ABSTRACT
Background: The procedure of wrapping a heat casualty in ice-water soaked bed sheets to reduce core temperature has received little investigation, despite the practice and recommendation for its use in some military settings. Thus, the purpose of this study was to investigate the cooling efficacy of ice-sheet cooling (ISC) following exertional hyperthermia. Methods: 13 (11 males, 2 females) participants (age = 23 ± 3 years, height = 176.5 ± 10.3 cm, mass = 78.6 ± 15.3 kg, body fat = 19.6 ± 8.6%, and body surface area = 1.95 ± 0.22 m2) volunteered to complete 2 randomized, crossover design trials on an outdoor recreation field (34.4 ± 1.4°C, 54.4 ± 4.1% relative humidity). Each trial consisted of exercise (self-paced 400-m warm-up, 1,609-m run, and 100-m sprints) followed by 15 minutes of either lying supine in the shade with no treatment (control [CON]) or being treated with ice-water soaked sheets wrapped around their body (ISC). Physiological (rectal temperature [Tre], heart rate, mean-weighted skin temperature) and perceptual measures (thermal sensation, rating of perceived exertion) were assessed after each exercise protocol, every 3 minutes during treatment, and every 5 minutes during recovery. Findings: By design, there were no differences during exercise between ISC and CON for Tre (p = 0.16), skin temperature (p = 0.52), heart rate (p = 0.62), thermal sensation (p = 0.89), or rating of perceived exertion (p = 0.99). There were greater decreases in Tre at 3 (ISC 0.33 ± 0.26°C vs. CON 0.03 ± 0.30°C, p = 0.01) and 6 minutes (ISC 0.47 ± 0.27°C vs. CON 0.30 ± 0.19°C, p = 0.05) of treatment; however, the overall rate of cooling was not different between trials (CON 0.05 ± 0.02°C/min vs. ISC 0.06 ± 0.02°C/min, p = 0.72). Skin temperature (Tsk) was significantly reduced from 3 minutes (ISC 34.4 ± 1.7°C vs. CON 36.6 ± 0.5°C, p = 0.007) through 15 minutes (ISC 32.4 ± 1.5 vs. CON 36.1 ± 0.4°C, p < 0.001) of treatment. There was a trend for lower heart rate with ISC (p = 0.051). Thermal sensation was reduced from 3 minutes of treatment (ISC 3.5 ± 0.9 vs. CON 4.5 ± 0.6, p = 0.002) through 15 minutes (ISC 2.8 ± 1.0 vs. CON 3.9 ± 0.4, p = 0.005). Discussion: ISC does not provide effective reduction in Tre following exertional hyperthermia compared to no treatment. However, perceptual benefits may warrant the use of ISC in settings where rapid reductions in core temperature are not a concern (i.e., recovery from exercise). Thus, clinicians should continue to utilize validated techniques (i.e., cold-water immersion) for the treatment of exertional heat illnesses.
BACKGROUND
Exertional heat illness threatens those conducting exercise in thermally straining environments (e.g., athletes, military, and firefighters). In 2015, there were 1,933 military members treated for heat-related injuries with 417 cases of heat stroke, most of which occurred in the southeastern region of the United States where ambient temperatures and humidity tend to be elevated.1 Exertional heat illnesses occur on a severity continuum, ranging from heat cramps, to exertional heat stroke, which is characterized by a rectal temperature (Tre) >40°C and central nervous system dysfunction.2 The rapid recognition and treatment of exertional heat stroke is vital to survival and the prevention of sequela.
The current criterion standard and recommended treatment for exertional heat stroke is whole body cold-water immersion, producing the fastest cooling rates (i.e., reductions in core temperature), thus providing the greatest potential to reduce core temperature below critical levels.2,–5 Furthermore, if implemented correctly, utilization of this technique has been documented to have a 100% survival rate.6,7 Despite these reputable results with cold-water immersion, the feasibility of this modality may be limited as a result of availability of an immersion tub and ice water in field or combat settings.
