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Heat stroke is a serious health concern globally, which is associated with high mortality. Newer treatments must be designed to improve outcomes. The aim of this study is to evaluate the effect of variations in ambient temperature and wind speed on the rate of cooling in a simulated heat stroke subject using the dynamic model of Wissler. We assume that a 60-year-old 70-kg female suffers classic heat stroke after walking fully exposed to the sun for 4 h while the ambient temperature is 40 °C, relative humidity is 20%, and wind speed is 2.5 m/s -1 . Her esophageal and skin temperatures are 41.9 and 40.7 °C at the time of collapse. Cooling is accomplished by misting with lukewarm water while exposed to forced airflow at a temperature of 20 to 40 °C and a velocity of 0.5 or 1 m/s -1 . Skin blood flow is assumed to be either normal, one-half of normal, or twice normal. At wind speed of 0.5 m/s -1 and normal skin blood flow, the air temperature decreased from 40 to 20 °C, increased cooling, and reduced time required to reach to a desired temperature of 38 °C. This relationship was also maintained in reduced blood flow states. Increasing wind speed to 1 m/s -1 increased cooling and reduced the time to reach optimal temperature both in normal and reduced skin blood flow states. In conclusion, evaporative cooling methods provide an effective method for cooling classic heat stroke patients. The maximum heat dissipation from the simulated model of Wissler was recorded when the entire body was misted with lukewarm water and applied forced air at 1 m/s at temperature of 20 °C.

This paper presents an updated and augmented version of the Wissler human thermoregulation model that has been developed continuously over the past 50 years. The existing Fortran code is translated into C with extensive embedded commentary. A graphical user interface (GUI) has been developed in Python to facilitate convenient user designation of input and output variables and formatting of data presentation. Use of the code with the GUI is described and demonstrated. New physiological elements were added to the model to represent the hands and feet, including the unique vascular structures adapted for heat transfer associated with glabrous skin. The heat transfer function and efficacy of glabrous skin is unique within the entire body based on the capacity for a very high rate of blood perfusion and the novel capability for dynamic regulation of blood flow. The model was applied to quantify the absolute and relative contributions of glabrous skin flow to thermoregulation for varying levels of blood perfusion. The model also was used to demonstrate how the unique features of glabrous skin blood flow may be recruited to implement thermal therapeutic procedures. We have developed proprietary methods to manipulate the control of glabrous skin blood flow in conjunction with therapeutic devices and simulated the effect of these methods with the model.

For approximately 50 years, hepatic clearance of indocyanine green (ICG) has been used to assess liver function. Steady-state infusion of ICG with simultaneous measurement of arterial and hepatic venous ICG concentrations provides unambiguous measures of the extraction ratio for ICG and the hepatic blood flow rate, but also requires cannulation of a hepatic vein. Transient clearance following injection of a single bolus of ICG, which typically involves only measurement of arterial ICG concentration, is a more commonly used procedure. Since drawing blood from a hepatic vein is often impossible, and, in any event can be difficult, there has been considerable interest in the claim by Grainger et al. (Clin Sci 64:207-212, 1983) that a single-bolus, two-compartment model "enabled the hepatic extraction ratio (ER(ss)) of dye to be determined solely from the plasma disappearance curve". The principal purpose of this paper is to show that the claim by Grainger et al. is not valid because it ignores the fact that a finite fraction of ICG entering the liver passes directly into hepatic veins without being sequestered in the liver. A valid relationship between ER(ss) and parameters determined from single-bolus clearance data is derived in this paper. For individuals with normally functioning livers, the single-bolus method of Grainger et al. yields an extraction ratio approximately 20% too large, but in cirrhotic patients with extensive intrahepatic shunting, the extraction ratio evaluated using the single-bolus method of Grainger et al. may be too large by a factor of two.

