COMPENSATORY RESERVE INDEX: NORMATIVE VALUES OF A NEW VITAL SIGN SURROUNDING EXERCISE IN THE HEAT

Abstract

Hypovolemic shock is a threat to the health and safety of active individuals when exposed to extreme environmental conditions. Non-invasive measurement of central volume loss, opposed to traditional vital signs, has been shown to be a stronger indicator of the onset of hypovolemic shock resulting from dehydration or hemorrhage. Compensatory reserve index (CRI), a scale ranging from 0-1.0, categorizes the stages of shock as a predictive measure of the remaining reserve of blood volume before compensation is diminished. Utilization of CRI in a clinical setting has been validated, while normative values surrounding exercise have yet to be established. PURPOSE: To assess normative CRI values surrounding exercise in the heat in active adults. METHODS: Data was collected on 41 active adults who completed a 11 km road race. Age (mean ± standard deviation [SD]): 44.7 ± 15.7 years; VO2max: 42.7 ± 9.2 ml/kg/min; percent body fat: 22.4 ± 9.6%. CRI was assessed at baseline for 10 minutes in a supine position in a thermoneutral environment. At the road race, CRI was assessed for 2 minutes pre-race and post-race in the supine position. Heart rate and oxygen saturation were assessed alongside CRI. Environmental conditions were captured surrounding the race. Core temperature was assessed post-race. Descriptive statistics (mean ± SD) were calculated and paired-samples t-tests were utilized to compare baseline to pre-race, baseline to post-race, and pre-race to post-race. RESULTS: Post-race CRI (mean ± SD: 0.70 ± 0.32) significantly diminished compared to baseline values (0.91 ± 0.07; p < 0.001). Post-race CRI was significantly diminished (p < 0.001) compared to pre-race CRI measures (mean ± SD: 0.88 ± 0.09). Resting heart rate increased from baseline (mean ± SD: 59.6 ± 10.4 bpm) to pre-race (65.4 ± 11.2 bpm) and to post-race (85.1 ± 16.9 bpm). Runners were characterized as hyperthermic following the race (core temperature: 38.90 ± 1.20 °C). Environmental conditions upon finishing the race were 22.78 °C, 53% RH, and 21.67 °C WBGT. CONCLUSIONS: Following physically demanding exercise in the heat, CRI monitoring may be able to detect changes resulting from increased physiological stress and may be utilized to prevent collapse. Future studies should assess the extent to which thermal stress is correlated to changes in CRI.