Validation of alternative cooling procedures to cold-water immersion is vital to afford the best possible treatment for victims of exertional heat stroke. One such method, often used in military settings, involves wrapping an individual in an ice-water soaked sheet, ensuring to replace the sheet once it becomes warm. Despite the use of the ice-sheet cooling (ISC) method, there is a lack of literature assessing the efficacy of this technique.8,–12 Ferris et al13 reported that heat stroke patients survived and recovered following treatment with ISC when Tre were <41.1°C, though fatalities occurred in those treated with initial temperatures >41.1°C. However, these were predominately older individuals suffering from classic heat stroke induced during a heat wave, thus the applicability to exertional heat stroke is limited. Sonna et al9 reported the use of ISC for the treatment of exertional heat injury in four U.S. marine recruits, though cooling rates were not provided. Armstrong et al14 compared treatment of distance runners by cold-water immersion or wrapping their torso and extremities in wet ice towels, demonstrating faster cooling with immersion (0.20°C/min) compared to ice towels (0.11°C/min). DeMartini et al15 rotated seven ice towels soaked in 14°C water across the torso, extremities, head, and neck, reporting reductions in Tre of 0.58°C after 10 minutes. The authors only rotated the ice towels every 5 minutes, possibly limiting the cooling capacity of this technique. The perceptual and heart rate responses with ice towels were significantly improved, therefore, ice towels may be beneficial for cardiovascular and perceptual recovery.15 Cooling with ice towels has demonstrated varied success,14,–16 yet these findings are not directly comparable to ISC given the differences in material and body surface area covered. Thus, given the lack of literature related to the effectiveness of ISC in the treatment of exertional heat stroke, the purpose of this study was to investigate the cooling efficacy of ISC following exertional hyperthermia.
METHODS
Participants
Thirteen (11 male, 2 female) participants (age = 23 ± 3 years, height = 176.5 ± 10.3 cm, mass = 78.6 ± 15.3 kg, body fat = 19.6 ± 8.6%, and body surface area = 1.95 ± 0.22 m2) participated in two randomized, counter-balanced, crossover trials separated by 3 days. All trials were completed outdoors on recreation fields in the heat (34.2°C, 54.9% relative humidity on day 1; 34.7°C, 53.5% relative humidity on day 2). The University of Arkansas Institutional Review Board approved all procedures and informed consent was obtained from all individuals before participation. All participants were physically active (at least three times per week for at least 30 minutes), had no history of chronic disease or illness, were not suffering from injury or current illness, and had not experienced heat exhaustion or exertional heat stroke in the past 3 years. All participants were advised to refrain from alcohol use and exercise for 24 hours, and caffeine use for 12 hours before each trial. For 24 hours before each trial, participants recorded their food intake on a standard diet log and were instructed to consume an additional 473 mL of fluid the night before and morning of their trials to ensure arrival in a euhydrated state. During the second visit, three-site skinfolds were collected before activity to estimate body density17,18 and subsequently body fat percentage.19 Body surface area was also calculated via Du Bois and Du Bois.20
Instrumentation
Tre was assessed using a rectal thermocouple inserted at least 15 cm past the anal sphincter (Physitemp Instruments Inc, Clifton, New Jersey). Heart rate was measured using a heart rate monitor (Timex Run Trainer 2.0, Middlebury, Connecticut) and mean weighted Tsk was collected by placing thermochrons (iButton, Maxim Integrated, San Jose, California) on the right anterior thigh, lateral calf, upper arm, and chest.21 Participants provided a urine sample to assess hydration status using urine specific gravity via a handheld refractometer (Master-SUR/NM, Atago, Japan). If an individual presented with suboptimal hydration (urine specific gravity >1.020),22 they were provided fluid until proper hydration was attained. Perceptual responses to the exercise and treatment were quantified with rating of perceived exertion (6–20)23 and thermal sensation (0.0–8.0)24 scales.
