Focused Clinical Question
Do cold-water immersion (CWI) rectal temperature (Trec) cooling rates differ between hyperthermic males and females?
Summary of Search, “Best Evidence” Appraised, and Key Findings
- •We searched for studies that used CWI (water temperature <20°C), the gold standard treatment for exertional heatstroke,1 to treat hyperthermic males and females (eg, Trec ≥39.5°C).
- •Nine studies met the search criteria; however, 6 studies were excluded because data were not separated by gender. Three studies2–4 met our predetermined inclusion and exclusion criteria for this critically appraised topic (CAT) study.
- •Two studies2,3 observed clinically meaningful differences, with females having higher Trec cooling rates. The third study4 observed no differences in Trec cooling rates between males and females (Table 1). The average effect size for all studies analyzed in this CAT2–4 was 2.4 (ranged from 0 to 3.9).
- •The average Trec cooling rates across all examined studies2–4 were 0.18 (0.05) and 0.24 (0.03)°C/min for males and females, respectively. Although hyperthermic females cooled ∼33% faster than males, both genders cooled faster than recommendations for exertional heat stroke (EHS) victims (ie, 0.15°C/min).5
Characteristics of Included Studies
| Authors | Armstrong et al3 | DeMartini et al4 | Lemire et al2 |
|---|---|---|---|
| Patients, n | 8000 athletes. 21 (35 [3] y) were diagnosed with extreme hyperthermia and either heat exhaustion or EHS. 17 males (age = 36 [12] y); 4 females (age = 32 [7] y). *Subject anthropomorphics not reported. | 180,000 athletes (∼10,000 each year). 258 athletes had documented EHS signs and symptoms (179 EHS cases reported by gender). 110 males (age = 34 [13] y); 69 females (age = 36 [12] y). *Subject anthropomorphics not reported. | 10 males (age = 25 [4] y, ht = 177.5 [5.8] m, mass = 73.6 [9.4] kg, body fat = 13.8% [2.9%], LBM = 63.4 [7.2] kg, BSA/M = 261 [16] cm2/kg, BSA/LBM = 302 [15] cm2/kg). 9 females (age = 24 [3] y, ht = 165.4 [6.8] m, mass = 64.2 [9.3] kg, body fat = 23.9% [6.1%], LBM = 48.4 [4.5] kg, BSA/M = 268 [19] cm2/kg, BSA/LBM = 353 [17] cm2/kg). |
| Experimental design and methods | Retrospective cohort study. – Subjects ran ∼11 km in the Falmouth Road Race in August (24.4°C [0.8°C], 67% [4%] humidity). – Trec ≥ 39.4°C for all subjects prior to treatment. | Retrospective cohort study. – Subjects ran ∼11 km in the Falmouth Road race in August (23.3°C [2.5°C], 70% [16%] humidity). – Race participants were included as subjects if Trec ≥ 40°C and an EHS diagnosis was documented. | Quasi-experimental study. – Subjects ran on a treadmill at 65% VO2 max in heat (40.0°C, ∼18% humidity) until Trec reached 39.5°C. |
| Cooling intervention | 13 males and 1 female were treated by IWI (1–3°C) until Trec reached 39.0°C. – The subjects’ torso and upper legs were completely immersed. – No mention of water circulation during treatment. | All subjects were treated by CWI (10°C) in a 50-gal tub until Trec reached 38.8°C. No noted specifications on water circulation. – Subjects were immersed in a seated position. If arms or legs were not completely submersed, then wet ice towels were used to cover the arms, legs, neck, and head. | All subjects were immersed in a circulated water bath (2°C) until Trec reached 37.5°C. – Subjects were immersed in a recumbent position (head and chest were not fully immersed). |
| Results | Trec post exercise: female = 41.9°C; male = 41.6°C (0.6°C). Immersion duration: female = 11 min; male = 17 (8) min. Trec cooling rate: female = 0.27°C/min; male = 0.20 (0.07)°C/min. | Trec post exercise: female = 41.2°C (0.7°C); male = 41.4°C (0.7°C). Immersion duration: afemale = 10.90 min; male = 10.45 min. Trec cooling rate: female = 0.22 (0.10)°C/min; male = 0.22 (0.12)°C/min. | Trec post exercise: female = 39.56°C (0.07°C); male = 39.57°C (0.06°C). Immersion duration: female = 10.89 (4.49) min; male = 18.13 (4.47) min. Trec cooling rate: female = 0.22 (0.07)°C/min; male = 0.12 (0.03)°C/min. |
| Conclusions | The female with EHS cooled 35% faster than the males with EHS. | No differences were found for cooling rates between male and female athletes suffering from EHS. | Hyperthermic females cooled 83% faster than hyperthermic males. |
Abbreviations: BSA/LBM, body surface area-to-lean body mass ratio; BSA/M, body surface area-to-mass ratio; CWI, cold-water immersion; EHS, exertional heat stroke; ht, height; IWI, ice water immersion; LBM, lean body mass; Trec, rectal temperature; VO2max, maximum oxygen consumption. Note: All data are reported as mean (SD) with the exception of Armstrong et al3.
