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. 48 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 [WBGT]: 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 (RMSSD, high-frequency (HF) power, and DFAα1) to predict whether participants 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 DFAα1 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 non-invasive cardiac monitoring for predicting work tolerance in healthy individuals exposed to occupational heat stress. HR and percentage of HR reserve were observed to predict 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.