Expanded prediction equations of human sweat loss and water needs.

The Institute of Medicine expressed a need for improved sweating rate (msw) prediction models that calculate hourly and daily water needs based on metabolic rate, clothing, and environment. More than 25 years ago, the original Shapiro prediction equation (OSE) was formulated as msw (g.m(-2).h(-1))=27.9.Ereq.(Emax)(-0.455), where Ereq is required evaporative heat loss and Emax is maximum evaporative power of the environment; OSE was developed for a limited set of environments, exposures times, and clothing systems. Recent evidence shows that OSE often overpredicts fluid needs. Our study developed a corrected OSE and a new msw prediction equation by using independent data sets from a wide range of environmental conditions, metabolic rates (rest to 500 observations) by using a variety of metabolic rates over a range of environmental conditions (ambient temperature, 15-46 degrees C; water vapor pressure, 0.27-4.45 kPa; wind speed, 0.4-2.5 m/s), clothing, and equipment combinations and durations (2-8 h). Data are expressed as grams per square meter per hour and were analyzed using fuzzy piecewise regression. OSE overpredicted sweating rates (P<0.003) compared with observed msw. Both the correction equation (OSEC), msw=147.exp (0.0012.OSE), and a new piecewise (PW) equation, msw=147+1.527.Ereq-0.87.Emax were derived, compared with OSE, and then cross-validated against independent data (21 males and 9 females; >200 observations). OSEC and PW were more accurate predictors of sweating rate (58 and 65% more accurate, P<0.01) and produced minimal error (standard error estimate<100 g.m(-2).h(-1)) for conditions both within and outside the original OSE domain of validity. The new equations provide for more accurate sweat predictions over a broader range of conditions with applications to public health, military, occupational, and sports medicine settings.

Pentadecanoic acid in serum as a marker for intake of milk fat: relations between intake of milk fat and metabolic risk factors.

BACKGROUND: The fatty acid composition of the diet is known to be partially reflected by the fatty acid composition of serum lipids. OBJECTIVE: We examined whether pentadecanoic acid (15:0) in serum lipids can be used as a marker for intake of milk fat, the major dietary source of 15:0. We also investigated the relations between intake of milk fat and cardiovascular disease risk factors. DESIGN: Sixty-two 70-y-old men completed 7-d dietary records. The intake of milk products was studied in relation to the proportions of 15:0 in serum cholesterol esters and phospholipids, as well as to the clinical characteristics of these men, by using Spearman's rank correlation. RESULTS: The proportions of 15:0 in serum cholesterol esters were positively related to butter intake (r = 0.36. P = 0.004) and to the total amount of fat from milk products (r = 0.46, P < 0.0001): 15:0 in phospholipids was related to the amount of fat from milk and cream (r = 0.34, P = 0.008) and to the total amount of fat from milk products (r = 0.34, P = 0.008). Inverse associations were found between intake of milk products and body mass index, waist circumference, LDL-HDL ratio, HDL triacylglycerols, and fasting plasma glucose, whereas relations to HDL cholesterol and apolipoprotein A-I tended to be positive. CONCLUSIONS: The results suggest that 15:0 in serum can be used as a marker for intake of milk fat. The explanation for the inverse associations between the intake of milk products and certain cardiovascular risk factors is not known.

Evaporative water losses through a temporary wound dressing under simulated wound conditions.

