Cross validation of USARIEM heat strain prediction models. U.S. ARMY Research Institute of Environmental Medicine.

HYPOTHESIS: This study was a cross validation of three heat strain prediction models developed at the U.S. Army Research Institute of Environmental Medicine: the ARIEM, HSDA, and ARIEM-EXP models ability to predict core temperature. METHODS: Seven heat-acclimated subjects completed twelve experimental tests, six in each of two hot climates, at three exercise intensities and two uniform configurations in each climate. RESULTS: Experimental results showed physiological responses as expected with heat strain increasing with work load and level of protective clothing, but with similar heat strain between the two environments matched for wet bulb, globe index. Neither the ARIEM or HSDA model closely predicted core temperatures over the course of the experiment, due mostly to an abrupt initial rise in core temperature in both models. A proportionality constant in the ARIEM-EXP buffered some of this abrupt rise. CONCLUSIONS: Comparisons of the core temperature and tolerance times data with the three models led to the conclusions that for healthy males: 1) the ARIEM and HSDA models provide conservative safety limits as a result of predicting rapid initial increases in core temperature; 2) the ARIEM-EXP most closely represents core temperature responses; 3) the ARIEM-EXP requires modifications with an alternate proportionality coefficient to increase accuracy for low metabolic cost exercise; 4) all of the models require additional input from existing research on tolerance to heat strain to better predict tolerance times; and 5) additional models should be examined to investigate the transient state of the body as it is affected by environment, clothing and exercise.

Characterization of a three-phase response in gloved cold-stressed fingers.

Seven gloves were studied worn by eight sedentary subjects (six men and two women) exposed to cold-dry, C D, (mean dry bulb temperature Tdb = -17.2 degrees C; mean dew point temperature Tdp = -25.1 degrees C), and cold-wet, C-W, (Tdh = 0 degrees C; Tdp = -8.4 degrees C) conditions. Mean endurance times were 75 min for the C-D and 162 min for the C W conditions. A three-phase response pattern of the temperature in the fingers was characterized. Phase I comprised an initial period during which finger temperature remained close to the pre-exposed level, due to delayed vasoconstriction in the finger. Phase II involved an exponential-like decrease of finger temperature indicative of the onset of vasoconstriction in the finger. Phase III manifested periodic finger temperature changes due to cold induced vasodilatation (CIVD). Mean wave patterns for phase III indicated approximately 3.5 waves x h(-1) in the C D but only about 2 waves x h(-1) in the C-W condition. Extension of endurance time, due to CIVD, was defined as the difference in time between the actual end of the experiment and the time the finger-tip would have reached the set temperature endurance limit as extrapolated by a continued exponential drop. Three overall response patterns of fingers in the cold were characterized: type A exhibiting all 3 phases; type B1 or B2 exhibiting either phases I+ II or phases II+ III; and type C showing only phase II. Considerable inter- and intra-subject variability was found. In both test conditions the final physiological thermal states of the subjects were between comfortable and slightly uncomfortable but acceptable and thus did not correlate with the responses in the fingers.

Simulation of a cold-stressed finger including the effects of wind, gloves, and cold-induced vasodilatation.

The thermal response of fingers exposed to cold weather conditions has been simulated. Energy balance equations were formulated, in a former study, for the tissue layers and the arterial, venous, and capillary blood vessels. The equations were solved by a finite difference scheme using the Thomas algorithm and the method of alternating directions. At this stage of development the model does not include any autonomic control functions. Model simulations assumed an electrical heating element to be embedded in the glove layers applied on the finger. A 1.3 W power input was calculated for maintaining finger temperatures at their pre-cold exposure level in a 0 degree C environment. Alternate assumptions of nutritional (low) and basal (high) blood flows in the finger demonstrated the dominance of this factor in maintaining finger temperatures at comfortable levels. Simulated exposures to still and windy air, at 4.17 m/s (15 km/h), indicated the profound chilling effects of wind on fingers in cold environments. Finally, the effects of variable blood flow in the finger, known as "cold-induced vasodilatation," were also investigated. Blood flow variations were assumed to be represented by periodic, symmetric triangular waves allowing for gradual opening-closing cycles of blood supply to the tip of the finger. Results of this part of the simulation were compared with measured records of bare finger temperatures. Good conformity was obtained for a plausible pattern of change in blood flow, which was assumed to be provided in its entirety to the tip of the finger alone.

