Comparison of Physiological Strain Index and Core Temperature Rise for Classifying Heat Tolerance Among Warfighters: 2081 Board #237 May 30 2:00 PM - 3:30 PM

The heat tolerance test (HTT) is used by the military to assist with return-to-duty decisions for heat-injured warfighters. Criteria for determining heat tolerance are generally accepted as core temperature (Tcore) < 38.6°C and heart rate (HR) < 160 bpm during 120 min of treadmill walking in a compensable heat stress environment. Additional information for classifying heat tolerance may be gained by evaluating the final 60 min of an HTT, as Druyan et al. (2013) determined a limit of < 0.45°C Tcore increase during the final 60 min (ΔTcoreF60). Use of calculations that encompass both Tcore and HR, such as the physiological strain index (PSI), may also provide useful information to aid in return-to-duty decisions. PURPOSE: To determine the validity of PSI increase during the final 60 min (ΔPSIF60) of an HTT for classifying heat tolerance and compare it with the ΔTcoreF60 criterion established by Druyan et al. METHODS: Using traditional HR and Tcore criteria, 15 males were classified as heat-tolerant (HT) (age: 27 ± 5 yrs, height: 177.2 ± 6.5 cm, weight: 82.4 ± 10.1 kg) and 15 males were classified as heat-intolerant (HI) (age: 27 ± 7 yrs, height: 177.3 ± 7.6 cm, weight: 86.7 ± 14.5 kg). ΔPSIF60 values were calculated for all subjects and compared with a previously determined ΔPSIF60 limit of 1.82 to confirm validity. Additionally, ΔPSIF60 was calculated for a sample of 77 male warfighters (age: 25 ± 5 yrs, height: 178.5 ± 7.1 cm, weight: 84.8 ± 10.1 kg) who were classified as HT/HI according to ΔPSIF60 and ΔTcoreF60 thresholds. The number of misclassifications for ΔPSIF60 and ΔTcoreF60 (Druyan et al.) were then compared. RESULTS: ΔPSIF60 threshold of 1.82 was validated by comparing it to ΔPSIF60 for HT and HI warfighters (HT: 0.54 ± 0.64, p < .001; HI: 1.95 ± 0.72, p = .318). Misclassifications of HT/HI for ΔPSIF60 and ΔTcoreF60 were 8 (10%) and 7 (9%), respectively. Sensitivity and specificity of the proposed ΔPSIF60 HT/HI criterion were 97% and 54%, respectively, versus 95% and 70% for ΔTcoreF60. CONCLUSION: Findings suggest that 1.82 ΔPSIF60 and 0.45°C ΔTcoreF60 thresholds to determine heat tolerance yield a similar number of misclassifications. Future work should aim to refine these techniques to reduce the number of HT/HI misclassifications.