Skin temperature feedback optimizes microclimate cooling.

INTRODUCTION: A novel pulsed cooling paradigm (PCskin) integrating mean skin temperature (Tsk) feedback was compared with constant cooling (CC) or time-activated pulsed cooling (PC). METHODS: Eight males exercised while wearing personal protective equipment (PPE) in a warm, dry environment (dry bulb temperature: 30 degrees C; dew-point temperature: 11 degrees C) in each of the tests. Treadmill exercise was performed (approximately 225 W x m(-2)) for 80 min. A liquid cooling garment (LCG) covered 72% of the body surface area. Core temperature (Tc), local skin temperatures, heart rate, inlet and outlet LCG perfusate temperatures, flow, and electrical power to the LCG and metabolic rate were measured during exercise. RESULTS: At 75 min of exercise Tsk was higher (33.9 +/- 0.2 degrees C) in PCskin, than in PC (33.1 +/- 0.5 degrees C) or CC (32.0 +/- 0.6 degrees C) and PC > CC. The changes in Tc and heart rate during the tests were not different. Tc at 75 min was not different among the cooling paradigms (37.6 +/- 0.3 degrees C in PCskin, 37.6 +/- 0.2 degrees C in PC and 37.6 +/- 0.2 degrees C in CC). Heart rate averaged 124 +/- 10 bpm in PCskin, 120 +/- 9 bpm in PC and 117 +/- 9 bpm in CC. Total body insulation (degrees C x W(-1) x m(-2)) was significantly reduced in PCskin (0.020 +/- 0.003) and PC (0.024 +/- 0.004) from CC (0.029 +/- 0.004). Electrical power in PCskin was reduced by 46% from CC and by 28% from PC. DISCUSSION/CONCLUSION: Real-time Tsk feedback to control cooling optimized LCG efficacy and reduced electrical power for cooling without significantly changing cardiovascular strain in exercising men wearing PPE.

Epidemiology of hospitalizations and deaths from heat illness in soldiers.

PURPOSE: Serious heat illness has received considerable recent attention due to catastrophic heat waves in the United States and Europe, the deaths of high-profile athletes, and military deployments. METHODS: This study documents heat illness hospitalizations and deaths for the U.S. Army from 1980 through 2002. Hospitalization data were obtained from the Total Army Injury Health Outcomes Database (TAIHOD) coded according to the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM). North Atlantic Treaty Organization Standardization Agreement codes were searched for heat injuries in an effort to detect cases that were not found during the ICD-9-CM search. RESULTS: Five-thousand two-hundred forty-six soldiers were hospitalized, and 37 died due to heat illness. Our results indicate: 1) approximately 60% reduction in hospitalization rates (fewer heat exhaustion cases) over the 22-yr period; 2) fivefold increase in heat stroke hospitalization rates (1.8 per 100,000 in 1980 to 14.5 per 100,000 in 2001); 3) heat stroke cases were associated with dehydration (17%), rhabdomyolysis (25%), and acute renal failure (13%); 4) lower hospitalizations rates among African and Hispanic Americans compared with Caucasians (incidence density ratio, 0.76 [95% confidence interval, 0.71-0.82]; 5) greater rates of hospitalizations and heat strokes among recruits from northern than southern states (incidence density ratio, 1.69 [95% confidence interval, 1.42-1.90]; and 6) greater rates of hospitalizations and heat strokes among women than men (incidence density ratio, 1.18 [95% confidence interval, 1.09-1.27]). CONCLUSIONS: Exertional heat illness continues to be a military problem during training and operations. Whereas the hospitalization rate of heat illness is declining, heat stroke has markedly increased.

Efficacy of intermittent, regional microclimate cooling.