Numerical analysis of an extremity in a cold environment including countercurrent arterio-venous heat exchange.

A model of the thermal behavior of an extremity, e.g., a finger, is presented. The model includes the effects of heat conduction, metabolic heat generation, heat transport by blood perfusion, heat exchange between the tissue and the large blood vessels, and arterio-venous heat exchange. Heat exchange with the environment through a layer of thermal insulation, depicting thermal handwear, is also considered. The tissue is subdivided into four concentric layers simulating, from the center outward, core, muscle, fat, and skin. Differential heat balance equations are formulated for the tissue and for the major artery and the major vein traversing the finger. These coupled equations are solved numerically by a finite-difference, alternating direction method employing a Thomas algorithm. The numerical scheme was extensively tested for its stability and convergence. This paper presents the model equations and results of the convergence tests, and shows plots of blood and tissue temperatures along the axis of the model for combinations of parameters including the effect of countercurrent heat exchange between the artery and the vein.

Lumped-parameter tissue temperature-blood perfusion model of a cold-stressed fingertip.

A lumped-parameter model of a fingertip is presented. The semispherical model includes the effects of heat storage, heat exchange with the environment, and heat transport by blood perfusion. The thermal insulation on the surface of the fingertip is represented by the overall heat transfer coefficient that is calculated by common engineering formulas. The model is solved analytically for the simple case of constant blood perfusion rate. The general case of variable blood perfusion rates is solved by an Euler finite difference technique. At this stage, the model does not include active control mechanisms of blood perfusion. Thus the effects of cold-induced vasodilatation have to be superimposed and are modeled by symmetrical triangular waveforms because these were found to best depict the behavior of fingers exposed to cold environments. Results of this model were compared with experimental data obtained in two separate studies. One included 60-min infrared thermograms of the dorsal surface of bare hands of sedentary subjects horizontally suspended on a fish net in a 0 degree C environment. Another study, on gloved finger temperatures, involved 0 and -6.7 degrees C environments. Fingertip (nail bed) temperatures of both these studies were compared with model predictions. Blood perfusion rates were assumed and adjusted within physiologically reasonable limits. Comparison of measured and computed temperature records showed very good conformity in both cases studied.

Validation and adjustment of the mathematical prediction model for human rectal temperature responses to outdoor environmental conditions.

Models to predict rectal temperature (Tre) have been based on indoor laboratory studies. The present study was conducted to validate and adjust a previously suggested model for outdoor environmental conditions. Four groups of young male volunteers were exposed to three different climatic conditions (30 degrees C, 65% rh; 31 degrees C, 41% rh; 40 degrees C, 20% rh). They were tested both in shaded and open field areas (radiation: 80 and 900 W.m-2, respectively) at different work loads (100, 300 and 450 watt). Exercise consisted of two bouts of 10 minutes rest and 50 minutes walking on a treadmill, at a constant speed (1.4 m.s-1) and different grades. The subjects were tested wearing cotton fatigues and protective garments. Their Tre and heart rate were monitored every 5 min and skin temperature every 15 min, oxygen uptake was measured towards the end of each bout of exercise; concomitantly, ambient temperature, relative humidity and solar load were monitored. We concluded that: (a) the corrected model to predict rectal temperature overestimates the actual measurements when applied outdoors; (b) radiative and convective heat exchanges should be considered separately when using the model outdoors; (c) radiative heat exchange should also be considered separately for short-wave radiation (solar radiation) and long-wave emission from the body to the atmosphere. Finally, an adjusted model to be used outdoors was suggested.

Validation and adjustment of the mathematical prediction model for human sweat rate responses to outdoor environmental conditions.