Patients with burns lose large amounts of water through evaporation from open wounds. Because the wound covering is the first line of defense for maintenance of body fluid balance in these patients, quantification of the evaporative water loss through wound coverings at the bedside would improve the accuracy of estimations of body water loss. The present experiment evaluates the use of a small ventilated capsule system automated with miniature resistance-type dew-point sensors for measurement of evaporative water loss through biologic dressings under simulated wound conditions. Evaporative water loss from wounds was simulated by pilocarpine-induced profuse sweating on the forearm. Evaporative water loss through uncovered skin was compared with that of skin covered with commercially available temporary wound dressings. Compared with an adjacent unstimulated area, forearm dew-point temperature in the capsule (Tcdp) and sweat rate increased immediately after pilocarpine exposure and remained significantly elevated and relatively constant for an additional 60 minutes. Evaporative water loss of the forearm was 29 +/- 4.8 gm/m2/hr (mean +/- SE) at baseline and rose significantly to 275 +/- 18.2 gm/m2/hr after pilocarpine exposure. The pilocarpine-stimulated sweat rate and Tcdp at neutral conditions were similar to those obtained from walking on a treadmill for 60 minutes in a 30 degrees C room. Compared with pilocarpine-induced evaporative water loss of the uncovered skin, temporary wound dressings significantly reduced evaporative water loss by 40% to 60%. No significant differences were observed between varieties of temporary wound dressings differing in thickness and/or porosity.(ABSTRACT TRUNCATED AT 250 WORDS)

Predicted thermophysiological responses of humans to MRI fields.

A relatively simple two-node model of human thermoregulation was developed to predict response changes during MRI procedures. Subsequent modifications of the model simulated impairments in cardiovascular function in terms of altered skin blood flow. In the present work, the model was programmed to predict the consequences of certain procedures used in the clinic, namely, precooling of the patient to the prevailing environment and covering the patient with a light blanket. Some of the fundamental predictions of the model during 20-min MRI scans at a low SAR were tested on two male subjects in the clinical setting. The following conclusions may be drawn: (1) Precooling of the patient for 20 min to the prevailing ambient conditions, whether inadvertent or deliberate, has little value in terms of preventing a rise in body temperatures. At the conclusion of a subsequent 20-min MRI scan, even at SARs as low as 2 W/kg, the modest effects of precooling are all but eliminated. Thus, inadvertent precooling should be no cause for concern; deliberate precooling carries little advantage for the patient and wastes valuable time. (2) Use of a blanket during an MRI scan should be discouraged in the normal clinical setting except when the SAR is 2 W/kg or less. At higher SARs, this added insulation impedes convective and radiative heat loss through evaporation of sweat. The result is an increase of heat storage in the body and a greater rise in core temperature than would occur otherwise. (3) Clinical tests on two normal male subjects have provided limited confirmation of the predictions of the two-node model. During 20-min MRI scans at a whole-body SAR of 1.2 W/kg, core and skin temperatures, sweat rate, and judgments of thermal sensation and discomfort were very similar to predicted values. Unexpected findings of an incremental increase in core temperature with successive scans and a sweating rebound following each scan may be important for future investigation. (4) Although pleased with the limited confirmation of our predictions, we are constantly aware of the limitations of the two-node model to accurately predict thermoregulatory responses of patients undergoing clinical MRI of various body parts. It is essential to keep in mind that the simulations are based on RF exposure of the whole body; thus, the predicted increase in core temperature will be proportionately higher than would be the case if only a portion of the body were exposed within the MRI device.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiological and behavioral temperature regulation of men in simulated nonuniform thermal environments between 18 and 30 degrees C.

Human thermoregulatory responses to nonuniform thermal environments was studied by simulating the situation with altered clothing distribution. Clothing was symmetrically or asymmetrically distributed over the body surface. Esophageal and local skin temperatures, metabolism, skin heat flux, evaporative heat loss and subjective responses of six sedentary men were measured at air temperatures between 18 and 30 degrees C. Clothing distribution significantly (p less than 0.05) influenced thermoregulatory responses only at 18 degrees C. At 18 degrees C, the bilaterally asymmetric clothing resulted in a higher (p less than 0.05) esophageal temperature compared to the symmetric condition. Mean skin temperatures did not differ with clothing distribution over the range of air temperatures studied, but at 18 degrees C whole body thermal sensation was warmer (p less than 0.05) for the asymmetric compared to the symmetric group. This increased perception of warm thermal sensation was significantly correlated to the difference in skin temperature across the body.

