The thermophysiology of uncompensable heat stress. Physiological manipulations and individual characteristics

The effect of cool water ingestion on gastrointestinal pill temperature

Telemetric gastrointestinal (GI) temperature pills are now commonly used to measure core body temperature and could minimize the risk of heat illness while maximizing operational effectiveness in workers subject to high levels of thermal strain.

PURPOSE

To quantify the effect of repeated cool water ingestion on the accuracy of GI pill temperature.

METHODS

Ten operational firefighters ingested a pill to measure GI temperature (T1int) before overnight sleep. Two hours following breakfast and 11.5 h after ingesting T1int, the firefighters ingested a second pill (T2int) before performing 8.5 h of intermittent activity (repetitive cycles of 30 min of seated rest followed by 30 min of general firefighter duties). During the first 2 min of each 30-min rest period, the firefighters consumed 250 mL of chilled water (5-8 degrees C).

RESULTS

Water ingestion had a highly variable effect both within and between subjects in transiently (32 +/- 10 min) reducing the temperature of T2int in comparison with T1int. In general, this transient reduction in T2int became progressively smaller as time following ingestion increased. In some firefighters, the difference between T1int and T2int became negligible (+/- 0.1 degrees C) after 3 h, whereas in two others, large differences (peaking at 2.0 degrees C and 6.3 degrees C) were still observed when water was consumed 8 h after pill ingestion.

CONCLUSION

These results show that a GI pill ingested immediately prior to physical activity cannot be used to measure core body temperature accurately in all individuals during the following 8 h when cool fluids are regularly ingested. This makes GI temperature measurement unsuitable for workers who respond to emergency deployments when regular fluid consumption is recommended operational practice.

Hydration knowledge, behaviours and status of staff at the residential camp of a fly-in/fly-out minerals extraction and processing operation in tropical North-Eastern Australia

A study was conducted with staff at a fly-in/fly-out mine camp to determine the: hydration knowledge, perceptions and behaviours; hydration status and needs; and perceived taste of potable water. A structured questionnaire was used to survey the self-reported hydration behaviour, knowledge and perception of. The study of the hydration status and needs of day shift staff while at the residential camp comprised measurements at 1800 h and 0600 h of weight, urine specific gravity and fluid intake. Staff rated the perceived taste of bottled, filtered potable and unfiltered potable waters on a seven point visual analogue scale. The mean correct responses to the knowledge items surveyed (n=78) was 9.2 out of 12. Three-quarters (n=15) of the 20 responses to an open-ended question suggested improved water taste to increase water consumption. In the hydration status study (n=46), the mean urine specific gravities at 1800 h and 0600 h were both 1.022, and the median fluid intake and loss rates were both 2.1 ml.kg(-1).h(-1). Staff (n=105) rated unfiltered water (median 4.0) as tasting significantly worse than bottled (median 6.0) or filtered (median 6.0) waters (Friedman test, p<0.001). While dehydration knowledge appears adequate, the observed fluid intakes and specific gravities demonstrate that external factors such as perceived taste of water influence hydration behaviour.

Thermoregulation during Exercise in the Heat

As a result of the inefficiency of metabolic transfer, >75% of the energy that is generated by skeletal muscle substrate oxidation is liberated as heat. During exercise, several powerful physiological mechanisms of heat loss are activated to prevent an excessive rise in body core temperature. However, a hot and humid environment can significantly add to the challenge that physical exercise imposes on the human thermoregulatory system, as heat exchange between body and environment is substantially impaired under these conditions. This can lead to serious performance decrements and an increased risk of developing heat illness. Fortunately, there are a number of strategies that athletes can use to prevent and/or reduce the dangers that are associated with exercise in the heat. In this regard, heat acclimatisation and nutritional intervention seem to be most effective. During heat acclimatisation, the temperature thresholds for both cutaneous vasodilation and the onset of sweating are lowered, which, in combination with plasma volume expansion, improve cardiovascular stability. Effective nutritional interventions include the optimisation of hydration status by the use of fluid replacement beverages. The latter should contain moderate amounts of glucose and sodium, which improve both water absorption and retention.

