Sex Differences in Sympathetic Responses to Lower Body Negative Pressure

INTRODUCTION

Trauma-induced hemorrhage is a leading cause of death in prehospital settings. Experimental data demonstrate that females have a lower tolerance to simulated hemorrhage (i.e., central hypovolemia). However, the mechanism(s) underpinning these responses are unknown. Therefore, this study aimed to compare autonomic cardiovascular responses during central hypovolemia between the sexes. We hypothesized that females would have a lower tolerance and smaller increase in muscle sympathetic nerve activity (MSNA) to simulated hemorrhage.

METHODS

Data from 17 females and 19 males, aged 19-45, were retrospectively analyzed. Participants completed a progressive lower-body negative pressure (LBNP) protocol to presyncope to simulate hemorrhagic tolerance with continuous measures of MSNA and beat-to-beat hemodynamic variables. We compared responses at baseline, at two LBNP stages (40 mmHg and 50 mmHg), and at immediately before presyncope. In addition, we compared responses at relative percentages (33%, 66%, and 100%) of hemorrhagic tolerance, calculated via the cumulative stress index (i.e., the sum of the product of time and pressure at each LBNP stage).

RESULTS

Females had lower tolerance to central hypovolemia (female: 561 ± 309 vs. male: 894 ± 304 min*mmHg [time*LBNP]; p = 0.003). At LBNP 40 mmHg and 50 mmHg, females had lower diastolic blood pressures (main effect of sex: p = 0.010). For the relative LBNP analysis, females exhibited lower MSNA burst frequency (main effect of sex: p = 0.016) accompanied by a lower total vascular conductance (sex: p = 0.028; main effect of sex).

CONCLUSIONS

Females have a lower tolerance to central hypovolemia, which was accompanied by lower diastolic blood pressure at 40 and 50 mmHg LBNP. Notably, females had attenuated MSNA responses when assessed as relative LBNP tolerance time.

Burn size and environmental conditions modify thermoregulatory responses to exercise in burn survivors

This project tested the hypothesis that burn survivors can perform mild/moderate-intensity exercise in temperate and hot environments without excessive elevations in core body temperature. Burn survivors with low (23 ± 5%TBSA; N = 11), moderate (40 ± 5%TBSA; N = 9), and high (60 ± 8%TBSA; N = 9) burn injuries performed 60 minutes of cycle ergometry exercise (72 ± 15 watts) in a 25°C and 23% relative humidity environment (ie, temperate) and in a 40°C and 21% relative humidity environment (ie, hot). Absolute gastrointestinal temperatures (TGI) and changes in TGI (ΔTGI) were obtained. Participants with an absolute TGI of >38.5°C and/or a ΔTGI of >1.5°C were categorized as being at risk for hyperthermia. For the temperate environment, exercise increased ΔTGI in all groups (low: 0.72 ± 0.21°C, moderate: 0.42 ± 0.22°C, and high: 0.77 ± 0.25°C; all P < .01 from pre-exercise baselines), resulting in similar absolute end-exercise TGI values (P = .19). Importantly, no participant was categorized as being at risk for hyperthermia, based upon the aforementioned criteria. For the hot environment, ΔTGI at the end of the exercise bout was greater for the high group when compared to the low group (P = .049). Notably, 33% of the moderate cohort and 56% of the high cohort reached or exceeded a core temperature of 38.5°C, while none in the low cohort exceeded this threshold. These data suggest that individuals with a substantial %TBSA burned can perform mild/moderate intensity exercise for 60 minutes in temperate environmental conditions without risk of excessive elevations in TGI. Conversely, the risk of excessive elevations in TGI during mild/moderate intensity exercise in a hot environment increases with the %TBSA burned.

Attrition of Well-Healed Burn Survivors to a 6-Month Community-Based Exercise Program: A Retrospective Evaluation

The purpose of this study was to evaluate whether burn survivors have lower adherence compared to non-burned control individuals during a 6-month community-based exercise program. In burn survivors, we sought to answer if there was a relation between the size of the burn injury and dropout frequency. Fifty-two burn survivors and 15 non-burned controls (n = 67) were recruited for a 6-month community-based (ie, non-supervised), progressive, exercise training program. During the exercise program, 27% (ie, 4 of the 15 enrolled) of the non-burned individuals dropped out of the study, while 37% (ie, 19 of the 52) of the burn survivors dropped out from the study. There was no difference in the percentage of individuals who dropped out between groups (P = .552). There was no difference in size of the burn injury, expressed as percent body surface area burned (%BSA) between the burn survivors that dropped out versus those who completed the exercise regimen (P = .951). We did not observe a relation between %BSA burned and dropouts (log odds = -0.15-0.01(%BSA), B = -0.01, SE = 0.015, P = .541). There was no effect of %BSA burned on the probability of dropout [Exp (B) = 0.991, 95% CI (0.961, 1.020)] and there were no differences in the percentage of individuals who dropped out of the study based on %BSA burned (χ2(1) = 0.44, P = .51). These data demonstrate that burn survivors have similar exercise adherence relative to a non-burned group and the extent of a burn injury does not affect exercise program adherence.

Key Exercise Concepts in the Rehabilitation from Severe Burns

This article presents information on the benefits of exercise in counteracting the detrimental effects of bed rest, and/or severe burns. Exercise is key for maintaining physical function, lean body mass, metabolic recovery, and psychosocial health after major burn injuries. The details of an exercise training program conducted in severely burned persons are presented, as well as information on the importance of proper regulation of body temperature during exercise or physical activity. The sections on exercise and thermoregulation are followed by a section on the role of exercise in scarring and contractures. Finally, gaps in the current knowledge of exercise, thermoregulation, and contractures are presented.

Baroreflex sensitivity following acute upper-body exercise in the cold among stable coronary artery disease patients

Background: A cold environment and exercise separately affect the autonomic nervous system (ANS), baroreflex sensitivity (BRS), and blood pressure variability (BPV) but their combined effects on post-exercise recovery are not known. Our cross-over trial examined these responses following upper-body static and dynamic exercise performed in a cold and neutral environment in patients with coronary artery disease (CAD). Methods: 20 patients with stable coronary artery disease performed both graded static (10%-30% of maximal voluntary contraction) and dynamic (light, moderate and high perceived intensity) upper-body exercise at -15°C and +22°C for 30 min. Electrocardiogram and continuous blood pressure were measured to compute post-exercise (10 and 30 min after exercise) spectral powers of heart rate (HR), blood pressure variability and BRS at low (0.04-0.15 Hz) and high (0.15-0.4 Hz) frequencies. Results: Static upper-body exercise performed in a cold environment increased post-exercise high frequency (HF) spectral power of heart rate (HF RR) (p < 0.001) and reduced heart rate (p = 0.001) and low-to-high frequency (LF/HF) ratio (p = 0.006) more than in a neutral environment. In addition, post-exercise mean BRS (p = 0.015) and high frequency BRS (p = 0.041) increased more following static exercise in the cold than in a neutral environment. Dynamic upper-body exercise performed in a cold environment reduced post-exercise HF BRS (p = 0.019) and systolic blood pressure (p = 0.003). Conclusion: Static upper-body exercise in the cold increased post-exercise BRS and overall vagal activity but without reduced systolic blood pressure. Dynamic upper-body exercise in the cold reduced post-exercise vagal BRS but did not affect the other parameters. The influence of cold exposure on post-exercise autonomic and cardiovascular responses following static upper-body exercise require further studies. This information helps understanding why persons with cardiovascular diseases are vulnerable to low environmental temperature. ClinicalTrials.gov: NCT02855905 (04/08/2016).

