Phase-Dependent Autonomic Responses to Electric Fan Use During a Prolonged 8-hour Heat Exposure in Older Adults

Electric fans are commonly recommended for older adults to reduce heat strain, yet their effects on cardiac autonomic regulation during heat exposure remain unclear. This study examined phase-dependent heart rate variability (HRV) responses to fan use during prolonged heat exposure in older adults. Sixteen participants (6 females; 65-85 yr) completed three 8-h heat exposures (36°C, 45% relative humidity) with fan-generated air speeds of 0, 2, or 4 m/s. Core (rectal) temperature and a 3-lead electrocardiogram were continuously acquired. Based on temporal core temperature responses, heat exposure was divided into Initial Ramp-Up Phase (0-4 h) and Plateau Phase (4-8 h). Core temperature increased from 37.1 (0.3)°C to 38.0 (0.3)°C ( P<0.001) during Ramp-Up and remained stable at 38.1 (0.3)°C ( P≥0.627) during Plateau Phase. During Ramp-Up, in all conditions, heart rate (HR) increased ( P<0.001), while vagally mediated HRV indices including RMSSD, high-frequency power, and CVI decreased ( P≤0.001). During Plateau Phase, HR was lower and RMSSD was higher in 2 m/s compared to 0 m/s condition ( P≤0.035), while RMSSD and CVI were higher in 2 m/s compared to 4 m/s condition ( P≤0.021). We show that HRV responses to prolonged heat exposure in older adults are phase-dependent. Progressive heat storage during Ramp-Up induces HRV responses reflective of vagal withdrawal, regardless of fan use, whereas low-level airflow (2 m/s) during Plateau Phase modestly attenuated reductions in HRV. These findings suggest that the physiological effectiveness of fan use during extreme indoor heat may depend on air flow intensity and temporal progression of heat strain in older adults.