TY - JOUR
AU - PhD, Roberta Pohlman,
AB - ABSTRACT Objective: The goals were to review the effectiveness of current cooling technologies used on the battlefield to reduce or to prevent heat illness in soldiers and to discuss possible alternative or improved cooling methods. Methods: A search of the literature for 1990–2007 was performed by using the Air Force Institute of Technology and Air Force Research Laboratory search engines. Results: Several current cooling technologies are modestly effective in attenuating brain and core body temperatures, but the cooling effects are not sustained and the devices present operational problems. This review indicates that some current cooling devices are effective in lowering perceived efforts and lengthening maximal exercise time but are incompatible with current demands. Conclusions: Many of the cooling methods and devices detailed in the literature are impractical for use in the field. Future research should focus on cooling technologies that are practical in the battlefield and have sustainable cooling effects. INTRODUCTION During war and training, battlefield soldiers are routinely exposed to a variety of environmental conditions that can alter judgment and physical performance and even result in death. The physical demands during battle, the types of military clothing and protective equipment worn, and dehydration may contribute to heat illness in soldiers. For example, exertional heat stroke may result and may be complicated by renal and hepatic failure, multiorgan dysfunction syndrome, and systemic inflammatory responses.1 The principal cause of heat stroke is failure of the hypothalamic temperature regulatory system. The failure of the hypothalamus to regulate body temperature results in the body's inability to dissipate heat, which induces an immediate excessive increase in core body temperature.1,2 A high core body temperature is defined as >41°C, which could result in dry skin, nervous system dysfunction, mental confusion, delirium, convulsions, and unconsciousness.3,4 The estimated death rate for subjects suffering from heat stroke is 20%.1 For this review, the scientific literature was reviewed to learn more about the current status of cooling options available to troops in the field. The literature search focused on the effects of heat on soldiers' work capacity and on current body-cooling technologies used by athletes and soldiers who must perform strenuous exercise in hot humid environments. The secondary purpose of this literature review was to introduce future cooling technologies that could prove useful to soldiers and athletes in combating heat illness and enhancing physical work capacity. Currently, practical cooling devices that provide sustained cooling effects for battlefield soldiers are not available. Failure to train troops correctly in hot humid environments or to use the appropriate cooling devices may result in mission failures. For instance, when a troop member suffers from a heat-related illness of any kind, an estimated four members of the platoon are needed to carry them any distance.4 Therefore, the prevention of heat illness among military troop members seems necessary for reducing battlefield mishaps and improving field performance.4 PHYSIOLOGICAL FEATURES OF HEAT STRESS Soldiers suffering from heat illness may experience reductions in physical performance that decrease their work capacity. Reductions in work capacity could severely limit soldiers' ability to adequately sustain and to satisfactorily complete specified duties required during intense ground operations. Physiological stressors caused by heat stress are expressed in individuals as reductions in cardiac output and reduced blood flow to splanchnic, skeletal muscle, brain, and skin tissues. Prolonged work (e.g., 8 hours) in hot, humid environments may also cause dehydration resulting from excessive sweat and associated sodium chloride losses in working skeletal muscle tissue, which collectively result in physical and mental exhaustion.5,–8 Basic physical training techniques exist that may aid in the prevention of heat stress, including heat acclimatization and ingestion of fluids before and during vigorous physical activity. Physical training techniques should be incorporated into a military training program that aids in reducing resting core body temperature and in conserving blood plasma volume.8 For example, acclimatization to the heat involves the subject engaging in intense exercise for a minimum of 6 days. Very fit subjects may require less time (e.g., 4 days) to become acclimated to the heat.1,8 Once subjects are acclimated to the heat, 3–27% increases in blood plasma volume may result. The increases in blood plasma volume help maintain stroke volume and sweating capacity.1,8 Other physiological benefits may also occur, such as reduced blood flow to the skin