Hydration characteristics and electrical resistivity of stratum corneum using a noninvasive four-point microelectrode method.

We have used a four-microelectrode probe to study the relationship between electrical resistivity (p), water content (wf) water binding, and electrolyte levels in samples of plantar stratum corneum. Untreated samples characteristically showed an exponential relationship between log p and wf. This characteristic was retained but altered quantitatively by factors which increase and decrease the ratio of bound to unbound water, by lipid extraction, and (following extraction) by electrolyte presoak. The theoretical aspects of obtaining similar data from intact stratum corneum are discussed.

Physiological consequences of intermittent exercise during compensable and uncompensable heat stress.

Time-weighted averaging is a traditional method used in heat stress analyses to approximate, in terms of a single continuous level of heat production, the rate of heat production from complex intermittent exercise patterns. Physiological responses during intermittent and continuous exercise were studied in four subjects exposed to heat stress in which evaporation was either free or severely restricted. Intermittent work consisted of repeated 10-min exercise-rest patterns. Continuous work was at the time-weighted average of intermittent exercise: 3.3 mets. When heat stress was uncompensable, intermittent work induced more physiological strain than continuous work: endurance time was 14 min less (P less than 0.05); core temperature at 60 min was 0.40 degrees C higher (P less than 0.05); and, after 30 min of exposure, the rate of core temperature rise was 33% greater. The difference in the rate of heat storage was not satisfactorily explained by a discrepancy in the average rate of heat production or in the calculated rate of surface heat loss. Alternatively, the results may be partially explained by interruptions in the usual rate of heat transport via the cutaneous circulation. These interruptions may be caused by nonthermal factors associated with postural and work load transitions. Although the mechanisms are not totally understood, it is clear that application of the time-weighted averaging method can lead to erroneous overprediction of endurance time and should be applied with discretion.

Physiologic tolerance to uncompensable heat: intermittent exercise, field vs laboratory.

PURPOSE: This study determined whether exercise (30 min)-rest (10 min) cycles alter physiologic tolerance to uncompensable heat stress (UCHS) when outdoors in the desert. In addition, the relationship between core temperature and exhaustion from heat strain previously established in laboratory studies was compared with field studies. METHODS: Twelve men completed four trials: moderate intensity continuous exercise (MC), moderate intensity exercise with intermittent rest (MI), hard intensity continuous exercise (HC), and hard intensity exercise with intermittent rest (HI). UCHS was achieved by wearing protective clothing and exercising (estimated at 420 W or 610 W) outdoors in desert heat. RESULTS: Heat Stress Index values were 200%, 181%, 417%, and 283% for MC, MI, HC, and HI, respectively. Exhaustion from heat strain occurred in 36 of 48 trials. Core temperatures at exhaustion averaged 38.6 +/- 0.5 degrees, 38.9 +/- 0.6 degrees, 38.9 +/- 0.7 degrees, and 39.0 +/- 0.7 degrees C for MC, MI, HC, and HI, respectively. Core temperature at exhaustion was not altered (P > 0.05) by exercise intensity or exercise-rest cycles and 50% of subjects incurred exhaustion at core temperature of 39.4 degrees C. These field data were compared with laboratory and field data collected over the past 35 years. Aggregate data for 747 laboratory and 131 field trials indicated that 50% of subjects incurred exhaustion at core temperatures of 38.6 degrees and 39.5 degrees C, respectively. When heat intolerant subjects (exhaustion < 38.3 degrees C core temperature) were removed from the analysis, subjects from laboratory studies (who underwent short-term acclimation) still demonstrated less (0.8 degrees C) physiological tolerance than those from field studies (who underwent long-term acclimatization). CONCLUSION: Exercise-rest cycles did not alter physiologic tolerance to UCHS. In addition, subjects from field studies demonstrate greater physiologic tolerance than subjects from laboratory studies.

Modeling analysis of women litter bearers during heat stress.

BACKGROUND: The majority of military physiological studies have been conducted on men. Consequently most physiological modeling is based on male performance. This study obtained data on women performing military tasks under hot-dry field conditions. Results for rectal temperature (Tre) and heart rate (HR) were compared with values generated by the SCENARIO physiological model. METHODS: Three skin surface temperatures, Tre, and HR were collected while four female subjects participated in walking or two-person litter carriage (68 kg load) in regular battledress and with full chemical protection at Yuma Proving Ground, AZ. Metabolic rates were measured prior to the test days. Meteorological data were also recorded. RESULTS: Overall, the potential for thermal stress decreased over the four test days, with air temperature dropping from 39.4 degrees to 35.5 degrees C. Whole body metabolic values were 364 W (walking), 91 W (standing), and 492 W (litter carriage). Endurance times (160 min maximum) for walking were 140 min in Mission Oriented Protective Posture (MOPP)-0 and 29 min in MOPP-4. For litter carriage, the average times were 59 min in MOPP-0 and 43 min in MOPP-4. A comparison of root mean squared deviation (RMSD) to the mean standard deviation for Tre demonstrated a good fit between the model and mean observed values for the MOPP-0 condition. Observed HRs were greater than predicted by the model. CONCLUSIONS: The modeling results support a conclusion that factors other than heat stress limited litter carriage in MOPP-0. Analysis of calculated values for Tre using RMSD supports an assumption that the male-based SCENARIO may be applicable to females.