Neuropsychological determinants of exercise tolerance in the heat

Traditionally, exercise in the heat has been assumed to be primarily limited by cardiovascular constraints. However, an evolutionary perspective suggests that psychological safeguards should also protect individuals prior to catastrophic hyperthermia, and exposure to hot environments or elevated body temperature may directly attenuate central drive for exercise even well before the attainment of a critical limiting central temperature. Voluntary exercise tolerance or pacing may be influenced by a complex integration of peripheral and central thermal afferents, with regional differences in thermosensitivity across the skin surface and individual variability due to age and fitness. Despite the risk of accidents from impairments in mental function, heat exposure guidelines are commonly driven by physiological parameters, and the incorporation of a psychological component should be an important focus in occupational health and safety. In directly counteracting the effects of heat stress, the face and head is a region of high sudomotor and thermal sensitivity, and may thereby serve as an effective site for reducing perceptual and/or physiological heat strain via improvements in ventilation, airflow, or active cooling.

Challenges to temperature regulation when working in hot environments

The focus of this review is upon acute exposure to hot environments and the accompanying physiological changes. The target audience includes physiologists, physicians and occupational health and safety practitioners. Using the principles of thermodynamics, the avenues for human heat exchange are explored, leading to an evaluation of some methods used to assess thermally-stressful environments. In particular, there is a critique of the wet-bulb globe temperature (WBGT) index, and an overview of an alternative means by which such assessments may be undertaken (the heat stress index). These principles and methods are combined to illustrate how one may evaluate the risk of heat illness. Three general areas of research are briefly reviewed: the physiological impact of wearing thermal protective clothing, heat adaptation (acclimation) and whole-body pre-cooling. These topics are considered as potential pre-exposure techniques that may be used to reduce the threat of hyperthermia, or to enhance work performance in the heat.

The menstrual cycle and sport performance

The female sex steroid hormones estrogen and progesterone have potential effects on exercise capacity and performance through numerous mechanisms, such as substrate metabolism, cardiorespiratory function, thermoregulation, psychologic factors, and injuries. Consequently, hormone level changes may theoretically lead to either improved or decreased performance at various times throughout the menstrual cycle. Numerous methodological issues and a paucity of studies have precluded evidence-based conclusions in almost every area of research in this field. In addition, there appears to be a great degree of inter- and intraindividual variability in these hormonal responses. Using oral contraceptives may be advantageous for female athletes who are negatively affected by their menstrual cycle, as they may provide a stable yet controllable hormonal milieu for training and competition.

Design and control optimization of microclimate liquid cooling systems underneath protective clothing

The use of protective clothing, whether in space suits, hazardous waste disposal, or sporting equipment, generally increases the risk of heat stress and hyperthermia by impairing the capacity for evaporative heat exchange from the body to the environment. To date the most efficient method of microclimate cooling underneath protective clothing has been via conductive heat exchange from circulating cooling fluid next to the skin. In order to make the use of liquid microclimate cooling systems ((LQ)MCSs) as portable and practical as possible, the physiological and biomedical engineering design goals should be towards maximizing the efficiency of cooling to maintain thermal comfort/neutrality with the least cooling possible to minimize coolant and power requirements. Meeting these conditions is an extremely complex task that requires designing for a plethora of different factors. The optimal fitting of the (LQ)MCSs, along with placement and design of tubing and control of cooling, appear to be key avenues towards maximizing efficiency of heat exchange. We review the history and major design constraints of (LQ)MCSs, the basic principles of human thermoregulation underneath protective clothing, and explore potential areas of research into tubing/fabric technology, coolant distribution, and control optimization that may enhance the efficiency of (LQ)MCSs.

