Research Letter
Climate Change and Health
December 21, 2023
Body Core Temperature After Foot Immersion and Neck Cooling in Older Adults Exposed to Extreme Heat
Robert D. Meade, Emma R. McCourt, James J. McCormick, et al
JAMA. Published online December 21, 2023. doi:10.1001/jama.2023.24417
As heat waves increase in regularity and severity, sustainable cooling interventions for preventing heat-related mortality and morbidity are needed.1 Based on research in young adults,2 immersing the feet in water has been recommended for protecting the health of vulnerable persons (eg, in aged care homes).1 Advantages of foot immersion include that it is inexpensive, can be used in emergencies (eg, heat-induced blackouts),1,2 and can be easily combined with other commonly recommended interventions like applying wet towels to the head or neck (eg, by soaking a towel before immersing the feet).3,4 However, its efficacy has not been evaluated in older adults, who exhibit declines in thermoregulatory function and are at elevated risk of heat-related adverse health events.5 We evaluated the hypothesis that foot immersion applied alone or with supplemental neck cooling mitigates increases in core temperature in older adults exposed to environmental conditions simulating recent deadly heat waves in North America (eg, 2021 Pacific Northwest Heat Dome).6
Following approval of the study by the University of Ottawa Research Ethics Board, adults aged 65 to 85 years from the Ottawa, Canada, region volunteered and provided written informed consent for the study, which ran from September 2022 to May 2023. Supplement 1 details the full protocol. Participants completed 3 randomized, 6-hour exposures to 38 °C and 35% relative humidity (separated by ≥5 days) differing only in the cooling intervention applied: no cooling (control), submersion of the feet to mid-calf in 20 °C water for the last 40 minutes of each hour (foot immersion), or foot immersion with a wet towel (20 °C) draped around the neck (foot immersion with neck cooling). Drinking water (≈15-20 °C) was available ad libitum.
The primary outcome was core (rectal) temperature. A reduction of 0.2 °C or more was chosen as the minimal clinically meaningful effect. A sample of at least 15 participants was required to detect this effect with 84% power. Of 14 prespecified secondary outcomes, 7 are reported here: heart rate, systolic and diastolic blood pressures (oscillometry), rate pressure product (heart rate × systolic pressure), fluid consumption, net fluid loss (% body mass change), and thermal discomfort (visual analog scale, anchors at 0 [extremely comfortable], 50 [neutral], and 100 mm [extremely uncomfortable]). Post hoc exploratory outcomes included mean hourly sweat rate (change in body weight corrected for urination and food/fluid consumption). Changes from baseline to end of exposure in each outcome were compared between trial groups using linear mixed-effects models (baseline-adjusted). Between-group comparisons for each outcome were Bonferroni-corrected (2-tailed α = .05). Data were analyzed with R version 4.2.0 (R Foundation).
Of 25 individuals assessed for eligibility, 17 participated (53% female; median age, 72 [IQR, 69–74] years) (Table 1). Mean core temperature increased by 1.1 °C (SD, 0.4) in control, 1.1 °C (SD, 0.4) in foot immersion, and 1.2 °C (SD, 0.4) in foot immersion with neck cooling and reached 38.0 °C (SD, 0.2) at end of exposure in each group. There were no statistically significant differences in core temperature between groups (all pairwise differences, 0.0 °C [95% CI, −0.1 to 0.1]; P > .99) (Table 2).
Elevations in heart rate, sweat rate, and fluid consumption were statistically significantly lower with foot immersion compared with control (Table 2). Heart rate, sweat rate, fluid consumption, fluid loss, and thermal discomfort were statistically significantly reduced in foot immersion and neck cooling compared with control. Sweat rate and fluid loss were statistically significantly reduced in foot immersion with neck cooling compared with foot immersion alone. There were no significant differences in blood pressure or rate pressure product.
Foot immersion with and without neck cooling did not lessen increases in core temperature in older adults in this small study conducted in a controlled experimental setting. Although some effects on heart rate, sweat rate, and fluid consumption were observed, they were small and of questionable clinical importance. Heart rate reductions did not exceed previous suggestions for clinical significance (≥5/min),2 and elevated sweating requirements without cooling were almost entirely offset by small increases in fluid consumption (≈½ cup hourly). These data do not support foot immersion and neck cooling as efficacious strategies for limiting increases in core temperature in older adults. Study generalizability is limited by the homogeneous sample and specific environmental conditions.
