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Standard methods of core temperature (Tc) measurement, in the oesophagus (Tes) or rectum (Tre), are poorly suited to athletic, occupational or military application, hence the desire for development of less obtrusive indices. Gastro-intestinal radio-pill (Tgi), infra-red tympanic (Tty) and insulated skin (Tinskin) temperatures may provide a solution but are not faultless. The measurement of Tgi is expensive and incurs problems with sequential application. Tty is readily contaminated by ambient temperature if methodology is inadequate. Tinskin shows some promise as a surrogate index of Tc (Taylor, et al., 1998), but has not been fully validated. We examined the accuracy of Tinskin as a surrogate measure of Tc in military applications - including periods of rising, falling and static Tc, under various environmental conditions. Subjects were thirteen heat-acclimatised, euhydrated, healthy volunteers from the Australian Army (mean±SD: age = 25±5 y; height = 173±11 cm; mass = 74±12 kg). Following two familiarisation sessions, subjects participated in experimental sessions with various environmental conditions; Wet-Bulb Globe Temperature (WBGT) = 21.2°C (Dry Bulb (DB)=25°C), 25.9°C (DB=30°C), 29.7°C (DB=40°C) and/or 32.2°C (DB=35°C). Each session was conducted at least one week apart and consisted of 15-min seated rest (REST), 45-min treadmill walking (5-6 kph at 5-10% grad; WALK1), 15-min manual load handling (repeatedly lifting and carrying a 20-kg crate; LOAD), a second walk, of up to 60 min (5-7 kph at 0-10% grad; WALK2) and 20-min seated recovery (RECOV). Subjects wore standard army combat uniform and carried a 20-kg pack during walk phases. Tc was measured at 1-min intervals from Tre, Tes, Tgi and Tinskin (positioned over spine at T2-T4), and from Tty at 15-min intervals. Tinskin showed a stronger association with Tes (r=0.68, n=3424, p<0.01) than with Tre (r=0.64, n=3957, p<0.01), independently of environmental condition. When separated by environmental condition the associations become stronger with increasing heat stress. For example, the relationship between Tes and Tinskin improved with greater heat stress (WBGT: 21.2°C, r=0.42; 25.9°C, r=0.65; 29.7°C, r=0.78; 32.2°C, r=0.80). When separated by exercise phase, Tinskin predicted Tc poorly during REST (eg. Tes: WBGT 32.2°C, r=0.04, n=136, p>0.05, SEE=0.13) and LOAD carriage (eg. Tes: WBGT 32.2°C, r=0.4, n=113, p<0.01, SEE=0.37), even under increased heat stress. However during WALK1, WALK2 and RECOV, Tes and Tinskin associations ranged from moderate to strong: r=0.67, n=382, p<0.01; r=0.56, n=107, p<0.01; r=0.86, n=127, p<0.01, respectively, depending on phase. Tgi had a stronger association with Tre (r=0.92, n=3198, p<0.01; Tre = 0.922Tgi + 2.84, SEE=0.25) than with Tes (r=0.83, n=2652, p<0.01; Tes = 0.746Tgi + 9.19, SEE=0.34), whereas Tty tended to be a poorer predictor of both Tre (r=0.69, n=334, p<0.01; Tre = 0.47Tty + 20.0, SEE=0.49) and Tes (r=0.77, n=310, p<0.01; Tes = 0.47Tty + 19.5, SEE=0.39). In summary, Tinskin represented Tes with more confidence than Tre. The relationship between Tinskin and Tcs improved with increasing heat stress. Exercise phases where Tc remains relatively constant displayed an uncoupling of Tinskin and Tcs, whereas epochs with increasing or decreasing Tc produce moderate to strong Tc to Tinskin dependence. Tgi by radio-pill thermometry generally had a stronger association with standard measures of Tc than did Tty by infra-red thermometry.
Taylor, N.A.S., Wilsmore, B.R., Amos, D., Takken, T. & Komen, T. (1998) Insulated skin temperatures: Indirect indices of human body-core temperature. DSTO-TR-0752. Melb.