Exercise Protocol
Participants arrived to the recreation fields between 1,400 and 1,700 hours (time of day matched between trials), and completed a urine sample and body mass wearing minimal-clothing, followed by instrumentation for Tre, heart rate, and Tsk. Participants were familiarized with the perceptual scales, and baseline physiological and perceptual measures were recorded. Participants then began exercise, which consisted of a light intensity, self-paced 400-m warm up followed by a moderate–high intensity, self-paced 1,609-m run around the recreation field. During the first trial, participants were instructed to complete the 1,609-m run at a pace that was comfortable and reproducible, but still intense enough to cause elevation in effort (rating of perceived exertion ∼16). During the second trial, participants repeated the 400-m warm up and 1,609-m run at the same pace as that recorded in the first trial (on the basis of pace from Global Positioning System watch).
Following the 1,609-m run, participants completed a 5-minute standing rest period during which physiological and perceptual measures were recorded. The participants consumed water ad libitum during this time with volumes recorded and matched identically during the subsequent trial. The temperature of the fluids was not controlled; however, these liquids were only consumed during the exercise portion of the trial and were replicated between trials to limit any effect on treatment response.
The participants then began a series of repeated 100-m sprints (50 m out-and-back), with ∼30 to 45 seconds rest between sprints, until Tre ≥39°C. After every five sprints, participants completed a 3-minute standing rest to assess Tre and allow participants to recover to delay volitional exhaustion. The number of sprints, total time (including sprints and rest), and heart rate were recorded to allow for reproducibility during the subsequent trial. Once Tre of ≥39.0°C was reached, the participants completed a 5-minute walking cool down (participants were instructed to match pace between trials) and physiological and perceptual measures were obtained. A body mass was once again obtained and participants moved to initiate the treatment portion of the trial.
Treatment Protocol
Treatment consisted of either ISC or control (CON) in randomized, crossover order. The CON condition involved lying supine in the shade for 15 minutes without external cooling. During the ISC trial, participants were wrapped in ice-water (2.8 ± 3.3°C) soaked bed sheets (200 thread count, twin flat sheet, 60% cotton, 40% polyester, Mainstays, Bentonville, Arkansas) for 15 minutes while lying supine in the shade. The participant initially assumed a supine position, followed by researchers moving the participant to their side and spreading the sheet out beneath them. The researchers returned the participant to the supine position and the sheet was wrapped so it covered their entire body, leaving only the face exposed. The ice-water soaked sheets were rotated every 2 minutes with freshly soaked sheets to augment cooling and standardize treatment between participants. The time to switch each sheet was <15 seconds (on the basis of pilot trial practice). To standardize the methods used to cool participants, the researchers were familiarized with the sheet replacement method and timing before trials and the same researchers conducted the cooling portion of both trials. Furthermore, the temperature of the water soaking the sheets was measured 2 to 3 times during individual treatments and ice or water were added to standardize between participants. During both CON and ISC trials, physiological measures and thermal sensation were obtained every 3 minutes.
Following the treatment protocols, the participants moved to another region in the shaded area to sit upright for an additional 15-minute recovery period. Physiological measures and thermal sensation were assessed every 5 minutes. Immediately following the 15-minute recovery portion, participants removed instrumentation and completed the trial.
Data Analysis
Tre cooling rates were calculated using the following equation: ([precooling Tre − end-cooling Tre]/cooling time). Mean weighted Tsk was calculated using the following equation ([0.3 × Tchest] + [0.3 × Tarm] + [0.2 × Tthigh] + [0.2 × Tleg]).21 Body surface area was calculated using the equation: 0.007184 × body mass (kg)0.425 × height (cm)0.725.20 Sweat rates were calculated using the following equation ([preexercise body mass – post exercise body mass] + fluid consumed)/time.