aImmersion duration calculated using the following formula: (mean Trec postexercise − 38.8°C)/mean cooling rate.
Clinical Bottom Line
There is moderate evidence that hyperthermic females cool faster than males. In the studies analyzed,2–4 hyperthermic males typically needed to be immersed in cold water for 4.3 (3.0) minutes longer than hyperthermic females. Despite the apparent cooling differences between genders, CWI continues to be the gold standard treatment for all EHS patients, male or female.5
Terms Used to Guide Search Strategy
- •Patient/Client group: Hyperthermic adults
- •Intervention/Assessment: Gender (males), sex
- •Comparison: Gender (females), sex
- •Outcome: Rectal temperature cooling rate
Sources of Evidence Searched
- •PubMed
- •CINAHL
- •SPORTSDiscus
- •Cochrane Library
- •ERIC
- •Additional articles were obtained by review of reference lists
Predetermined Inclusion and Exclusion Criteria
Inclusion
- •Following exercise, subjects were hyperthermic with a Trec ≥ 39.5°C (103.1°F).
- •Studies used CWI (≤20°C [68°F]) to treat hyperthermia.
- •Body core temperature was measured with Trec since this is the only valid, clinically useful body temperature site for an exercising human.6
- •Limited to studies published in English in the last 15 years (1993–2018).
- •Limited to studies classified as Oxford Centre for Evidence-Based Medicine level 3 or higher.
Exclusion
- •Studies that did not test both male and female subjects or differentiate cooling rates by gender.
- •Studies that measured body core temperature by oral, axillary, esophageal, tympanic, or temporal thermometry.
- •Studies that cooled hyperthermic subjects by other means (eg, cooling vests, passive cooling, ice towels).
- •Studies that used percooling (ie, cooling during exercise) or precooling (ie, cooling before exercise).
Results of Search
Two retrospective cohort studies3,4 on EHS victims and 1 quasi-experimental laboratory study2 met the predetermined inclusion and exclusion criteria. Each study was reviewed and independently categorized using the 2011 Oxford Centre for Evidence-Based Medicine (Table 2).
Best Evidence
The studies in Table 2 were identified as the best evidence and selected for inclusion in this CAT. These studies were included because they were considered level 3 evidence or higher and included data to allow comparison of gender differences in CWI Trec cooling rates.
Implications for Practice, Education, and Future Research
Two main clinical observations can be derived from this CAT. First, hyperthermic male and female athletes do not have similar Trec cooling rates. Hyperthermic females included in this CAT cooled ∼33% faster than males. Males required CWI for 4.3 (3.0) minutes longer than females. Second, Trec cooling rates for men and women were either acceptable (0.08–0.15°C/min) or ideal (>0.15°C/min).5 All 3 studies2–4 observed ideal cooling rates for females, whereas only 2 studies3,4 observed ideal cooling rates in males. In 1 study,2 males cooled at only acceptable Trec cooling rates. These results suggest that no additional precautions need to be taken based on gender; however, clinicians should be aware that males may cool slightly slower than females when being treated with CWI.