Effect of thermal conditions on the acceptability of respiratory protective devices on humans at rest.

The physiological and subjective responses of six sedentary subjects wearing half-facepiece respirators were observed over a wide range of room and respirator air conditions. Room air and dew-point (Ta:Tdp) temperatures were 25:11 degrees, 30:13 degrees, and 35:16 degrees C in still air. Respirator air temperatures were maintained independently of room conditions at 27 degrees, 30 degrees, 33 degrees, and 36 degrees C with relative humidity levels of 47% and 73%. Physiological measurements included local skin and dew-point temperatures. Subjective judgments of acceptability, thermal sensation, degree of discomfort, sense of skin moisture, and difficulty of breathing were recorded separately for the thermal environment in the room and inside the respirator. Respirator temperatures cooler than 33 degrees C were always comfortable and 100% acceptable; respirator air temperatures above 33 degrees C or higher humidity levels decreased respirator acceptability. Acceptability of the respirator environment decreased as lip temperature increased above 34.5 degrees C or when respirator dew-point temperature increased above 20 degrees C. Increased respirator air temperature and humidity often made breathing seem "slightly hard." The respirator conditions influenced the subjects' judgment of the acceptability of the surrounding thermal environment.

Thermoregulatory consequences of cardiovascular impairment during NMR imaging in warm/humid environments.

A simple model of physiological thermoregulation, previously adapted to predict the thermoregulatory consequences of exposure to the nuclear magnetic resonance (NMR) imaging environment, has been further adapted to simulate impaired cardiovascular function. Restrictions on the rate of skin blood flow (SkBF), ranging from 0 to 89% of normal, were studied. Predictions of physiological heat loss responses in real time were generated as a function of ambient temperature (Ta), relative humidity (RH) and rate of whole-body radiofrequency (RF) energy deposition (SAR). Under conditions that are desirable in the clinic (Ta = 20 degrees C, 50% RH, still air), moderate restrictions (up to 67%) of SkBF yield tolerable increases in core temperature (delta Tco less than or equal to 1 degree C) during NMR exposures (SAR less than or equal to 4 W/kg) of 40 min or less. Increased Ta and RH exacerbate the thermal stress imposed by absorbed RF energy; severely impaired SkBF encourages short NMR exposures (e.g., 20 min or less) at SARs less than or equal to 3 W/kg. In warm/humid environments, sweating is predicted to be profuse and evaporative cooling curtailed, yielding a state of extreme thermal discomfort. Added insulation (e.g., a blanket) is discouraged. Some guidelines, incorporating SkBF restrictions, Ta, RH, and insulation, are offered for the prediction of tolerable NMR exposure conditions.

The effect of temperature and humidity levels in a protective mask on user acceptability during exercise.

Subjective and physiological responses were obtained from six subjects wearing a ventilated face mask while exercising (3.8 met) for 15 min on a bicycle ergometer. Different combinations of ambient air temperatures (7 degrees, 16 degrees, 25 degrees C) and mask air temperatures (22 degrees, 27 degrees, 33 degrees C) were studied together with two different air humidities inside the mask (61% and 86% RH). Control experiments were performed without the mask at the same ambient temperatures. Skin temperatures, heart rates and skin wettedness were monitored during exercise. The subject's acceptance of the mask and thermal environment, thermal sensation, sensations of discomfort, sweating and skin wettedness, and their judgment of the work of breathing were assessed at the end of the 15 min exercise period. The acceptance of both the ambient thermal environment and of the thermal microclimate in the mask primarily was determined by the ambient air temperature, but it was influenced by the air temperature and humidity inside the mask. At ambient temperatures of 7 degrees C and 25 degrees C, the acceptance of the thermal work conditions decreased. In the warm environment a mask air temperature less than or equal to 27 degrees C was 100% acceptable and increased the acceptance of thermal environment. In the cool environment, a mask air temperature greater than or equal to 27 degrees C was 100% acceptable. The humidity content of the mask air was only important when the mask air was warm. Warm humid air significantly decreased acceptance of the mask conditions.