Statistical Analysis
All statistical analyses were conducted using SPSS v. 23 (IBM Corporation, Somers, NY). Tre, heart rate, Tsk, and thermal sensation were analyzed between conditions and over time via two-way repeated measures analysis of variance. Post hoc analyses with appropriate Bonferroni corrections were applied to identify significant time point differences. Paired samples t tests were used to analyze differences in exercise times, cooling rates, baseline body mass, urine specific gravity, body mass changes, sweat rates, and ambient environmental conditions. A Pearson product correlation was conducted to assess the relationship between cooling rate with ISC and body surface area. The alpha level was set to 0.05 a priori. All values are expressed as mean ± standard deviation.
FINDINGS
Wet bulb globe temperatures were similar between days (day 1, 30.7 ± 0.9 vs. day 2, 31.9 ± 5.2°C, p = 0.59). Urine specific gravity was not different between trials (CON 1.011 ± 0.007 vs. ISC 1.013 ± 0.006, p = 0.49). Furthermore, there were no differences in body mass at baseline (CON 78.6 ± 15.8 vs. 78.6 ± 15.5 kg) or postexercise (CON 78.1 ± 15.6 vs. 78.1 ± 15.4 kg, p = 0.67). Sweat rates were also similar between the CON (1.7 ± 0.6 L/hr) and ISC (1.7 ± 0.5 L/hr, p = 0.83) trials.
Exercise
By design, there were no differences in the total exercise time (CON 0.52 ± 0.09 hours vs. ISC 0.53 ± 0.10 hours, p = 0.65), 1,609-m run time (CON 8.17 ± 1.74 vs. ISC 8.36 ± 1.76 minutes, p = 0.22), and sprints completed (CON 8 ± 4 vs. ISC 7 ± 4 repetitions, p = 0.39). Furthermore, Tre increased independent of treatment (p < 0.001); however, there were no differences in Tre between treatments (p = 0.16) at baseline (CON 37.42 ± 0.30 vs. ISC 37.56 ± 0.28°C), after the 1,609-m run (CON 38.47 ± 0.34 vs. ISC 38.60 ± 0.42°C), or after the sprint protocol (CON 39.14 ± 0.22 vs. ISC 39.22 ± 0.26°C). Exercise Tsk was also similar between treatment trials independent of time (p = 0.32); however, there were differences across time independent of treatment (p = 0.002) from baseline (CON 36.2 ± 0.5; ISC 36.1 ± 0.3°C), to the 1,609-m (CON 36.6 ± 0.3; ISC 36.5 ± 0.4°C), and sprints (CON 36.8 ± 0.3; ISC 36.5 ± 0.8°C), with no interaction (p = 0.52). Similarly, heart rate increased throughout the exercise protocol independent of treatment (p < 0.001), and was not different at any exercise time point (p = 0.62) between CON (baseline 89 ± 13, 1,609-m run 184 ± 13, and sprints 182 ± 13 bpm) and ISC (baseline 91 ± 15, 1,609-m run 181 ± 17, and sprints 180 ± 7 bpm). Ratings of perceived exertion were different across the exercises (p < 0.001) between the 1,609-m run (CON 16 ± 2; ISC 16 ± 2), sprints (CON 17 ± 1; ISC 18 ± 1), and cooldown (CON 13 ± 2; ISC 13 ± 2); however, there was no difference between treatment trials (p = 0.99), with no interaction (p = 0.26). Thermal sensation was also increased from baseline (CON 4.9 ± 0.6; ISC 5.0 ± 0.9) to the end of sprints (CON 6.7 ± 0.6; ISC 6.5 ± 0.7) independent of treatment (p < 0.001); however, there were no differences between treatment trials (p = 0.89), with no interaction of time and treatment (p = 0.76).