Aging Increases Enterocyte Damage during a 3-Hour Exposure to Very Hot and Dry Heat: A Preliminary Study

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-FABPlog), interleukin-8 (IL-8log), and tissue factor (TFlog) 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-FABPlog 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-FABPlog 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 IFABPlog. There was no change in IL-8log in the younger group (p = 0.193, d = 0.23) following heat exposure, but we observed a decrease in IL-8log in the older group (p = 0.047, d = 0.48). TFlog 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-FABPlog 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.

Age alters the thermoregulatory responses to extreme heat exposure with accompanying activities of daily living

Older adults are at greater risk for heat-related morbidity and mortality, due in part to age-related reductions in heat dissipating capabilities. Previous studies investigating the impact of age on responses to heat stress used approaches that lack activities of daily living and therefore may not accurately depict the thermal/physiological strain that would occur during actual heatwaves. We sought to compare the responses of young (18-39 yr) and older (≥65 yr) adults exposed to two extreme heat simulations. Healthy young (n = 20) and older (n = 20) participants underwent two 3-h extreme heat exposures on different days: 1) DRY (47°C and 15% humidity) and 2) HUMID (41°C and 40% humidity). To mimic heat generation comparable with activities of daily living, participants performed 5-min bouts of light physical activity dispersed throughout the heat exposure. Measurements included core and skin temperatures, heart rate, blood pressure, local and whole body sweat rate, forearm blood flow, and perceptual responses. Δ core temperature (Young: 0.68 ± 0.27°C vs. Older: 1.37 ± 0.42°C; P < 0.001) and ending core temperature (Young: 37.81 ± 0.26°C vs. Older: 38.15 ± 0.43°C; P = 0.005) were greater in the older cohort during the DRY condition. Δ core temperature (Young: 0.58 ± 0.25°C vs. Older: 1.02 ± 0.32°C; P < 0.001), but not ending core temperature (Young: 37.67 ± 0.34°C vs. Older: 37.83 ± 0.35°C; P = 0.151), was higher in the older cohort during the HUMID condition. We demonstrated that older adults have diminished thermoregulatory responses to heat stress with accompanying activities of daily living. These findings corroborate previous reports and confirm epidemiological data showing that older adults are at a greater risk for hyperthermia.NEW & NOTEWORTHY 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.

Six Months of Exercise Training Improves Ventilatory Responses during Exercise in Adults with Well-Healed Burn Injuries

INTRODUCTION

Pulmonary function is lower after a severe burn injury, which could influence ventilatory responses during exercise. It is unclear whether exercise training improves pulmonary function or ventilatory responses during exercise in adults with well-healed burn injuries. Therefore, we tested the hypothesis that exercise training improves pulmonary function and ventilatory responses during exercise in adults with well-healed burn injuries.

METHODS

Thirty-nine adults (28 with well-healed burn injuries and 11 non-burn-injured controls) completed 6 months of unsupervised, progressive exercise training including endurance, resistance, and high-intensity interval components. Before and after exercise training, we performed comprehensive pulmonary function testing and measured ventilatory responses during cycling exercise. We compared variables using two-way ANOVA (group-time; i.e., preexercise/postexercise training (repeated factor)).

RESULTS

Exercise training did not increase percent predicted spirometry, lung diffusing capacity, or airway resistance measures (time: P ≥ 0.14 for all variables). However, exercise training reduced minute ventilation ( V̇E ; time: P ≤ 0.05 for 50 and 75 W) and the ventilatory equivalent for oxygen ( V̇E /V̇O 2 ; time: P < 0.001 for 75 W) during fixed-load exercise for both groups. The ventilatory equivalent for carbon dioxide ( V̇E /V̇CO 2 ) during exercise at 75 W was reduced after exercise training (time: P = 0.04). The percentage of age-predicted maximum heart rate at the ventilatory threshold was lower in adults with well-healed burn injuries before ( P = 0.002), but not after ( P = 0.22), exercise training. Lastly, exercise training increased V̇E and reduced V̇E /V̇O 2 during maximal exercise (time: P = 0.005 for both variables).

CONCLUSIONS

These novel findings demonstrate that exercise training can improve ventilatory responses during exercise in adults with well-healed burn injuries.

Inhibiting regional sweat evaporation modifies the ventilatory response to exercise: interactions between core and skin temperature

In humans, elevated body temperatures can markedly increase the ventilatory response to exercise. However, the impact of changing the effective body surface area (BSA) for sweat evaporation (BSAeff) on such responses is unclear. Ten healthy adults (9 males, 1 female) performed eight exercise trials cycling at 6 W/kg of metabolic heat production for 60 min. Four conditions were used where BSAeff corresponded to 100%, 80%, 60%, and 40% of BSA using vapor-impermeable material. Four trials (one at each BSAeff) were performed at 25°C air temperature, and four trials (one at each BSAeff) at 40°C air temperature, each with 20% humidity. The slope of the relation between minute ventilation and carbon dioxide elimination (V̇E/V̇co2 slope) assessed the ventilatory response. At 25°C, the V̇E/V̇co2 slope was elevated by 1.9 and 2.6 units when decreasing BSAeff from 100 to 80 and to 40% (P = 0.033 and 0.004, respectively). At 40°C, V̇E/V̇co2 slope was elevated by 3.3 and 4.7 units, when decreasing BSAeff from 100 to 60 and to 40% (P = 0.016 and P < 0.001, respectively). Linear regression analyses using group average data from each condition demonstrated that end-exercise mean body temperature (integration of core and mean skin temperature) was better associated with the end-exercise ventilatory response, compared with core temperature alone. Overall, we show that impeding regional sweat evaporation increases the ventilatory response to exercise in temperate and hot environmental conditions, and the effect is mediated primarily by increases in mean body temperature.NEW & NOTEWORTHY Exercise in the heat increases the slope of the relation between minute ventilation and carbon dioxide elimination (V̇E/V̇co2 slope) in young healthy adults. An indispensable role for skin temperature in modulating the ventilatory response to exercise is noted, contradicting common belief that internal/core temperature acts independently as a controller of ventilation during hyperthermia.

Evidence for Chronotropic Incompetence in Well-healed Burn Survivors

Due to various pathophysiological responses associated with a severe burn injury, we hypothesized that burn survivors exhibit chronotropic incompetence. To test this hypothesis, a graded peak oxygen consumption (V̇O2peak) test was performed in 94 adults (34 nonburned, 31 burn survivors with 14-35% body surface area grafted, and 29 burn survivors with >35% body surface area grafted). The threshold of 35% body surface area grafted was determined by receiver operating characteristic (ROC) curve analysis. Peak exercise heart rates (HRmax) were compared against age-predicted HRmax within each group. The proportion of individuals not meeting their age-predicted HRmax (within 5 b/min) were compared between groups. Age-predicted HRmax was not different from measured HRmax in the nonburned and moderate burn groups (P = .09 and .22, respectively). However, measured HRmax was 10 ± 6 b/min lower than the age-predicted HRmax in those with a large burn injury (P < .001). While 56 and 65% of individuals in the nonburned and moderate burn group achieved a measured HRmax within 5 b/min or greater of age-predicted HRmax, only 21% of those in the large burn group met this criterion (P < .001). These data provide preliminary evidence of chronotropic incompetence in individuals with severe burn injury covering >35% body surface area.

Impact of the Insoluble Gas Concentration on Measured Stroke Volume at Rest and Submaximal Exercise Using the Innocor Device

INTRODUCTION

The Innocor® device uses an insoluble gas (SF 6 ) to estimate lung volume and the rate of disappearance of a soluble gas (nitrous oxide) to measure pulmonary blood flow (PBF), which approximates cardiac output assuming no shunt. We sought to identify error in the measurement of the insoluble gas in an effort to reduce variation in Innocor® measurement.