Measurement of sweating rate with capacitance sensors.

A new forced-evaporation type skin capsule for measuring local sweat gland activity in humans is described and details of its construction and problems of calibration are presented. The capsule is self-contained, portable, and inexpensive making it well suited for use in arrays to make multiple simultaneous determinations on adjacent skin areas. Sweat is evaporated from the skin surface by a stream of dry nitrogen gas; subsequent changes in capsule relative humidity and temperature are measured by a commercially available thin-film capacitance sensor and a solid-state current regulating device, respectively. These changes are measured within the capsule itself so no external water vapor analyzer is needed. Data acquisition and evaporation rate calculations are handled by an on-line microcomputer.

A gas-stream heater to control local skin temperature under evaporative sweat capsules.

A compact 4-W gas-stream heater for use with ventilated sweating capsules is described. The heating element is constructed from two modified 75-omega power resistors connected in parallel and contained in the capsule gas inlet tube. Heater power is controlled with integrated circuits which compare the output of a thermocouple, measuring either capsule skin or air temperature, with a predetermined target temperature. The unit responds fairly slowly to sudden large changes in target temperature, but once a steady state is achieved, maintains target temperature levels despite changes in evaporative heat loss. The device is well suited for studies in which local skin temperature or capsule air temperature must be elevated and clamped while the effects of other local stimuli or central thermoregulatory activity are studied, and in studies where the skin temperature effect on transepidermal water loss must be controlled.

Human thermoregulatory responses during heat exposure after artificially induced sunburn.

Thermoregulatory responses in the heat (ambient temperature 49 degrees C, 20% relative humidity, 1 m/s wind) were investigated in 10 unacclimated men during 50 min of cycle ergometer exercise (approximately 53% of maximal aerobic power) after a 10-min rest before as well as 24 h and 1 wk after twice the minimal erythemal dose of UV-B radiation that covered approximately 85% of the body surface area. In 7 subjects esophageal temperature (Tes) was recorded while in all 10 subjects five-site skin and rectal temperatures, heart rate, and back, left forearm, and shielded (12 cm2 area) right forearm sweating rates (msw) were recorded at 15-s intervals. Venous blood was collected before and after exercise-heat stress. Mean skin temperature, Tes, rectal temperature, heart rate, and total body sweating rate were not significantly (P greater than 0.05) affected by sunburn. Pre- and postexercise values of hematocrit, hemoglobin, plasma protein, plasma volume, and plasma osmolality were also not affected (P greater than 0.05) by sunburn. Analysis of presunburn and post-sunburn data showed that the Tes intercept for sweating (degrees C) was unaffected (P greater than 0.05), but msw/Tes and final msw from the left forearm (msw/Tes 0.24 +/- 0.02 vs. 0.17 +/- 0.01 mg.cm-2.min-1. degrees C-1, P less than 0.05; msw 0.60 +/- 0.05 vs. 0.37 +/- 0.02, mg.cm-2.min-1, P less than 0.05) and back (msw/Tes 0.43 +/- 0.03 vs. 0.36 +/- 0.01 mg.cm-2.min-1. degrees C-1, P = 0.052; msw 1.08 +/- 0.09 vs. 0.74 +/- 0.05 mg.cm-2.min-1, P less than 0.05) were significantly reduced 24 h postsunburn.(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of active cutaneous vasodilation associated with congenital absence of sweat glands in humans.

In the rare syndrome, hereditary anhidrotic ectodermal dysplasia (AED), sweat glands are congenitally absent. Assuming normal vasculature and normal central mechanisms, presence or absence of active cutaneous vasodilation (AVD) in hyperthermic subjects with AED critically tests the hypothesis that AVD is a consequence of sudomotor activity. Three men with full expression of the syndrome and a woman who is mosaic were heated in water-perfused suits until oral temperature was 1.4-1.7 degrees C above control. The men showed no sweat gland imprints on iodine-treated paper nor significant elevation in forearm blood flow (FBF, determined plethysmographically). In the woman, we observed sweat gland activity, approximately 9 and 22 glands/cm2, on the right and left side, respectively, and vasodilation, slight on the right and more on the left. Cutaneous vasoconstriction in response to negative pressure applied to the lower body was observed (3 subjects) and local FBF increased in response to local heating (2 subjects). Therefore, in AED, with apparently normal cutaneous vasculature and sympathetic innervation, AVD is absent as well as sweat glands.