Cooling
Tre at the end of exercise, but before treatment, was not different between trials (CON 39.10 ± 0.40 vs. ISC 39.27 ± 0.29°C, p = 0.08). The change in Tre during cooling was dependent on treatment (p = 0.05; Fig. 1A) with ISC inducing a greater change at 3 (p = 0.01) and 6 minutes (p = 0.05) compared to CON. Overall cooling rates were not different between CON (0.05 ± 0.02°C/min) and ISC (0.06 ± 0.02°C/min, p = 0.72). The cooling rates during the first 3 minutes of treatment were faster with ISC (0.11 ± 0.09°C/min) than CON (0.01 ± 0.10°C/min, p = 0.01), whereas CON elicited faster cooling (0.06 ± 0.03°C/min) during minutes 9 to 12 compared with ISC (0.03 ± 0.02°C/min, p = 0.007). Furthermore, there was no relationship between body surface area and cooling rate during ISC (r = −0.12, p = 0.69). Tsk changes over time were dependent on treatment (p = 0.01), exhibiting a reduction from 3 minutes through 15 minutes with ISC compared to CON (all p < 0.05, Fig. 1B). There was a trend for an interaction of treatment and time for heart rate (p = 0.051); however, there was no overall difference in heart rate between ISC and CON independent of time (p = 0.10, Fig. 1C), but heart rate decreased regardless of trial (p < 0.001). The thermal sensation changes were dependent on treatment over time (p = 0.03), revealing lower perceptions from 3 minutes (ISC 3.5 ± 0.9 vs. CON 4.5 ± 0.6, p = 0.002) through 15 minutes (ISC 2.8 ± 1.0 vs. CON 3.9 ± 0.4, p = 0.005) with ISC compared to CON.
(A) Changes in rectal temperature from pre-treatment, (B) skin temperature, and (C) heart rate responses to treatment with ice sheet cooling (ISC) and control (CON). aSignificant difference from CON (p < 0.05). bSignificant difference from Pre (p < 0.05). cSignificant difference from 3 minutes (p < 0.05). dSignificant difference from 6 minutes (p < 0.05). eSignificant difference from 9 minutes (p < 0.05). fSignificant difference from 12 minutes (p < 0.05).
Recovery
The change in Tre during recovery was dependent on treatment (p = 0.004), with greater Tre decreases during ISC compared to CON at minute 15 (p = 0.03; Fig. 2A). Tsk responses exhibited a dependence on treatment over time (p = 0.04) with ISC eliciting cooler Tsk throughout recovery compared to CON (p < 0.05, Fig. 2B). The interaction of time and treatment for heart rate during recovery (p = 0.04) demonstrated differences at the onset, 5, and 10 minutes (all p < 0.05, Fig. 2C). Thermal sensation changes over time were also dependent on treatment (p = 0.002); however, the only significant difference between CON (4.0 ± 0.4) and ISC (3.5 ± 0.5) occurred at the onset of recovery (p = 0.03).
(A) Changes in rectal temperature from baseline, (B) skin temperature, and (C) heart rate responses to recovery following ice sheet cooling (ISC) and control (CON). aSignificant difference from CON (p < 0.05). bSignificant difference from onset (p < 0.05). cSignificant difference from 5 minutes (p < 0.05).
DISCUSSION
This is the first study, to our knowledge, to investigate the cooling efficacy of using ice-water soaked sheets following exertional hyperthermia. The primary finding in this study demonstrated that ISC was not effective in the rapid reduction of exertional hyperthermia, as there were no differences in cooling rates compared to CON. When utilizing recommendations provided by McDermott et al,5 the cooling rates elicited by ISC (0.06°C/min) were within the unacceptable category (<0.078°C°/min) with the ideal rate ≥0.155°C/min. Given these findings, the use of the ISC method should not be recommended as the primary treatment for an individual suffering from exertional heat illness. However, ISC augmented physiological and perceptual recovery through greater reductions in Tsk and thermal sensation than CON, with a trend for improved heart rate recovery. Thus, if the goal of the treatment is to improve perceptual and short-term symptom recovery, but not to lower body temperature, then this modality may provide benefit following exercise in the heat. Therefore, ISC use will provide physiological benefit in scenarios in which exertional heat illness patients are being transported to, or are awaiting arrival of, a more effective treatment (i.e., cold-water immersion) if no other means of cooling are available. Yet, if current emergency action plans for heat casualties include ISC as the primary or only treatment option; these should be changed to reflect evidence-based guidelines and the inclusion of cold-water immersion.
