Body Bag Cooling with Two Different Water Temperatures for the Treatment of Hyperthermia

METHODS

A randomized (order of testing), crossover, counterbalanced, repeated measures design guided data collection in this study. The independent variables were water temperature (Ten or Fifteen) and time (factor levels varied according to the dependent variable). We chose Ten and Fifteen for our water temperatures to emulate a situation where a clinician lacked access to large quantities of ice and had to use cold water rather than ice water in the body bag. Ten and Fifteen met this criteria while also still falling within professional recommendations for water temperature for EHS victims.²

The dependent variables were T_REC cooling rates and T_REC nadir. T_REC was measured every 5 min during exercise, every 0.5 min during cooling, and every 5 min during recovery. We also measured environmental chamber temperature and relative humidity, pre-exercise hydration status, and exercise duration for consistency between testing days.

Subjects

Sample size was estimated a priori using the following assumptions: an alpha value of 0.05, a difference in cooling rate of 0.10°C/min, 80% power, and a standard deviation of 0.06°C/min. Based on these assumptions, we needed 10 subjects to observe statistically significant differences in cooling rates.

We tested a convenience sample of 13 healthy, physically active, college-age men and women. One subject discontinued testing due to the difficulty of the exercise protocol. A total of 12 subjects completed testing (subject demographics can be found in Table I). Individuals were excluded from participating if they self-reported: 1) an injury or illness which impaired their ability to exercise; 2) any neurological, respiratory, gastrointestinal, esophageal, or cardiovascular diseases; 3) taking any medications that may have affected fluid balance or temperature regulation; 4) a sedentary lifestyle (defined as exercising <30 min three times per week)²⁴; 5) a history of heat-related illness in the 6 mo preceding data collection; 6) current pregnancy; 7) cold allergy; or 8) positive COVID test result within 14 d of testing days. All women were tested within the follicular phase of their menstrual cycle (i.e., first 14 d after the onset of menstruation) and none of the women reported taking a hormone-based birth control. All subjects signed a written informed consent prior to testing and all procedures were approved by Central Michigan University Institutional Review Board.

Table I.Subject Demographics and Hydration Information.

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Procedures

Procedures for this study were similar to our prior study.¹³ Subjects reported for 2 d of testing between 08:00 and 17:00. Subjects were instructed to abstain from exercise (24 h) and stimulants (e.g., caffeine) and depressants (e.g., alcohol) for at least 12 h. They were instructed to drink water regularly throughout the day preceding testing to ensure their urine was clear or light yellow. Compliance with these instructions was self-reported prior to testing.

Approximately 45 min prior to the subjects’ arrival, we began filling six 37.8-L (10-gal) water coolers. Due to the specificity of each water temperature in this study, we mixed ice and tap water as necessary until the water in the middle of the cooler was ∼9.8°C or ∼14.8°C since we anticipated the water would warm slightly while the participant exercised. No ice was visible in the coolers on each day once preparation was completed. Each cooler lid was numbered so we could average the temperatures of the water from the coolers used during treatment in the event we did not use the water from all six coolers for a participant.

Upon participants’ arrival, they voided their bladders completely and a spot urine-specific gravity assessed hydration status (SUR-Ne refractometer, Atago USA Inc., Bellevue, WA, United States). If urine specific gravity indicated subjects were hypohydrated (i.e., >1.020),²¹ subjects consumed ∼1 L of water and urine specific gravity was reassessed ∼45 min later. If subjects were still hypohydrated, they were rescheduled. If euhydrated, subjects were weighed nude (Defender #5000, Ohaus Corp, Parsippany, NJ, United States). Then they dressed in undergarments (sports bras also for women), shorts, socks, and t-shirts. We measured skinfolds at the chest, abdomen, and thigh (men) and the triceps brachii, abdomen, and thigh (women) in triplicate per Pollack, Schmidt, and Jackson¹⁹ (baseline skinfold caliper #12-1110, Fabricated Enterprises, Inc, White Plains, NY, United States). Skinfolds were averaged at each site and summed to estimate body density⁸ and percent body fat.²² Body surface area was estimated using Dubois and Dubois’s equation.⁴

Subjects donned a heart rate monitor (Polar Electro, Inc, Lake Success, NY, United States) and self-inserted a rectal thermistor (#401, Advanced Industrial Systems; Prospect, KY, United States) 15 cm past the anal sphincter.¹⁵ They entered an environmental chamber and we recorded the environmental temperature and humidity (Kestrel Heat Stress Tracker #4400, Nielsen-Kellerman, Boothwyn, PA, United States). T_REC was recorded and they stood on a treadmill for 10 min to acclimate to the heat. Subjects performed an exercise protocol on a treadmill consisting of walking for 3 min at 3 mph and running at approximately 90% of their age-predicted maximum heart rate for 2 min (0% incline). When subjects’ T_REC reached ∼38.2°C, an assistant stirred the water in each cooler and measured the water temperatures by placing a #401 thermistor approximately halfway (30.5 cm; 12 in) in the center of the cooler. Then, we moved the coolers from our main laboratory (∼22°C) inside the environmental chamber.

