Utility of the Heat Index in defining the upper limits of thermal balance during light physical activity (PSU HEAT Project).

Extreme heat events and consequent detrimental heat-health outcomes have been increasing in recent decades and are expected to continue with future climate warming. While many indices have been created to quantify the combined atmospheric contributions to heat, few have been validated to determine how index-defined heat conditions impact human health. However, this subset of indices is likely not valid for all situations and populations nor easily understood and interpreted by health officials and the public. In this study, we compare the ability of thresholds determined from the National Weather Service's (NWS) Heat Index (HI), the Wet Bulb Globe Temperature (WBGT), and the Universal Thermal Climate Index (UTCI) to predict the compensability of human heat stress (upper limits of heat balance) measured as part of the Pennsylvania State University's Heat Environmental Age Thresholds (PSU HEAT) project. While the WBGT performed the best of the three indices for both minimal activities of daily living (MinAct; 83 W·m-2) and light ambulation (LightAmb; 133 W·m-2) in a cohort of young, healthy subjects, HI was likewise accurate in predicting heat stress compensability in MinAct conditions. HI was significantly correlated with subjects' perception of temperature and humidity as well as their body core temperature, linking perception of the ambient environment with physiological responses in MinAct conditions. Given the familiarity the public has with HI, it may be better utilized in the expansion of safeguard policies and the issuance of heat warnings during extreme heat events, especially when access to engineered cooling strategies is unavailable.

Analysis of human thermoregulatory mechanisms using 2-D computational model.

A numerical investigation is reported comparing various human thermoregulation mechanisms under hot and cold stress. The passive system of the developed model consists of 12 spherical/cylindrical segments and is modeled using the Pennes bioheat equation and finite difference method. The active system accounts for all regulatory responses, including the counter current heat exchange between veins and arteries; the respiratory heat loss; and threshold, gain, and maximum intensity of response mechanisms. The developed code analyzes various thermoregulatory defense mechanisms under hot and cold environments. Results indicate that shivering and sweating are more effective than other defense mechanisms under cold and hot conditions, respectively. Suppressing shivering will be more effective than the stoppage of vasoconstriction for inducing therapeutic hypothermia. The normal basal metabolic heat generation is essential to maintain a constant body temperature. It is seen that the changes in threshold temperatures of thermoregulatory mechanisms significantly affect the core more than the peripheral regions. The result may be helpful for better management of therapeutic hypothermia, hot/cold stress management, and design of drug protocols.

Human thermophysiological models: Quantification of uncertainty in the output quantities of the passive system due to uncertainties in the control equations of the active system via the Monte Carlo method.

Uncertainty propagation analysis in the Fiala thermophysiological model is performed by the Monte Carlo Method. The uncertainties of the output quantities of the passive system, due to imported uncertainties in the coefficients of the control equations of the active system, caused by the variation of the experimental data, are computed. The developed and implemented in-house code is accordingly validated. The effect of the input uncertainties, in each of the four main responses (shivering, vasodilatation, vasoconstriction, sweating) of the active system, is separately examined by simulating the human exposure from neutral conditions to cold and hot environments. It is predicted that the maximum output uncertainties of the response mechanisms may be of the same order of magnitude as the imported ones, while the corresponding maximum uncertainties in core and skin temperatures always remain less than 2%. The maximum absolute deviations of the rectal (core) temperatures from their estimated mean values may be up to 0.72 °C and 0.22 °C, due to input uncertainties in shivering and sweating respectively, while the corresponding deviations due to uncertainties in vasomotion processes are negligible. The deviations, particularly the ones due to shivering, are significant, since differences of a few tenths of a degree may have large impact in human health. The maximum absolute deviations of the skin temperatures are 0.42 °C in the hands due to uncertainties in shivering and 0.69 °C in the feet due to uncertainties in vasodilatation. These deviations are less significant than the core ones, but they may still affect human thermal sensation and comfort. The present analysis provides a better insight in the dynamic response of the model and indicates which response mechanism needs to be further investigated by more accurate estimates in order to improve model reliability. It can be also applied in other human thermophysiological models.

New neurophysiological human thermal model based on thermoreceptor responses.

A new neurophysiological human thermal model based on thermoreceptor responses, the NHTM model, has been developed to predict regulatory responses and physiological variables in asymmetric transient environments. The passive system is based on Wissler's model, which is more complex and refined. Wissler's model segments the human body into 21 cylindrical parts. Each part is divided into 21 layers, 15 for the tissues and 6 for clothes, and each layer is divided into 12 angular sectors. Thus, we have 3780 nodes for the tissues and 1512 for clothes. The passive system simulates heat exchange within the body and between the body and the surroundings. The active system is composed of the thermoregulatory mechanisms, i.e., skin blood flow, shivering thermogenesis, and sweating. The skin blood flow model and the shivering model are based on thermoreceptor responses. The sweating model is that of Fiala et al. and is based on error signals. The NHTM model was compared with Wissler's model, and the results showed that a calculation based on neurophysiology can improve the performance of the thermoregulation model. The NHTM model was more accurate in the prediction of mean skin temperature, with a mean absolute error of 0.27 °C versus 0.80 °C for the original Wissler model. The prediction accuracy of the NHTM model for local skin temperatures and core temperature could be improved via an optimization method to prove the ability of the new thermoregulation model to fit with the physiological characteristics of different populations.

