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

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.

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.

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.

Metabolism- and sex-dependent critical WBGT limits at rest and during exercise in the heat

Critical environmental limits are environmental thresholds above which heat gain exceeds heat loss and body core temperature (T_c) cannot be maintained at equilibrium. Those limits can be represented as critical wet-bulb globe temperature (WBGT_crit), a validated index that represents the overall thermal environment. Little is known about WBGT_crit at rest and during low-to-moderate intensity exercise, or sex differences in WBGT_crit, in unacclimated young adults. The following hypotheses were tested: 1) WBGT_crit progressively decreases as metabolic heat production (M_net) increases, 2) no sex differences in WBGT_crit occur at rest, and 3) WBGT_crit is lower during absolute-intensity exercise but higher at relative intensities in women than in men. Thirty-six participants [19 men (M)/17 women (W); 23 ± 4 yr] were tested at rest, during light, absolute-intensity exercise (10 W), or during moderate, relative-intensity exercise [30% maximal oxygen consumption (V̇o_2max)] in an environmental chamber. Dry-bulb temperature was clamped as relative humidity or ambient water vapor pressure was increased until an upward inflection was observed in T_c (rectal or esophageal temperature). Sex-aggregated WBGT_crit was lower during 10 W (32.9°C ± 1.7°C, P < 0.0001) and 30% V̇o_2max (31.6°C ± 1.1°C, P < 0.0001) exercise versus at rest (35.3°C ± 0.8°C), and lower at 30% V̇o_2max versus 10 W (P = 0.01). WBGT_crit was similar between sexes at rest (35.6°C ± 0.8°C vs. 35.0°C ± 0.8°C, P = 0.83), but lower during 10 W (31.9°C ± 1.7°C vs. 34.1°C ± 0.3°C, P < 0.01) and higher during 30% V̇o_2max (32.4°C ± 0.8°C vs. 30.8°C ± 0.9°C, P = 0.03) exercise in women versus men. These findings suggest that WBGT_crit decreases as M_net increases, no sex differences occur in WBGT_crit at rest, and sex differences in WBGT_crit during exercise depend on absolute versus relative intensities.

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.

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.