BRIEF REVIEW
NOVEL COOLING STRATEGIES FOR MILITARY TRAINING AND OPERATIONS JASON K.W. LEE,1,2,3 ROBERT W. KENEFICK,4
AND
SAMUEL N. CHEUVRONT4
1
Combat Protection and Performance, Defense Medical and Environmental Research Institute, DSO National Laboratories, Singapore, Singapore; 2Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore; 3Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, Singapore; and 4US Army Research Institute of Environmental Medicine, Thermal and Mountain Medicine Division, Natick, Massachusetts ABSTRACT
Lee, JKW, Kenefick, RW, and Cheuvront, SN. Novel cooling strategies for military training and operations. J Strength Cond Res 29(11S): S77–S81, 2015—The deleterious effects of environmental heat stress, combined with high metabolic loads and protective clothing and equipment of the modern Warfighter, impose severe heat strain, impair task performance, and increase risk of heat illness, thereby reducing the chance for mission success. Despite the implementation of heat-risk mitigation procedures over the past decades, task performance still suffers and exertional heat illness remains a major military problem. We review 3 novel heat mitigation strategies that may be implemented in the training or operational environment to reduce heat strain and the risk of exertional heat illness. These strategies include ingestion of ice slurry, arm immersion cooling, and microclimate cooling. Each of these strategies is suitable for use in different scenarios and the choice of cooling strategy is contingent on the requirements, circumstances, and constraints of the training and operational scenario. Ingestion of ice slurry and arm immersion cooling are practical strategies that may be implemented during training scenarios; ice slurry can be ingested before and during exercise, whereas arm immersion cooling can be administered after exercise-heat exposure. In the operational environment, existing microclimate cooling can be implemented with retrofitted vehicles and as an unmounted system, and it has the potential for use in many military occupational scenarios. This review will discuss the efficacy, limitations, and practical considerations for field implementation of each strategy.
KEY WORDS ice slurry, arm immersion, microclimate cooling, performance
Address correspondence to Jason K.W. Lee, [email protected]. 29(11S)/S77–S81 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
INTRODUCTION
T
he combined effects of environmental heat stress and metabolic heat production during military training and operations can impose significant heat strain on the Warfighter. The protective clothing/ equipment worn during training and combat can further restrict heat loss. Without sufficient heat dissipation, hyperthermia (high body core and skin temperatures) can threaten mission success by impairing task performance and increasing the risk of exertional heat illness. Although extensive heat-risk mitigation procedures (e.g., aerobic conditioning, heat acclimatization, proper hydration, and work-rest cycles) have been implemented to attenuate heat strain (18,26), exertional heat illness remains a major concern among military personnel (1,2). As such, auxiliary cooling strategies are warranted to reduce heat strain, maintain task performance, and protect the health of the Warfighter. Yet, existing field-cooling strategies (water misting and showers) have severe limitations (17,28). Therefore, the purpose of this article is to review 3 novel cooling methods (i.e., ingestion of ice slurry, arm immersion cooling, and microclimate cooling) that are currently in use or in development for use by military personnel in both the training and operational environment. The efficacy, limitations, and practical considerations for field implementation of each strategy will be discussed.