Under outdoor conditions this model was over estimating sweat loss response in shaded (low solar radiation) environments, and underestimating the response when solar radiation was high (open field areas). The present study was conducted in order to adjust the model to be applicable under outdoor environmental conditions. Four groups of fit acclimated subjects participated in the study. They were exposed to three climatic conditions (30 degrees, 65% rh; 31 degrees C, 40% rh; and 40 degrees C, 20% rh) and three levels of metabolic rate (100, 300 and 450 W) in shaded and sunny areas while wearing shorts, cotton fatigues (BDUs) or protective garments. The original predictive equation for sweat loss was adjusted for the outdoor conditions by evaluating separately the radiative heat exchange, short-wave absorption in the body and long-wave emission from the body to the atmosphere and integrating them in the required evaporation component (Ereq) of the model, as follows: Hr = 1.5SL0.6/I(T) (watt) H1 = 0.047Me.th/I(T) (watt), where SL is solar radiation (W.m-2), Me.th is the Stephan Boltzman constant, and I(T) is the effective clothing insulation coefficient. This adjustment revealed a high correlation between the measured and expected values of sweat loss (r = 0.99, p < 0.0001).

Predicting human heat strain and performance with application to space operations.

This Institute has developed a USARIEM Heat Strain Prediction Model for predicting physiological responses and soldier performance in the heat, which has been programmed for use by hand-held calculators and personal computers, and incorporated into the development of a heat strain decision aid. This model is demonstrated to predict accurately (generally within +/- 1 SD/SEM) rectal temperature (Tre) responses for soldiers wearing various military clothing ensembles during U.S. or non-U.S. military scenarios in the heat at home or abroad. The value of this model is shown presently for three NASA scenarios involving the Launch and Entry Suit (LES). The LES (ventilated or unventilated) is modeled during pre-launch/launch, re-entry/landing, and emergency egress after re-entry/landing scenarios, predominately to evaluate heat acclimation and hydration state effects. During the pre-launch/launch scenario, predicted final Tre closely agrees with observed values suggesting minimal heat strain (Tre approximately 38.0 degrees C). In contrast, dehydrated (3%) unacclimated individuals show moderate levels of heat strain (Tre approximately 38.5 degrees C) for this same scenario. During the re-entry/landing and emergency egress scenarios, dehydrated unacclimated individuals are predicted to exhibit excessive heat strain (Tre > 39.0 degrees C). Thermal tolerance time is predicted to be only 6 min during emergency egress if individuals are dehydrated and unacclimated to heat while wearing the LES. If heat transfer values for space operations clothing are known, NASA can use this prediction model to help avoid undue heat strain involving astronauts for most scenarios during spaceflight.

Quantification of conservative endurance times in thermally insulated cold-stressed digits.

The estimation of endurance times of the digits exposed to cold weather is performed by an analytical, one-dimensional cylindrical model. Blood perfusion effects are lumped into a volumetric heat-generation term. Cold-induced vasodilatation (CIVD) effects are not included in the present analysis. Endurance times, defined by a drop in cylinder tip temperature to 5 degrees C, were evaluated. Parameters included in this evaluation were 1) environmental temperatures, 2) thermal insulation applied on the cylinder, 3) length of the cylinder, and 4) diameter of the cylinder. It was found that the lower the ambient temperature, the longer the finger, and the smaller its diameter, then the shorter the endurance time for the same thermal insulation. Results of the model were compared with measured data for a subject not exhibiting CIVD response to cold stress. Conformity of results calculated for an adjusted value of the volumetric heat-generation term and measured data was very good, with a maximum deviation of less than 10% at only one particular point in time. This model facilitates the conservative estimation of lower bounds to thermally insulated fingers and toes exposed to cold stress.

Predicting metabolic cost of running with and without backpack loads.

