Agreement among common core temperature indices in defining critical environmental limits for human thermoregulation: A secondary analysis

Global intensification of heat extremes has heightened interest in defining critical environmental limits for human thermoregulation. These limits are commonly estimated from the inflection point in core temperature during progressive heat stress protocols. Esophageal temperature (T es ) is the criterion core temperature index, as it reflects right atrial blood temperature and responds rapidly to changes in the body’s thermal state. However, measurements of rectal (T re ) and gastrointestinal temperature (T gi ) are more commonly used due to better participant tolerability. We evaluated agreement between inflection points of these core temperature indices using data from two previously published progressive heat stress protocols. Study 1 measured T es and T re in 12 resting young adults (3 female). Study 2 evaluated T re and T gi in 16 resting or cycling young adults (8 females). Ambient conditions at the identified inflection points were converted to wet-bulb temperature (T wb ) and analyzed with Bland-Altman analysis (±1.3°C T wb acceptable agreement range). In Study 1, both the mean bias (0.3°C [95% CI: 0.0 to 0.6°C]) and the individual limits of agreement (LOA: −0.6 to 1.3°C) for the T es -T re comparison were within ±1.3 °C T wb . In Study 2, the mean bias was also within the acceptable range (0.8°C [0.5 to 1.1°C]), whereas the limits of agreement (0.6 to 1.7°C T wb ) exceeded this threshold. Although larger confirmatory trials are needed, these findings suggest that T re inflection identified during a progressive humidity protocol closely aligns with that identified using T es . While T re and T gi demonstrated acceptable group-level agreement, caution is warranted when interpreting individual responses.