METHODS

We enrolled 28 participants from the Dallas Heart Study (mean age, 63 yr; 57% men; 43% White). Stroke volume was measured at rest and at submaximal (20 and 40 W) exercise using both echocardiography (Philips iE33) and the Innocor® device. We defined a priori peak and equilibrium SF 6 measurement errors as greater or less than 20% of the mean observed value. Three Innocor measurements were obtained at rest ( n = 27) for a total of 81 measurements. Of these, 22% had SF 6 measurements that fell outside of the a priori range.

RESULTS

Resting Innocor® stroke volume measures with peak SF 6 measured above a priori range (>0.12%) was associated with larger stroke volumes compared with stroke volume measures without peak SF 6 error (101.4 [26.8] vs 64.9 [8.7] mL; P = 0.006) and overestimated stroke volume when compared with stroke volume by echo (101.4 [26.8] vs 59.9 [16.3] mL; P = 0.017). A similar pattern was observed at submaximal exercise. In contrast, there was no consistent association between variation in equilibrium SF 6 concentrations and measured stroke volume.

CONCLUSIONS

Variability in peak SF 6 concentration is common while using the Innocor® device and results in overestimated stroke volume. These findings have implications for research protocols using this device.

Cardiac remodeling in well-healed burn survivors after 6 months of unsupervised progressive exercise training

Aerobic exercise is important in the rehabilitation of individuals with prior burn injuries, but no studies have examined whether adult burn survivors demonstrate cardiac remodeling to long-term aerobic exercise training. In this study, we tested the hypothesis that 6 months of progressive exercise training improves cardiac magnetic resonance imaging-based measures of cardiac structure and function in well-healed burn survivors. Secondary analyses explored relations between burn surface area and changes in cardiac structure in the cohort of burn survivors. V̇o2peak assessments and cardiac magnetic resonance imaging were performed at baseline and following 6 months of progressive exercise training from 19 well-healed burn survivors and 10 nonburned control participants. V̇o2peak increased following 6 months of training in both groups (Control: Δ5.5 ± 5.8 mL/kg/min; Burn Survivors: Δ3.2 ± 3.6 mL/kg/min, main effect of training, P < 0.001). Left ventricle (LV) mass (Control: Δ1.7 ± 3.1 g/m2; Burn survivors: Δ1.8 ± 2.7 g/m2), stroke volume (Control: Δ5.8 ± 5.2 mL/m2; Burn Survivors: Δ2.8 ± 4.2 mL/m2), and ejection fraction (Control: Δ2.4 ± 4.0%; Burn Survivors: Δ2.2 ± 4.3%) similarly increased following 6 months of exercise training in both cohorts (main effect of training P < 0.05 for all indexes). LV end-diastolic volume increased in the control group (Δ6.5 ± 4.5 mL/m2) but not in the cohort of burn survivors (Δ1.9 ± 2.7 mL/m2, interaction, P = 0.040). Multiple linear regression analyses revealed that burn surface area had little to no effect on changes in ventricular mass or end-diastolic volumes in response to exercise training. Our findings provide initial evidence of physiological cardiac remodeling, which is not impacted by burn size, in response to exercise training in individuals with well-healed burn injuries.NEW & NOTEWORTHY Aerobic exercise is important in the rehabilitation of individuals with prior burn injuries, but no studies have examined whether adult burn survivors demonstrate cardiac remodeling to long-term aerobic exercise training. In this study, we tested the hypothesis that 6 months of progressive exercise training would improve cardiac magnetic resonance imaging-based measures of cardiac structure and function in well-healed burn survivors. Our findings highlight the ability of exercise training to modify cardiac structure and function in well-healed burn survivors and nonburned sedentary controls alike.

Altered cardiac β1 responsiveness in hyperthermic older adults

Compared with younger adults, passive heating induced increases in cardiac output are attenuated by ∼50% in older adults. This attenuated response may be associated with older individuals' inability to maintain stroke volume through ionotropic mechanisms and/or through altered chronotropic mechanisms. The purpose of this study was to identify the interactive effect of age and hyperthermia on cardiac responsiveness to dobutamine-induced cardiac stimulation. Eleven young (26 ± 4 yr) and 8 older (68 ± 5 yr) participants underwent a normothermic and a hyperthermic (baseline core temperature +1.2°C) trial on the same day. In both thermal conditions, after baseline measurements, intravenous dobutamine was administered for 12 min at 5 µg/kg/min, followed by 12 min at 15 µg/kg/min. Primary measurements included echocardiography-based assessments of cardiac function, gastrointestinal and skin temperatures, heart rate, and mean arterial pressure. Heart rate responses to dobutamine were similar between groups in both thermal conditions (P > 0.05). The peak systolic mitral annular velocity (S'), i.e., an index of left ventricular longitudinal systolic function, was similar between groups for both thermal conditions at baseline. While normothermic, the increase in S' between groups was similar with dobutamine administration. However, while hyperthermic, the increase in S' was attenuated in the older participants with dobutamine (P < 0.001). Healthy, older individuals show attenuated inotropic, but maintained chronotropic responsiveness to dobutamine administration during hyperthermia. These data suggest that older individuals have a reduced capacity to increase cardiomyocyte contractility, estimated by changes in S', via β1-adrenergic mechanisms while hyperthermic.

Human temperature regulation under heat stress in health, disease, and injury

The human body constantly exchanges heat with the environment. Temperature regulation is a homeostatic feedback control system that ensures deep body temperature is maintained within narrow limits despite wide variations in environmental conditions and activity-related elevations in metabolic heat production. Extensive research has been performed to study the physiological regulation of deep body temperature. This review focuses on healthy and disordered human temperature regulation during heat stress. Central to this discussion is the notion that various morphological features, intrinsic factors, diseases, and injuries independently and interactively influence deep body temperature during exercise and/or exposure to hot ambient temperatures. The first sections review fundamental aspects of the human heat stress response, including the biophysical principles governing heat balance and the autonomic control of heat loss thermoeffectors. Next, we discuss the effects of different intrinsic factors (morphology, heat adaptation, biological sex, and age), diseases (neurological, cardiovascular, metabolic, and genetic), and injuries (spinal cord injury, deep burns, and heat stroke), with emphasis on the mechanisms by which these factors enhance or disturb the regulation of deep body temperature during heat stress. We conclude with key unanswered questions in this field of research.

Six months of unsupervised exercise training lowers blood pressure during moderate, but not vigorous, aerobic exercise in adults with well-healed burn injuries

Exercise training reduces cardiovascular disease risk, partly due to arterial blood pressure (BP) lowering at rest and during fixed-load exercise. However, it is unclear whether exercise training can reduce BP at rest and during exercise in adults with well-healed burn injuries. Therefore, the purpose of this investigation was to test the hypothesis that 6 mo of unsupervised exercise training reduces BP at rest and during lower-body cycle ergometry in adults with well-healed burn injuries. Thirty-nine adults (28 with well-healed burn injuries and 11 controls) completed 6 mo of unsupervised, progressive exercise training including endurance, resistance, and high-intensity interval components. Before and after exercise training, we measured BP at rest, during fixed-load submaximal exercise (50 and 75 W), during fixed-intensity submaximal exercise (40% and 70% of V̇o2peak), and during maximal exercise on a lower-body cycle ergometer. We compared cardiovascular variables using two-way ANOVA (group × pre/postexercise training [repeated factor]). Adults with well-healed burn injuries had higher diastolic BP at rest (P = 0.04), which was unchanged by exercise training (P = 0.26). Exercise training reduced systolic, mean, and diastolic BP during fixed-load cycling exercise at 75 W in adults with well-healed burn injuries (P ≤ 0.03 for all), but not controls (P ≥ 0.67 for all). Exercise training also reduced mean and diastolic BP during exercise at 40% (P ≤ 0.02 for both), but not at 70% (P ≥ 0.18 for both), of V̇o2peak. These data suggest that a 6-mo unsupervised exercise training program lowers BP during moderate, but not vigorous, aerobic exercise in adults with well-healed burn injuries.NEW & NOTEWORTHY Adults with well-healed burn injuries have greater cardiovascular disease morbidity and all-cause mortality compared with nonburn-injured adults. We found that exercise training reduced blood pressure (BP) during fixed-load cycling at 75 W and during moderate, but not vigorous, intensity cycling exercise in adults with well-healed burn injuries. These data suggest that 6 mo of unsupervised exercise training provides some degree of cardioprotection by reducing BP responses during submaximal exercise in well-healed burn-injured adults.