Virtual population-based assessment of the impact of 3 Tesla radiofrequency shimming and thermoregulation on safety and B1 + uniformity.

PURPOSE: To assess the effect of radiofrequency (RF) shimming of a 3 Tesla (T) two-port body coil on B1 + uniformity, the local specific absorption rate (SAR), and the local temperature increase as a function of the thermoregulatory response. METHODS: RF shimming alters induced current distribution, which may result in large changes in the level and location of absorbed RF energy. We investigated this effect with six anatomical human models from the Virtual Population in 10 imaging landmarks and four RF coils. Three thermoregulation models were applied to estimate potential local temperature increases, including a newly proposed model for impaired thermoregulation. RESULTS: Two-port RF shimming, compared to circular polarization mode, can increase the B1 + uniformity on average by +32%. Worst-case SAR excitations increase the local RF power deposition on average by +39%. In the first level controlled operating mode, induced peak temperatures reach 42.5°C and 45.6°C in patients with normal and impaired thermoregulation, respectively. CONCLUSION: Image quality with 3T body coils can be significantly increased by RF shimming. Exposure in realistic scan scenarios within guideline limits can be considered safe for a broad patient population with normal thermoregulation. Patients with impaired thermoregulation should not be scanned outside of the normal operating mode. Magn Reson Med 76:986-997, 2016. © 2015 Wiley Periodicals, Inc.

Review on modeling heat transfer and thermoregulatory responses in human body.

Several mathematical models of human thermoregulation have been developed, contributing to a deep understanding of thermal responses in different thermal conditions and applications. In these models, the human body is represented by two interacting systems of thermoregulation: the controlling active system and the controlled passive system. This paper reviews the recent research of human thermoregulation models. The accuracy and scope of the thermal models are improved, for the consideration of individual differences, integration to clothing models, exposure to cold and hot conditions, and the changes of physiological responses for the elders. The experimental validated methods for human subjects and manikin are compared. The coupled method is provided for the manikin, controlled by the thermal model as an active system. Computational Fluid Dynamics (CFD) is also used along with the manikin or/and the thermal model, to evaluate the thermal responses of human body in various applications, such as evaluation of thermal comfort to increase the energy efficiency, prediction of tolerance limits and thermal acceptability exposed to hostile environments, indoor air quality assessment in the car and aerospace industry, and design protective equipment to improve function of the human activities.

Thermal conditions in freezing chambers and prediction of the thermophysiological responses of workers.

The present work is dedicated to the assessment of the cold thermal strain of human beings working within freezing chambers. To obtain the present results, both field measurements and a numerical procedure based on a modified version of the Stolwijk thermoregulation model were used. Eighteen freezing chambers were considered. A wide range of physical parameters of the cold stores, the workers clothing insulation, and the working and recovering periods were observed. The combination of these environmental and individual parameters lead to different levels of thermal stress, which were grouped under three categories. Some good practices were observed in the field evaluations, namely situations with appropriate level of clothing protection and limited duration of exposure to cold avoiding unacceptable level of hypothermia. However, the clothing ensembles normally used by the workers do not provide the minimum required insulation, which suggests the possibility of the whole body cooling for levels higher than admissible. The numerical predictions corroborate the main conclusions of the field survey. The results obtained with both methodologies clearly show that, for the low temperature of the freezing chambers, the clothing insulation is insufficient, the exposure periods are too long, and the recovering periods are inadequate. Thus, high levels of physiological strain can indeed be reached by human beings under such working environments.

Fatal pediatric hyperthermia: A forensic review.

This paper examines a pediatric hyperthermia homicide in which the decedent was placed into a room with only a diaper on and left unattended overnight. There were no furnishings in the room except for a 1500-W space heater and a stroller. The following morning, emergency personnel were summoned to the residence. A caretaker said the decedent was playing normally 5 min before making the 911 call. The decedent's initial rectal temperature was 42.2°C. Law enforcement asked how long the child had to be exposed to a high temperature in order to induce fatal hyperthermia in an empty bedroom. The scene was reconstructed using the child's residence and the same heater. Environmental data were gathered over a 16-h period. The thermal parameters of the room and environment were analyzed using a lumped-element thermal model. These parameters were then fed into an adapted Gagge's two-node model of human thermal regulation, which provided a time-window of exposure necessary to elicit hyperthermia, which in this case, depending on certain variables, ranged from 45 min to 4 h.

