Heart rate variability during rest and exercise in various environmental conditions

Abstract

The beat-to-beat variation in time between heart beats is known as heart rate variability (HRV). The effect that temperature and humidity have on this function of heart rate (HR) control remains largely unclear. Therefore, the current study examined the effects of different environmental conditions on HRV during rest and moderate exercise. Seven healthy male volunteers (age 20.6, range 19 - 23 yrs) participated in the study. Participants completed the protocol under four environmental conditions in randomised order within a climate control chamber. These conditions were: thermoneutral (TN; 20°C, 50% relative humidity [rh]), cold/dry (CD; 5°C, 35% rh), hot/dry (HD; 40°C, 35% rh) and hot/humid (HH; 32°C, 75% rh). Variables were recorded during supine rest, sitting and treadmill exercise of 50% maximum TN HR. Two-way repeated measures ANOVA (p<0.05) showed VO2, ventilation and RPE were significantly higher during rest compared to exercise but there was no effect of condition. RER was significantly higher in TN and CD compared to HH. Exercise and condition had no effect on core temperature. Thermal discomfort was significantly higher in CD compared to the other conditions. Skin temperatures (head, arm, back, chest) were significantly higher in HH and HD compared to CD with chest and back being significantly lower in CD compared to TN. Hot/dry conditions resulted in significantly greater HR and reduced HRV compared to CD. In the current study, HD induced greater central and peripheral cardiovascular stress compared with CD despite greater thermal discomfort in CD.