An assessment of the validity and reliability of the P022-P version of e-Celsius core temperature capsules.

Core temperature monitoring is a research tool used in many studies, though most popularly, heat strain. Ingestible core temperature capsules are a non-invasive and increasingly popular choice for measuring core body temperature, particularly considering the well-established validation of capsule systems. A newer version of the e-Celsius ingestible core temperature capsule has been released since the preceding validation study resulting in a paucity of validated research for the current version of capsules, P022-P, used by researchers. Using a 1:1 ratio of propylene glycol to water in a circulating water bath, and a reference thermometer with resolution and uncertainty of 0.01 °C, we assessed the validity and reliability of 24 P022-P e-Celsius capsules in three groups of eight, at seven temperature plateaus between 35 °C, and 42 °C in a test-retest format. The systematic bias of these capsules across all 3360 measurements was found to be -0.038 °C ± 0.086 °C (p < .001), The TEST-RETEST evaluation revealed excellent reliability by way of a minute overall mean difference of 0.0095 °C ± 0.048 °C (p < .001), and an intraclass correlation coefficient of 1.00 for each of TEST and RETEST conditions. Although quite small, differences in systematic bias across temperature plateaus were observed for both the OVERALL bias-between 0.00066 °C and 0.041°C-and TEST/RETEST bias-between 0.00010 °C and 0.016 °C. We found that the new capsule version outperforms manufacturer guarantees, with half of the systematic bias observed in a validation study of the previous capsule version. Though these capsules tend to slightly underestimate temperature, we find they possess excellent validity and reliability between 35 °C and 42 °C.

Firefighter salivary cortisol responses following rapid heat stress.

This research evaluated the impact of rapid heat stress on the rate of salivary cortisol appearance. We hypothesized that rapid heat stress would result in an increased rate of salivary cortisol appearance. Fourteen adult male participants performed an incremental exercise test to a termination criterion (volitional maximum, core temperature = 39.5 °C, or a 2-h time maximum time) with or without firefighting gear in a laboratory with an ambient temperature of 25-26 °C. Salivary cortisol was collected at each 0.5 °C increase in core temperature. We observed a significant increase (p ≤ 0.01) in the rate of cortisol appearance when the subjects were wearing the firefighting gear; no change was observed without firefighting gear. Our results demonstrate that rapid heat stress and the resulting physiological stress cause a rapid increase in the rate of salivary cortisol appearance. Our results also support previous research demonstrating that cortisol is a sensitive strain metric of heat intolerance.