Once subjects’ T_REC reached 39.5°C, they removed their shoes and lay supine inside the body bag (Polar Life Pod^®, Polar Products, Inc.; Fig. 1). We purposefully used water temperatures at the higher end of the manufacturers⁶ and expert recommendations² and we did not follow the manufacturer recommendation⁶ to add ice or colder water to the body bag if water temperature exceeded 15°C over the course of treatment to test our hypothesis. For shorter subjects, we folded the end of the body bag closest to the subjects’ feet to minimize water accumulation at the end of the unit. One investigator poured the prepared water into the body bag so subjects’ torso, arms, legs, and neck were covered. Subjects’ heads rested on a pillow included with the unit to ensure airway patency during cooling. A separate #401 thermistor was placed next to the subjects’ neck into the water so we could monitor the water temperature in the body bag during cooling. The body bag’s zipper was closed and the straps were secured. We recorded the volume of water initially added. The body bag was shaken continuously side-to-side during cooling. The body bag water temperature was also monitored and recorded once subjects’ T_REC reached 38°C.

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T_REC was recorded every 0.5 min during cooling. A standard stopwatch was started when we began pouring water on top of subjects. The stopwatch was stopped when subjects’ T_REC reached 38°C. Cooling rates were calculated by taking the difference in body temperatures from the end of exercise to the end of treatment and dividing it by the amount of time necessary to reduce T_REC to 38°C.

Subjects self-reported shivering onset during cooling so we could ascertain if shivering-induced thermogenesis affected cooling duration. Once T_REC was 38°C, subjects exited the body bag and towel dried their arms and legs. They sat in the environmental chamber for 10 min to recover and we recorded environmental conditions. After recovery, subjects exited the chamber, removed the rectal thermistor, towel dried, were weighed nude a second time, and excused.

No fluids were given to subjects once they entered the environmental chamber. Subjects completed their second testing day at approximately the same time of day (± 3 h) and at least 48 h after the first testing day.

RESULTS

All subjects self-reported compliance with testing instructions each day. Subjects were euhydrated before exercise [t₍₁₁₎ = 0.63, P = 0.54, Table I] and exercised for similar durations [t₍₁₁₎ = 0.16, P = 0.44, Table II]. Environmental chamber temperature [t₍₁₁₎ = 0.76, P = 0.46] and humidity [t₍₁₁₎ = 1.1, P = 0.30] were similar between days (Table II).

Table II.Exercise and Cooling Data.

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Subjects’ T_REC were consistent during exercise each day and everyone discontinued exercise when T_REC was 39.5°C (Fig. 2, P > 0.05). However, T_REC cooling rates [t₍₁₁₎ = 2.0, P = 0.03, Table II] and T_REC nadir differed between water temperatures [t₍₁₁₎ = 2.6, P = 0.01, Fig. 2, Table II].

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DISCUSSION

This is the second study we completed examining the effectiveness of body bag cooling as a tool for treating hyperthermia. In our first study,¹³ we observed using 202.7 ± 23.8 L of 3.2°C ice water in a body bag quickly cooled subjects (0.28 ± 0.09°C/min). However, our original study presumed clinicians had access to modest quantities of ice (24 gal, 91 L) to achieve ice-water temperatures. In the current study, we intentionally filled the body bag with water at temperatures at the higher end of the manufacturer⁶ and professional recommendations² to determine if body bag cooling would meet expert¹² recommendations for ideal or acceptable cooling. Our main observation was body bag cooling was still able to meet expert recommendations for ideal cooling with Ten and acceptable cooling with Fifteen.¹² Clinically, this means clinicians who do not have access to large quantities of ice or only have access to cold tap water can still use body bag cooling effectively so long as the water temperatures fall within official recommendations.² These results are encouraging because they indicate body bags can be an effective tool for treating hyperthermia, and possibly EHS, even when optimal parameters for its use are not present.

The body bag cooling rates in the current study were comparable to or exceeded the average cooling rates of several other studies examining portable CWI techniques. When authors used 2.1°C and 9°C water with TACO, they observed cooling rates of 0.14°C/min and 0.17°C/min.^7,11 More recently, Klous et al.¹⁰ used 80 L (21 gal) of 27.2°C water and TACO and noted acceptable cooling of 0.12 ± 0.03°C/min. When unknown volumes of ice and water were put in a medical body bag up to a patient’s midaxillary line, emergency room physicians observed an oral temperature cooling rate of 0.16°C/min.⁹

We acknowledge our current and prior¹³ body bag results are significantly faster than Nye et al.,¹⁸ who observed body bag cooling rates of 0.04 ± 0.08°C/min. The slow cooling rates in their study¹⁸ were likely due to subjects being only mildly hyperthermic (T_REC averaged 38.4°C), the potential lack of convective cooling during treatment, and water likely accumulating at the end of the unit due to a failure to adjust the size of the body bag based on subjects’ height. Regardless, the current data and those from others^7,9,11 suggest clinicians have several effective portable CWI tools available to treat EHS. Therefore, clinicians should consider athlete size, number of people available to help treat EHS victims, tool cost, and terrain when designing heat illness policy and procedure documents if these portable CWI tools are going to be used to treat EHS.

