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 (Tes) 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 (Tre) and gastrointestinal temperature (Tgi) 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 Tes and Tre in 11 resting young adults (3 female). Study 2 evaluated Tre and Tgi in 16 resting or cycling young adults (8 females). Ambient conditions at the identified inflection points were converted to wet-bulb temperature (Twb) and analyzed with Bland-Altman analysis (±1.3°C Twb acceptable agreement range). In Study 1, both the mean bias [0.3°C (95% CI: 0.0°C to 0.6°C)] and the individual limits of agreement (LOA: -0.6°C to 1.3°C) for the Tes-Tre comparison were within ±1.3°C Twb. In Study 2, the mean bias was also within the acceptable range [0.8°C (0.5°C to 1.1°C)], whereas the limits of agreement (0.6°C to 1.7°C Twb) exceeded this threshold. Although larger confirmatory trials are needed, these findings suggest that Tre inflection identified during a progressive humidity protocol closely aligns with that identified using Tes. Tre and Tgi demonstrated acceptable group-level agreement but caution is warranted when interpreting individual responses.NEW & NOTEWORTHY We assessed agreement in inflection points for esophageal (Tes), rectal (Tre), and gastrointestinal (Tgi) temperatures in young adults during progressive humidity protocols. Tes and Tre exhibited good agreement on both group (study) and individual levels. Although group-level agreement between Tgi and Tre was acceptable, individual agreement was poor. These findings indicate that all three techniques are suitable for studies of critical environmental limits, though Tes or Tre are preferred when assessing individual thresholds.