(0.7) °C, P<0.01]. The total number of behaviors [8(3) vs. 10(5), P = 0.04] and cumulative cooling time [850(323) vs. 1230(616) seconds, P = 0.02] were lower in Handgrip compared to Button. These data indicate that when physical effort is required, the incidence and duration of cooling behavior during heat exposure is attenuated compared to when behaving requires minimal physical effort.

Kidney physiology and pathophysiology during heat stress and the modification by exercise, dehydration, heat acclimation and aging

The kidneys' integrative responses to heat stress aid thermoregulation, cardiovascular control, and water and electrolyte regulation. Recent evidence suggests the kidneys are at increased risk of pathological events during heat stress, namely acute kidney injury (AKI), and that this risk is compounded by dehydration and exercise. This heat stress related AKI is believed to contribute to the epidemic of chronic kidney disease (CKD) occurring in occupational settings. It is estimated that AKI and CKD affect upwards of 45 million individuals in the global workforce. Water and electrolyte disturbances and AKI, both of which are representative of kidney-related pathology, are the two leading causes of hospitalizations during heat waves in older adults. Structural and physiological alterations in aging kidneys likely contribute to this increased risk. With this background, this comprehensive narrative review will provide the first aggregation of research into the integrative physiological response of the kidneys to heat stress. While the focus of this review is on the human kidneys, we will utilize both human and animal data to describe these responses to passive and exercise heat stress, and how they are altered with heat acclimation. Additionally, we will discuss recent studies that indicate an increased risk of AKI due to exercise in the heat. Lastly, we will introduce the emerging public health crisis of older adults during extreme heat events and how the aging kidneys may be more susceptible to injury during heat stress.

Increased skin wetness independently augments cool-seeking behaviour during passive heat stress

KEY POINTS

Skin wetness occurring secondary to the build-up of sweat on the skin provokes thermal discomfort, the precursor to engaging in cool-seeking behaviour. Associative evidence indicates that skin wetness stimulates cool-seeking behaviour to a greater extent than increases in core and mean skin temperatures. The independent contribution of skin wetness to cool-seeking behaviour during heat stress has never been established. We demonstrate that skin wetness augments cool-seeking behaviour during passive heat stress independently of differential increases in skin temperature and core temperature. We also identify that perceptions of skin wetness were not elevated despite increases in actual skin wetness. These data support the proposition that afferent signalling from skin wetness enhances the desire to engage in cool-seeking behaviour during passive heat stress.

ABSTRACT

This study tested the hypothesis that elevations in skin wetness augments cool-seeking behaviour during passive heat stress. Twelve subjects (6 females, age: 24 ± 2 y) donned a water-perfused suit circulating 34 °C water and completed two trials resting supine in a 28.5 ± 0.4 °C environment. The trials involved a 20 min baseline period (26 ± 3% relative humidity (RH)), 60 min while ambient humidity was maintained at 26±3% RH (LOW) or increased to 67 ± 5% RH (HIGH), followed by 60 min passive heat stress (HS) where the water temperature in the suit was incrementally increased to 50 °C. Subjects were able to seek cooling when their neck was thermally uncomfortable by pressing a button. Each button press initiated 30 s of -20 °C fluid perfusing through a custom-made device secured against the skin on the dorsal neck. Mean skin (T_skin ) and core (T_core ) temperatures, mean skin wetness (W_skin ) and neck device temperature (T_device ) were measured continuously. Cool-seeking behaviour was determined from total time receiving cooling (TT_cool ) and cumulative button presses. T_skin and T_core increased during HS (P < 0.01) but were not different between conditions (P ≥ 0.11). W_skin was elevated in HIGH vs. LOW during HS (60 min: by + 0.06 ± 0.07 a.u., P ≤ 0.04). T_device was lower in HIGH vs. LOW at 40-50 min of HS (P ≤ 0.01). TT_cool was greater for HIGH (330 ± 172 s) vs. LOW (225 ± 167 s, P < 0.01), while the number of cumulative button presses was greater from 40-60 min in HS for HIGH vs. LOW (P ≤ 0.04). Increased skin wetness amplifies the engagement in cool-seeking behaviour during passive heat stress.