Overall resistances for heat and vapor transport in a multilayer garment depend on the properties of individual layers and the thickness of any air space between layers. Under uncomplicated, steady-state conditions, thermal and mass fluxes are uniform within the garment, and the rate of transport is simply computed as the overall temperature or water concentration difference divided by the appropriate resistance. However, that simple computation is not valid under cool ambient conditions when the vapor permeability of the garment is low, and condensation occurs within the garment. Several recent studies have measured heat and vapor transport when condensation occurs within the garment (Richards et al. in Report on Project ThermProject, Contract No. G6RD-CT-2002-00846, 2002; Havenith et al. in J Appl Physiol 104:142-149, 2008). In addition to measuring cooling rates for ensembles when the skin was either wet or dry, both studies employed a flat-plate apparatus to measure resistances of individual layers. Those data provide information required to define the properties of an ensemble in terms of its individual layers. We have extended the work of previous investigators by developing a rather simple technique for analyzing heat and water vapor transport when condensation occurs within a garment. Computed results agree well with experimental results reported by Richards et al. (Report on Project ThermProject, Contract No. G6RD-CT-2002-00846, 2002) and Havenith et al. (J Appl Physiol 104:142-149, 2008). We discuss application of the method to human subjects for whom the rate of sweat secretion, instead of the partial pressure of water on the skin, is specified. Analysis of a more complicated five-layer system studied by Yoo and Kim (Text Res J 78:189-197, 2008) required an iterative computation based on principles defined in this paper.

Various aspects of skin blood flow (SkBF) in human beings have been studied experimentally for more than seven decades. While reasonably complete phenomenological descriptions of individual factors have emerged from those investigations, little effort has been devoted to assembling the component parts into a coherent description of the entire system. This paper describes an effort to do that. Although the result is essentially a mathematical model of human SkBF, the model is firmly based on empirical data and not merely an abstract theoretical construct. We found that experimental data for human forearm blood flow (FBF) from many sources are well represented by an equation in which the rate of cutaneous blood flow (q (s)) is defined by the equation q (s) = q (s,r) AVD x CVCM x CVCL x CVCE. The coefficient q (s,r) is the perfusion rate at a reference state, and the four component factors are defined as follows: AVD defines centrally mediated active vasodilation as a function of central temperature (T (c)), mean skin temperature (T(s))d intensity of exercise (V(o)(2)) CVCM defines reflexly mediated cutaneous vasoconstriction as a function of (T(s)) CVCL defines locally mediated cutaneous vasoconstriction as a function of local skin temperature (T (s)); and CVCE defines the effect of exercise on cutaneous vasoconstriction and mean arterial pressure. The definition of each component function is based on experimental data. Two conclusions are particularly significant. One is that the study provides a rational explanation, based on the role of (T(s)), for previously disparate opinions about the non-thermal effect of exercise on active cutaneous vasodilation. The other is that it establishes that the four factors combine multiplicatively, and not additively, as previous investigators have suggested.

Background: Thermoregulation is critical for survival in space, especially during contingencies demanding of human cognitive and physical performance. A review of the negative feedback human thermoregulatory system is provided. The Advanced Crew Escape Suit is worn by astronauts during ascent and descent on the Space Shuttle to provide active cooling for nominal and contingency operations and protection from loss of cabin pressure mishaps. Failure of a thermal system control element during a recent Shuttle flight resulted in a single point failure that could have elevated cabin temperature, possibly resulting in cognitive deficits of the pilot during the reentry and landing phases., Methods: The efficacy of the existing cooling equipment and procedures for maintaining crew thermal comfort in the event of such a failure was assessed. The Wissler and 41-node thermoregulatory models were used to conduct a parametric study of Shuttle cabin temperatures and resulting thermal effects on crew., Results: Under high metabolic loads, crewmember core temperatures and heat storage are shown to increase beyond allowable limits using this analysis. Resulting levels of thermal stress may exceed standardized limits, after which cognitive performance and manual tracking ability are diminished., Discussion: The operational procedure for entry and landing during this failure scenario may result in significant thermal compromise to crewmembers, including cognitive and manual performance deficits. Revision of the flight rule governing crew actions during compromise of cabin thermal control has been undertaken to minimize thermal stress on returning Shuttle crewmembers. Modifications to the crew thermal protection system for the Shuttle are suggested.