and increased central blood volume.1 Although these training techniques may work to improve physiological functions of subjects exposed to hot humid conditions, the effects can be short lived when a soldier or athlete performs sustained, high-intensity, physical activity in the heat. For example, excessive exposure to heat with impeded heat removal through the cerebral circulation can lead to accumulation of heat in the brain.9,10 When heat accumulation exceeds removal in the brain, temperatures may exceed 40°C, potentially impairing the ability of soldiers and athletes to sustain normal work intensity.11 With ongoing exertion, there may be restricted availability of glucose in the brain, invoking central fatigue. Continued bouts of high-intensity activity impose strains on the cardiovascular and locomotive systems and also the brain, ultimately leading to increases in soldiers' perceptions of effort.12,13 Brain temperature is a critical factor affecting motor activity during exercise in hot conditions. During exercise under high temperatures, there is impaired ability to remove heat from the blood and there is estimated to be 7 ± 2% greater heat production in the brain.13 On the basis of those results, the average brain temperature is estimated to be 0.2°C higher than the core body temperature during sustained aerobic exercise in either normal or hot humid conditions.9,14 Although soldiers are normally acclimated to extreme environmental conditions before battlefield exposure, acclimation is not complete unless physical training is performed at high exercise intensities in the heat. For example, optimal acclimatization requires subjects to elevate their core body temperature and to produce an hourly sweat rate of 400–600 mL at temperatures of > 30°C for a minimum of 5 days.1,12 Many soldiers do not have the opportunity to experience such exposure durations before deployment to a battlefield zone. Therefore, they may be prone to developing heat-related illnesses, which may place limitations on their cognitive and physical performance. MECHANISMS OF COOLING DEVICES Cooling devices have been introduced to combat heat illness. They are often applied to the neck and head region, on the basis of the concept that brain temperature can be controlled through regulation of cerebral blood flow. Cerebral blood flow can be regulated through active cooling of the head and neck regions.13,–16 Cooling results in a strong vasodilatory response because of the large supply of arteries to the brain, in contrast to heating, which elicits a strong constrictive response. The degree of constriction within the carotid arteries is related to the temperature. Active cooling of the neck enhances the vasodilatory response of the carotid arteries, effectively cooling the cerebral supply vessels.9,14,–16 The method of actively cooling the carotid artery to effectively lower brain temperature is known as selective brain cooling, which can be accomplished by applying an ice pack to the neck surface during heat stress.10,11 The supply of blood to the brain comes from the carotid arteries, which divide at the upper larynx into the right and left common carotid arteries.14,15 Under resting conditions, a blood flow rate in the common carotid artery for humans is 250–375 mL/minute.15 For total blood supply, 70% of blood flow is in the internal carotid artery and 30% is in the external carotid artery. Moderate physical exertion may increase cerebral blood flow rates by 30%, to support neural metabolic activity.14 Cooling of the large blood supply flowing through the carotid arteries induces a powerful vasodilatory response and effectively cools the blood flowing through the carotid arteries into the brain. The ability to effectively cool the blood flowing into the brain through the carotid arteries is limited by the brain's high perfusion rates and low surface area. However, application of ice packs to the lateral surface of the neck can still favorably reduce brain temperatures between 0.2°C and 0.5°C, contributing to reductions in heat illness.10,14,15 Local cooling of the carotid arteries during high-intensity exercise has been found to attenuate increasing brain temperatures, to reduce core temperatures, and to improve perceptions of physical efforts.13 Studies suggest that cooling of the neck region (e.g., right and left carotid arteries) and head may attenuate heat-related illnesses during strenuous physical effort in hot environments and may improve physical performance (work capacity).10,14,16,–18 A typical treatment for someone suffering from heatstroke is to provide rapid cooling to reduce the core body temperature, which can help normalize the patient's mental status and minimize organ damage.1,2 The head and neck are some of the most efficacious regions of the body to cool to prevent heat illness.13,14 For example, one key