Rectal Temperature Cooling Using 2 Cold-Water Immersion Preparation Strategies

When body core temperature exceeds 40.5°C (105°F) and central nervous system dysfunction occurs, exertional heatstroke (EHS) is a possibility.^1,2 Exertional heatstroke is a dangerous condition that has resulted in an average of 3 deaths per year since 1995, mainly in secondary school football players.^3,4 The morbidity and mortality of EHS are directly related to how long a person’s body temperature remains >40.5°C.^1,5,6

National best-practice EHS treatment recommendations include rectal temperature (T_REC) assessment to confirm an EHS diagnosis, followed by cold-water immersion (CWI) within 30 minutes of patient collapse.^1,2 Cold-water immersion is the criterion standard treatment for individuals with EHS because it has the fastest body temperature cooling rate of all cooling modalities as a result of its high heat capacity, thermal conductivity, and body surface area coverage.^1,7,8 Collectively, these factors effectively allow blood to return flow to the heart and reduce the hypermetabolic state of the internal organs.⁹ Importantly, when best practices are performed, EHS survival rates are 100%.⁵

The National Athletic Trainers’ Association (NATA)¹ recommended that patients with EHS be immersed in water between 1.7°C (35°F) and 15°C (59°F). Several research studies^7,8 have shown CWI cooling rates exceeding 0.20°C/min, but authors of many of these studies used circulated or temperature-controlled whirlpools. In the field, clinicians may not have access to circulating, temperature-controlled whirlpools. Instead, noncirculating stationary tubs, kiddie pools, or tarps are often used for CWI to treat EHS.^2,10

Among secondary school athletic trainers (ATs), 52% prepared immersion tubs before practice as part of their EHS policy and procedures.¹⁰ Consequently, it is possible that the water temperature in these baths rises beyond expert-recommended temperatures¹ for patients with EHS by the time they are needed, especially if exposed to hot, humid conditions and solar radiation for several hours. Anecdotally, many ATs immerse patients with EHS in the baths and plan to add ice in the event of an emergency. To our knowledge, no researchers have examined if adding ice to a water bath that is warmer than NATA water bath temperature recommendations¹ still results in cooling rates consistent with best practices.^5,11

The purposes of our study were 2-fold. First, we compared T_REC cooling rates of 2 water bath preparation strategies. The first strategy used water kept at a temperature of 10°C (no ice added; CON). Our second strategy (ICE) used water starting at 17°C, but 75.7 L (20 gal) of ice was added when the individual entered the bath. Next, we determined if ICE cooling rates met the definition of ideal (ie, >0.15°C/min) or acceptable (0.08°C/min to 0.15°C/min).¹¹ We hypothesized that ICE cooling rates would be slower than CON cooling rates and that both water bath preparation strategies would cool at ideal rates.¹¹

METHODS

Participants

A convenience sample of 16 healthy men and women were recruited for this study. Three participants discontinued testing because of heat intolerance and 1 discontinued because of CWI intolerance on the CON day. Therefore, 12 participants finished our study (Table 1). Individuals were not permitted to participate if they self-reported the following: (1) a cold allergy; (2) a previous physician–diagnosed gastrointestinal, cardiac, musculoskeletal, respiratory, or neurologic illness; (3) use of medications that affect thermoregulation, fluid balance, or blood pressure; (4) a history of serious heat illness in the 6 months before the study; (5) a sedentary lifestyle (exercising fewer than 3 times a week for 30 minutes)¹²; (6) an injury that prevented them from exercising; (7) a COVID-19 diagnosis within 14 days of testing; or (7) pregnancy. All women were tested in their follicular phase of menses (ie, first 14 days after the onset of menstruation). All procedures were approved by Central Michigan University’s institutional review board, and individuals provided written consent before participation.

Table 1 Participant Demographics and Hydration Information (N = 12)

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RESULTS

All participants self-reported compliance with testing instructions each day. They were well hydrated before exercise (t₁₁ = 0.61, P = .56; Table 1). They exercised for similar durations (Z₁₁ = 0.94, P = .38) and in similar environmental heat (t₁₁ = 1.6, P = .15) and relative humidity (t₁₁ = 1.0, P = .33; Table 2) each day.

Table 2 Exercise and Cooling Data^a

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Rectal temperatures were comparable between conditions during exercise, and all participants finished exercise when their T_REC reached 39.5°C (Figure 1). They cooled at ideal rates¹¹ in both conditions (Table 2), but no differences in cooling rates were observed between water bath preparation strategies (t₁₁ = 0.72, P = .49).

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Thermal sensation differed between conditions over time (F_3,33 = 3.8, P = .02; Figure 2). Both CON and ICE thermal sensations at pre-exercise differed from all other measurement times in their respective conditions. Thermal sensation was higher immediately postexercise compared with when T_REC was 38.75°C and postimmersion on each day. Postimmersion thermal sensation was also higher than when T_REC was 38.75°C for each condition (P < .05).

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The ESQ scores were similar between conditions over time (F_3,33 = 0.6, P = .62; Table 3), and we did not observe a main effect of water bath preparation condition (F_1,11 = 2.1, P = .18). However, we did note a main effect of time (F_3,33 = 30.1, P < .001). The ESQ scores at pre-exercise and postexercise were different than those at all other measurement times (P values < .05).

