Comparison of Compensatory Reserve Index during Lower Body Negative Pressure and Hemorrhage

Abstract

A machine learning algorithm has been developed to detect reductions in central blood volume during lower body negative pressure (LBNP), a model of hemorrhage (Hem) which causes shifts in blood to the legs and elicits central hypovolemia. The algorithm calculates compensatory reserve index (CRI) where CRI=1 represents normovolemia and CRI=0 represents hypovolemia at the point of cardiovascular decompensation. The purpose of this study was to test the hypothesis that actual Hem and matching LBNP applications will elicit similar CRI values. This study compared CRI during Hem and matching LBNP in baboons. CRI was calculated from the arterial pressure waveform measured in baboons during Hem and LBNP (n=13). Blood was removed in four steps: 6.25%, 12.5%, 18.75%, and 25% of total blood volume. Shed blood was returned and the animals were allowed to recover. Four weeks after the Hem protocol, the same animals were subjected to four levels of LBNP which matched cardiovascular responses during Hem. Statistical analysis was performed using two way repeated measures ANOVA and linear regression analysis. Data are shown below as mean±95% confidence interval. CRI decreased significantly during each step of Hem and LBNP (p 蠄 0.003). CRI was not different between Hem and LBNP. Linear regression analysis between Hem CRI and LBNP CRI revealed slope of 1.03 and an adjusted R2 of 0.98. Hem Volume (ml/kg) Hem CRI LBNP (mm Hg) LBNP CRI 0 0.69 ± 0.10 0 0.76 ± 0.05 4.5 ± 0.1 0.57 ± 0.09 -22 ± 4 0.66 ± 0.08 9.1 ± 0.1 0.36 ± 0.08 -41 ± 4 0.36 ± 0.13 13.6 ± 0.2 0.16 ± 0.08 -54 ± 6 0.23 ± 0.11 17.8 ± 0.4 0.08 ± 0.03 -71 ± 5 0.14 ± 0.09 Conclusions Stepwise Hem and matching LBNP levels which are used as surrogates for actual blood loss elicited the same values of CRI. This study supports the validity of CRI as an index of central hypovolemia associated with Hem. FUNDING: US Army MRMC Combat Casualty Care Research Program