treatment involves the use of a neck-cooling collar, which has proven to be an effective therapeutic strategy for normal heatstroke patients because it enhances vasodilation by cooling the cerebral blood vessels.13 However, the neck-cooling collar has not been tested with field troops. Although cooling collars may be physiologically sound, they are viewed as impractical for battlefield troops who are routinely exposed to hot humid conditions for long periods and need to perform at high levels. We suspect that cooling collars could impede or restrict head movement and thus degrade battlefield performance. Cooling devices, including the aforementioned neck-cooling collars, cooling bandanas, cooling vests, neck-cooling devices, and clothing containing cooling micro-gels, have been effectively used to reduce core temperature. This cooling may contribute to enhanced work capacity. However, these cooling vests, bandanas, and neck-cooling devices may be bulky and frequently require ice or gels, which are impractical for many troops who must carry all of their gear for long distances. Supplies become a burden for logisticians. Traditional cooling treatments (such as immediate hydration or placing overheated individuals in a cool room) remove the troops suffering from heat illness from their relevant wartime duties, which places greater physical demands on other troop members. Those may not be options in some circumstances. Clearly, there is a need for effective portable methods of cooling. COOLING DEVICE EFFECTIVENESS During strenuous exercise in the heat, blood perfusion to the brain is reduced and heat removal rates are decreased, which may invoke rapid central fatigue.11 Central fatigue is closely associated with reduced (∼20%) cerebral blood flow during strenuous bouts of physical exertion. Various cooling devices have been developed for athletes and soldiers routinely exposed to extremely hot humid conditions, to attenuate rapid fatigue and heat illness. Many countermeasures have been extrapolated from physiological and mathematical models to facilitate military operations in extremely hot humid environments. However, research is limited regarding the countermeasures' effectiveness for operators in the field.17 Heat acclimatization strategies and procedures cannot be applied to all branches of service because, within each branch, there are different physical operation parameters and objectives. For example, an ice cooling jacket worn by a flight deck seaman or a crew member operating a rotary wing aircraft may be practical and useful in combating heat illness, because ice and supplies can be stored and reached easily aboard ship. However, this same system may not be practical for special operations involving Marines in ground warfare, because they are often mobile and have access only to what they carry on their persons.5 The development of practical cooling devices that can be worn by soldiers in the field is relevant because of the number of members who routinely suffer from heat-related illnesses during training or in battle. For example, U.S. Army recruits and especially reservists routinely suffer from heat injury during deployments and field training. In a 1-week training period, 33–49 cases of heat-related injuries per 10,000 Army troops are reported; over a 2-week training period, 210–455 cases of heat-related injuries per 10,000 troops are reported.18 Few data showing the effectiveness of cooling methods were found. Subjective ratings of perceived exertion were reportedly lowered with use of a cooling vest. For example, a group of researchers reported that subjects cycled a mean of 1.1 minute longer (332 vs. 341 W), compared with a control trial.19 However, subjects' blood lactate levels, exercise heart rates, and rectal temperatures remained unchanged during the cooling trial.19 Although the ice jackets might have improved the mean cycling time to exhaustion and decreased subjects' subjective feelings of exertion, many of the physiological measures remained unchanged. An ice vest was also tested with Canadian soldiers performing heavy exercise while wearing chemical gear in the heat, to determine work tolerance time. The cooling vest did not improve work tolerance time and did not prevent increases in core body temperature or sweat loss when the chemical gear was worn.13,19 Tests of a cooling vest to be worn under body armor showed that, although the cooling vest might delay fatigue, increase energy, and improve performance, there were several different parts that required assembly and the cooling effects were not sustained. In fact, Marines who drank cold tap water actually received equal or greater benefit in cooling the body, compared with wearing a cooling vest.20 A neck-cooling device made of terry cloth and a chamber containing ice was developed.11 An estimated 250 g of ice is placed