High-fructose corn syrup-sweetened soft drink consumption increases vascular resistance in the kidneys at rest and during sympathetic activation

We first tested the hypothesis that consuming a high-fructose corn syrup (HFCS)-sweetened soft drink augments kidney vasoconstriction to sympathetic stimulation compared with water (study 1). In a second study, we examined the mechanisms underlying these observations (study 2). In study 1, 13 healthy adults completed a cold pressor test, a sympathoexcitatory maneuver, before (preconsumption) and 30 min after drinking 500 mL of decarbonated HFCS-sweetened soft drink or water (postconsumption). In study 2, venous blood samples were obtained in 12 healthy adults before and 30 min after consumption of 500 mL water or soft drinks matched for caffeine content and taste, which were either artificially sweetened (Diet trial), sucrose-sweetened (Sucrose trial), or sweetened with HFCS (HFCS trial). In both study 1 and study 2, vascular resistance was calculated as mean arterial pressure divided by blood velocity, which was measured via Doppler ultrasound in renal and segmental arteries. In study 1, HFCS consumption increased vascular resistance in the segmental artery at rest (by 0.5 ± 0.6 mmHg·cm^-1·s^-1, P = 0.01) and during the cold pressor test (average change: 0.5 ± 1.0 mmHg·cm^-1·s^-1, main effect: P = 0.05). In study 2, segmental artery vascular resistance increased in the HFCS trial (by 0.8 ± 0.7 mmHg·cm^-1·s^-1, P = 0.02) but not in the other trials. Increases in serum uric acid were greater in the HFCS trial (0.3 ± 0.4 mg/dL, P ≤ 0.04) compared with the Water and Diet trials, and serum copeptin increased in the HFCS trial (by 0.8 ± 1.0 pmol/L, P = 0.06). These findings indicate that HFCS acutely increases vascular resistance in the kidneys, independent of caffeine content and beverage osmolality, which likely occurs via simultaneous elevations in circulating uric acid and vasopressin.

Renal and segmental artery hemodynamic response to acute, mild hypercapnia

Profound increases (>15 mmHg) in arterial carbon dioxide (i.e., hypercapnia) reduce renal blood flow. However, a relatively brief and mild hypercapnia can occur in patients with sleep apnea or in those receiving supplemental oxygen therapy during an acute exacerbation of chronic obstructive pulmonary disease. We tested the hypothesis that a brief, mild hypercapnic exposure increases vascular resistance in the renal and segmental arteries. Blood velocity in 14 healthy adults (26 ± 4 yr; 7 women, 7 men) was measured in the renal and segmental arteries with Doppler ultrasound while subjects breathed room air (Air) and while they breathed a 3% CO₂, 21% O₂, 76% N₂ gas mixture for 5 min (CO₂). The end-tidal partial pressure of CO₂ ([Formula: see text]) was measured via capnography. Mean arterial pressure (MAP) was measured beat to beat via the Penaz method. Vascular resistance in the renal and segmental arteries was calculated as MAP divided by blood velocity. [Formula: see text] increased with CO₂ (Air: 45 ± 3, CO₂: 48 ± 3 mmHg, P < 0.01), but there were no changes in MAP (P = 0.77). CO₂ decreased blood velocity in the renal (Air: 35.2 ± 8.1, CO₂: 32.2 ± 7.3 cm/s, P < 0.01) and segmental (Air: 24.2 ± 5.1, CO₂: 21.8 ± 4.2 cm/s, P < 0.01) arteries and increased vascular resistance in the renal (Air: 2.7 ± 0.9, CO₂: 3.0 ± 0.9 mmHg·cm^-1·s, P < 0.01) and segmental (Air: 3.9 ± 1.0, CO₂: 4.4 ± 1.0 mmHg·cm^-1·s, P < 0.01) arteries. These data provide evidence that the kidneys are hemodynamically responsive to a mild and acute hypercapnic stimulus in healthy humans.

Both hyperthermia and dehydration during physical work in the heat contribute to the risk of acute kidney injury