Low-dose morphine reduces tolerance to central hypovolemia in healthy adults without affecting muscle sympathetic outflow

Hemorrhage is a leading cause of preventable battlefield and civilian trauma deaths. Low-dose (i.e., an analgesic dose) morphine is recommended for use in the prehospital (i.e., field) setting. Morphine administration reduces hemorrhagic tolerance in rodents. However, it is unknown whether morphine impairs autonomic cardiovascular regulation and consequently reduces hemorrhagic tolerance in humans. Thus, the purpose of this study was to test the hypothesis that low-dose morphine reduces hemorrhagic tolerance in conscious humans. Thirty adults (15 women/15 men; 29 ± 6 yr; 26 ± 4 kg·m-2, means ± SD) completed this randomized, crossover, double-blinded, placebo-controlled trial. One minute after intravenous administration of morphine (5 mg) or placebo (saline), we used a presyncopal limited progressive lower-body negative pressure (LBNP) protocol to determine hemorrhagic tolerance. Hemorrhagic tolerance was quantified as a cumulative stress index (mmHg·min), which was compared between trials using a Wilcoxon matched-pairs signed-rank test. We also compared muscle sympathetic nerve activity (MSNA; microneurography) and beat-to-beat blood pressure (photoplethysmography) during the LBNP test using mixed-effects analyses [time (LBNP stage) × trial]. Median LBNP tolerance was lower during morphine trials (placebo: 692 [473-997] vs. morphine: 385 [251-728] mmHg·min, P < 0.001, CI: -394 to -128). Systolic blood pressure was 8 mmHg lower during moderate central hypovolemia during morphine trials (post hoc P = 0.02; time: P < 0.001, trial: P = 0.13, interaction: P = 0.006). MSNA burst frequency responses were not different between trials (time: P < 0.001, trial: P = 0.80, interaction: P = 0.51). These data demonstrate that low-dose morphine reduces hemorrhagic tolerance in conscious humans. Thus, morphine is not an ideal analgesic for a hemorrhaging individual in the prehospital setting.NEW & NOTEWORTHY In this randomized, crossover, placebo-controlled trial, we found that tolerance to simulated hemorrhage was lower after low-dose morphine administration. Such reductions in hemorrhagic tolerance were observed without differences in MSNA burst frequency responses between morphine and placebo trials. These data, the first to be obtained in conscious humans, demonstrate that low-dose morphine reduces hemorrhagic tolerance. Thus, morphine is not an ideal analgesic for a hemorrhaging individual in the prehospital setting.

Low-Dose Morphine Reduces Pain Perception and Blood Pressure, but Not Muscle Sympathetic Outflow, Responses During the Cold Pressor Test

Our knowledge about how low-dose (analgesic) morphine affects autonomic cardiovascular regulation is primarily limited to animal experiments. Notably, it is unknown if low-dose morphine affects human autonomic cardiovascular responses during painful stimuli in conscious humans. Therefore, we tested the hypothesis that low-dose morphine reduces perceived pain and subsequent sympathetic and cardiovascular responses in humans during an experimental noxious stimulus. Twenty-nine participants (14F/15M; 29±6 y; 26±4 kg•m^-2, mean ± SD) completed this randomized, crossover, placebo-controlled trial during two laboratory visits. During each visit, participants completed a cold pressor test (CPT; hand in ~0.4 °C ice bath for two minutes) before and ~35 minutes after drug/placebo administration (5 mg IV morphine or saline). We compared pain perception (100 mm visual analog scale), muscle sympathetic nerve activity (MSNA; microneurography; 14 paired recordings), and beat-to-beat blood pressure (BP; photoplethysmography) between trials (at both pre- and post-drug/placebo time points) using paired, two-tailed t-tests. Before drug/placebo infusion, perceived pain (p=0.92), Δ MSNA burst frequency (n=14, p=0.21), and Δ mean BP (p=0.39) during the CPT were not different between trials. After the drug/placebo infusion, morphine versus placebo attenuated perceived pain (morphine: 43±20 vs. placebo: 57±24 mm,p<0.001) and Δ mean BP (morphine: 10±7 vs. placebo: 13±8 mmHg,p=0.003), but not Δ MSNA burst frequency (morphine: 10±11 vs. placebo: 13±11 bursts/minute,p=0.12), during the CPT. Reductions in pain perception and Δ mean BP were only weakly related (r=0.34,p=0.07; post-morphine CPT minus post-placebo CPT). These data provide valuable information regarding how low-dose morphine affects autonomic cardiovascular responses during an experimental painful stimulus.

Adults with well-healed burn injuries have lower pulmonary function values decades after injury

Sub-acute (e.g., inhalation injury) and/or acute insults sustained during a severe burn injury impairs pulmonary function. However, previous work has not fully characterized pulmonary function in adults with well-healed burn injuries decades after an injury. Therefore, we tested the hypothesis that adults with well-healed burn injuries have lower pulmonary function years after recovery. Our cohort of adults with well-healed burn-injuries (n = 41) had a lower forced expiratory volume in one second (Burn: 93 ± 16 vs. Control: 103 ± 10%predicted, mean ± SD; d = 0.60, p = 0.04), lower maximal voluntary ventilation (Burn: 84 [71-97] vs. Control: 105 [94-122] %predicted, median [IQR]; d = 0.84, p < 0.01), and a higher specific airway resistance (Burn: 235 ± 80 vs. Control: 179 ± 40%predicted, mean ± SD; d = 0.66, p = 0.02) than non-burned control participants (n = 12). No variables were meaningfully influenced by having a previous inhalation injury (d ≤ 0.44, p ≥ 0.19; 13 of 41 had an inhalation injury), the size of the body surface area burned (R²  ≤ 0.06, p ≥ 0.15; range of 15%-88% body surface area burned), or the time since the burn injury (R²  ≤ 0.04, p ≥ 0.22; range of 2-50 years post-injury). These data suggest that adults with well-healed burn injuries have lower pulmonary function decades after injury. Therefore, future research should examine rehabilitation strategies that could improve pulmonary function among adults with well-healed burn injuries.