Exertional heat illness treatment via ISC has been reported in military and civilian settings,9,10,13 and has been recommended as an alternative, or transient, cooling modality in the absence of cold-water immersion 8,12,25 despite the paucity of research related to its efficacy. Anecdotal observations of cooling rates up to 7°C per hour with ISC have been described.26 Ferris et al13 demonstrated that ISC was effective to treat a population of primarily older individuals suffering from classic heat stroke induced by a heat wave when Tre <41.1°C, yet this method was not able to cool patients effectively when Tre >41.1°C, resulting in fatalities. The variability in these accounts of the effectiveness of ISC may be attributed to age differences, modality application technique, and the addition of other factors to optimize cooling (e.g., spraying with water, fanning). Barner et al27 demonstrated cooling rates of 0.08 and 0.27°C/min using fanning and water dousing in combination; however, other studies have produced mixed results using water dousing with and without fanning.5,27,–29 Nevertheless, requiring additional material (e.g., fan, extra water for dousing) negates a quality characteristic of ISC in allowing for treatment of EHS in remote settings with limited access to supplies.
A similar technique using ice towels has also received investigation.14,–16 DeMartini et al15 examined the effectiveness of ice towels on hyperthermic (∼38.7°C) individuals, demonstrating decreases in Tre of 0.58°C after 10 minutes of cooling. Interestingly, this matches our cooling rates elicited by ISC. In contrast, Armstrong et al14 demonstrated a cooling rate of 0.11°C/min using ice towels (covering the limbs and torso) to treat hyperthermic runners following a road race. It is likely that the differences between the cooling responses of our participants and those of Armstrong et al14 are the result of variability in the methodologies or ambient temperatures (Armstrong et al = 24°C vs. this study = 34.2°C). Ice towels may be thicker than the sheets in this study, resulting in a greater capacity to hold water and store heat. The frequency of rotation and temperature of the water used to cool the ice towels was not stated14; therefore, it is difficult to ascertain any dissimilarities. The cooler ambient temperatures may have also augmented the cooling capacity of the ice towels by providing a continued heat transfer gradient from the skin to towel to air.
The reductions in heart rate and thermal sensation found with ISC were as expected, since the water temperatures used to soak the sheets induced significant reductions in Tsk. The reduced Tsk likely induced cutaneous vasoconstriction, consequently returning a greater blood volume to the central vasculature from the periphery, causing increased cardiac filling and a decreased heart rate.30 Similarly, reductions in heart rate (−67 bpm) and thermal sensation were also documented using ice towels.15
The lower Tsk induced by the ISC may have attributed to the decreased efficacy of this technique. The water temperature used to soak the sheets was similar to that used to provide the fastest cooling rates via cold-water immersion (∼2°C).3,31 However, the cutaneous vasoconstriction found with immersion,32,33 is overridden by continuously circulated water carrying heat away from the skin.2 Therefore, a constant core-to-skin-to-water gradient allows for greater heat dissipation. During ISC, the initial application of the ice sheet likely elicited marked cold-induced cutaneous vasoconstriction, therefore attenuating blood flow for the removal of heat. Additionally, the water in the sheets was limited, and as such, began to warm immediately after placement on the participants (anecdotal observation). Thus, the temperature gradient between the skin and sheet gradually decreased, restricting heat loss. Even with continuous sheet replacement in this study (standardized at 2 minutes), Tre reductions were not substantial enough to make ISC an effective treatment option for exertional heat stroke. Furthermore, the after-drop in Tre following ISC was significantly greater than in the CON trial; however, the magnitude of these differences was small. After-drop in Tre with cold-water immersion is much greater, hence the recommendation to remove exertional heat stroke victims from immersion while hyperthermic to prevent a continued decrease into hypothermia.3,34