Occupational heat stress increases the risk of acute kidney injury (AKI) and kidney disease. This study tested the hypothesis that attenuating the magnitude of hyperthermia (i.e., increase in core temperature) and/or dehydration during prolonged physical work in the heat attenuates increases in AKI biomarkers. Thirteen healthy adults (3 women, 23 ± 2 yr) exercised for 2 h in a 39.7 ± 0.6°C, 32 ± 3% relative-humidity environmental chamber. In four trials, subjects received water to remain euhydrated (Water), continuous upper-body cooling (Cooling), a combination of both (Water + Cooling), or no intervention (Control). The magnitude of hyperthermia (increased core temperature of 1.9 ± 0.3°C; P < 0.01) and dehydration (percent loss of body mass of -2.4 ± 0.5%; P < 0.01) were greatest in the Control group. There were greater increases in the urinary biomarkers of AKI in the Control trial: albumin (increase of 13 ± 11 μg/mL; P ≤ 0.05 compared with other trials), neutrophil gelatinase-associated lipocalin (NGAL) (increase of 16 ± 14 ng/dL, P ≤ 0.05 compared with Cooling and Water + Cooling groups), and insulin-like growth factor-binding protein 7 (IGFBP7) (increase of 227 ± 190 ng/mL; P ≤ 0.05 compared with other trials). Increases in IGFBP7 in the Control trial persisted after correcting for urine production/concentration. There were no differences in the AKI biomarker tissue inhibitor of metalloproteinase 2 (TIMP-2) between trials (P ≥ 0.11). Our findings indicate that the risk of AKI is highest with greater magnitudes of hyperthermia and dehydration during physical work in the heat. Additionally, the differential findings between IGFBP7 (preferentially secreted in proximal tubules) and TIMP-2 (distal tubules) suggest the proximal tubules as the location of potential renal injury.NEW & NOTEWORTHY We demonstrate that the risk for acute kidney injury (AKI) is higher in humans with greater magnitudes of hyperthermia and dehydration during physical work in the heat and that alleviating the hyperthermia and/or limiting dehydration equally reduce the risk of AKI. The biomarker panel employed in this study suggests the proximal tubules as the location of potential renal injury.

Reliability and agreement of human renal and segmental artery hemodynamics measured using Doppler ultrasound

To optimize study design and data interpretation, there is a need to understand the reliability of Doppler ultrasound-derived measures of blood velocity (BV) measured in the renal and segmental arteries. Thus, this study tested the following two hypotheses: 1) renal and segmental artery BV measured over the current standard of three cardiac cycles have good agreement with measurements over nine cardiac cycles (study 1); and 2) renal and segmental artery BV measurements have relatively poor day-to-day reliability (study 2). In study 1, there was excellent agreement between measurements over three and nine cardiac cycles for BV in both the renal and segmental arteries, as evidenced by BV measurements that were not statistically different (P ≥ 0.68), were highly consistent (r ≥ 0.99, P < 0.01), had a coefficient of variation ≤2.5 ± 1.8%, and 97% (renal artery) and 92% (segmental artery) of the individual differences fell within the 95% limits of agreement. In study 2, there was relatively good day-to-day reliability in renal artery BV as evidenced by no differences between three separate days (P ≥ 0.30), an intraclass correlation coefficient (ICC) of 0.92 (0.78, 0.98), and 7.4 ± 5.5% coefficient of variation. The day-to-day reliability was relatively poor in the segmental artery with an ICC of 0.77 (0.41, 0.93) and 9.0 ± 5.6% coefficient of variation. These findings support measuring renal and segmental artery hemodynamics over three cardiac cycles and the utility in reporting renal BV across days. However, because of the variation across days, hemodynamic responses in the segmental arteries should be reported as changes from baseline when making comparisons across multiple days.NEW & NOTEWORTHY The present study indicates that Doppler ultrasound-derived measures of renal and segmental artery hemodynamics over three cardiac cycles have excellent agreement with those over nine cardiac cycles. These findings support the current practice of measuring renal and segmental artery blood velocity over three cardiac cycles. This study also demonstrates that there is excellent day-to-day reliability for measures of renal artery blood velocity, which supports reporting absolute values of renal artery blood velocity across days. However, it was also found that the day-to-day reliability of segmental artery measurements is relatively poor. Thus, to account for this variability, we suggest that segmental artery hemodynamics be compared as relative changes from baseline across separate days.