INGESTION
OF
ICE SLURRY
Ingestion of ice slurry is an internal cooling strategy that has recently received considerable attention, as it is more practical and applicable in the field than most external cooling methods (24). Ice slurry is a beverage containing a mixture of tiny ice particles and liquid water. Ingestion of ice slurry provides cooling by acting as an internal heat sink to absorb body heat. Although consuming cold fluids alone can lower heat strain more effectively than warm water (15), the associated large volumes required and the low cooling capacity can be unfavorable (20). However, by adding ice to the drink, the cooling effect is enhanced, as ice absorbs a larger amount of heat energy than cool water during its transition into liquid water without a change in temperature VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 |
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Novel Heat Mitigation for the Military (enthalpy of fusion) (25). Thus, for a given volume of beverage, the ice slurry would absorb more heat energy and provide a greater cooling effect than cold fluids alone (29). The benefits of ingesting ice slurry before and during exercise in the heat have been shown (11,28). Siegel et al. (25) recorded a ;0.668 C decrease in pre-exercise body core temperature after ingesting ice slurry, which resulted in a 19% increase in running duration. Similarly, a field study by Yeo et al. (33) showed a 0.58 C decrease in pre-exercise gastrointestinal temperature and better performance in a running time-trial. Recently, we also showed that ice slurry ingestion during a 15-km route march (5 work-rest cycles) in the heat reduced the gastrointestinal temperature and led to a prolonged work tolerance time (Tan PMS (BSc.), Ang WH (MSc.), Teo YS (BSc.) and Lee JKW (PhD.), unpublished data, March 2015.). Collectively, results from these studies support the use of ice slurry ingestion as a means to alleviate heat strain and enhance the work performance of military personnel working in hot environments. Yet, its efficacy may be limited by the lack of simultaneous skin cooling (25). A smaller core-to-skin temperature gradient results when the core temperature reduces more than the skin temperature, reducing heat transfer from the core (14), which increases heat strain (21). Despite the lack of temperature gradient between the skin and core, ice slurry ingestion would still induce a net reduction in heat strain from internal cooling alone because of its high latent heat of fusion due to the phase change from ice to water (25). Ice slurry can be used in the military training environment where it can be consumed in place of water before and during
training exercises, because ice slurry will also hydrate soldiers. Nevertheless, the implementation of ice slurry ingestion depends on having access to an ample supply of ice and water in the training location and a power supply to operate the ice slurry machine (24). Ice slurry ingestion may also cause gastric discomfort and brain freeze (sphenopalatine ganglioneuralgia) (13,25), although these effects are temporary. Furthermore, the need to maintain the integrity of the drink for prolonged periods also limits its application during exercise. The development of a lightweight and highly-insulated water bottle to store the ice slurry might solve this problem.
ARM IMMERSION COOLING
Unlike ice slurry ingestion, arm immersion cooling is an external cooling method that involves immersing the arms (hands and forearms) in a bath of cool water. Research indicates that the optimal water temperature is between 10 and 208 C (8,9,23). This strategy of arm immersion can be effective because the heat transfer coefficient of water is 25 times greater compared with that of air at the same temperature (5). The forearms are highly vascularized with a large surface area:mass ratio. Because peripheral vasodilation and enhanced skin blood flow occur when body temperature is high (12), arm immersion is a convenient and effective means for effectively reducing body temperature (8). When the arms are immersed in cool water, convection and conduction removes heat from the arms and the warm blood perfusing them (8). The cooled blood then flows directly back to the core through superficial veins due to vasodilated arteriovenous anastomoses (30), thus cooling the body core. Although the impact of arm immersion cooling on physical work or exercise performance has not been studied to date, the efficacy of arm immersion in reducing thermal strain and prolonging the work tolerance time in the heat is well established. Immersing the arms in 208 C water for 10 minutes reduced body core temperature by 0.78 C (9) (Figure 1). In a later study, Selkirk et al. (2004) also showed a slower rate of rise in body core temperature when the arms were immersed in 178 C water compared with the nocooling control condition, which led to a 66 and 62% increase in tolerance time and Figure 1. Body temperature at each time point without arm immersion (control) and during arm immersion in 10, work time during subsequent 20, and 308 C water. Adapted from (9). Adaptations are themselves works protected by copyright. So in order to heat exposure, respectively. publish this adaptation, authorization must be obtained both from the owner of the copyright in the original work These results imply that arm and from the owner of copyright in the translation or adaptation. immersion is a fast and effective