Low-dose fentanyl reduces pain perception, muscle sympathetic nerve activity responses, and blood pressure responses during the cold pressor test

Our knowledge about how low-dose (analgesic) fentanyl affects autonomic cardiovascular regulation is primarily limited to animal experiments. Notably, it is unknown if low-dose fentanyl influences human autonomic cardiovascular responses during painful stimuli in humans. Therefore, we tested the hypothesis that low-dose fentanyl reduces perceived pain and subsequent sympathetic and cardiovascular responses in humans during an experimental noxious stimulus. Twenty-three adults (10 females/13 males; 27 ± 7 yr; 26 ± 3 kg·m^-2, means ± SD) completed this randomized, crossover, placebo-controlled trial during two laboratory visits. During each visit, participants completed a cold pressor test (CPT; hand in ∼0.4°C ice bath for 2 min) before and 5 min after drug/placebo administration (75 μg fentanyl or saline). We compared pain perception (100-mm visual analog scale), muscle sympathetic nerve activity (MSNA; microneurography, 11 paired recordings), and beat-to-beat blood pressure (BP; photoplethysmography) between trials (at both pre- and postdrug/placebo timepoints) using paired, two-tailed t tests. Before drug/placebo administration, perceived pain (P = 0.8287), ΔMSNA burst frequency (P = 0.7587), and Δmean BP (P = 0.8649) during the CPT were not different between trials. After the drug/placebo administration, fentanyl attenuated perceived pain (36 vs. 66 mm, P < 0.0001), ΔMSNA burst frequency (9 vs. 17 bursts/min, P = 0.0054), and Δmean BP (7 vs. 13 mmHg, P = 0.0174) during the CPT compared with placebo. Fentanyl-induced reductions in pain perception and Δmean BP were moderately related (r = 0.40, P = 0.0641). These data provide valuable information regarding how low-dose fentanyl reduces autonomic cardiovascular responses during an experimental painful stimulus.

Low-dose fentanyl does not alter muscle sympathetic nerve activity, blood pressure, or tolerance during progressive central hypovolemia

Hemorrhage is a leading cause of battlefield and civilian trauma deaths. Several pain medications, including fentanyl, are recommended for use in the prehospital (i.e., field setting) for a hemorrhaging solider. However, it is unknown whether fentanyl impairs arterial blood pressure (BP) regulation, which would compromise hemorrhagic tolerance. Thus, the purpose of this study was to test the hypothesis that an analgesic dose of fentanyl impairs hemorrhagic tolerance in conscious humans. Twenty-eight volunteers (13 females) participated in this double-blinded, randomized, placebo-controlled trial. We conducted a presyncopal limited progressive lower body negative pressure test (LBNP; a validated model to simulate hemorrhage) following intravenous administration of fentanyl (75 µg) or placebo (saline). We quantified tolerance as a cumulative stress index (mmHg·min), which was compared between trials using a paired, two-tailed t test. We also compared muscle sympathetic nerve activity (MSNA; microneurography) and beat-to-beat BP (photoplethysmography) during the LBNP test using a mixed effects model [time (LBNP stage) × trial]. LBNP tolerance was not different between trials (fentanyl: 647 ± 386 vs. placebo: 676 ± 295 mmHg·min, P = 0.61, Cohen's d = 0.08). Increases in MSNA burst frequency (time: P < 0.01, trial: P = 0.29, interaction: P = 0.94) and reductions in mean BP (time: P < 0.01, trial: P = 0.50, interaction: P = 0.16) during LBNP were not different between trials. These data, the first to be obtained in conscious humans, demonstrate that administration of an analgesic dose of fentanyl does not alter MSNA or BP during profound central hypovolemia, nor does it impair tolerance to this simulated hemorrhagic insult.

Edward F. Adolph Distinguished Lecture. It's more than skin deep: thermoregulatory and cardiovascular consequences of severe burn injuries in humans

Each year, within the United States, tens of thousands of individuals are hospitalized for burn-related injuries. The treatment of deep burns often involves skin grafts to accelerate healing and reduce the risk of infection. The grafting procedure results in a physical disruption between the injured and subsequently debrided host site and the skin graft placed on top of that site. Both neural and vascular connections must occur between the host site and the graft for neural modulation of skin blood flow to take place. Furthermore, evaporative cooling from such burn injured areas is effectively absent, leading to greatly impaired thermoregulatory responses in individuals with large portions of their body surface area burned. Hospitalization following a burn injury can last weeks to months, with cardiovascular and metabolic consequences of such injuries having the potential to adversely affect the burn survivor for years postdischarge. With that background, the objectives of this article are to discuss 1) our current understanding of the physiology and associated consequences of skin grafting, 2) the effects of skin grafts on efferent thermoregulatory responses and the associated consequences pertaining to whole body thermoregulation, 3) approaches that may reduce the risk of excessive hyperthermia in burn survivors, 4) the long-term cardiovascular consequences of burn injuries, and 5) the extent to which burn survivors can "normalize" otherwise compromised cardiovascular responses. Our primary objective is to guide the reader toward an understanding that severe burn injuries result in significant physiological consequences that can persist for years after the injury.

Rehabilitative Exercise Training for Burn Injury

Due to improvements in acute burn care over the last few decades, most patients with severe burns (up to 90% of the total body surface) survive. However, the metabolic and cardiovascular complications that accompany a severe burn can persist for up to 3 years post injury. Accordingly, there is now a greater appreciation of the need for strategies that can hasten recovery and reduce long-term morbidity post burn. Rehabilitation exercise training (RET) is a proven effective treatment to restore lean body mass, glucose and protein metabolism, cardiorespiratory fitness, and muscle strength in burn survivors. Despite this, very few hospitals incorporate RET in programs to aid the rehabilitation of patients with severe burns. Given that RET is a safe and efficacious treatment that restores function and reduces post-burn morbidity, we propose that a long-term exercise prescription plan should be considered for all patients with severe burns. In this literature review, we discuss the current understanding of burn trauma on major organ systems, and the positive benefits of incorporating RET as a part of the long-term rehabilitation of severely burned individuals. We also provide burn-specific exercise prescription guidelines for clinical exercise physiologists.

Thermoregulatory Responses with Size-matched Simulated Torso or Limb Skin Grafts

Skin grafting following a burn injury attenuates/abolishes sweat production within grafted areas. It is presently unknown whether the thermoregulatory consequences of skin grafting depend on anatomical location.

PURPOSE

To test the hypothesis that a simulated burn injury on the torso will be no more or less detrimental to core temperature control than on the limbs during uncompensable exercise-heat stress.

METHODS

Nine non-burned individuals (7 males, 2 females) completed the protocol. On separate occasions, burn injuries of identical surface area (0.45 ± 0.08 m2 or 24.4% ± 4.4% of total body surface area) were simulated on the torso or the arms/legs using an absorbent, vapor-impermeable material that impedes sweat evaporation in those regions. Participants performed 60 min of treadmill walking at 5.3 km·h-1 and a 4.1% ± 0.8% grade, targeting 6 W·kg-1 of metabolic heat production in 40.1°C ± 0.2°C and 19.6% ± 0.6% relative humidity conditions. Rectal temperature, heart rate, and perceptual responses were measured.

RESULTS

Rectal temperature increased to a similar extent with simulated injuries on the torso and limbs (condition-by-time interaction: P = 0.86), with a final rectal temperature 0.9 ± 0.3°C above baseline in both conditions. No differences in heart rate, perceived exertion, or thermal sensation were observed between conditions (condition-by-time interactions: P ≥ 0.50).

CONCLUSION

During uncompensable exercise-heat stress, sized-matched simulated burn injuries on the torso or limbs evoke comparable core temperature, heart rate, and perceptual responses, suggesting that the risk of exertional heat illness in such environmental conditions is independent of injury location.

Local Passive Heat for the Treatment of Hypertension in Autonomic Failure

Background

Supine hypertension affects a majority of patients with autonomic failure; it is associated with end‐organ damage and can worsen daytime orthostatic hypotension by inducing pressure diuresis and volume loss during the night. Because sympathetic activation prevents blood pressure (BP) from falling in healthy subjects exposed to heat, we hypothesized that passive heat had a BP‐lowering effect in patients with autonomic failure and could be used to treat their supine hypertension.