Physical work limits for Toronto firefighters in warm environments

This study examined the relationship between time to reach critical end points (tolerance time [TT] and metabolic rate for three different environmental temperatures (25 degrees C, 30 degrees C, and 35 degrees C, 50% relative humidity), while wearing firefighting protective clothing (FPC) and self-contained breathing apparatus (SCBA). Thirty-seven Toronto firefighters (33 male and 4 female) were divided into four work groups defined as Heavy (H, n = 9), Moderate (M, n = 9), Light (L, n = 10), and Very Light (VL, n = 9). At 25 degrees C, 30 degrees C, and 35 degrees C, TT (min) decreased from 56 to 47 to 41 for H, 92 to 65 to 54 for M, 134 to 77 to 67 for L, and 196 to 121 to 87 for VL. Significant differences in TT were observed across all group comparisons, excluding M versus L at 30 degrees C and 35 degrees C, and H versus M at 35 degrees C. Comparing 25 degrees C to 30 degrees C, M, L, and VL had significant decreases in TT, whereas only VL had a significant decrease when 30 degrees C was compared to 35 degrees C. For 25 degrees C to 30 degrees C, the relative change in TT was significantly greater for L (37%) and VL (41%) compared with H (16%) and M (26%). For 30 degrees C to 35 degrees C, the relative change among the groups was similar and approximately 17%. During passive recovery at 35 degrees C, rectal temperature (T(re)) continued to increase 0.5 degrees C above T(re final), whereas heart rate declined significantly. These findings show the differential impact of environmental conditions at various metabolic rates on TT while wearing FPC and SCBA. Furthermore, these findings reveal passive recovery may not be sufficient to reduce T(re) below pre-recovery levels when working at higher metabolic rates in hot environments.

Physiological responses to the menstrual cycle: implications for the development of heat illness in female athletes

Fluctuations in estrogen and progesterone during the menstrual cycle can cause changes in body systems other than the reproductive system. For example, progesterone is involved in the regulation of fluid balance in the renal tubules and innervation of the diaphragm via the phrenic nerve. However, few significant changes in the responses of the cardiovascular and respiratory systems, blood lactate, bodyweight, performance and ratings of perceived exertion are evident across the cycle. Nevertheless, substantial evidence exists to suggest that increased progesterone levels during the luteal phase cause increases in both core and skin temperatures and alter the temperature at which sweating begins during exposure to both ambient and hot environments. As heat illness is characterised by a significant increase in body temperature, it is feasible that an additional increase in core temperature during the luteal phase could place females at an increased risk of developing heat illness during this time. In addition, it is often argued that physiological gender differences such as oxygen consumption, percentage body fat and surface area-to-mass ratio place females at a higher risk of heat illness than males. This review examines various physiological responses to heat exposure during the menstrual cycle at rest and during exercise, and considers whether such changes increase the risk of heat illness in female athletes during a particular phase of the menstrual cycle.

Diagnosis of heat stroke in forensic medicine. Contribution of thermophysiology

The diagnosis of heat stroke is difficult to establish in forensic medicine due to the absence of observations made on the victims whilst alive. A recent case, concerning the death of two children in a vehicle, was restudied by taking into account calculations of thermophysiology. The results obtained allowed some assumptions of the investigation to be checked, and adds to the data provided previously in literature on the subject.

Influence of aerobic fitness and body fatness on tolerance to uncompensable heat stress

This study examined the independent and combined importance of aerobic fitness and body fatness on physiological tolerance and exercise time during weight-bearing exercise while wearing a semipermeable protective ensemble. Twenty-four men and women were matched for aerobic fitness and body fatness in one of four groups (4 men and 2 women in each group). Aerobic fitness was expressed per kilogram of lean body mass (LBM) to eliminate the influence of body fatness on the expression of fitness. Subjects were defined as trained (T; regularly active with a peak aerobic power of 65 ml x kg LBM(-1) x min(-1)) or untrained (UT; sedentary with a peak aerobic power of 53 ml x kg LBM(-1) x min(-1)) with high (High; 20%) or low (Low; 11%) body fatness. Subjects exercised until exhaustion or until rectal temperature reached 39.5 degrees C or heart rate reached 95% of maximum. Exercise times were significantly greater in T(Low) (116 +/- 6.5 min) compared with their matched sedentary (UT(Low); 70 +/- 3.6 min) or fatness (T(High); 82 +/- 3.9 min) counterparts, indicating an advantage for both a high aerobic fitness and low body fatness. However, similar effects were not evident between T(High) and UT(High) (74 +/- 4.1 min) or between the UT groups (UT(Low) and UT(High)). The major advantage attributed to a higher aerobic fitness was the ability to tolerate a higher core temperature at exhaustion (the difference being as great as 0.9 degrees C), whereas both body fatness and rate of heat storage affected the exercise time as independent factors.