Y. Shapiro

Exertional heat stroke : a case series

UNLABELLED Exertional heat stroke (EHS) is a state of extreme hyperthermia that occurs when excess heat that is generated by muscular exercise exceeds the bodys ability to dissipate it at the same rate. EHS is thought to coincide with previously healthy, highly motivated, and relatively untrained individuals exerting in hot environments for long periods. PURPOSE To establish this notion, the present study was aimed to follow the trends in the incidence of EHS in the period 1988-1996. METHODS During these years, 150 cases of male soldiers (age = 20+/-3 yr) were reported to our institute as suffering from heat illnesses. According to the files, 82 cases were definitely diagnosed as EHS. RESULTS More than 50% of the cases occurred during the first 6 months in service. Most of the cases occurred during the summer season (June-September), but 30% of the cases occurred during the spring. EHS was not related to time in the day. Many cases occurred during the night or early morning, even under mild heat load. Forty percent of the cases occurred during very short activities, and about 60% occurred already during the first 2 h of exercise. The results were discussed in view of the regulations which prevail in the Israeli army. CONCLUSIONS It seems that almost all EHS cases occurred when regulations were not strictly followed.

Predicting Sweat Loss Response to Exercise, Environment and Clothing

SummaryMetabolic heat production (M), clothing heat transfer characteristics, and the environment dictate a required evaporative cooling (Ereq) from the body to maintain thermal balance. However, the maximal evaporative capacity (Emax) is dictated by vapor transfer properties of the clothing and environment. Relationships between metabolic load, environmental conditions, clothing and sweat loss were studied in 34 heat-acclimatized males categorized into four groups (eight, eight, eight, and ten subjects) and exposed to various environmental conditions (ambient temperature, 20–54‡ C, and relative humidity, 10–90%), three levels of metabolic rate (resting; walking 1.34 m·s−1, level; or walking 1.34 m·s−1, 5% grade) while wearing various clothing ensembles (shorts and T-shirts, fatigues, fatigues plus overgarment, or sweat suit). Individual groups were not exposed to all combinations. Exposures lasted 120 min: either 10 min rest — 50 min exercise — 10 min rest — 50 min exercise, or 120 min at rest. Physiological measurements included heart rate, rectal temperature, mean skin temperature, energy expenditure and sweat loss (δmsw). Emax and Ereq were calculated from environmental conditions, metabolism, clothing insulation and permeability. The ratio Ereq/δmsw was found to correlate with Emax and not with M. The predictive equation for sweat loss was: δmsw=18.7×Ereq×(Emax)−0.455 within the limits 50

Cardio-Respiratory Physical Training in Water and on Land,

SummaryFifteen unconditioned young men, who were similar in maximal aerobic power (VO2 max), were divided into three groups (n=5 each) and physically trained for one month on a cycle ergometer either on land (I) or immersed to the neck in water of either 32‡ C (II) or 20‡ C (III) to determine if physical training (PT) in water and air differ. PT consisted of one-hour daily exercise, 5 times/wk, with exercise intensity readjusted each week to maintain a constant training stimulus of ~ 75% VO2 max (determined on land). Throughout the training period, heart rates (fc) of III averaged 20 and 10 beats·min−1 less than I and II, respectively, despite working at the same VO2 and % VO2 max. Training elicited a 16% increase in VO2 max in I compared to increases of 13 and 15% for II and III, respectively. It was concluded that PT in water produces similar physiological adaptations as does training on land. In cold water, VO2 max is improved despite training with fc significantly lower than that on land.

An Environmental Stress Index (ESI) as a Substitute for the Wet Bulb Globe Temperature (WBGT)

Abstract The purpose of this study was to develop a new environmental stress index (ESI) based on different parameters relating to heat stress. Meteorological measurements were taken in three climatic zones (hot/wet, hot/dry, and extremely hot/dry) for 60 days, and a new stress index based on these databases was developed as follows: ESI=0.63 T a − 0.03RH+0.002SR+0.0054( T a · RH) − 0.073(0.1+SR) −1 , where Ta is the ambient temperature (°C), RH the relative humidity (%), and SR the solar radiation (W·m−1). The correlation coefficients between ESI and wet bulb globe temperature (WBGT) were very high (R2>0.981). Therefore, we conclude that ESI, based on fast response and the more commonly used accurate climatic microsensors (Ta, RH, SR) which can be combined in a small portable device, has the potential to be a practical alternative to the WBGT.