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may present a potential biohazard issue. Yet, despite the fairly low logistical burden, issues such as obtaining adequate ice and water in the field may limit its applicability. It may also be cumbersome to set up and disassemble the AICS in the field, especially when time is limited. Therefore, the use of AICS may be most practical when high-intensity training activities are performed in the heat, when heat illness risk is greatest (31). Indeed, the effects of arm immersion cooling on the incidence of exertional heat illness have been examined (6). Exertional heat illness incidence rates, injury severity, and treatment cost data from the U.S. Army Ranger School were compared with and without Figure 2. Relationship between microclimate cooling and endurance times at selected metabolic rates when wearing nuclear-biological-chemical protective clothing in a tropical environment. Adapted from (22). Adaptations arm immersion cooling during are themselves works protected by copyright. So in order to publish this adaptation, authorization must be the summer months of 2007– obtained both from the owner of the copyright in the original work and from the owner of copyright in the 12. Although incidence rates translation or adaptation. were not different between those who did and did not use arm immersion cooling, way to cool heat-stressed Warfighters and improve work AICS use was associated with a decrease in exertional heat tolerance. Of note, immersing the arms in cold water may illness severity and associated medical treatment costs (6). cause reflex cutaneous vasoconstriction, which would MICROCLIMATE COOLING reduce peripheral blood flow and compromise cooling (5,23). That said, this may not affect hyperthermic individPersonal microclimate cooling systems refer to cooling uals, as a high heat strain greatly weakens the vasocongarments typically worn under protective clothing. Besides strictor response (10,16). Moreover, previous data suggest the high physical workloads and hot environmental that arm immersion in 108 C water did not induce significant conditions encountered during military operations, vasoconstriction (8). In normothermic individuals, however, Warfighters are also required to wear protective clothing cutaneous vasoconstriction readily occurs in response to or body armor that is heavy, impermeable, and insulative cold-water arm immersion (4). Therefore, arm immersion (22). Thus, not only more metabolic heat is generated but should be used during breaks in-between training also its dissipation is compromised (19). In such cases, activities, for instance, during the rest periods of military cooling garments can be used to promote the loss of body work-rest cycles. heat and reduce thermal strain. The efficacy of microcliThe U.S. Army Research Institute of Environmental mate cooling systems for reducing heat strain is very well Medicine is in the process of improving the design of an acknowledged (22). Arm Immersion Cooling System (AICS) for use by Several types of cooling garments using different systems Warfighters during military training. At present, proto(e.g., air-cooled, liquid-cooled, ice-packet vests, and phase types of the current version of the AICS have been change materials) have been developed. The choice of implemented in several military training units in the cooling modality must be an educated one, as no single United States (5). The AICS features a lightweight design cooling solution is optimal under all circumstances. Of the and can accommodate up to 6 Warfighters at one time. available systems, liquid-cooled garments are regarded as the Care should be taken when individuals with small cuts or most beneficial by the military due to the higher heat abrasions received during training use the AICS, as this transfer potential of circulating water compared with air or VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 |
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Novel Heat Mitigation for the Military ice and its general use compatibility within a contaminated environment (7). In liquid-cooled garments, a batteryoperated pump constantly circulates cool liquid over the skin through a network of tubes embedded within the garment. Heat from the skin is transferred to the cool liquid by conduction. The warmed liquid then returns to a cooler where it is chilled again. As such, skin temperature is kept low and the core-to-skin temperature gradient is maintained. This facilitates convective heat transfer from the core to the skin, thus lowering body heat content. Covering more body surface areas can increase the cooling effect of liquid-cooled garments (7,22), but the use of full-body cooling suits is not practical, as it restricts mobility. This is also true with garments covering the limbs (arms and thighs). As such, torso cooling vests afford a relatively large potential surface area for heat extraction (20–25% body surface area [BSA]) and represent the most practical location for body cooling on the battlefield. Liquid-cooled garments can also prolong work performance (22) (Figure 2). Liquid-cooling systems have been implemented for use in mounted Warfighters (22), but very few microclimate cooling systems have been fielded despite their widely demonstrated efficacy, in part because of the need to retrofit vehicles to accommodate and power the systems. No systems have been fielded for the dismounted Warfighter or foot soldier. The typical weight of a system used on the go (e.g., 2.8-kg system weight; 1.4-kg battery) imposes additional metabolic load demands. The noise signature and the limited battery life (#4 hours) also reduce their relevance during sustained military operations on the ground. The current method of continuous cooling is also not the most efficient practice, especially in situations of changing metabolic activity and hence cooling required by the wearer (7). The resulting cutaneous vasoconstriction impairs conductive heat transfer from the skin, reducing cooling efficiency and effectiveness of the system (3,26,32). Recent research has been undertaken to improve current liquid-cooling systems and increase their field application to dismounted Warfighters (3,26,31). The use of intermittent cooling incorporating skin temperature feedback can reduce power requirements by almost 50% (26), thus extending battery life and reducing the number of batteries carried. Novel tubing construction can also improve cooling efficiency by increasing surface area contact between the skin and cooling tubes, thus enhancing conductive heat transfer (7,22). The more widespread availability and use of personal microclimate cooling could improve work performance and reduce the risk of heat illness, particularly for some military occupational specialities more than others. For example, most heat illnesses occur in the infantry and gun crew (2) where power, weight, and noise signatures are constraining. However, the second highest incidence rate of heat casualties occurs in service and supply crews (2) where the same power, weight, and noise constraints are absent.
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CONCLUSIONS The conditions (environmental, metabolic, and clothing) that military personnel are routinely exposed to during military training and operations can severely impede normal heat dissipation, necessitating auxiliary cooling methods. The 3 cooling strategies reviewed in this article are effective in reducing heat strain, and may improve work tolerance and reduce heat illness risks in Warfighters during training/ operations in the heat. In the military training environment, ingestion of ice slurry and arm immersion cooling are practical strategies. Ice slurry can be ingested before and during exercise, whereas arm immersion cooling can only be administered during exercise following heat exposure. In the operational environment, existing microclimate cooling (i.e., liquid-cooled vests) can be implemented with properly retrofitted vehicles and has potential for use in many different military occupational specialties as an unmounted system. Each of these strategies is suitable for use in different scenarios. The choice of cooling strategy is therefore contingent on the requirements, circumstances, and constraints of the training and operational scenario.
ACKNOWLEDGMENTS The authors thank Ms. Beverly Tan and Ms. Elizabeth Caruso for technical and editorial assistance. The opinions or assertions contained herein are the private views of the authors and should not be construed as official or reflecting the views of the Army or the Department of Defense. Citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations.
REFERENCES 1. Armed Forces Health Surveillance Center. Update: Heat injuries, active component, U.S. Armed Forces. MSMR 21(3): 10–13, 2014. 2. Carter, R III, Cheuvront, SN, Williams, JO, Kolka, MA, Stephenson, LA, Sawka, MN, and Amoroso, PJ. Epidemiology of hospitalizations and deaths from heat illness in soldiers. Med Sci Sports Exerc 37: 1338–1344, 2005. 3. Cheuvront, SN, Kolka, MA, Cadarette, BS, Montain, SJ, and Sawka, MN. Efficacy of intermittent, regional microclimate cooling. J Appl Physiol (1985) 94: 1841–1848, 2003. 4. Davies, N. The Control of Peripheral Circulation of the Hands and Forearms and the Effect of Local Cooling in Hyperthermic Subjects. Final Year Project Report, BSc Applied Biological Sciences. Bristol, England: University of the West England, 1995. 5. DeGroot, DW, Gallimore, RP, Thompson, SM, and Kenefick, RW. Extremity cooling for heat stress mitigation in military and occupational settings. J Therm Biol 38: 305–310, 2013. 6. DeGroot, DW, Kenefick, RW, and Sawka, MN. Impact of arm immersion cooling on incidences of exertional heat illness during summer ranger training. Mil Med 2015. In Press. 7. Flouris, AD and Cheung, SS. Design and control optimization of microclimate liquid cooling systems underneath protective clothing. Ann Biomed Eng 34: 359–372, 2006.
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8. Giesbrecht, GG, Jamieson, C, and Cahill, F. Cooling hyperthermis firefighters by immersing forearms and hands in 108 C and 208 C water. Aviat Space Environ Med 78: 561–567, 2007.
21. Pandolf, KB and Goldman, RF. Convergence of skin and rectal temperatures as a criterion for heat tolerance. Aviat Space Environ Med 49: 1095–1101, 1978.
9. House, J. Extremity cooling as a method for reducing heat strain. J Defence Sci 3: 108–114, 1998.
22. Pandolf, KB, Gonzalez, JA, and Sawka, MN. Tri-service perspectives on microclimate cooling of protective clothing in the heat. In: Technical Report T95-10. Natick, MA: U.S. Army Research Institute of Environmental Medicine, 1995.
10. House, JR, Lunt, H, Magness, A, and Lyons, J. Testing the effectiveness of techniques for reducing heat strain in Royal Navy nuclear, biological and chemical cleansing stations’ teams. J R Nav Med Serv 89: 27–34, 2003. 11. James, CA, Richardson, AJ, Watt, PW, Gibson, OR, and Maxwell, NS. Physiological responses to incremental exercise in the heat following internal and external precooling. Scand J Med Sci Sports 25: 190–199, 2015. 12. Johnson, JM and Park, MK. Reflex control of skin blood flow by skin temperature: Role of core temperature. J Appl Physiol Respir Environ Exerc Physiol 47: 1188–1193, 1979. 13. Kaufman, CE. Ice slurry and cold drink reduces exercise induced physiological strain in the heat. PhD Thesis, The University of Montana, Missoula, 2012. 14. Kenefick, RW, Cheuvront, SN, Palombo, LJ, Ely, BR, and Sawka, MN. Skin temperature modifies the impact of hypohydration on aerobic performance. J Appl Physiol (1985) 109: 79–86, 2010. 15. Lee, JK, Shirreffs, SM, and Maughan, RJ. Cold drink improves exercise endurance capacity in the heat. Med Sci Sports Exerc 40: 1637–1644, 2008. 16. Livingstone, SD, Nolan, RW, and Cattroll, SW. Heat loss caused by immersing the hands in water. Aviat Space Environ Med 60: 1166– 1171, 1989. 17. McDermott, BP, Casa, DJ, Ganio, MS, Lopez, RM, Yeargin, SW, Armstrong, LE, and Maresh, CM. Acute whole-body cooling for exercise-induced hyperthermia: A systematic review. J Athl Train 44: 84–93, 2009. 18. Montain, SJ, Latzka, WA, and Sawka, MN. Fluid replacement recommendations for training in hot weather. Mil Med 164: 502– 508, 1999. 19. Montain, SJ, Sawka, MN, Cadarette, BB, and Quigley, MD. Physiological tolerance to uncompensable heat stress: Effects of exercise intensity, protective clothing and climate. J Appl Physiol (1985) 77: 218–222, 1994. 20. Mundel, T, King, J, Collacott, E, and Jones, DA. Drink temperature influences fluid intake and endurance capacity in men during exercise in a hot, dry environment. Exp Physiol 91: 925–933, 2006.
23. Selkirk, GA, McLellan, TM, and Wong, J. Active versus passive cooling during work in warm environments while wearing firefighting protective clothing. J Occup Environ Hyg 1: 521–531, 2004. 24. Siegel, R and Laursen, PB. Keeping your cool: Possible mechanisms for enhanced exercise performance in the heat with internal cooling methods. Sports Med 42: 89–98, 2012. 25. Siegel, R, Mate, J, Brearley, MB, Watson, G, Nosaka, K, and Laursen, PB. Ice slurry ingestion increases core temperature capacity and running time in the heat. Med Sci Sports Exerc 42: 717– 725, 2010. 26. Stephenson, LA, Vernieuw, CR, Leammukda, W, and Kolka, MA. Skin temperature feedback optimizes microclimate cooling. Aviat Space Environ Med 78: 377–382, 2007. 27. Tan, PM and Lee, JK. The role of fluid temperature and form on endurance performance in the heat. Scand J Med Sci Sports 25: 39–51, 2015. 28. TBMED 507. Heat stress control and heat casualty management. In: Technical Bulletin Medical 507/Air Force Pamphlet. Washington, DC: Department of the Army and Air Force, 2003. pp. 48–152. 29. Vanden Hoek, TL, Kasza, KE, Beiser, DG, Abella, BS, Franklin, JE, Oras, JJ, Alvardo, JP, Anderson, T, Son, H, Wardrip, CL, Zhao, D, Wang, H, and Becker, LB. Induced hypothermia by central venous infusion: Saline ice slurry versus chilled saline. Crit Care Med 32: S425–S431, 2004. 30. Vanggaard, L and Gjerloff, CC. A new simple technique of rewarming in hypothermia. Int Rev Army Navy Air Force Med Serv 52: 427–430, 1979. 31. Wallace, RF, Kriebel, D, Punnett, L, Wegman, DH, Wenger, CB, Gardner, JW, and Gonzalez, RR. The effects of continuous hot weather training on risk of exertional heat illness. Med Sci Sports Exerc 37: 84–90, 2005. 32. Xu, X, Berglund, LG, Cheuvront, SN, Endrusick, TL, and Kolka, MA. Model of human thermoregulation for intermittent regional cooling. Aviat Space Environ Med 75: 1065–1069, 2004. 33. Yeo, ZW, Fan, PW, Nio, AQ, and Lee, JK. Ice slurry in outdoor running performance in heat. Int J Sports Med 33: 859–866, 2012.
VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 |
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Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
NOVEL COOLING STRATEGIES FOR MILITARY TRAINING AND OPERATIONS JASON K.W. LEE,1,2,3 ROBERT W. KENEFICK,4
AND
SAMUEL N. CHEUVRONT4
1
Combat Protection and Performance, Defense Medical and Environmental Research Institute, DSO National Laboratories, Singapore, Singapore; 2Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore; 3Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, Singapore; and 4US Army Research Institute of Environmental Medicine, Thermal and Mountain Medicine Division, Natick, Massachusetts ABSTRACT
Lee, JKW, Kenefick, RW, and Cheuvront, SN. Novel cooling strategies for military training and operations. J Strength Cond Res 29(11S): S77–S81, 2015—The deleterious effects of environmental heat stress, combined with high metabolic loads and protective clothing and equipment of the modern Warfighter, impose severe heat strain, impair task performance, and increase risk of heat illness, thereby reducing the chance for mission success. Despite the implementation of heat-risk mitigation procedures over the past decades, task performance still suffers and exertional heat illness remains a major military problem. We review 3 novel heat mitigation strategies that may be implemented in the training or operational environment to reduce heat strain and the risk of exertional heat illness. These strategies include ingestion of ice slurry, arm immersion cooling, and microclimate cooling. Each of these strategies is suitable for use in different scenarios and the choice of cooling strategy is contingent on the requirements, circumstances, and constraints of the training and operational scenario. Ingestion of ice slurry and arm immersion cooling are practical strategies that may be implemented during training scenarios; ice slurry can be ingested before and during exercise, whereas arm immersion cooling can be administered after exercise-heat exposure. In the operational environment, existing microclimate cooling can be implemented with retrofitted vehicles and as an unmounted system, and it has the potential for use in many military occupational scenarios. This review will discuss the efficacy, limitations, and practical considerations for field implementation of each strategy.
KEY WORDS ice slurry, arm immersion, microclimate cooling, performance
Address correspondence to Jason K.W. Lee, [email protected]. 29(11S)/S77–S81 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
INTRODUCTION
T
he combined effects of environmental heat stress and metabolic heat production during military training and operations can impose significant heat strain on the Warfighter. The protective clothing/ equipment worn during training and combat can further restrict heat loss. Without sufficient heat dissipation, hyperthermia (high body core and skin temperatures) can threaten mission success by impairing task performance and increasing the risk of exertional heat illness. Although extensive heat-risk mitigation procedures (e.g., aerobic conditioning, heat acclimatization, proper hydration, and work-rest cycles) have been implemented to attenuate heat strain (18,26), exertional heat illness remains a major concern among military personnel (1,2). As such, auxiliary cooling strategies are warranted to reduce heat strain, maintain task performance, and protect the health of the Warfighter. Yet, existing field-cooling strategies (water misting and showers) have severe limitations (17,28). Therefore, the purpose of this article is to review 3 novel cooling methods (i.e., ingestion of ice slurry, arm immersion cooling, and microclimate cooling) that are currently in use or in development for use by military personnel in both the training and operational environment. The efficacy, limitations, and practical considerations for field implementation of each strategy will be discussed.
INGESTION
OF
ICE SLURRY
Ingestion of ice slurry is an internal cooling strategy that has recently received considerable attention, as it is more practical and applicable in the field than most external cooling methods (24). Ice slurry is a beverage containing a mixture of tiny ice particles and liquid water. Ingestion of ice slurry provides cooling by acting as an internal heat sink to absorb body heat. Although consuming cold fluids alone can lower heat strain more effectively than warm water (15), the associated large volumes required and the low cooling capacity can be unfavorable (20). However, by adding ice to the drink, the cooling effect is enhanced, as ice absorbs a larger amount of heat energy than cool water during its transition into liquid water without a change in temperature VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 |
S77
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Novel Heat Mitigation for the Military (enthalpy of fusion) (25). Thus, for a given volume of beverage, the ice slurry would absorb more heat energy and provide a greater cooling effect than cold fluids alone (29). The benefits of ingesting ice slurry before and during exercise in the heat have been shown (11,28). Siegel et al. (25) recorded a ;0.668 C decrease in pre-exercise body core temperature after ingesting ice slurry, which resulted in a 19% increase in running duration. Similarly, a field study by Yeo et al. (33) showed a 0.58 C decrease in pre-exercise gastrointestinal temperature and better performance in a running time-trial. Recently, we also showed that ice slurry ingestion during a 15-km route march (5 work-rest cycles) in the heat reduced the gastrointestinal temperature and led to a prolonged work tolerance time (Tan PMS (BSc.), Ang WH (MSc.), Teo YS (BSc.) and Lee JKW (PhD.), unpublished data, March 2015.). Collectively, results from these studies support the use of ice slurry ingestion as a means to alleviate heat strain and enhance the work performance of military personnel working in hot environments. Yet, its efficacy may be limited by the lack of simultaneous skin cooling (25). A smaller core-to-skin temperature gradient results when the core temperature reduces more than the skin temperature, reducing heat transfer from the core (14), which increases heat strain (21). Despite the lack of temperature gradient between the skin and core, ice slurry ingestion would still induce a net reduction in heat strain from internal cooling alone because of its high latent heat of fusion due to the phase change from ice to water (25). Ice slurry can be used in the military training environment where it can be consumed in place of water before and during
training exercises, because ice slurry will also hydrate soldiers. Nevertheless, the implementation of ice slurry ingestion depends on having access to an ample supply of ice and water in the training location and a power supply to operate the ice slurry machine (24). Ice slurry ingestion may also cause gastric discomfort and brain freeze (sphenopalatine ganglioneuralgia) (13,25), although these effects are temporary. Furthermore, the need to maintain the integrity of the drink for prolonged periods also limits its application during exercise. The development of a lightweight and highly-insulated water bottle to store the ice slurry might solve this problem.
ARM IMMERSION COOLING
Unlike ice slurry ingestion, arm immersion cooling is an external cooling method that involves immersing the arms (hands and forearms) in a bath of cool water. Research indicates that the optimal water temperature is between 10 and 208 C (8,9,23). This strategy of arm immersion can be effective because the heat transfer coefficient of water is 25 times greater compared with that of air at the same temperature (5). The forearms are highly vascularized with a large surface area:mass ratio. Because peripheral vasodilation and enhanced skin blood flow occur when body temperature is high (12), arm immersion is a convenient and effective means for effectively reducing body temperature (8). When the arms are immersed in cool water, convection and conduction removes heat from the arms and the warm blood perfusing them (8). The cooled blood then flows directly back to the core through superficial veins due to vasodilated arteriovenous anastomoses (30), thus cooling the body core. Although the impact of arm immersion cooling on physical work or exercise performance has not been studied to date, the efficacy of arm immersion in reducing thermal strain and prolonging the work tolerance time in the heat is well established. Immersing the arms in 208 C water for 10 minutes reduced body core temperature by 0.78 C (9) (Figure 1). In a later study, Selkirk et al. (2004) also showed a slower rate of rise in body core temperature when the arms were immersed in 178 C water compared with the nocooling control condition, which led to a 66 and 62% increase in tolerance time and Figure 1. Body temperature at each time point without arm immersion (control) and during arm immersion in 10, work time during subsequent 20, and 308 C water. Adapted from (9). Adaptations are themselves works protected by copyright. So in order to heat exposure, respectively. publish this adaptation, authorization must be obtained both from the owner of the copyright in the original work These results imply that arm and from the owner of copyright in the translation or adaptation. immersion is a fast and effective
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Journal of Strength and Conditioning Research
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
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may present a potential biohazard issue. Yet, despite the fairly low logistical burden, issues such as obtaining adequate ice and water in the field may limit its applicability. It may also be cumbersome to set up and disassemble the AICS in the field, especially when time is limited. Therefore, the use of AICS may be most practical when high-intensity training activities are performed in the heat, when heat illness risk is greatest (31). Indeed, the effects of arm immersion cooling on the incidence of exertional heat illness have been examined (6). Exertional heat illness incidence rates, injury severity, and treatment cost data from the U.S. Army Ranger School were compared with and without Figure 2. Relationship between microclimate cooling and endurance times at selected metabolic rates when wearing nuclear-biological-chemical protective clothing in a tropical environment. Adapted from (22). Adaptations arm immersion cooling during are themselves works protected by copyright. So in order to publish this adaptation, authorization must be the summer months of 2007– obtained both from the owner of the copyright in the original work and from the owner of copyright in the 12. Although incidence rates translation or adaptation. were not different between those who did and did not use arm immersion cooling, way to cool heat-stressed Warfighters and improve work AICS use was associated with a decrease in exertional heat tolerance. Of note, immersing the arms in cold water may illness severity and associated medical treatment costs (6). cause reflex cutaneous vasoconstriction, which would MICROCLIMATE COOLING reduce peripheral blood flow and compromise cooling (5,23). That said, this may not affect hyperthermic individPersonal microclimate cooling systems refer to cooling uals, as a high heat strain greatly weakens the vasocongarments typically worn under protective clothing. Besides strictor response (10,16). Moreover, previous data suggest the high physical workloads and hot environmental that arm immersion in 108 C water did not induce significant conditions encountered during military operations, vasoconstriction (8). In normothermic individuals, however, Warfighters are also required to wear protective clothing cutaneous vasoconstriction readily occurs in response to or body armor that is heavy, impermeable, and insulative cold-water arm immersion (4). Therefore, arm immersion (22). Thus, not only more metabolic heat is generated but should be used during breaks in-between training also its dissipation is compromised (19). In such cases, activities, for instance, during the rest periods of military cooling garments can be used to promote the loss of body work-rest cycles. heat and reduce thermal strain. The efficacy of microcliThe U.S. Army Research Institute of Environmental mate cooling systems for reducing heat strain is very well Medicine is in the process of improving the design of an acknowledged (22). Arm Immersion Cooling System (AICS) for use by Several types of cooling garments using different systems Warfighters during military training. At present, proto(e.g., air-cooled, liquid-cooled, ice-packet vests, and phase types of the current version of the AICS have been change materials) have been developed. The choice of implemented in several military training units in the cooling modality must be an educated one, as no single United States (5). The AICS features a lightweight design cooling solution is optimal under all circumstances. Of the and can accommodate up to 6 Warfighters at one time. available systems, liquid-cooled garments are regarded as the Care should be taken when individuals with small cuts or most beneficial by the military due to the higher heat abrasions received during training use the AICS, as this transfer potential of circulating water compared with air or VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 |
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Novel Heat Mitigation for the Military ice and its general use compatibility within a contaminated environment (7). In liquid-cooled garments, a batteryoperated pump constantly circulates cool liquid over the skin through a network of tubes embedded within the garment. Heat from the skin is transferred to the cool liquid by conduction. The warmed liquid then returns to a cooler where it is chilled again. As such, skin temperature is kept low and the core-to-skin temperature gradient is maintained. This facilitates convective heat transfer from the core to the skin, thus lowering body heat content. Covering more body surface areas can increase the cooling effect of liquid-cooled garments (7,22), but the use of full-body cooling suits is not practical, as it restricts mobility. This is also true with garments covering the limbs (arms and thighs). As such, torso cooling vests afford a relatively large potential surface area for heat extraction (20–25% body surface area [BSA]) and represent the most practical location for body cooling on the battlefield. Liquid-cooled garments can also prolong work performance (22) (Figure 2). Liquid-cooling systems have been implemented for use in mounted Warfighters (22), but very few microclimate cooling systems have been fielded despite their widely demonstrated efficacy, in part because of the need to retrofit vehicles to accommodate and power the systems. No systems have been fielded for the dismounted Warfighter or foot soldier. The typical weight of a system used on the go (e.g., 2.8-kg system weight; 1.4-kg battery) imposes additional metabolic load demands. The noise signature and the limited battery life (#4 hours) also reduce their relevance during sustained military operations on the ground. The current method of continuous cooling is also not the most efficient practice, especially in situations of changing metabolic activity and hence cooling required by the wearer (7). The resulting cutaneous vasoconstriction impairs conductive heat transfer from the skin, reducing cooling efficiency and effectiveness of the system (3,26,32). Recent research has been undertaken to improve current liquid-cooling systems and increase their field application to dismounted Warfighters (3,26,31). The use of intermittent cooling incorporating skin temperature feedback can reduce power requirements by almost 50% (26), thus extending battery life and reducing the number of batteries carried. Novel tubing construction can also improve cooling efficiency by increasing surface area contact between the skin and cooling tubes, thus enhancing conductive heat transfer (7,22). The more widespread availability and use of personal microclimate cooling could improve work performance and reduce the risk of heat illness, particularly for some military occupational specialities more than others. For example, most heat illnesses occur in the infantry and gun crew (2) where power, weight, and noise signatures are constraining. However, the second highest incidence rate of heat casualties occurs in service and supply crews (2) where the same power, weight, and noise constraints are absent.
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CONCLUSIONS The conditions (environmental, metabolic, and clothing) that military personnel are routinely exposed to during military training and operations can severely impede normal heat dissipation, necessitating auxiliary cooling methods. The 3 cooling strategies reviewed in this article are effective in reducing heat strain, and may improve work tolerance and reduce heat illness risks in Warfighters during training/ operations in the heat. In the military training environment, ingestion of ice slurry and arm immersion cooling are practical strategies. Ice slurry can be ingested before and during exercise, whereas arm immersion cooling can only be administered during exercise following heat exposure. In the operational environment, existing microclimate cooling (i.e., liquid-cooled vests) can be implemented with properly retrofitted vehicles and has potential for use in many different military occupational specialties as an unmounted system. Each of these strategies is suitable for use in different scenarios. The choice of cooling strategy is therefore contingent on the requirements, circumstances, and constraints of the training and operational scenario.
ACKNOWLEDGMENTS The authors thank Ms. Beverly Tan and Ms. Elizabeth Caruso for technical and editorial assistance. The opinions or assertions contained herein are the private views of the authors and should not be construed as official or reflecting the views of the Army or the Department of Defense. Citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations.
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