The vasomotor response to cold may compromise the capacity for microclimate cooling (MCC) to reduce thermoregulatory strain. This study examined the hypothesis that intermittent, regional MCC (IRC) would abate this response and improve heat loss when compared with constant MCC (CC) during exercise heat stress. In addition, the relative effectiveness of four different IRC regimens was compared. Five heat-acclimated men attempted six experimental trials of treadmill walking ( approximately 225 W/m(2)) in a warm climate (dry bulb temperature = 30 degrees C, dewpoint temperature = 11 degrees C) while wearing chemical protective clothing (insulation = 2.1; moisture permeability = 0.32) with a water-perfused (21 degrees C) cooling undergarment. The six trials conducted were CC (continuous perfusion) of 72% body surface area (BSA), two IRC regimens cooling 36% BSA by using 2:2 (IRC(1)) or 4:4 (IRC(2)) min on-off perfusion ratios, two IRC regimens cooling 18% BSA by using 1:3 (IRC(3)) or 2:6 (IRC(4)) min on-off perfusion ratios, and a no cooling (NC) control. Compared with NC, CC significantly reduced changes in rectal temperature ( approximately 1.2 degrees C) and heart rate ( approximately 60 beats/min) (P < 0.05). The four IRC regimens all provided a similar reduction in exercise heat strain and were 164-215% more efficient than CC because of greater heat flux over a smaller BSA. These findings indicate that the IRC approach to MCC is a more efficient means of cooling when compared with CC paradigms and can improve MCC capacity by reducing power requirements.

Branched-chain amino acid supplementation and human performance when hypohydrated in the heat.

The serotonin system may contribute to reduced human performance when hypohydrated in the heat. This study determined whether branched-chain amino acid (BCAA) supplementation could sustain exercise and cognitive performance in the heat (40 degrees C dry bulb, 20% relative humidity) when hypohydrated by 4% of body mass. Seven heat-acclimated men completed two experimental trials, each consisting of one preparation and one test day. On day 1, a low-carbohydrate diet was eaten and subjects performed exhaustive cycling (morning) and treadmill exercise in the heat (afternoon) to lower muscle glycogen and achieve the desired hypohydration level. On day 2, subjects consumed an isocaloric BCAA and carbohydrate (BC) or carbohydrate-only drink during exercise. Experimental trials included 60 min of cycle ergometry (50% peak oxygen uptake) followed by a 30-min time trial in the heat. A cognitive test battery was completed before and after exercise, and blood samples were taken. BC produced a 2.5-fold increase (P < 0.05) in plasma BCAA and lowered (P < 0.05) the ratios of total tryptophan to BCAA and large neutral amino acid. Blood prolactin, glucose, lactate, and osmolality were not different between trials but increased over time. Cardiovascular and thermoregulatory data were also similar between trials. BC did not alter time-trial performance, cognitive performance, mood, perceived exertion, or perceived thermal comfort. We conclude that BCAA does not alter exercise or cognitive performance in the heat when subjects are hypohydrated.

Model of human thermoregulation for intermittent regional cooling.

INTRODUCTION: A prospective approach to save energy expenditure for a liquid cooling garment (LCG) system is to provide intermittent regional cooling (IRC) to the human body instead of continuous cooling. In order to gain insight into IRC mechanisms, a mathematical model was developed to simulate thermal interaction between the human and IRC. METHODS: Human thermoregulatory responses were simulated by a previously validated six-cylinder mathematical model. Two equations were derived from the energy balance principle to estimate LCG heat removal during ON (coolant circulation) and OFF (no coolant circulation) periods. The heat removal equations were incorporated into the boundary equations of the human model. The augmented model was used to predict human thermal responses under different IRC conditions. RESULTS AND CONCLUSIONS: The model was evaluated against experimental results with IRC in warm environments. The comparison demonstrated that the model predictions of the core temperature and mean skin temperature were reliable within root mean square deviations of +/- 0.10 degrees C and 0.44 degrees C, respectively. Simulation analysis showed that IRC has the potential to reduce power requirements. Modeling is an effective alternative to predict efficacy when actual responses cannot be attempted. A systematic approach, consisting of manikin measurements, physiological experiments, and mathematical modeling can expedite the research and development of LCG systems.

Heat strain reduction by ice-based and vapor compression liquid cooling systems with a toxic agent protective uniform.