Heat Intolerance in Former Heatstroke Patients

Nine young men who had suffered from heatstroke on previous occasions (heat-intolerant subjects) and 10 young volunteers (control subjects) were examined to determine their physiologic responses to exercise in temperate (23 degrees C) and hot environments (40 degrees C). The tests included an orthostatic test, work loads of 40 W and 80 W, and oxygen consumption (Vo2) determination. Although all the control subjects completed the exercise under severe heat load (3 h), none of the heat-intolerant subjects succeeded in completing this test due to high rectal temperatures and high heart rates. Sweat rates were similar in both groups, with Vo2 slightly higher in the control subjects. Orthostatic responses were similar in each group. The results suggest that inefficient thermoregulation, possibly due to decreased heat conductance from core to periphery, contributes to heat intolerance in former heatstroke patients.

Can gender differences during exercise-heat stress be assessed by the physiological strain index?

A physiological strain index (PSI) based on rectal temperature (Tre) and heart rate (HR) was recently suggested to evaluate exercise-heat stress. The purpose of this study was to evaluate PSI for gender differences under various combinations of exercise intensity and climate. Two groups of eight men each were formed according to maximal rate of O2 consumption (VO2 max). The first group of men (M) was matched to a group of nine women (W) with similar (P > 0.001) VO2 max (46.1 +/- 2.0 and 43.6 +/- 2.9 ml. kg-1. min-1, respectively). The second group of men (MF) was significantly (P < 0. 001) more fit than M or W with VO2 max of 59.1 +/- 1.8 ml. kg-1. min-1. Subjects completed a matrix of nine experimental combinations consisting of three different exercise intensities for 60 min [low, moderate, and high (300, 500, and 650 W, respectively)] each at three climates (comfortable, hot wet, and hot dry [20 degrees C 50% relative humidity (RH), 35 degrees C 70% RH, and 40 degrees C 35% RH, respectively]). No significant differences (P > 0.05) were found between matched genders (M and W) at the same exposure for sweat rate, relative VO2 max (%VO2 max), and PSI. However, MF had significantly (P < 0.05) lower strain than M and W as reflected by %VO2 max and PSI. In summary, PSI applicability was extended for exercise-heat stress and gender. This index continues to show potential for wide acceptance and application.A physiological strain index (PSI) based on rectal temperature (Tre) and heart rate (HR) was recently suggested to evaluate exercise-heat stress. The purpose of this study was to evaluate PSI for gender differences under various combinations of exercise intensity and climate. Two groups of eight men each were formed according to maximal rate of O2 consumption (V˙o 2 max). The first group of men (M) was matched to a group of nine women (W) with similar ( P > 0.001)V˙o 2 max (46.1 ± 2.0 and 43.6 ± 2.9 ml ⋅ kg-1 ⋅ min-1, respectively). The second group of men (MF) was significantly ( P < 0.001) more fit than M or W with V˙o 2 max of 59.1 ± 1.8 ml ⋅ kg-1 ⋅ min-1. Subjects completed a matrix of nine experimental combinations consisting of three different exercise intensities for 60 min [low, moderate, and high (300, 500, and 650 W, respectively)] each at three climates {comfortable, hot wet, and hot dry [20°C 50% relative humidity (RH), 35°C 70% RH, and 40°C 35% RH, respectively]}. No significant differences ( P > 0.05) were found between matched genders (M and W) at the same exposure for sweat rate, relativeV˙o 2 max(%V˙o 2 max), and PSI. However, MF had significantly ( P < 0.05) lower strain than M and W as reflected by %V˙o 2 max and PSI. In summary, PSI applicability was extended for exercise-heat stress and gender. This index continues to show potential for wide acceptance and application.