Table 3 Environmental Symptoms Questionnaire Responses (N = 12) With 2 Water Bath Preparation Strategies^a

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DISCUSSION

To our knowledge, we are the first to examine whether a secondary school water bath preparation strategy can lower T_REC at expert-recommended rates.¹¹ Our main findings were that T_REC cooling rates for both CON and ICE were excellent, met the definition of ideal,¹¹ and were not different. Encouragingly, the cooling rates we demonstrated were consistent with the cooling rates of patients with EHS who were treated with CWI and survived without sequelae of injury.⁵ Moreover, our participants were able to cool effectively without any clinically meaningful differences in thermal sensation or ESQ scores. These results suggest that time- or resource-burdened clinicians can successfully treat patients with EHS even if their water baths are exposed to prolonged periods of high heat and humidity so long as ice is added to the water bath at the same time as the patient.

Our ICE protocol was designed to emulate a scenario in which a clinician could not maintain the water bath temperature according to NATA recommendations¹ (1.7°C–15°C) until it was needed to treat an individual with EHS. Despite using a higher starting water temperature of 17°C with approximately 76 L (20 gal) of ice, the ICE protocol quickly lowered T_REC (0.21°C ± 0.12°C/min). This cooling rate was faster than that of other researchers^19–21 who performed temperate water immersion to treat hyperthermia. Proulx et al⁸ used circulating water baths at 8°C, 14°C, or 20°C to treat hyperthermic participants; the cooling rates were 0.19°C ± 0.07°C/min, 0.15°C ± 0.06°C/min, and 0.19°C ± 0.10°C/min, respectively. Miller et al¹⁹ used 21°C water to cool individuals with hyperthermia wearing an American football uniform. With the uniform donned during cooling, the cooling rate was 0.12°C ± 0.05°C/min; it was only slightly faster when the uniform was removed (0.13°C ± 0.05°C/min). When hyperthermic participants (T_REC = 40.1°C ± 0.67°C) were immersed in a 26°C water bath, they cooled at 0.10°C ± 0.02°C/min.²¹ Collectively, these data^8,19,21 suggest that even if the starting water bath temperatures are somewhat higher than those in our study, the cooling rates of patients with EHS will still likely meet the definition of acceptable or ideal,¹¹ especially if ice is added when the patient is immersed.

For 3 reasons, our ICE cooling rates were likely higher than those of other investigators^8,19,21 who used temperate water immersion. First, ice gradually lowered the water bath temperature, creating a greater thermal gradient and increasing the conductive heat exchange. Second, ice requires approximately 80 times more energy to melt (ie, the latent heat of melting) and undergo a phase change than it does to increase water temperature. Third, some authors^21,22 have argued for using temperate water immersion instead of CWI to reduce shivering and vasoconstriction of blood vessels in the skin. Reducing shivering and vasoconstriction would enhance heat exchange because of less shivering-induced thermogenesis, promoting a beneficial thermal gradient to allow heat exchange. Our data support this, as more people shivered in CON than in ICE. Although most participants who shivered in CON also shivered in ICE (n = 6), 3 individuals shivered only in CON. None of our participants shivered only in ICE. Moreover, shivering onset was delayed in ICE, which may have also contributed to the quick cooling rates observed in that condition. Importantly, shivering did not impair cooling effectiveness in any meaningful way, and shivering is considered only a minor factor in cooling because of the larger conductive and convective heat loss from the CWI.⁹ However, our shivering data should be interpreted cautiously, because of their self-reported and descriptive nature.

We also noted excellent cooling with CON. Clinicians capable of monitoring water baths and keeping them consistent with NATA recommendations¹ in hot conditions are to be commended. In several studies^8,15,23,24 and systematic reviews,^7,11 the researchers agreed that CWI resulted in some of the fastest T_REC cooling rates and has saved lives in real EHS scenarios.^5,25 For example, patients with EHS who had an average initial T_REC of 41.11°C ± 0.63°C cooled at rates of 0.22°C ± 0.11°C/min when immersed in ice and water. Therefore, clinicians who have access to ice and the resources to provide CWI according to the NATA’s recommendations for water temperature¹ will cool these individuals quickly.

Our study had several additional strengths. We used standardized water and ice volumes and temperatures on each day of testing with little variability in preimmersion bath temperatures. We also recorded participants’ perceptions of thermal sensation and ESQ scores. Both subjective indicators were important for understanding how participants felt before, during, and after exercise and CWI. The thermal sensation and ESQ data suggested that they felt similarly in both CON and ICE during water immersion and postcooling. Thus, we infer that the body encountered no additional stress when ice was added to the water bath at the time of immersion and that ICE was experienced in much the same way as in cold water without ice.

We recognize the limitations of our work. First, for safety reasons, we were not able to study the effects of ICE protocol on participants with higher body temperatures or individuals with impaired central nervous systems. Second, we were not able to simulate the radiative effects of the sun on our water bath when determining the ICE starting water bath temperature. However, we used weather data from a state that is a location with many cases of EHS (ie, Florida)^26,27 to see how the temperature and humidity conditions affected the water bath temperature. Third, we did not measure the skin temperature of participants during exercise and cooling, which prevented us from performing heat balance and exchange estimates during each condition. Yet all individuals completed the exercise in similar environmental conditions with similar exercise durations and to the same internal body core temperature. Finally, we used the rough volume indicators inside our 10-gal coolers to determine how much ice to add for the ICE condition. Even though we used crushed ice, air was likely present between the ice cubes. Hence, the volume of water in the ICE stationary tub may have been slightly less than that in CON.

CONCLUSIONS

Despite using water that started 7°C warmer than CON, our ICE water-bath preparation strategy cooled participants with hyperthermia at ideal cooling rates¹¹ for patients with EHS. If clinicians lack the resources to maintain water bath temperature consistent with NATA guidelines,¹ they can add ice to the water while the patient is immersed. The added ice will increase the thermal gradient between the bath and patient while also absorbing more energy than water alone. Future authors should examine whether adding ice to even warmer water (>17°C) can still elicit ideal cooling rates of hyperthermic individuals.