Sensitivity and specificity of using exercise heart rate in a thermoneutral environment to predict heat tolerance status.

Objectives: Following heat illness, a return to activity may require passing a heat tolerance test (HTT). However, there are several logistical limitations to the widespread use of the HTT. Thus, it would be advantageous to develop a test that could be conducted in a thermoneutral (~22°C) environment to predict heat tolerance status. The purpose of the current study was to determine the sensitivity and specificity of using the criteria of a heart rate (HR) ≥130 bpm following 30 min of thermoneutral exercise in detecting heat-intolerant and heat-tolerant individuals.

Material and methods: Sixty-five subjects visited the lab on 3 separate days. The first visit consisted of completing a maximal oxygen uptake (VO2 max) test to assess cardiovascular fitness. For lab visits 2 and 3, subjects randomly completed a 2-hour walking treadmill test in either a hot (40°C, 40% relative humidity [RH]) or thermoneutral (22°C, 40% RH) environment.

Results: Forty-eight subjects were classified as heat-intolerant and 17 subjects as heat-tolerant. Using the criterion of a HR ≥130 bpm at 30 min of exercise in the thermoneutral environment, specificity (54%) and sensitivity (100%) of passing the HTT was calculated. Secondary analysis using multiple regression revealed 3 significant variables for predicting ending HR during the HTT. They were: 1) absolute VO2 max (l/min), 2) age, and 3) HR at 30 min of exercise during thermoneutral exercise.

Conclusions: Exercise in a thermoneutral environment had a positive predictive value of 100%, thus, if a subject has a HR ≥130 bpm at 30 min of exercise in a thermoneutral environment, they are very likely to fail a subsequent 2-hour HTT in the heat and be classified as heat-intolerant. Therefore, prior screening has the potential to save time and money, along with providing safety to a heat-intolerant subject. Int J Occup Med Environ Health. 2023;36(2):192-200.