Background: Estimating the probability of survival during accidental immersion in cold water presents formidable challenges for both theoreticians and empirics. A number of theoretical models have been developed assuming that death occurs when the central body temperature, computed using a mathematical model, falls to a certain level. This paper describes a different theoretical approach to estimating the probability of survival., Method: The human thermal model developed by Wissler is used to compute the central temperature during immersion in cold water. Simultaneously, a survival probability function is computed by solving a differential equation that defines how the probability of survival decreases with increasing time. The survival equation assumes that the probability of occurrence of a fatal event increases as the victim's central temperature decreases. Generally accepted views of the medical consequences of hypothermia and published reports of various accidents provide information useful for defining a "fatality function" that increases exponentially with decreasing central temperature., Results: The particular function suggested in this paper yields a relationship between immersion time for 10% probability of survival and water temperature that agrees very well with Molnar's empirical observations based on World War II data., Discussion: The method presented in this paper circumvents a serious difficulty with most previous models--that one's ability to survive immersion in cold water is determined almost exclusively by the ability to maintain a high level of shivering metabolism.

The Wissler human thermoregulation model was augmented to incorporate simulation of a space suit thermal control system that includes interaction with a liquid cooled garment (LCG) and ventilation gas flow through the suit. The model was utilized in the design process of an automatic controller intended to maintain thermal neutrality of an exercising subject wearing a liquid cooling garment. An experimental apparatus was designed and built to test the efficacy of specific physiological state measurements to provide feedback data for input to the automatic control algorithm. Control of the coolant inlet temperature to the LCG was based on evaluation of transient physiological parameters that describe the thermal state of the subject, including metabolic rate, skin temperatures, and core temperature. Experimental evaluation of the control algorithm function was accomplished in an environmental chamber under conditions that simulated the thermal environment of a space suit and transient metabolic work loads typical of astronaut extravehicular activity (EVA). The model was also applied to analyze experiments to evaluate performance of the automatic control system in maintaining thermal comfort during extensive transient metabolic profiles for a range of environmental temperatures. Finally, the model was used to predict the efficacy of the LCG thermal controller for providing thermal comfort for a variety of regiments that may be encountered in future space missions. Simulations with the Wissler model accurately predicted the thermal interaction between the subject and LCG for a wide range of metabolic profiles and environmental conditions and matched the function of the automatic temperature controller for inlet cooling water to the LCG.

Background: Thermal control in the EVA spacesuit requires attention from the astronaut which is not always desirable or feasible. Improvements in thermal control involve implementation of an automatic thermal control system operating independently of the knowledge of the working astronaut., Methods: A control system was designed, developed, and tested to automatically maintain a subject's thermal neutrality while wearing a liquid cooling garment (LCG). Measurement of CO2 production as an indication of metabolic rate was used as a signal to initiate the control response. Mean body temperature, computed as a function of ear canal temperature and mean skin temperature, provided feedback to account for the thermal state of subjects as they were being cooled by the LCG. The control algorithm was tested on nine subjects, six males and three females, who performed a varying 90-min metabolic profile using an arm cranking ergometer. A total of 27 tests, three for each subject, were conducted in a thermal chamber at three different environmental temperatures: 10 degrees C, 18.3 degrees C and 26.7 degrees C., Results and Conclusions: Evaluation of subjective comfort rating and quantitative energy storage indicates good performance of the controller in maintaining thermal neutrality for the subject over a wide range of environmental and transient metabolic states. Measurements of metabolic rate effectively initiated controller response, and feedback of mean body temperature to the controller proved very capable of accounting for various steady-state environmental conditions and inter-subject variability.