Thermal Behavior Augments Heat Loss Following Low Intensity Exercise

We tested the hypothesis that thermal behavior alleviates thermal discomfort and accelerates core temperature recovery following low intensity exercise. Methods: In a 27 0 C, 48 6% relative humidity environment, 12 healthy subjects (six females) completed 60 min of exercise followed by 90 min of seated recovery on two occasions. Subjects wore a suit top perfusing 34 ± 0 °C water during exercise. In the control trial, this water continually perfused throughout recovery. In the behavior trial, the upper body was maintained thermally comfortable by pressing a button to receive cool water (3 2 °C) perfusing through the top for 2 min per button press. Results: Physiological variables (core temperature, p ≥ 0.18; mean skin temperature, p = 0.99; skin wettedness, p ≥ 0.09; forearm skin blood flow, p = 0.29 and local axilla sweat rate, p = 0.99) did not differ between trials during exercise. Following exercise, mean skin temperature decreased in the behavior trial in the first 10 min (by -0.5 0.7 °C, p < 0.01) and upper body skin temperature was reduced until 70 min into recovery (by 1.8 1.4 °C, p < 0.05). Core temperature recovered to pre-exercise levels 17 31 min faster (p = 0.02) in the behavior trial. There were no differences in skin blood flow or local sweat rate between conditions during recovery (p ≥ 0.05). Whole-body thermal discomfort was reduced (by -0.4 0.5 a.u.) in the behavior trial compared to the control trial within the first 20 min of recovery (p ≤ 0.02). Thermal behavior via upper body cooling resulted in augmented cumulative heat loss within the first 30 min of recovery (Behavior: 288 92 kJ; Control: 160 44 kJ, p = 0.02). Conclusions: Engaging in thermal behavior that results in large reductions in mean skin temperature following exercise accelerates the recovery of core temperature and alleviates thermal discomfort by promoting heat loss.

Thermal behavior alleviates thermal discomfort during steady-state exercise without affecting whole body heat loss

We tested the hypothesis that thermal behavior resulting in reductions in mean skin temperature alleviates thermal discomfort and mitigates the rise in core temperature during light-intensity exercise. In a 27 ± 0°C, 48 ± 6% relative humidity environment, 12 healthy subjects (6 men, 6 women) completed 60 min of recumbent cycling. In both trials, subjects wore a water-perfused suit top continually perfusing 34 ± 0°C water. In the behavior trial, subjects maintained their upper body thermally comfortable by pressing a button to perfuse cool water (2.2 ± 0.5°C) through the top for 2 min per button press. Metabolic heat production (control: 404 ± 52 W, behavior: 397 ± 65 W; P = 0.44) was similar between trials. Mean skin temperature was reduced in the behavior trial (by -2.1 ± 1.8°C, P < 0.01) because of voluntary reductions in water-perfused top temperature (P < 0.01). Whole body (P = 0.02) and local sweat rates were lower in the behavior trial (P ≤ 0.05). Absolute core temperature was similar (P ≥ 0.30); however, the change in core temperature was greater in the behavior trial after 40 min of exercise (P ≤ 0.03). Partitional calorimetry did not reveal any differences in cumulative heat storage (control: 554 ± 229, behavior: 544 ± 283 kJ; P = 0.90). Thermal behavior alleviated whole body thermal discomfort during exercise (by -1.17 ± 0.40 arbitrary units, P < 0.01). Despite lower evaporative cooling in the behavior trial, similar heat loss was achieved by voluntarily employing convective cooling. Therefore, thermal behavior resulting in large reductions in skin temperature is effective at alleviating thermal discomfort during exercise without affecting whole body heat loss.NEW & NOTEWORTHY This study aimed to determine the effectiveness of thermal behavior in maintaining thermal comfort during exercise by allowing subjects to voluntarily cool their torso and upper limbs with 2°C water throughout a light-intensity exercise protocol. We show that voluntary cooling of the upper body alleviates thermal discomfort while maintaining heat balance through convective rather than evaporative means of heat loss.

Renal and segmental artery hemodynamics during whole body passive heating and cooling recovery

High environmental temperatures are associated with increased risk of acute kidney injury, which may be related to reductions in renal blood flow. The susceptibility of the kidneys may be increased because of heat stress-induced changes in renal vascular resistance (RVR) to sympathetic activation. We tested the hypotheses that, compared with normothermia, increases in RVR during the cold pressor test (CPT, a sympathoexcitatory maneuver) are attenuated during passive heating and exacerbated after cooling recovery. Twenty-four healthy adults (22 ± 2 yr; 12 women, 12 men) completed CPTs at normothermic baseline, after passive heating to a rise in core temperature of ~1.2°C, and after cooling recovery when core temperature returned to ~0.2°C above normothermic baseline. Blood velocity was measured by Doppler ultrasound in the distal segment of the right renal artery (Renal, n = 24 during thermal stress, n = 12 during CPTs) or the middle portion of a segmental artery (Segmental, n = 12). RVR was calculated as mean arterial pressure divided by renal or segmental blood velocity. RVR increased at the end of CPT during normothermic baseline in both arteries (Renal: by 1.0 ± 1.0 mmHg·cm−1·s, Segmental: by 2.2 ± 1.2 mmHg·cm−1·s, P ≤ 0.03), and these increases were abolished with passive heating ( P ≥ 0.76). At the end of cooling recovery, RVR in both arteries to the CPT was restored to that of normothermic baseline ( P ≤ 0.17). These data show that increases in RVR to sympathetic activation during passive heating are attenuated and return to that of normothermic baseline after cooling recovery.

