The impact of age, type 2 diabetes and hypertension on heart rate variability during rest and exercise at increasing levels of heat stress.

Abstract

In older adults with type 2 diabetes (T2D) and hypertension (HTN), cardiac autonomic modulation is markedly attenuated during exercise-heat stress. However, the extent to which this impairment is evident under increasing levels of heat stress remains unknown. We examined heart rate variability (HRV), a surrogate of cardiac autonomic modulation, during incremental exercise-heat stress exposures in young (20-30years) and middle-aged-to-older individuals (50-70years) without and with T2D and HTN. Thirteen young and healthy (Young, n = 13) and 37 older men without (Older, n = 14) and with HTN (n = 13) or T2D (n = 10) performed 180-min treadmill walking at a fixed metabolic rate (~ 200W/m2; ~ 3.5 METs) in a differing wet-bulb globe temperature (WBGT; 16°C, 24°C, 28°C, and 32°C). Electrocardiogram (ECG) and core temperature measurements were recorded throughout. Data were analysed using 5-min averaged epochs following 60-min exercise, which represented the last common timepoint across groups and conditions. Ageing did not significantly reduce HRV during increasing exercise-heat stress (all p > 0.050). However, T2D and HTN modified HRV during exercise-heat stress such that Detrended Fluctuation Analysis (DFA) α1 (p = 0.012) and the cardiac sympathetic index (p = 0.037) were decreased compared to Older in all except the warmest WBGT condition (32°C). Our unique observations indicate that, relative to their younger counterparts, HRV in healthy older individuals is not perturbed during exercise heat-stress. However, relative to their age-matched healthy counterparts, HRV is reduced during exercise-heat stress in individuals with age-associated chronic conditions, indicative of cardiac autonomic dysfunction.