inside a zipper chamber, and the device is then snugly wrapped around the subject's neck with attached Velcro strips.11 It was tested with 10 healthy male athletes who performed strenuous exercise by completing a 45-minute run on an indoor track. The subjects reported reductions in thermal perception of effort, with a slight reduction in tympanic membrane temperature. However, exercise heart rates and ratings of perceived exertion were not significantly changed. The neck wrap did effectively lower sweat rates and rectal temperatures. Those investigators were the first to show that carotid artery cooling by means of a neck wrap could reduce the effects of thermal stress during strenuous exercise in a thermoneutral environment. A head-cooling device was tested to determine the physical and psychological effects of head cooling on performance-related parameters in a group of 14 male distance runners in a hot environment during exercise and rest.13 Experimental test subjects completed four testing sessions (one per week) while wearing a head-cooling device in an environmental chamber (33°C, with 55% relative humidity). The cooling hood provided a cooling effect by perfusing water through the hood. The improvement in head comfort was not significant (p > 0.05).13 Body comfort and perceived head temperature were improved only when cooling took place during exercise. The cooling hood did lower the runners' perception of thermal discomfort, heart rate, sweat rate, and core body temperature, but only during the rest period. There were no improvements in performance (e.g., maximal aerobic capacity).13 Cooling devices such as the cooling vest can add weight to the backpack, and the support assembly (tubing, pump, and saline cooling bladder) makes this system inflexible for use with troop members' backpacks. FUTURE COOLING TECHNOLOGY NEEDS A carotid artery-cooling cuff based on theoretical cooling technology was introduced and tested.15 The cooling cuff is a 3-cm-long cooling cuff that could be wrapped internally around the carotid artery. This technology may provide rapid cooling to arterial blood flowing into the brain and may provide effective protection from brain ischemia and traumatic injury. A mathematical model that predicted the temperature of the tissue surrounding the carotid artery was developed. These predicted results indicated a 3°C decrease along that tissue region.15 This model indicated that future technology involving an artery-cooling cuff may prove useful in selectively cooling brain tissue, especially for patients suffering from brain injury. However, because of the intrusive nature of the artery-cooling cuff, this technology would not be useful for athletes or soldiers who are routinely exposed to hot humid conditions. We propose a cooling technology known as the carotid artery-cooling patch (CACP), based on the research of Zhu,15 who investigated the effects of countercurrent heat exchange on selective brain cooling in human subjects. Zhu developed a mathematical model (using a 5-cm-long cooling cuff) based on the temperature field along the carotid artery and the surrounding tissue region. Although there has been no formal testing or development of this proposed cooling patch concept, future researchers could examine whether the CACP offers the potential of a low-profile, effective, cooling method based on Zhu's15 mathematical model. For example, on the basis of the physiological effects of carotid artery cooling, perhaps the CACP could be placed directly over the carotid artery, to reduce the likelihood of heat illness, thereby improving the physical work capacity of soldiers in the field. The future development of the CACP also could be beneficial because it could be used by a large number of soldiers in the field because of its ease of use, in contrast to current cooling technology (e.g., cooling vests, which are heavy and cumbersome when carried in the backpacks of soldiers). As previously mentioned, on the basis of the physiological effects of carotid artery cooling and other technologies such as the cooling collar, future researchers should direct their efforts toward the development of technologies that can provide sustained cooling effects, can be used by large numbers of soldiers in the field, do not require maintenance, and do not add weight to soldiers' backpacks. CONCLUSIONS Several heat acclimatization strategies should be introduced to troop members before they are placed in hot humid environments. For soldiers who are not routinely exposed to hot humid conditions before deployment and those who fail to partake in specified physical training protocols that enhance heat acclimatization rates, noticeable decrements in work performance requiring maximal or submaximal efforts may result. Performing proper physical training with exposure to hot humid environments, managing