The primary reason for differences in Trec cooling rates between genders is likely related to body surface area (BSA), lean body mass (LBM), and body surface area-to-lean body mass (BSA/LBM) ratios. Women, typically, have higher BSA than males.7 Thus, females may have a greater capacity to dissipate heat by nonevaporative mechanisms during CWI. Conversely, males are typically heavier and have more LBM than females,2,7 which would tend to favor higher metabolic activity and slower cooling rates during CWI. Collectively, this means BSA/LBM ratios tend to be higher in women than men. Several investigators2,8 have noted relationships between the BSA/LBM ratio and Trec cooling rates of hyperthermic subjects. Friesen et al8 observed male subjects with a high BSA/LBM cooled 1.7 times faster than males with a low BSA/LBM. Similarly, Lemire et al2 hypothesized that females cooled faster than males because of differences in BSA/LBM ratios, since the body surface area-to-mass (BSA/M) ratio and BSA were matched between genders. In fact, BSA/LBM was highly correlated with the CWI Trec cooling rate (r = .7) when gender data was pooled.2 Future research should confirm Friesen et al’s8 observations in female subjects to provide greater evidence for the contribution of the BSA/LBM ratio to Trec cooling rates.
Two other minor variables may explain why females cooled faster than males. First, females, in general, have higher adiposity than males.2,7,9 However, simply having more adipose tissue is an unlikely explanation for differences in cooling rate between genders. A difference of 10% adiposity did not affect Trec cooling rates in hyperthermic men cooled in 8°C water (13% body fat = 0.23 [0.09]°C/min; 23% body fat = 0.20 [0.09]°C/min).10 However, individuals with more fat mass tend to shiver less during cold exposure, which may result in less metabolic activity in the lean tissue and may result in faster cooling rates.2 The influence of adiposity on Trec cooling rates in hyperthermic men and women with >25% body fat has not yet been tested. Second, increases in circulating plasma hormones, especially during the luteal phase of menses, may impact the sensitivity of the shivering response, skin vasoconstrictor response, and/or metabolic activity in women.11 Experiments that attempt to control resting body core temperature typically test females during their follicular phase to decrease variability and these hormonal responses. Thus, Trec cooling rates of women tested during the follicular phase are likely to be less impacted by menses than those of women tested in the luteal phase.
We acknowledge 2 main limitations of our CAT. First, only one of our studies2 reported subjects’ mass, body fat, BSA/M ratio, and BSA/LBM ratio. Therefore, it was not possible to compare the effects of anthropometric factors as they relate to gender between all 3 studies in our CAT. Because only 1 study2 controlled for anthropomorphics between genders, and these factors greatly influence Trec cooling,8,7 we placed greater weight on the results of this study2 for the final conclusion in our CAT. Second, while Armstrong et al3 had 4 females diagnosed with EHS in their study, only 1 was included in the CWI treatment group. Consequently, we were only able to compare the Trec cooling rates of the lone female subject to the 13 male EHS victims who were also treated with CWI.
In conclusion, hyperthermic males and females (some with EHS) have different CWI Trec cooling rates. Hyperthermic females cooled faster than males, which is likely attributed to their higher BSA/LBM ratio. However, future research may wish to answer the following questions: (1) Do females with different BSA/LBM ratios cool at different rates like their male counterparts?8 (2) When BSA/LBM ratios are matched between men and women, are Trec cooling rates equivocal? (3) What is the effect of >25% body fat on Trec cooling rates in men and women? Regardless of differences in Trec cooling rates between genders, the cooling rates were either acceptable or ideal5 in all studies examined. The fact that 100% of the men and women who experienced EHS (n = 295) in both retrospective cohort studies3,4 survived without any noted complications attests to the life-saving capabilities of immediate CWI. Therefore, we conclude CWI is highly effective, regardless of gender, and must be the clinical gold standard for treating EHS victims. This CAT should be reviewed and revised upon the completion of additional studies as they relate to Trec cooling rates between genders.
References
- 7.↑
Anderson GS, Ward R, Mekjavic IG. Gender differences in physiological reactions to thermal stress. Eur J Appl Physiol. 1995;71:95–101. doi:10.1007/BF00854965
- 11.↑
Castellani JW, Young AJ. Human physiological responses to cold exposure: acute responses and acclimitization to prolonged exposure. Auto Neuro. 2016;196:63–74. doi:10.1016/j.autneu.2016.02.009