Methods and Results

In Protocol 1 (n=22), the acute effects of local heat (40–42°C applied with a heating pad placed over the abdomen for 2 hours) versus sham control were assessed in a randomized crossover fashion. Heat acutely decreased systolic BP by −19±4 mm Hg (versus 3±4 with sham, P<0.001) owing to decreases in stroke volume (−18±5% versus −4±4%, P=0.013 ) and cardiac output (−15±5% versus −2±4%, P=0.013). In Protocol 2 (proof‐of‐concept overnight study; n=12), we compared the effects of local heat (38°C applied with a water‐perfused heating pad placed under the torso from 10 pm to 6 am) versus placebo pill. Heat decreased nighttime systolic BP (maximal change −28±6 versus −2±6 mm Hg, P<0.001). BP returned to baseline by 8 am. The nocturnal systolic BP decrease correlated with a decrease in urinary volume (r=0.57, P=0.072) and an improvement in the morning upright systolic BP (r=−0.76, P=0.007).

Conclusions

Local heat therapy effectively lowered overnight BP in patients with autonomic failure and supine hypertension and offers a novel approach to treat this condition. Future studies are needed to assess the long‐term safety and efficacy in improving nighttime fluid loss and daytime orthostatic hypotension.

Registration

URL: https://www.clinicaltrials.gov; Unique identifiers: NCT02417415 and NCT03042988.

Burn Injury Does Not Exacerbate Heat Strain during Exercise while Wearing Body Armor

INTRODUCTION

Although evaporative heat loss capacity is reduced in burn-injured individuals with extensive skin grafts, the thermoregulatory strain due to a prior burn injury during exercise-heat stress may be negligible if the burn is located underneath protective clothing with low vapor permeability.

PURPOSE

This study aimed to test the hypothesis that heat strain during exercise in a hot-dry environment while wearing protective clothing would be similar with and without a simulated torso burn injury.

METHODS

Ten healthy individuals (8 men/2 women) underwent three trials wearing: uniform (combat uniform, tactical vest, and replica torso armor plates), uniform with a 20% total body surface area simulated torso burn (uniform + burn), or shorts (and sports bra) only (control). Exercise consisted of treadmill walking (5.3 km·h; 3.7% ± 0.9% grade) for 60 min at a target heat production of 6.0 W·kg in 40.0°C ± 0.1°C and 20.0% ± 0.6% relative humidity conditions. Measurements included rectal temperature, heart rate, ratings of perceived exertion (RPE), and thermal sensation.

RESULTS

No differences in rectal temperature (P ≥ 0.85), heart rate (P ≥ 0.99), thermal sensation (P ≥ 0.73), or RPE (P ≥ 0.13) occurred between uniform + burn and uniform trials. In the control trial, however, core temperature, heart rate, thermal sensation, and RPE were lower compared with the uniform and uniform + burn trials (P ≤ 0.04 for all).

CONCLUSIONS

A 20% total body surface area simulated torso burn injury does not further exacerbate heat strain when wearing a combat uniform. These findings suggest that the physiological strain associated with torso burn injuries is not different from noninjured individuals when wearing protective clothing during an acute exercise-heat stress.

Interaction of Exercise Intensity and Simulated Burn Injury Size on Thermoregulation

PURPOSE

This study aimed to test the hypothesis that the elevation in internal body temperature during exercise in a hot environment is influenced by the combination of exercise intensity and BSA burned.

METHODS

Ten healthy participants (8 males, 2 females; 32 ± 9 yr; 75.3 ± 11.7 kg) completed eight exercise trials on a cycle ergometer, each with different combinations of metabolic heat productions (low, 4 W·kg-1; moderate, 6 W·kg-1) and simulated BSA burn in a hot environmental chamber (39.9°C ± 0.3°C, 20.1% ± 1.5% RH). Burns were simulated by covering 0%, 20%, 40%, or 60% of participants' BSA with a highly absorbent, vapor-impermeable material. Gastrointestinal temperature (TGI) was recorded, with the primary analysis being the increase in TGI after 60 min of exercise.

RESULTS

We identified an interaction effect for the increase in TGI (P < 0.01), suggesting TGI was influenced by both intensity and simulated burn BSA. Regardless of the percentage BSA burn simulated, the increase in TGI was similar across low-intensity trials (0.70°C ± 0.26°C, P > 0.11 for all). However, during moderate-intensity exercise, the increase in TGI was greater for the 60% (1.78°C ± 0.38°C, P < 0.01) and 40% BSA coverage trials (1.33°C ± 0.44°C, P = 0.04), relative to 0% (0.82°C ± 0.36°C). There were no differences in TGI responses between 0% and 20% trials.

CONCLUSION

These data suggest that exercise intensity influences the relationship between burn injury size and thermoregulatory responses in a hot environment.

Low-dose ketamine affects blood pressure, but not muscle sympathetic nerve activity, during progressive central hypovolemia without altering tolerance

KEY POINTS

Haemorrhage is the leading cause of battlefield and civilian trauma deaths. Given that a haemorrhagic injury on the battlefield is almost always associated with pain, it is paramount that the administered pain medication does not disrupt the physiological mechanisms that are beneficial in defending against the haemorrhagic insult. Current guidelines from the US Army's Committee on Tactical Combat Casualty Care (CoTCCC) for the selection of pain medications administered to a haemorrhaging soldier are based upon limited scientific evidence, with the clear majority of supporting studies being conducted on anaesthetized animals. Specifically, the influence of low-dose ketamine, one of three analgesics employed in the pre-hospital setting by the US Army, on haemorrhagic tolerance in humans is unknown. For the first time in conscious males and females, the findings of the present study demonstrate that the administration of an analgesic dose of ketamine does not compromise tolerance to a simulated haemorrhagic insult. Increases in muscle sympathetic nerve activity during progressive lower-body negative pressure were not different between trials. Despite the lack of differences for muscle sympathetic nerve activity responses, mean blood pressure and heart rate were higher during moderate hypovolemia after ketamine vs. placebo administration.

ABSTRACT

Haemorrhage is the leading cause of battlefield and civilian trauma deaths. For a haemorrhaging soldier, there are several pain medications (e.g. ketamine) recommended for use in the prehospital, field setting. However, the data to support these recommendations are primarily limited to studies in animals. Therefore, it is unknown whether ketamine adversely affects physiological mechanisms responsible for maintenance of arterial blood pressure (BP) during haemorrhage in humans. In humans, ketamine has been demonstrated to raise resting BP, although it has not been studied with the concomitant central hypovolemia that occurs during haemorrhage. Thus, the present study aimed to test the hypothesis that ketamine does not impair haemorrhagic tolerance in humans. Thirty volunteers (15 females) participated in this double-blinded, randomized, placebo-controlled trial. A pre-syncopal limited progressive lower-body negative pressure (LBNP; a validated model for simulating haemorrhage) test was conducted following the administration of ketamine (20 mg) or placebo (saline). Tolerance was quantified as a cumulative stress index and compared between trials using a paired, two-tailed t test. We compared muscle sympathetic nerve activity (MSNA; microneurography), beat-to-beat BP (photoplethysmography) and heart rate (electrocardiogram) responses during the LBNP test using a mixed effects model (time [LBNP stage] × drug). Tolerance to the LBNP test was not different between trials (Ketamine: 635 ± 391 vs. Placebo: 652 ± 360 mmHg‧min, p = 0.77). Increases in MSNA burst frequency (time: P < 0.01, trial: p = 0.27, interaction: p = 0.39) during LBNP stages were no different between trials. Despite the lack of differences for MSNA responses, mean BP (time: P < 0.01, trial: P < 0.01, interaction: p = 0.01) and heart rate (time: P < 0.01, trial: P < 0.01, interaction: P < 0.01) were higher during moderate hypovolemia after ketamine vs. placebo administration (P < 0.05 for all, post hoc), but not at the end of LBNP. These data, which are the first to be obtained in conscious humans, demonstrate that the administration of low-dose ketamine does not impair tolerance to simulated haemorrhage or mechanisms responsible for maintenance of BP.

