Efficacy Of Limb And Neck Cooling For Reducing Thermal And Cardiovascular Strain In Older Adults

Low-cost cooling interventions are being increasingly recommended as sustainable alternatives to air conditioning during heat waves. These include interventions exploiting the thermal properties of water, such as lower limb immersion and neck cooling, which carry the added benefit that they can be used in low resource settings or during power outages. However, there is currently a lack of empirical evidence to support their efficacy for limiting physiological strain in heat-vulnerable older adults (e.g., in aged care homes). PURPOSE: In this randomized trial (NCT05601713), we evaluated the effects of limb immersion with and without neck cooling on core temperature, cardiovascular strain, and hydration status in older adults during heat exposure. METHODS: Eight older adults (4 women, 72 (SD 5) yrs) completed 3 randomized 6-hour exposures to 38 °C and 35% relative humidity. Trials differed in the cooling strategy employed: no cooling (CTRL), lower limb (mid-calf) immersion in 20 °C water (LIMB), or limb immersion with a damp towel (20 °C) placed around the neck (LIMB+NECK). Cooling was applied intermittently (20 min off, 40 min on). Core temperature (rectal), heart rate, and systolic blood pressure were monitored throughout. Hourly fluid consumption and cumulative fluid loss (% body mass change) were also recorded. Mixed model estimated marginal means were compared between conditions. RESULTS: After 6 hours of heat exposure, core temperature was not different between CTRL (37.9 (0.4)°C) and either LIMB or LIMB+NECK (mean diff and 95% CI vs CTRL: both 0.0 °C [-0.1, 0.2], P ≥ 0.69). By contrast, heart rate was reduced 7 bpm [4, 10] and 5 bpm [2, 8] from CTRL (68 (10) bpm) in LIMB and LIMB+NECK, respectively (P ≤ 0.003). No between-condition differences in systolic pressure were detected (P ≥ 0.37; 115 (10) mm Hg). Compared to CTRL, fluid consumption was reduced 53 mL/hour [9, 96] in LIMB and 71 mL/hour [28, 115] LIMB+NECK (P ≤ 0.018). There were no differences in the change in body mass (P ≥ 0.09; -0.5 (0.9)% body mass). CONCLUSION: Our preliminary data suggest that lower limb immersion with or without neck cooling does not reduce core temperature in older adults resting in the heat. We did, however, observe small but statistically significant reductions in heart rate and improvements in fluid maintenance. FUNDING: Health Canada