Panting as a human heat loss thermoeffector.

The human autonomic nervous system participates in the control of thermoregulatory responses that are employed to regulate core temperature following deviations of skin temperature and/or core temperature from their respective resting values. This permits a regulation of the core temperature (TC) at 37.0 ± 1°C with superimposed circadian variations in both sexes and menstrual cycle-associated variations in premenopausal women. When rendered hyperthermic, passively by heat exposure while at rest or actively during exercise, humans engage heat loss or thermolytic responses, including eccrine sweating and cutaneous vasodilatation. A third, less studied, human thermolytic response is thermal panting, and this response is the focus of this review. Human thermal panting was first described over a century ago. It has since been shown to be a reproducible response showing some similar patterns of breathing in species that employ panting as their sole thermolytic heat loss response. The contribution of human panting as a thermolytic response, however, remains controversial. This review highlights both past and recent evidence supporting that hyperthermic humans have a panting pattern of breathing that plays an important role in human thermoregulation.

An integrated approach to develop, validate and operate thermo-physiological human simulator for the development of protective clothing.

Following the growing interest in the further development of manikins to simulate human thermal behaviour more adequately, thermo-physiological human simulators have been developed by coupling a thermal sweating manikin with a thermo-physiology model. Despite their availability and obvious advantages, the number of studies involving these devices is only marginal, which plausibly results from the high complexity of the development and evaluation process and need of multi-disciplinary expertise. The aim of this paper is to present an integrated approach to develop, validate and operate such devices including technical challenges and limitations of thermo-physiological human simulators, their application and measurement protocol, strategy for setting test scenarios, and the comparison to standard methods and human studies including details which have not been published so far. A physical manikin controlled by a human thermoregulation model overcame the limitations of mathematical clothing models and provided a complementary method to investigate thermal interactions between the human body, protective clothing, and its environment. The opportunities of these devices include not only realistic assessment of protective clothing assemblies and equipment but also potential application in many research fields ranging from biometeorology, automotive industry, environmental engineering, and urban climate to clinical and safety applications.

Computer Simulation of Individualized Human Thermal Response during General Anesthesia / 医用生物力学

Objective To analyze the effects of anesthesia-induced thermoregulatory system impairment and low temperature environment of the operating room on the perioperative thermoregulation of individualized patients by constructing a computer simulation model. Methods A simple anesthesia model was proposed and then incorporated into the self-developed individualized thermoregulatory model, in which human body was represented as a cylinder with two layers of the core and the skin. The integrated model could be used to assess the effects of individualized characteristics such as age, obesity, and cardiovascular diseases on thermoregulation by modifying different physiological parameters involving sweating, shivering and cutaneous vasomotion. Simulation of the general anesthesia effects on human thermoregulation could be achieved by reducing basal metabolic rate and thresholds for vasoconstriction and shivering. Results The elderly people showed lower core temperature but higher skin temperature, compared with the young people. In a low temperature environment, an increase in fat thickness or an increase in severity degree of the left ventricular failure (LVF) might alleviate the decrease in core temperature, while an increase in wind speed or relative humidity could result in a decrease in core temperature. When the threshold setting of vasoconstriction was reduced by 0-5-3 ℃, the core temperature showed a significant decrease. Conclusions By comparing model simulations with experimental measurements, the reliability and validity of the model in predicting human transient thermal responses during varying external thermal environment was verified. The individualized characteristics of human body had an important influence on human body temperature in a low temperature environment. Moreover, the combination of individualized characteristics of human body and general anesthesia further complicated the body′s thermoregulation and posed significant challenges for clinicians.

Interoception and autonomic nervous system reflexes thermoregulation.

Important conceptual changes concerning human thermoregulation have occurred in the last decade. While the hypothalamus maintains its central role in sensing core temperature and providing connectivity to orchestrate heat loss and cold defense autonomic neuronal mechanisms, it is now regarded as one of multiple, independent thermoeffector pathways that control core body temperature. Recent research in primate central and peripheral thermosensitivity has emphasized the importance of temperature-activated transient receptor potential (TRP) channels and afferent neuronal pathways from peripheral thermosensors that are activated by unique combinations of core and shell temperature. The interoceptive aspects of behavioral thermoregulation have been emphasized including the primary importance of shell (skin) temperature, the concept of thermal discomfort and the important contribution of orbitofrontal, insular, somatosensory, and amygdala cortical regions deployed to anticipate and avoid thermal stress. Clinical testing of human thermoregulation requires afferent stimuli to activate the independent thermoeffector loops while monitoring an efferent response. Patterns of sweat gland activation, amount of sweat produced, and areas of anhidrosis demonstrated by the thermoregulatory and axon reflex sweat testing provide diagnostic information about neurological and medical disorders of the autonomic nervous system.