Significance Of Body Surface Area-to-mass Ratio In The Analysis Of Heat Tolerance Test Results

A heat tolerance test (HTT) is commonly used to evaluate an individual's ability to tolerate heat stress. This functional test is advocated as part of the decision on return to duty/practice (RTD/P) of individuals who have recently recovered from exertional heat illness (EHI). PURPOSE: To evaluate the relationship of body surface area-to-mass ratio (BSA/M) to the results of a standard Israel Defence Forces (IDF) HTT. We hypothesized that individuals defined as heat tolerant (HT) would have on average higher BSA/M compared to heat intolerant (HI) individuals, indicating a higher potential of evaporative heat loss over metabolic heat production. METHODS: We analysed the IDF-HTT results of 517 soldiers (age: 18-38 yrs., M/F: 95.7/4.3%). HI was defined when body-core temperature did not plateau during the second hour of the test; accordingly, 6.7% were HI. We performed logistic regression analysis adjusted for potential confounders (age, sex) to evaluate the predictive capacity of BSA/M for heat intolerance. Spline model was applied for the logistic regression analysis. RESULTS: BSA/M of HI individuals was lower than for HT individuals (250 vs. 260 cm2/kg, p < 0.05). In the analysis of women alone (n = 22), a similar trend was identified (262 vs. 273 cm2/kg) although not statistically significant (p > 0.05), probably because of a small sample size. The OR for heat intolerance for every unit increase in BSA/M was 0.97 (CI 95% 0.95-0.99). The spline model demonstrated a concave curve that plateaus above BSA/M of 275 cm2/kg (see figure). CONCLUSIONS: In our cohort, the likelihood of HI decreased with every unit increase in BSA/M. These results have implications for clinical decision-making on RTD/P of soldiers, athletes and others who have recovered from EHI. The association of BSA/M to HI in females was consistent with that of males, but likely due to small sample size was not statistically significant.