We believe the differences in cooling rates between portable CWI studies can be explained primarily by experimental differences in water volume and water temperature. Water temperature is one of the primary factors affecting cooling rates of hyperthermic individuals and EHS patients.^20,25 Proulx et al.²⁰ observed some of the fastest T_REC cooling rates reported in the literature when hyperthermic subjects (T_REC = 40°C) were immersed up to their clavicles in 2°C water (0.35 ± 0.14°C/min). When warmer water temperatures of 8°C, 14°C, and 20°C were used, cooling rates slowed by 46% (0.19 ± 0.07°C/min), 57% (0.15 ± 0.06°C/min), and 46% (0.19 ± 0.10°C/min), respectively. Similarly, Miller et al.¹⁷ observed an average cooling rate of 0.12 ± 0.05°C/min when subjects were immersed up to the neck in 21°C water. Taylor et al.²³ also observed temperate water (26°C) was capable of cooling subjects within acceptable ranges (0.10 ± 0.02°C/min). In the current study, because we did not replace or add new, colder water or ice during cooling, we observed substantial increases in water temperature within the body bag of 3.72°C (Ten) and 3.42°C (Fifteen) by the time subjects finished cooling. Even with these temperature increases, the body bag was still capable of holding enough cold water to cool at acceptable rates.

Water volume and, in turn, body surface area covered during cooling are two other important factors affecting cooling rates.¹ One advantage of body bags is water can cover the chest and most of the body (minus the face) because they can be closed around the patient. This is advantageous since it ensures greater body surface area coverage. In two of the TACO studies, authors used 30–40 gal (113 L to 151 L) of water to treat hyperthermic subjects. The smaller water volumes and fact that water often accumulates over the torso rather than fully covering the entire body with TACO is one consideration clinicians need to factor in when considering which portable tool to use. Conversely, body bags are able to accommodate about 40% more water while covering the entire body. One concern, however, is water can leak from the bag during treatment, especially from the area around the head. Regardless, having more water is desirable because, as hyperthermic subjects cool, the water inside the body bag will warm and the thermal gradient between the body and water will be reduced, leading to slower cooling rates. This explains the ∼3.5°C increase in water temperatures at the end of cooling in the body bag. We intentionally did not add any new, colder water or ice to the body bag once we began immersion to test the efficacy of the body bag with several constraints. However, if clinicians followed the manufacturer recommendation⁶ to add ice or colder water if the water in the body bag exceeded 15°C, it is likely the thermal gradient would be maintained or enhanced and the cooling rates could exceed those reported in this study.

A secondary aim in this study was to determine if T_REC nadir could be improved with warmer water to reduce the risk of overcooling and hypothermia. While T_REC nadir in Ten and Fifteen were statistically different in this study, the minor difference of 0.2°C is unlikely to be clinically meaningful. However, T_REC nadir was ∼0.5°C higher than our first body bag study, which used ice water (∼3°C). Moreover, shivering onset was delayed 3–6 min by using Ten and Fifteen compared to our first study.¹³ Consequently, using warmer water in the body bag allowed for less afterdrop and was tolerated much more effectively than ice water. Thus, it is likely fewer rewarming efforts or cold-water shock responses would occur if clinicians use Ten or Fifteen in a body bag.

We acknowledge our study’s limitations. First, our subjects likely had normal thermoregulatory capabilities because none of our subjects experienced EHS. This is a common limitation of hyperthermia studies done within the context of a university laboratory setting. Studying the effect of body bag cooling in patients with EHS is necessary in the future to confirm these results. Second, the volume of water poured into the body bag for each participant varied since some water leaked out of the system, mostly near the head and rectal thermometer port, during cooling. Thus, reported immersion volumes must be considered rough estimates. Third, subjects were not immediately immersed in the body bag each day. We estimate it took ∼2 min to pour the water onto subjects, close the zipper, and secure the straps of the body bag device. Finally, we only measured T_REC for 10 min post-immersion. It is possible T_REC nadir would have been lower had the recovery period been longer. However, the cooling rates from 5 min to 10 min post-immersion in both conditions were <0.03°C/min. Consequently, we do not feel this limitation changes our clinical interpretation of the data.

In conclusion, using Ten or Fifteen in the body bag produced acceptable to ideal T_REC cooling rates¹² and reduced T_REC afterdrop. Consequently, clinicians with limited access to ice or ice-cold water can still use body bags to treat hyperthermia. However, the fastest cooling rates will occur by following professional recommendations² and using the coldest water. Overall, body bags may be another effective tool, like TACO,^7,10,11 for clinicians to consider when developing their heat illness policy and procedures. Future research on the effectiveness of body bags in treating EHS is still required, but these results and those of others^9,13 show promise that body bag cooling could be another life-saving technique to combat EHS mortality.