On the thermoregulatory consequences of NMR imaging.

A simple model of physiological thermoregulation has been adapted to predict the thermoregulatory consequences of exposure to the nuclear magnetic resonance (NMR) imaging environment. Based on our knowledge of thermoregulatory processes and how heat is exchanged between a person and the environment, the model can predict physiological heat loss responses in real time as a function of selected ambient temperature (Ta), air movement (v), and rate of whole-body radiofrequency (RF) energy deposition (SAR). Assuming a criterion elevation in deep body temperature (delta Tco) of 0.6 degree C, Ta = 20 degrees C and v = 0.8 m/sec, a 70 kg patient could undergo an NMR exposure of infinite duration at SAR less than or equal to 5 W/kg. Lowering Ta or increasing v permits a rise in permissible SAR for a given delta Tco. More restrictive delta Tco criteria result in lower permissible SARs and shorter exposure durations. The limiting response under all conditions tested was found to be the rate of peripheral blood flow, although sweating played a significant role in preventing excessive delta Tco. Some guidance for the clinical application of the predictions is offered.

Indices of thermoregulatory strain for moderate exercise in the heat.

The effect of varying humidity and dry bulb temperatures was studied on five normal male unclothed subjects while exercising (40-45 min) at 28% VO2max. Air movement was 0.75 m.s-1. The initial test and the 16th test on each subject both done at 50 degrees C and 30 Torr (32% rh). Each subject did the intervening 14 experiments twice per day at varying ambient temperature (Ta) and water vapor pressure (Pa) levels, so selected to progressively increase skin wettedness levels. Mean skin temperature (Tsk) and esophageal temperature (Tes), heart rate (HR), skin evaporative heat loss (Esk), and warm discomfort were continuously observed. Skin wettedness (w) was evaluated as the ratio of the observed Esk to the maximum evaporative capacity of the environment. A rational effective temperature (ET) is defined as the dry bulb temperature at 50% rh in which the total heat exchange from skin surface would be the same as in the test environment, described by the observed Ta and Pa. The results showed that 1) during steady state both HR and Tes were unaffected by Ta from 26 to 41 degrees C responding to the level of exercise intensity, when Pa less than or equal to 20 Torr; 2) both mean body temperature, found by weighting Tsk:Tes by 1:9, and ET were each significant indicators of physiological strain when Pa greater than 20 Torr; 3) a level of strain, caused by skin wettedness values greater than 0.5, is suggested as a primary condition necessary for inducing heat acclimation.

Evaporation of sweat from sedentary man in humid environments.

UNLABELLED: Physiological and sensory responses were observed in four male subjects while they were seated on a balance. The chamber temperature always equaled mean skin (Tsk)(34-36 degrees C), thus eliminating all sensible heat transfer. Ambient water vapor pressure (Pa) was increased in steps from 10 to 40 Torr. Tests at air velocities from 0.1 to 2.4 m/s were conducted with subjects nude and clothed. Esophageal temperature (Tes) and Tsk, heat rate (HR) and weight loss (M) were measured throughout the 2.5-h tests. After each 25-min humidity step subjects recorded their warm discomfort, sensation of sweating, and thermal sensation. RESULTS: Tsk, HR, and M were unaffected by humidity until critical Pa (P a crit) was reached above which M decreased and Tsk and HR increased rapidly. P a crit decreased with clothing and decreasing velocity. Tes remainded relatively constant throughout the tests. Sweating, discomfort and temperature sensations increased gradually with increasing Pa. Above P a crit sweating and discomfort sensations accelerated markedly. Mass transfer coefficients and clothing vapor conductance factors were evaluated. Skin wettedness at Pa crit decreased from 74% at 0.1 m/s to 35% at 2.4 m/s.