Efficacy Of Limb And Neck Cooling For Limiting Heat-related Cardiac Autonomic Disfunction In Older Adults

Limb immersion and neck cooling have been advocated as sustainable cooling strategies during heat waves when air conditioning is unavailable. In a preliminary analysis, we found that these strategies reduced resting heart rate during heat exposure (Meade et al. ACSM Annual Meeting, 2023). However, their impacts on cardiac autonomic function both at rest and during postural changes remain unknown. PURPOSE: In this randomized trial, we evaluated the effect of limb immersion with and without neck cooling on resting heart rate variability as well as heart rate and systolic blood pressure (SBP) responses to standing in older adults during heat exposure. METHODS: 6 older adults (3 women, 72 (SD 5) yrs) completed 3 randomized 6-hour exposures to 38 °C and 35% relative humidity. Trials differed only in the cooling strategy applied: no cooling (CTRL), lower limb (mid-calf) immersion in 20 °C water (LIMB), or limb immersion with a damp towel (20 °C) around the neck (LIMB+NECK). Cooling was intermittent (20 min off, 40 min on). Core temperature was recorded continuously. Heart rate variability (RMSSD) was derived during paced breathing (15 breaths/min). Autonomic control of heart rate (30/15 ratio calculated from slowest HR between 25-35th beat divided by fastest HR between 10-20th beat after standing) and blood pressure (lowest SBP after 1 and 2 min of standing minus SBP at supine rest) were assessed during lying to standing tests. Mixed model estimated marginal means were compared between conditions. RESULTS: Following the 6-hour heat exposure, core temperature was not different between CTRL and either LIMB or LIMB+NECK (P ≥ 0.82; across-condition average: 37.8 (0.2)°C). RMSSD was elevated by 7.0 ms [95% CI: 0.7, 13.2] in LIMB (24.6 (18.1) ms) compared to CTRL (18.6 (8.9) ms; P = 0.03) but did not differ between CTRL and LIMB+NECK (21.7 (16.8) ms; P = 0.16). By contrast, neither the 30/15 ratio or the supine-to-stand change in SBP were different between CTRL (1.06 (0.13) and -2 (19) mm Hg, respectively) and either LIMB (1.04 (0.08) and -4 (12) mm Hg; both P ≥ 0.54) or LIMB+NECK (1.07 (0.04) and -8 (6) mm Hg; both P ≥ 0.10). CONCLUSION: Limb immersion may improve resting cardiac autonomic function during heat exposure. However, we observed no effect of cooling on subsequent heart rate and SBP responses to postural changes. FUNDING: Health Canada