In the past, a mathematical equation to predict the metabolic cost of standing or walking (Mw) was developed. However, this equation was limited to speeds less than 2.2 m.s-1 and overestimated the metabolic cost of walking or running at higher speeds. The purpose of this study was, therefore, to develop a mathematical model for the metabolic cost of running (Mr), in order to be able to predict the metabolic cost under a wide range of speeds, external loads and grades. Twelve male subjects were tested on a level treadmill under different combinations of speed and external load. Speed varied between 2.2 to 3.2 m.s-1 using 0.2 m.s-1 intervals and external loads between 0-30 kg with 10 kg intervals. Four of the subjects were also tested at 2 and 4% incline while speed and load remained constant (2.4 m.s-1, 20 kg). The model developed is based on Mw and is proportionately linear with external load (L) carried as follows: Mr = Mw-0.5 (1-0.01L)(Mw -15L-850), (watt) The correlation coefficient between predicted and observed values was 0.99 (P less than 0.01) with SER of 7.7%. The accuracy of the model was validated by its ability to predict the metabolic cost of running under different conditions extracted from the literature. A highly significant correlation (r = 0.95, P less than 0.02, SER = 6.5%) was found between our predicted and the reported values. In conclusion, the new equation permits accurate calculation of energy cost of running under a large range of speeds, external loads and inclines.

Prediction modeling of physiological responses and human performance in the heat.

Over the last two decades, our laboratory has been establishing the data base and developing a series of predictive equations for deep body temperature, heart rate and sweat loss responses of clothed soldiers performing physical work at various environmental extremes. Individual predictive equations for rectal temperature, heart rate and sweat loss as a function of the physical work intensity, environmental conditions and particular clothing ensemble have been published in the open literature. In addition, important modifying factors such as energy expenditure, state of heat acclimation and solar heat load have been evaluated and appropriate predictive equations developed. Currently, we have developed a comprehensive model which is programmed on a Hewlett-Packard 41 CV hand held calculator. The primary physiological inputs are deep body (rectal) temperature and sweat loss while the predicted outputs are the expected physical work--rest cycle, the maximum single physical work time if appropriate, and the associated water requirements. This paper presents the mathematical basis employed in the development of the various individual predictive equations of our heat stress model. In addition, our current heat stress prediction model as programmed on the HP 41 CV is discussed from the standpoint of propriety in meeting the Army's needs and therefore assisting in military mission accomplishment.

Modification of Otis-McKerrow valve for measurement of respiratory water loss.

An apparatus is described that allows a continuous measurement of inspired and expired gas dew-point temperature for the calculation of water loss (Eres) during ventilation. A rapid response dew-point temperature measurement method is described which is based on a small Peltier module. The compact structure with near zero system dead space minimizes potential errors inherent in many techniques used to measure Eres. The simple design and rugged construction permit the incorporation of the apparatus into many manual or personal computer controlled oxygen consumption systems. Collection of data may be done in a variety of ambient temperatures, altitudes, and activity levels. There is also the potential for creating a portable system for field use.

Three instruments for assessment of WBGT and a comparison wiwh WGT (Botsball).

Environmental heat stress, expressed as the Wet Bulb Globe Temperature (WBGT), was measured outdoors using three different instruments: a) the conventional shaded dry-bulb, 15.2-cm black globe and naturally convected wet bulb thermometers, b) a minaturized thermometer kit, and c) a commercial WBGT instrument using a thermistor sensors. The WBGT values were compared with the Wet Globe Temperature (WGT) measured with a Botsball. Measurements were made visually on the instruments at regular intervals and an automated data collection system also was used to obtain data from thermocouples attached to the instruments. Statisticallly significant differences in WBGT readings were found among the instruments; however, the difference for a given environment usually was less than 0.5 degrees C. Readings taken by visual observations resulted in WBGT values which differed by less than 0.3 degrees C from those calculated from the automated data collection system. By using an equation derived for the Botsball, WBGT = 1.044 WGT - 0.187 (in degrees C), it is possible to convert the Botsball thermometer dial to indicate the conventional WBGT for outdoor environments, thus making it a simple instrument for assessing environmental heat stress at the work site.