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

Intestinal epithelial injury and inflammation after physical work in temperate and hot environments in older men with hypertension or type 2 diabetes

We tested whether older adults with well-controlled type 2 diabetes or hypertension, compared with age-matched adults without chronic disease, exhibit greater intestinal damage, microbial translocation and inflammation during exertional heat stress. Twelve healthy men (age 59 years, SD 4 years), nine with type 2 diabetes (age 60 years, SD 5 years) and nine with hypertension (age 60 years, SD 4 years) walked for 180 min at 200 W/m2 in temperate conditions (wet-bulb globe temperature 16°C) and high-heat stress conditions (wet-bulb globe temperature 32°C). Serum intestinal fatty acid binding protein (IFABP), plasma soluble cluster of differentiation 14, lipopolysaccharide-binding protein (LBP), interleukin-6 and tumour necrosis factor-alpha were measured pre- and postexercise and after 60 min recovery. Total exercise duration was lower in men with hypertension and diabetes (p ≤ 0.049), but core temperature did not differ. All markers increased more in heat versus temperate conditions (p < 0.002). In the heat, individuals with type 2 diabetes had greater postexercise increases in IFABP [+545 pg/mL (95% confidence interval: 222, 869)] and LBP [+3.64 µg/mL (1.73, 5.56)] relative to healthy control subjects (p < 0.048), but these resolved after recovery. Despite reduced exercise duration, hypertensive individuals showed similar increases in IFABP and LBP to control subjects. Our findings suggest that older workers with well-controlled type 2 diabetes or hypertension might have greater vulnerability to heat-induced gastrointestinal barrier disturbance and downstream inflammatory responses when compared with otherwise healthy, age-matched adults during prolonged exercise in the heat.

The utility of heart rate and heart rate variability to identify limits of tolerance to moderate-intensity work in the heat: a secondary analysis

We investigated the utility of heart rate (HR) and heart rate variability (HRV) for identifying individuals who may terminate work early due to excessive heat strain. Forty-eight men and women (median = 36 years; Q1 = 20 years; Q3 = 54 years) attempted 180 min of moderate-intensity work at a fixed metabolic rate (∼200 W/m2; ∼3.5 METs) in a hot environment (wet-bulb globe temperature: 32 °C). Receiver operating characteristics (ROC) curves were used to identify the ability of indices of HR (absolute HR, percentage of maximum HR, percentage of HR reserve) and HRV (root-mean-square of successive differences (RMSSD), high-frequency power, and detrended fluctuation analysis component alpha 1 (DFA α1)) to discriminate between participants who completed the 180 min work bout or terminated prematurely. Participants who terminated work prematurely (n = 26) exhibited higher HR and percentage of HR measures, as well as reduced RMSSD and DFA α1 after the first hour of work compared to participants who completed the bout. The discriminative utility of HR and HRV indices was strongest within the first hour of work, with percentage of HR reserve demonstrating excellent discriminative power (ROC area under curve (AUC) of 0.8). Stratifying participants by age and sex improved ROC AUC point estimates for most indices, particularly in female participants. The study provides preliminary evidence supporting the use of noninvasive cardiac monitoring for predicting work tolerance in healthy individuals exposed to occupational heat stress. HR and percentage of HR reserve were suggested to discriminate work termination most effectively. Further investigations are warranted to explore the influence of individual factors and refine the discriminative thresholds for early identification of excessive occupational heat strain.

Cardiac autonomic modulation in individuals with controlled and uncomplicated hypertension during exercise-heat stress

TAKE-HOME MESSAGE

During short bouts of light-to-vigorous exercise in the heat, controlled and uncomplicated hypertension did not significantly modulate HRV in physically active individuals. These findings can be used to refine guidance on use of exercise for hypertension management in the heat.