The effect of caffeine ingestion on physical performance after prolonged exercise

SummaryThe purpose of this study was to determine the effect of caffeine ingestion on physical performance after prolonged endurance exercise. Twenty three trained male volunteers participated in a 40-km march and were divided into two groups, matched for caffeine clearance rate and aerobic capacity. The experimental group ingested, prior to the march, a caffeinated drink at a dose of 5 mg·kg−1 body mass and at the 3rd and 5th h of marching an additional drink at a dose of 2.5 mg·kg−1 body mass. The control group ingested a drink of equal volume at the same times. Upon termination of the march each subject performed a cycle ergometer test at an intensity of 90% maximal oxygen consumption. Time to exhaustion and rate of perceived exertion (RPE) were recorded. Blood samples were drawn predrink, at the 3rd and 5th h of marching and immediately after the cycle ergometer test, and were analysed for caffeine, free fatty acids (FFA), lactate and glucose levels. Plasma FFA levels increased during the march (p<0.05), with no significant difference between groups. Lactate levels increased in the experimental group (p<0.05), with no significant change in the control group. Glucose levels did not change significantly in either group. After the cycle ergometer test, lactate levels were significantly higher in the experimental, as compared to the control group (3.77±0.33 vs 2.52±0.35 mmol·l−1, respectively). There was no significant difference between treatments in the time to exhaustion on the cycle ergometer, but RPE was different (p<0.05). Under the conditions of this study, the results do not indicate caffeine ingestion as an ergogenic aid which will postpone exhaustion following prolonged endurance exercise.

Evaluation of the environmental stress index for physiological variables

Abstract A new environmental stress index (ESI), based on ambient temperature ( T a ), relative humidity (RH) and solar radiation (SR), was recently suggested as a potential substitute for the wet-bulb globe temperature (WBGT) index. The purpose of this study was to evaluate and validate ESI for three different physiological variables including rectal temperature ( T re ), heart rate (HR), and sweat rate ( m sw ). A database was taken from a previous study where 12 young men (21±1 y) served as subjects exposed to 120 min of 12 different combinations consisting of three metabolic rates (rest and treadmill walking at 5 km·h −1 at 0% and 5% grades), two clothing ensembles (BDU and protective MOPP gear) and two outdoor solar radiation levels (shade and open sky). ESI was calculated as follows: ESI=0.63 T a -0.03RH+0.002SR+0.0054( T a RH)-0.073(0.1+SR) −1 . Significant differences of about 2 units ( p p m sw , HR and T re when measured in the sun and in the shade during all the exercise exposures. Thus, very high correlations ( R 2 >0.838) were found between ESI and T re , HR, or m sw . These results indicate that ESI is strongly correlated to the physiological strain, whereby higher stress is reflected in higher strain. Therefore, evaluating heat stress by ESI, which uses the more common, fast response and accurate climatic measures, becomes more predominant.

Comparison between different auxiliary cooling devices in a severe hot/dry climate

Seven different cooling devices were compared under the same hot/dry climatic conditions (50°C, 30% r. h.). Using a Latin-square design, eight male subjects tested water or air-cooled garments (vests or hoods), ice-bag vests, zone cooling or a fan for their beneficial effects on physiological variables. A strain index (SI), computed from sweat rate, change in rectal temperature and heart rate was used to evaluate the physiological status of the subjects while using the different devices. According to the SI, the ice-bag vest had the highest cooling capacity (SI = 0·55), while cooling the torso either by a water- or air-cooled vest or using zone cooling were found to have the same effect (SI ∼ 1). Cooling the head was found to be less effective (SI > 2) while the fan had a minimal cooling effect when compared to control (SI = 3·11 and 3·40 respectively). The results are discussed with respect to the interaction between the cooling capacity of the devices, evaporative heat loss and environmental conditions.

The cumulative heat strain index--a novel approach to assess the physiological strain induced by exercise-heat stress.

Abstract. The cumulative heat strain index (CHSI) is a new approach for assessing the total physiological strain experienced by subjects exposed to an exercise-heat stress. The index is based on inherent physiological logic that combines the thermoregulatory strain, which is described by the area under the hyperthermic curve, and the circulatory strain, which is characterized by heart-beat count. According to this model, the index reflects the dynamics of changes in the thermoregulatory and cardiovascular components and accounts for the complementary nature of the interaction between them. Mathematically, the index is calculated as follows: