Skin Temperature Feedback Increases Thermoregulatory Efficiency and Decreases Required Microclimate Cooling Power

Personal protective equipment (PPE) markedly increases heat strain, reduces work performance, and increases the incidence of heat casualties. Microclimate cooling (MCC) technologies have been successfully used to alleviate this heat strain in mounted soldiers, but cooling limitations, power and weight restrictions do not currently make this technology applicable to dismounted soldiers. This composite of studies investigated the potential for intermittent-regional cooling and skin temperature feedback approaches to better enable MCC systems for the dismounted soldier. PURPOSE: The purposes of this study were to 1) determine, using a variety of intermittent cooling paradigms, the optimal skin temperature for maximizing thermoregulatory efficiency, and 2) examine the potential power savings associated with using biofeedback to maintain optimal skin temperature. METHODS: Two studies were conducted using the same facilities and test equipment. In study one, 5 male soldiers exercised moderately (~500W) in a warm environment (30°C, 30%rh) while wearing PPE (clo: 2.1; im/clo: .32) over a water-perfused (21oC) liquid MCC garment covering the head, chest, back, and legs (72% of body surface area, BSA). All four body regions were independently controlled. A matrix of six randomized trials was conducted in which conventional MCC (constant perfusion, CP), no MCC (NC), or 4 trials of intermittent and regional (IR1-4) MCC was provided. IR1-4 was time-activated and on:off cooling ratios and the % BSA cooled were systematically varied. In study two, 8 male soldiers were subjected to the same conditions as study one, but only three trials were performed to include CP, IR2 (2 min on: 2 min off, 72% BSA), and skin temperature feedback (STF, 72% BSA) using a skin temperature range of 33-35°C. Heart rate (HR), body core (Tc) and skin temperatures (Tsk) were measured at regular intervals in both studies. RESULTS: In study one, all IR1-4 paradigms significantly reduced physiological strain compared with NC (P<0.05) and were similar to CP (P>0.05). In CP, Tsk was lowered to ~32°C and tissue insulation increased. In NC, Tsk rose quickly to 36oC and HR increased exponentially. In IR1-4, Tsk fluctuated between 33-35oC, which improved thermoregulatory efficiency by maintaining heat flux similar to CP over a smaller average BSA (18 or 36%). The variety of time activated on:off ratios made little difference to the results. In study two, IR2 and STF again reduced physiological strain similar to CP (P>0.05), but the power required for STF was lowest (122±18 W), followed by IR2 (169±16 W), and CP (224±15 W) (P<0.05, successive). CONCLUSION: The use of STF to maintain Tsk between 33-35oC improves thermoregulatory efficiency and decreases MCC power requirements by 45%. This potential breakthrough has direct application for the dismounted soldier, Objective Force Warrior, and Homeland Defense. Authors’ views; not official U.S. Army or DoD policy. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE OCT 2009 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Skin Temperature Feedback Increases Thermoregulatory Efficiency and Decreases Required Microclimate Cooling Power 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) U.S. Army Research Institute of Environmental Medicine Thermal and Mountain Medicine Division Kansas Street, Building 42 Natick, MA 01760-5007 8. PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release, distribution unlimited 13. SUPPLEMENTARY NOTES See also ADA562561. RTO-MP-HFM-181 Human Performance Enhancement for NATO Military Operations (Science, Technology and Ethics) (Amelioration des performances humaines dans les operations militaires de l’OTAN (Science, Technologie et Ethique)). RTO Human Factors and Medicine Panel (HFM) Symposium held in Sofia, Bulgaria, on 5-7 October 2009., The original document contains color images. 14. ABSTRACT Personal protective equipment (PPE) markedly increases heat strain, reduces work performance, and increases the incidence of heat casualties. Microclimate cooling (MCC) technologies have been successfully used to alleviate this heat strain in mounted soldiers, but cooling limitations, power and weight restrictions do not currently make this technology applicable to dismounted soldiers. This composite of studies investigated the potential for intermittent-regional cooling and skin temperature feedback approaches to better enable MCC systems for the dismounted soldier. PURPOSE: The purposes of this study were to 1) determine, using a variety of intermittent cooling paradigms, the optimal skin temperature for maximizing thermoregulatory efficiency, and 2) examine the potential power savings associated with using biofeedback to maintain optimal skin temperature. METHODS: Two studies were conducted using the same facilities and test equipment. In study one, 5 male soldiers exercised moderately (~500W) in a warm environment (30°C, 30%rh) while wearing PPE (clo: 2.1; im/clo: .32) over a water-perfused (21oC) liquid MCC garment covering the head, chest, back, and legs (72% of body surface area, BSA). All four body regions were independently controlled. A matrix of six randomized trials was conducted in which conventional MCC (constant perfusion, CP), no MCC (NC), or 4 trials of intermittent and regional (IR1-4) MCC was provided. IR1-4 was time-activated and on:off cooling ratios and the % BSA cooled were systematically varied. In study two, 8 male soldiers were subjected to the same conditions as study one, but only three trials were performed to include CP, IR2 (2 min on: 2 min off, 72% BSA), and skin temperature feedback (STF, 72% BSA) using a skin temperature range of 33-35°C. Heart rate (HR), body core (Tc) and skin temperatures (Tsk) were measured at regular intervals in both studies. RESULTS: In study one, all IR1-4 paradigms significantly reduced physiological strain compared with NC (P<0.05) and were similar to CP (P>0.05). In CP, Tsk was lowered to ~32°C and tissue insulation increased. In NC, Tsk rose quickly to 36oC and HR increased exponentially. In IR1-4, Tsk fluctuated between 33-35oC, which improved thermoregulatory efficiency by maintaining heat flux similar to CP over a smaller average BSA (18 or 36%). The variety of time activated on:off ratios made little difference to the results. In study two, IR2 and STF again reduced physiological strain similar to CP (P>0.05), but the power required for STF was lowest (122±18 W), followed by IR2 (169±16 W), and CP (224±15 W) (P<0.05, successive). CONCLUSION: The use of STF to maintain Tsk between 33-35oC improves thermoregulatory efficiency and decreases MCC power requirements by 45%. This potential breakthrough has direct application for the dismounted soldier, Objective Force Warrior, and Homeland Defense. Authors views; not official U.S. Army or DoD policy. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT SAR 18. NUMBER OF PAGES 8 19a. NAME OF RESPONSIBLE PERSON a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 Skin Temperature Feedback Increases Thermoregulatory Efficiency and Decreases Required Microclimate Cooling Power 6 2 RTO-MP-HFM-181 1.0 INTRODUCTION Personal protective equipment (PPE) markedly increases heat strain, reduces work performance, and increases the incidence of heat casualties. Microclimate cooling (MCC) technologies have been successfully used to alleviate heat strain and sustain performance [4] (Figure 1) in mounted soldiers, but cooling limitations, power and weight restrictions do not currently make this technology applicable to dismounted soldiers. Not only does the provision of substantial MCC require a large power supply, but conventional (i.e., continuous) MCC approaches can result in constriction of the cutaneous vasculature. Overcooling increases tissue insulation (It), decreases convective heat transfer from the body core, and reduces the MCC garment – to – skin (Tsk) gradient, thus theoretically reducing MCC operating efficiency [1]. This is particularly true when the thermal demands of the wearer are not constant. Automated control of MCC garments using various biofeedback signals have been investigated [2] [3] for their potential to improve the practicality of MCC relative to changing needs in real-time, but no comparison of an automated approach to simpler and more conventional solutions for reducing heat strain had been attempted. A series of studies [1] [5] [6] [7] were undertaken to compare the bio-thermal responses to automated and non-automated MCC solutions for reducing heat strain. Particular interest was in understanding the potential for automated MCC to reduce the combined power and size of MCC systems for the dismounted soldier. The purposes of this research were to 1) use a variety of intermittent cooling paradigms in humans combined with modelling simulations to determine the optimal Tsk for maximizing thermoregulatory efficiency, and 2) examine the potential power savings associated with using biofeedback to maintain an optimal Tsk temperature range.