BACKGROUND: The purpose of this study was to compare a vapor compression microclimate cooling system (MCC) and a personal ice cooling system (PIC) for their effectiveness in reducing physiological strain when used with cooling garments worn under the impermeable self-contained toxic environment protective outfit (STEPO). A second comparison was done between the use of total body (TOTAL) and hooded shirt-only (SHIRT) cooling garments with both the MCC and PIC systems. It was hypothesized that the cooling systems would be equally effective, and total body cooling would allow 4 h of physical work in the heat while wearing STEPO. METHODS: Eight subjects (six men, two women) attempted four experiments at 38 degrees C (100 degrees F), 30% rh, 0.9 m x sec(-1) wind, while wearing the STEPO. Subjects attempted 4 h of treadmill walking (rest/exercise cycles of 10/20 min) at a time-weighted metabolic rate of 303 +/- 50 W. RESULTS: Exposure time was not different between MCC and PIC, but exposure time was greater with TOTAL (131 +/- 66 min) than with SHIRT (83 +/- 27 min) for both cooling systems (p < 0.05). Cooling rate was not different between MCC and PIC, but cooling rate while wearing TOTAL (362 +/- 52 W) was greater than with SHIRT (281 +/- 48 W) (p < 0.05). Average heat storage was lower with MCC (39 +/- 20 W x m(-2)) than with PIC (50 +/- 17 W x m(-2)) in both TOTAL and SHIRT (p < 0.05). Also, average heat storage while wearing TOTAL (34 +/- 19 W x m(-2)) was less than with SHIRT (55 +/- 13 W x m(-2)) for both cooling systems (p < 0.05). The Physiological Strain Index (PSI) was lower in MCC-TOTAL (2.4) than MCC-SHIRT (3.7), PIC-SHIRT (3.8), and PIC-TOTAL (3.3) after 45 min of heat exposure (p < 0.05). CONCLUSIONS: Total body circulating liquid cooling was more effective than shirt-only cooling under the impermeable STEPO uniform, providing a greater cooling rate, allowing longer exposure time, and reducing the rate of heat storage. The MCC and PIC systems were equally effective during heat exposure, but neither system could extend exposure for the 4 h targeted time.

Effectiveness of revised fluid replacement guidelines for military training in hot weather.

BACKGROUND: This study compared the revised U.S. Army fluid replacement guidelines (REV) with the old guidelines (OLD) on daily changes in serum sodium concentration (Na+) and body mass (BM) during Basic Combat Training at Fort Benning, GA during two successive summers. METHODS: Recruits (n = 550; OLD = 277, REV = 273) were evaluated before and after 8-12 h of outdoor military combat training in hot weather. The WBGT (mean +/- SD) averaged 26.6 +/- 1.7 degrees C for OLD and 27.4 +/- 0.9 degrees C for REV (NS). RESULTS: Serum Na+ decreased from 137.5 +/- 1.6 mEq x L(-1) to 137.0 +/- 2.1 mEq x L(-1) after outdoor military training in OLD (p < 0.05). Twenty-two recruits (8%) had serum sodium fall to below 135 mEq x L(-1) during OLD. Serum Na+ increased from 139.0 +/- 1.7 mEq x L(-1) to 139.4 +/- 2.1 mEq x L(-1) after outdoor military training in REV (p < 0.05). Only two recruits (1%) had serum Na+ fall to below 135 mEq x L(-1) during REV. BM increased an average of 1.3 +/- 1.4 kg (p < 0.05) in OLD and an average of 0.4 +/- 1.7 kg in REV (p < 0.05). CONCLUSIONS: The revised guidelines effectively reversed the decrease in serum sodium, reduced the increase in body mass, maintained hydration and minimized overdrinking during hot weather military training compared with the old fluid replacement guidelines.

Thermal comfort and sensation in men wearing a cooling system controlled by skin temperature.

OBJECTIVE: The study was done to determine whether thermal comfort (TC), thermal sensation (TS), and subjective factors gauging environmental stress were negatively affected with different cooling methods in men exercising in chemical protective clothing. BACKGROUND: Previous studies have reported that intermittent regional cooling improved the efficacy of cooling as compared with constant cooling (CC), but no studies have addressed whether there is any improvement in thermal comfort. METHODS: Eight male volunteers exercised at moderate work intensity (425 W) in three microclimate cooling tests. The circulating fluid in the cooling garment was provided during exercise to the head (6% body surface area [BSA]), torso (22% BSA), and thighs (44% BSA) and manipulated under three methods: (a) CC, (b) pulsed cooling (PC), and (c) PC activated by mean skin temperature (T(sk)) control (PC(skin)). TC and TS ratings were recorded every 20 min during the 80-min test. RESULTS: TC and TS ratings were not different for PC(skin) and CC; thus the participants perceived PC(skin) as being similar to CC. TS was significantly warmer with PC than with PC(skin) and CC (p < .001). In PC(skin), T(sk) was significantly higher than in PC and CC (p < .001), and PC(skin) was rated as being not as warm as PC according to TS. CONCLUSION: This indicates that the PC(skin) method was perceived as being as cool as CC and cooler than PC. APPLICATION: These findings indicate that the PC(skin) cooling method is an acceptable alternative to CC and PC based on human perceptions.