Exercise Training Improves Microvascular Function in Burn Injury Survivors

INTRODUCTION

Vasodilator function is impaired in individuals with well-healed burn injuries; however, therapeutic interventions that lessen or reverse this maladaptation are lacking. The purpose of this study was to test the hypothesis that a 6-month community-based exercise training program would increase microvascular dilator function in individuals with well-healed burn injuries, irrespective of the magnitude of the injured body surface area. Further, we hypothesize that macrovascular dilator function would remain unchanged posttraining.

METHODS

Microvascular function (forearm reactive hyperemia), macrovascular function (brachial artery flow-mediated dilation), and the maximal vasodilatory response after ischemic handgrip exercise (an estimate of microvascular remodeling) were assessed before and after exercise training in nonburned control subjects (n = 11) and individuals with burn injuries covering a moderate body surface area (26% ± 7%; n = 13) and a high body surface area (59% ± 15%; n = 19).

RESULTS

Peak vascular conductance and area under the curve during postocclusive reactive hyperemia increased from pretraining to posttraining in control and burn injury groups (both P < 0.05), the magnitude of which did not differ between groups (both P = 0.6). Likewise, the maximal vasodilatory response after ischemic handgrip exercise increased in all groups after exercise training (P < 0.05). Macrovascular dilator function did not differ across time or between groups (P = 0.8).

CONCLUSIONS

These data suggest that a community-based exercise training program improves microvascular function in individuals with well-healed burn injuries, which may be due in part to vascular remodeling.

Exercise Core Temperature Response with a Simulated Burn Injury: Effect of Body Size

Although the severity of a burn injury is often associated with the percentage of total body surface area burned (%TBSA), the thermoregulatory consequences of a given %TBSA injury do not account for the interactive effects of body morphology and metabolic heat production (Hprod).

PURPOSE

Using a simulated burn injury model to mimic the detrimental effect of a 40% TBSA injury on whole-body evaporative heat dissipation, core temperature response to exercise in physiologically uncompensable conditions between morphologically disparate groups were examined at (i) an absolute Hprod (W), and (ii) a mass-specific Hprod (W·kg).

METHODS

Healthy, young, nonburned individuals of small (SM, n = 11) or large (LG, n = 11) body size cycled for 60 min at 500 W or 5.3 W·kg of Hprod in 39°C and 20% relative humidity conditions. A 40% burn injury was simulated by affixing a highly absorbent, vapor-impermeable material across the torso (20% TBSA), arms (10% TBSA), and legs (10% TBSA) to impede evaporative heat loss in those regions.

RESULTS

Although the elevation in core temperature was greater in SM compared with LG at an Hprod of 500 W (SM, 1.69°C ± 0.26°C; LG, 1.05°C ± 0.26°C; P < 0.01), elevations in core temperature were not different at an Hprod of 5.3 W·kg between groups (SM, 0.99°C ± 0.32°C; LG, 1.05°C ± 0.26°C; P = 0.66).

CONCLUSIONS

These data suggest that among individuals with a 40% TBSA burn injury, a smaller body size leads to exacerbated elevations in core temperature during physical activities eliciting the same absolute Hprod (non-weight-bearing tasks) but not activities eliciting the same mass-specific Hprod (weight-bearing tasks).

Exercise Thermoregulation with a Simulated Burn Injury: Impact of Air Temperature

The U.S. Army's Standards of Medical Fitness (AR 40-501) states: "Prior burn injury (to include donor sites) involving a total body surface area of 40% or more does not meet the standard." However, the standard does not account for the interactive effect of burn injury size and air temperature on exercise thermoregulation.

PURPOSE

To evaluate whether the detrimental effect of a simulated burn injury on exercise thermoregulation is dependent on air temperature.

METHODS

On eight occasions, nine males cycled for 60 min at a fixed metabolic heat production (6 W·kg) in air temperatures of 40°C or 25°C with simulated burn injuries of 0% (Control), 20%, 40%, or 60% of total body surface area (TBSA). Burn injuries were simulated by covering the skin with an absorbent, vapor-impermeable material to impede evaporation from the covered areas. Core temperature was measured in the gastrointestinal tract via telemetric pill.

RESULTS

In 40°C conditions, greater elevations in core temperature were observed with 40% and 60% TBSA simulated burn injuries versus Control (P < 0.01). However, at 25°C, core temperature responses were not different versus Control with 20%, 40%, and 60% TBSA simulated injuries (P = 0.97). The elevation in core temperature at the end of exercise was greater in the 40°C environment with 20%, 40%, and 60% TBSA simulated burn injuries (P ≤ 0.04).

CONCLUSIONS

Simulated burn injuries ≥20% TBSA exacerbate core temperature responses in hot, but not temperate, air temperatures. These findings suggest that the U.S. Army's standard for inclusion of burned soldiers is appropriate for hot conditions, but could lead to the needless discharge of soldiers who could safely perform their duties in cooler training/operational settings.

Dietary nitrate supplementation does not influence thermoregulatory or cardiovascular strain in older individuals during severe ambient heat stress

NEW FINDINGS

What is the central question of this study? Does dietary nitrate supplementation with beetroot juice attenuate thermoregulatory and cardiovascular strain in older adults during severe heat stress? What is the main finding and its importance? A 7-day nitrate supplementation regimen lowered resting mean arterial pressure in thermoneutral conditions. During heat stress, core and mean skin temperatures, vasodilatory responses, sweat loss, heart rate and left ventricular function were unchanged, and mean arterial pressure was only transiently reduced, post-supplementation. These data suggest nitrate supplementation with beetroot juice does not mitigate thermoregulatory or cardiovascular strain in heat-stressed older individuals.

ABSTRACT

This study tested the hypothesis that dietary nitrate supplementation with concentrated beetroot juice attenuates thermoregulatory and cardiovascular strain in older individuals during environmental heat stress. Nine healthy older individuals (six females, three males; aged 67 ± 5 years) were exposed to 42.5 ± 0.1°C and 34.0 ± 0.5% relative humidity conditions for 120 min before (CON) and after 7 days of dietary nitrate supplementation with concentrated beetroot juice (BRJ; 280 ml, ∼16.8 mmol of nitrate daily). Core and skin temperatures, body mass changes (indicative of whole-body sweat loss), skin blood flow and cutaneous vascular conductance, forearm blood flow and vascular conductance, heart rate, arterial blood pressures and indices of cardiac function were measured. The 7-day beetroot juice regimen increased plasma nitrate/nitrite levels from 27.4 ± 15.2 to 477.0 ± 102.5 μmol l^-1 (P < 0.01) and lowered resting mean arterial pressure from 90 ± 7 to 83 ± 10 mmHg at baseline under thermoneutral conditions (P = 0.02). However, during subsequent heat stress, no differences in core and skin temperatures, skin blood flow and vascular conductance, forearm blood flow and vascular conductance, whole-body sweat loss, heart rate, and echocardiographic indices of systolic function and diastolic filling were evident following nitrate supplementation (all P > 0.05). Mean arterial pressure was lower in BRJ vs. CON during heat stress (treatment-by-time interaction: P = 0.02). Overall, these findings suggest that dietary nitrate supplementation with concentrated beetroot juice does not attenuate thermoregulatory or cardiovascular strain in older individuals exposed to severe ambient heat stress.