A paper published by Harry H. Pennes in Volume 1 of the Journal of Applied Physiology defined the theoretical basis for a considerable body of analysis performed by many investigators during the ensuing half century. However, during the past decade, the Pennes' model of heat transfer in perfused tissue has been criticized for various reasons, one of which is that his own experimental data seemed to be at variance with the model. More specifically, the shape of the mean temperature-depth relationship measured by Pennes was distinctly different from the shape of the theoretical curve. In this paper, I show that Pennes used an inappropriate procedure to analyze his data and that, when the data are analyzed in a more rigorous manner, they support his theory. Additional support for Pennes' theory is provided by the experimental data of H. Barcroft and O. G. Edholm [J. Physiol. (Lond.) 102: 5-20, 1942 and 104: 366-376, 1946], who had previously studied cooling of the forearm during immersion in water at various temperatures.

Commercially available sensors are being used by several investigators to measure thermal flux through the skin and skin temperature at a given site. Since these transducers place an additional thermal resistance into the system, they perturb the quantities that are being measured. This problem has been analyzed theoretically to obtain the following relatively simple equations: (Q0 - Q)/Q0 = EQ = QRt/(Ta - Te) and (Ts - Ts,0)/(Ta - Te) = [EQ/(1 - EQ)] [1 - EQ2 - (Ts - Te)/(Ta - Te)] in which Q = measured thermal flux; Rt = thermal resistance of the transducer; Ta, Ts, and Te = deep tissue, skin, and environmental temperatures, respectively; and the subscript 0 denotes unperturbed values. These equations can be rearranged easily to obtain improved estimates for the unperturbed values, Q0 and Ts,0, using the measured values, Q and Ts. Use of these relationships to estimate errors for various conditions previously reported in the literature reveals that the EQ can be as large as 10% to 20% for nude subjects in hyperbaric heliox or water, and the error in skin temperature can exceed 1 degree C. When used under a 1-clo garment, the transducer will perturb Q by 4% and Ts by 0.3 degrees C.

A finite element model of the bioheat transfer equation has been developed to simulate the temperature distribution in the head of a subhuman primate. Simulations were made of the induction of deep hypothermia and of subsequent hypothermic circulatory arrest (HCA). Simulations of the circulatory arrest phase were performed with different values of surface heat transfer coefficient and tissue metabolic heat generation. Numerical results were compared with experimental data for the same procedure. The simulations indicate the brain cools rapidly to a near isothermal condition in response to an infusion of cold arterial blood. However, extracerebral structures cool much more slowly. The bulk of heat gain by the brain during HCA is due to heat transfer from these warmer extra-cerebral tissues. These results suggest extended cooling by cardiopulmonary bypass (CPB) combined with surface cooling pads should reduce or even prevent the rise of brain temperatures during HCA.

Accidental immersion often involves the threat of death due to hypothermia. Clothing to control heat loss in water is generally selected to minimize bulk while providing the necessary protection. While water temperature (Tw) and possible immersion time are often considered, another relevant variable is the insulation provided by subcutaneous fat. This paper describes the use of a sophisticated computer model to explore the interactions among skinfold thickness (6-20 mm mean weighted value), clothing insulation (0.06-0.23 clo, immersed), and Tw (0-20 degrees C), in producing critical hypothermia (arterial temperature less than or equal to 34 degrees C). Results indicate that subcutaneous fat strongly affects heat loss even with heavy clothing. Discussion includes examples of the possible use of skinfold data to improve specification of protective clothing for groups and allow special clothing prescription for individuals.

Presented in this paper is a solution for countercurrent heat exchange between two parallel vessels embedded in an infinite medium with a linear temperature gradient along the axes of the vessels. The velocity profile within the vessel is assumed to be parabolic. This solution describes the temperature field within the vessels, as well as in the tissue, and establishes that the intravessel temperature is not uniform, as is generally assumed to be the case. An explicit expression for the intervessel thermal resistance based on the difference between cup-mixed mean temperatures is derived.