The portability of ISC contributes significantly to the appeal as a treatment method in place of cold-water immersion. However, this study has demonstrated that cooling efficacy is limited, and consideration for alternative treatment options is necessary. Recent investigations have shown success with the use of a portable immersion technique (i.e., tarp-assisted cooling with oscillations) to reduce exertional hyperthermia.35,36 The tarp-assisted cooling method uses a standard tarp and 20 to 40 gallons of ice water to create an improvised immersion tub.35,36 Average cooling rates with tarp assisted cooling range from 0.14 to 0.17°C/min, falling in the acceptable (0.078–0.154°C/min) and ideal (≥0.155°C/min) categories for cooling efficiency, respectively.5,35,36 Thus, the tarp-assisted cooling method provides a portable and effective substitute to a traditional immersion tub.35 In remote settings, where cold-water immersion is not feasible, the cooling rates elicited using the tarp-assisted cooling method may provide a superior treatment alternative as compared to the ISC method used in this study. As such, emergency action plans currently using ISC to treat exertional heat stroke patients should consider the addition or replacement of this technique with the tarp-assisted cooling method to provide victims with the best possible treatment.
There were several limitations with this study. The participants achieved asymptomatic exertional hyperthermia rather than exertional heat illness, thus the cooling efficacy of ISC for heat illness treatment cannot be exclusively stated at this time. However, studies utilizing cold-water immersion in exertional heat stroke victims have shown similar cooling rates as compared to studies utilizing exertional hyperthermia.3,5,6,31 Thus, clinicians who have utilized the ISC technique should be encouraged to provide proper analysis of the efficacy, or update current emergency action protocols to include cold-water immersion, or suitable treatment strategies. Furthermore, we did not investigate ISC as a temporary or initial treatment for exertional hyperthermia before documented effective treatments (e.g., immersion). It is plausible that ISC before immersion in cold water may facilitate more effective treatment compared to our CON condition of lying in the shade, but we did not test this with this study.
The cooling rate analysis may have also been limited by the small female sample size. Lemire et al37 identified faster cooling rates with female participants compared with males, though, the authors suggest this difference is likely attributed to the differences in lean body mass between genders. This is supported by the work of Friesen et al,38 establishing faster cooling in individuals with greater body surface area to lean body mass ratios. Regardless, the small female sample size prevented statistical analysis from identifying sex-dependent influences on cooling; however, observational analysis of the cooling rates revealed similar trends to the male participants.
The temperature of the water ingested during exercise was also not controlled, yet, researchers matched fluid volumes between trials, and the thermophysiological responses were not different during exercise or at the onset of treatment. The use of volitional walking pace for cooldown was also of concern. However, researchers provided verbal encouragement to move similarly between trials, and ratings of perceived exertion at the end of the cool down, and physiological or perceptual measures before treatment, were not different between trials. Therefore, any cooldown or fluid temperature differences were of negligible impact on the findings.
In conclusion, ISC did not provide effective treatment of exertional hyperthermia compared to no treatment. However, the physiological and perceptual benefits may warrant the use of ISC in settings not reliant on rapid reductions in core temperature (i.e., recovery from exercise, during transit to cold-water immersion location). Combining ISC with other modalities, such as water dousing or fanning, may improve the cooling effectiveness; however, controlled experimental protocols should investigate these allegations before use in clinical settings. Thus, clinicians should continue to use, or advocate for implementation of, successfully validated treatments (i.e., tarp assisted cooling method, cold-water immersion) for exertional heat stroke in every setting.