Intermittent microclimate cooling during exercise-heat stress in US army chemical protective clothing.

The effectiveness of intermittent, microclimate cooling for men who worked in US Army chemical protective clothing (modified mission-oriented protective posture level 3; MOPP 3) was examined. The hypothesis was that intermittent cooling on a 2 min on-off schedule using a liquid cooling garment (LCG) covering 72% of the body surface area would reduce heat strain comparably to constant cooling. Four male subjects completed three experiments at 30 degrees C, 30% relative humidity wearing the LCG under the MOPP 3 during 80 min of treadmill walking at 224 +/- 5 W . m(-2). Water temperature to the LCG was held constant at 21 degrees C. The experiments were; 1) constant cooling (CC); 2) intermittent cooling at 2-min intervals (IC); 3) no cooling (NC). Core temperature increased (1.6 +/- 0.2 degrees C) in NC, which was greater than IC (0.5 +/- 0.2 degrees C) and CC (0.5 +/- 0.3 degrees C) ( p < 0.05). Mean skin temperature was higher during NC (36.1 +/- 0.4 degrees C) than IC (33.7 +/- 0.6 degrees C) and CC (32.6 +/- 0.6 degrees C) and mean skin temperature was higher during IC than CC ( p < 0.05). Mean heart rate during NC (139 +/- 9 b . min(-1)) was greater than IC (110 +/- 10 b . min(-1)) and CC (107 +/- 9 b . min(-1)) ( p < 0.05). Cooling by conduction (K) during NC (94 +/- 4 W . m(-2)) was lower than IC (142 +/- 7 W . m(-2)) and CC (146 +/- 4 W . m(-2)) ( p < 0.05). These findings suggest that IC provided a favourable skin to LCG gradient for heat dissipation by conduction and reduced heat strain comparable to CC during exercise-heat stress in chemical protective clothing.

Upper body cooling during exercise-heat stress wearing the improved toxicological agent protective system for HAZMAT operations.

This study compared endurance in a U.S. Army developmental Occupational Safety and Health Administration Level B personal protective equipment (PPE) system against the toxicological agent protective (TAP) suit, the Army's former standard PPE for Level A and Level B toxic environments. The developmental system consisted of two variations: the improved toxicological agent protective (ITAP) suit with self-contained breathing apparatus (ITAP-SCBA), weight 32 kg, and the ITAP with blower (ITAP-B), weight 21 kg. Both ITAP suits included the personal ice cooling system (PICS). TAP (weight 9.5 kg) had no cooling. It was hypothesized that PICS would effectively cool both ITAP configurations, and endurance in TAP would be limited by heat strain. Eight subjects (six men, two women) attempted three 2-hour treadmill walks (0.89 m/sec, 0% grade, rest/exercise cycles of 10/20 min) at 38 degrees C, 30% relative humidity. Metabolic rate for TAP (222+/-35 W) was significantly less than either ITAP-SCBA (278+/-27 W) or ITAP-B (262+/-24 W) (p<0.05). Endurance time was longer in ITAP-SCBA (85+/-20 min) and ITAP-B (87+/-25 min) than in TAP (46+/-10 min) (p<0.05). Heat storage was greater in TAP (77+/-15 W.m(-2)) than in ITAP-SCBA (51+/-16 W.m(-2)) (p<0.05), which was not different from ITAP-B (59+/-14 W.m(-2)). Sweating rate was greater in TAP (23.5+/-11.7 g/min(1)) than in either ITAP-SCBA (11.1+/-2.9 g/min) or ITAP-B (12.8+/-3.5 g/min) (p<0.05). Endurance in ITAP was nearly twice as long as in PPE with no cooling, even though the PICS, SCBA tanks, and new uniform itself all served to increase metabolic cost over that in TAP. PICS could also be used with civilian Levels A and B PPE increasing work time and worker safety.