The benefits of an unsupervised exercise program in persons with well-healed burn injuries within the International Classification of Functioning, Disability and Health (ICF)

Vast improvements in the survival rates following burn injuries has led to a greater number of patients living with a wide range of long-term impairments, activity limitations, and participation constraints. Therefore, long-term care is critical in this clinical population and necessitates appropriate rehabilitation strategies to maximize an individual's overall health. The purpose of this study was to test the hypothesis that the extent to which outcomes within the International Classification of Functioning, Disability, and Health (ICF) framework are improved following 6 months of unsupervised exercise training is influenced by the severity of a burn injury (i.e., percent body surface area injured). Outcome variables representing the dimensions of the ICF, body functions & structure, activity, and participation, were collected pre- and post- 6 months of exercise training in three groups of participants: non-injured control subjects (N = 11), subjects with moderate-level well-healed burn injuries (N = 13, 26 ± 6% body surface area burned), and subjects with high-level well-healed burn injuries (N = 20, 58 ± 15% body surface area burned). Exercise training improved lower extremity strength (changes in peak torque/kg body mass at 90 degrees/sec flexion: 30 ± 5% and extension: 36 ± 4%, p < 0.05) and functional activities (changes in sit to stand: -9 ± 4% and ascend stairs: -4 ± 1%; p < 0.05) in all groups. For outcome variables representing ICF levels of body functions & structure and activity, there were no differences at baseline or improvements made between the groups after training. That said, with the exception of the domain of functional activity (reported 17 ± 34% improvement in the high-level burn cohort, p < 0.05), no changes were revealed in the participation level of ICF indexed by health-related quality of life questionnaires. These findings support the utilization of a 6-month unsupervised exercise training program in the long-term rehabilitation of individuals with burn injuries; that is, improvements in body functions & structure and activity can be achieved with an exercise regimen regardless of the severity of burn injury.

Cardiac Structure and Function in Well-Healed Burn Survivors

Long-term burn survivors have reduced aerobic capacity, placing them at increased risk for cardiovascular disease, morbidity, and mortality. However, the exact mechanism contributing to a reduced aerobic capacity remains incompletely understood, but may be related to adverse cardiovascular remodeling. Therefore, it was hypothesized that well-healed burn survivors would exhibit adverse left ventricular (LV) remodeling and impaired LV function. To test this hypothesis, 22 well-healed moderately burned individuals (age: 41 ± 14 years; BMI: 27.7 ± 5.4 kg/m2; male/female: 12/10; extent of burn: 37 ± 12 %BSA), 11 well-healed severely burned individuals (age: 43 ± 12 years; BMI: 29.5 ± 5.8 kg/m2; male/female: 8/3; extent of burn: 73 ± 11 %BSA), and 12 healthy, age-matched controls (age: 34 ± 9 years; BMI: 28.6 ± 5.2 kg/m2; male/female: 5/7) were enrolled in the study. All subjects were sedentary, performing less than 30 minutes of aerobic exercise per day, 3 days per week. LV morphology and function were assessed via cardiac magnetic resonance imaging. In contrast to the hypothesis, neither the presence nor severity of burn injury adversely affected LV morphology or function, when compared with equally sedentary nonburned controls. However, of note, LV mass of all three groups was in the lowest 5th percentile compared with normative values. Finally, group differences in LV morphology were largely explained by differences in aerobic capacity. Taken together, these data suggest a prior burn injury itself does not result in pathological remodeling of the LV and support a role for aerobic exercise training to improve cardiac function.

An assessment of hypercapnia-induced elevations in regional cerebral perfusion during combined orthostatic and heat stresses

We investigated that the effects of hypercapnia-induced elevations in cerebral perfusion during a heat stress on global cerebrovascular responses to an orthostatic challenge. Seven volunteers completed a progressive lower-body negative pressure (LBNP) challenge to presyncope during heat stress, with or without breathing a hypercapnic gas mixture. Administration of the hypercapnic gas mixture increased the partial pressure of end-tidal CO₂ greater than pre-heat stress alone, and increased both internal carotid artery (ICA) and vertebral artery (VA) blood flows (P < 0.05). During LBNP, both ICA and VA blood flows with the hypercapnic gas mixture remained elevated relative to the control trial (P < 0.05). However, at the end of LBNP due to pre-syncopal symptoms, both ICA and VA blood flows decreased to similar levels between trials. These findings suggest that hypercapnia-induced cerebral vasodilation is insufficient to maintain cerebral perfusion at the end of LBNP due to pre-syncope in either the anterior or posterior vascular beds.

Endothelial function in response to exercise in the cold in patients with coronary artery disease

BACKGROUND

Regular long-term physical exercise has favourable effects on endothelial function in patients with coronary artery disease (CAD). However, the effects of an acute exercise bout in the cold on endothelial function are not known.

METHODS

At first, the effects of moderate-intensity aerobic lower-body exercise were assessed in CAD patients (n = 16) in a neutral [+22°C] and cold [-15°C] environment. Secondly, responses to static and dynamic upper-body exercise in a neutral [+22°C] and cold [-15°C] environment were investigated in CAD patients (n = 15). All experiments were performed in a random order. Endothelial function was measured by flow-mediated dilation (FMD) of the brachial artery in response to reactive hyperaemia, before and after the exposures in a neutral environment.

RESULTS

No significant temperature*exercise*condition (pre-post) interaction was observed in FMD% when comparing rest versus aerobic exercise or static versus dynamic upper-body exercise. Relative reactive hyperaemia during FMD protocol, measured by changes in shear rate, was elevated after rest compared to aerobic exercise (p = .001) and after static compared to dynamic upper-body exercise (p < .001). However, no significant temperature*exercise*condition interaction was observed when FMD% was normalized for shear rate.

CONCLUSIONS

Endothelial function to an acute bout of exercise among CAD patients was not modified by the environmental temperature where the exercise was performed. The present findings argue against the hypothesis that exercise in cold environmental conditions impairs endothelial function in patients with CAD.

Keeping older individuals cool in hot and moderately humid conditions: wetted clothing with and without an electric fan

The present study evaluated whether wearing a water-soaked t-shirt, with or without electric fan use, mitigates thermal and cardiovascular strain in older individuals exposed to hot and moderately humid conditions. Nine healthy older individuals (68 ± 4 yr; five women) completed three 120-min heat exposures (42.4 ± 0.2°C, 34.2 ± 0.9% relative humidity) on separate days while wearing a dry t-shirt (CON), a t-shirt soaked with 500 ml of tap water (WET), or a t-shirt soaked with 500 ml of tap water while facing an electric fan (2.4 ± 0.4 m/s; WET+FAN). Measurements included core and skin temperatures, evaporative mass losses, heart rate, and blood pressure. In the WET condition, elevations in core temperature were attenuated compared with DRY from 30 to 120 min and compared with WET+FAN from 30 to 90 min (P < 0.05). Evaporative mass losses (inclusive of sweat and water losses from the shirt) were greatest in WET+FAN, followed by WET, and then DRY (P < 0.01). Sweat losses were lowest in WET, followed by DRY, and then WET+FAN (P < 0.01). Heart rate was lower only at 60 min in WET versus DRY (P = 0.01). No differences in mean arterial pressure were observed (P = 0.51). In conclusion, wearing a water-soaked t-shirt without, but not with, electric fan use is an effective heat management strategy to mitigate thermal strain and lower sweat losses in older individuals exposed to hot and moderately humid conditions.NEW & NOTEWORTHY In older individuals exposed to hot and moderately humid environments, electric fan use coupled with a water-soaked t-shirt exacerbates sweat losses without mitigating heat strain compared with a dry t-shirt. However, wearing a water-soaked t-shirt without fan use reduces sweat losses and attenuates heat strain compared with a dry t-shirt and a fan/water-soaked t-shirt combination. These findings suggest wearing a water-soaked t-shirt is an effective heat-management strategy for older individuals during heat waves when air conditioning is inaccessible.