amounts of heat stress by wearing lightweight clothing or cooling devices (e.g., cooling vests or cooling hats), and maintaining hydration levels during activity are simple techniques that can attenuate heat stress.1,5,11 The carotid artery is flexible and contains elastic properties that permit enhanced blood flow to the head and neck.12,15,16 Applying a form of cryotherapy (e.g., CACP) to the carotid artery could prevent heat illness and could be easily used for large groups of soldiers in hot conditions for long periods. The devices currently being used (e.g., cooling bandanas and cooling vests) restrict the movements of ground troop members and cannot be used by a large number of soldiers. Current cooling devices either must be frozen or contain a form of cooling crystals or gel packs that are enclosed within fabric and are activated by soaking in water or by pressing of a sensor attached to the cooling vest. The cooling effects provided are minimal and are not sustained, especially when ground troop soldiers are exposed to hot humid conditions for long periods. Furthermore, bandanas or neck straps connected to a core cooling vest are not strategically placed directly over the right or left common carotid artery. The cooling vest requires a generator to be worn in the backpack to sustain a continual cooling effect. The water or gel necessary for the cooling vest adds additional weight. Current devices to prevent heat illness are therefore ineffective for soldiers or athletes exposed to extremely hot humid conditions. Future research should focus on practical cooling devices that can provide rapid cooling effects to soldiers and can be used by entire ground units because of their ease of use in the battlefield. REFERENCES 1. Brooks GA, Fahey TD Baldwin KM: Exercise Physiology: Human Bioenergetics and Its Applications , Ed 4. Mountain View, CA, McGraw-Hill, 2005. 2. Gisolfi C, Robinson S Relations between physical training acclimatization and heat tolerance. J Appl Physiol  1969; 26: 530– 4. Google Scholar PubMed  3. Greer RJ, McCutcheon LJ Thermoregulatory adaptations associated with training and heat acclimation. Vet Clin North Am Equine Pract  1998; 14: 97– 120. Google Scholar CrossRef Search ADS PubMed  4. Sawka MN, Wenger CB, Pandolf KB Thermoregulatory responses to acute exercise-heat stress and heat acclimation. In: Handbook of Physiology, Section 4: Environmental Physiology , pp. 157– 8. Edited by Fregly MJ, Blatteis CM New York, NY: Oxford University Press, 1996. 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Dennis BH, Eberhart RC, Dulikravich GS, Radons SW Finite-element simulations of cooling of realistic 3-D human head and neck. ASME  2003; 125: 832– 40. 11. Gordon NF, Bogdanffy M, Wilkinson J Effect of a practical neck cooling device on core temperature during exercise. Med Sci Sports Exerc  1990; 22: 245– 9. Google Scholar PubMed  12. Rasmussen P, Stie H, Nybo L, Nielson B Heat induced fatigue and changes of the EEG is not related to reduced perfusion of the brain during prolonged exercise in humans. J Thermal Biol  2004; 731– 7. 13. Palmer CD, Sleivert GG, Cotter JD, Dunedin NZ The effects of head and neck cooling on thermoregulation, pace selection, and performance. Proc Aust Physiol Pharmcol Soc  2003; 32( Suppl 1): 122. 14. Nybo L, Secher NH, Nielsen B Inadequate heat release from the brain during prolonged exercise with hyperthermia. J Physiol  2002; 454: 697– 704. Google Scholar CrossRef Search ADS   15. Zhu L Theoretical evaluation of contributions of heat conduction and countercurrent heat exchange in selective brain cooling in humans. Ann Biomed Eng  2000; 28: 269– 77. Google Scholar CrossRef Search ADS PubMed  16. Mustafa S, Thulesius O, Ismael HN Hyperthermia-induced vasoconstriction of the carotid artery, a possible causative factor for heatstroke. J Appl Physiol  2004; 96: 1875– 8. Google Scholar CrossRef Search ADS PubMed  17. Burr RE Heat Illness: A Handbook for Medical Officers . Natick, MA, US Army Research Institute of Environmental Medicine, 1991. Technical Report 91-3. 18. Kerstein MD, Wright D, Connelly J, Hubbard R Heat illness in a hot/humid environment. Milit Med  1996; 161: 739– 42. 19. Smith JA, Yate K, Lee H, Thompon MW, Holcombe BV, Martin DT Precooling improves cycling performance in hot/humid conditions [Abstract]. Med Sci Sports and Exerc  1997; 29: S263. Google Scholar CrossRef Search ADS   20. 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TI - Current and Future Cooling Technologies Used in Preventing Heat Illness and Improving Work Capacity for Battlefield Soldiers: Review of the Literature
JF - Military Medicine
DO - 10.7205/MILMED.173.7.653
DA - 2008-07-01
UR - https://www.deepdyve.com/lp/oxford-university-press/current-and-future-cooling-technologies-used-in-preventing-heat-rX2IdNXJVJ
SP - 653
EP - 657
VL - 173
IS - 7
DP - DeepDyve
ER -