NEW & NOTEWORTHY Our data indicate that increases in renal vascular resistance to the cold pressor test (i.e., sympathetic activation) are attenuated during passive heating, but at the end of cooling recovery this response returns to that of normothermic baseline. Importantly, hemodynamic responses were assessed in arteries going to (renal artery) and within (segmental artery) the kidney, which has not been previously examined in the same study during thermal and/or sympathetic stressors.

The Potential for Renal Injury Elicited by Physical Work in the Heat

An epidemic of chronic kidney disease (CKD) is occurring in laborers who undertake physical work in hot conditions. Rodent data indicate that heat exposure causes kidney injury, and when this injury is regularly repeated it can elicit CKD. Studies in humans demonstrate that a single bout of exercise in the heat increases biomarkers of acute kidney injury (AKI). Elevations in AKI biomarkers in this context likely reflect an increased susceptibility of the kidneys to AKI. Data largely derived from animal models indicate that the mechanism(s) by which exercise in the heat may increase the risk of AKI is multifactorial. For instance, heat-related reductions in renal blood flow may provoke heterogenous intrarenal blood flow. This can promote localized ischemia, hypoxemia and ATP depletion in renal tubular cells, which could be exacerbated by increased sodium reabsorption. Heightened fructokinase pathway activity likely exacerbates ATP depletion occurring secondary to intrarenal fructose production and hyperuricemia. Collectively, these responses can promote inflammation and oxidative stress, thereby increasing the risk of AKI. Equivalent mechanistic evidence in humans is lacking. Such an understanding could inform the development of countermeasures to safeguard the renal health of laborers who regularly engage in physical work in hot environments.

Exercise intensity independently modulates thermal behavior during exercise recovery but not during exercise

We tested the hypothesis that thermal behavior is greater during and after high- compared with moderate-intensity exercise. In a 27°C, 20% relative humidity environment, 20 participants (10 women, 10 men) cycled for 30 min at moderate [53% (SD 6) peak oxygen uptake (V̇o2peak) or high [78% (SD 6) V̇o2peak] intensity, followed by 120 min of recovery. Mean skin and core temperatures and mean skin wettedness were recorded continuously. Participants maintained thermally comfortable neck temperatures with a custom-made neck device. Neck device temperature provided an index of thermal behavior. The weighted average of mean skin and core temperatures and mean skin wettedness provided an indication of the afferent stimulus to thermally behave. Mean skin and core temperatures were greater at end-exercise in high intensity ( P < 0.01). Core temperature remained elevated in high intensity until 70 min of recovery ( P = 0.03). Mean skin wettedness and the afferent stimulus were greater at 10–20 min of exercise in high intensity ( P ≤ 0.03) and remained elevated until 60 min of recovery ( P < 0.01). Neck device temperature was lower during exercise in high versus moderate intensity ( P ≤ 0.02). There was a strong relation between the afferent stimulus and neck device temperature during exercise (high: R2 = 0.82, P < 0.01; moderate: R2 = 0.95, P < 0.01) and recovery (high: R2 = 0.97, P < 0.01; moderate: R2 = 0.93, P < 0.01). During exercise, slope ( P = 0.49) and y-intercept ( P = 0.91) did not differ between intensities. In contrast, slope was steeper ( P < 0.01) and y-intercept was higher ( P < 0.01) during recovery from high-intensity exercise. Thermal behavior is greater during high-intensity exercise because of the greater stimulus to behave. The withdrawal of thermal behavior is augmented after high-intensity exercise.

NEW & NOTEWORTHY This is the first study to determine the effects of exercise intensity on thermal behavior. We show that exercise intensity does not independently modulate thermal behavior during exercise but is dependent on the magnitude of afferent stimuli. In contrast, the withdrawal of thermal behavior after high-intensity exercise is augmented. This may be a consequence of an attenuated perceptual response to afferent stimuli, which may be due to processes underlying postexercise hypoalgesia.