Comparison of compensatory reserve during lower-body negative pressure and hemorrhage in nonhuman primates

Abstract

Compensatory reserve was measured in baboons (n = 13) during hemorrhage (Hem) and lower-body negative pressure (LBNP) using a machine-learning algorithm developed to estimate compensatory reserve by detecting reductions in central blood volume during LBNP. The algorithm calculates compensatory reserve index (CRI) from normovolemia (CRI = 1) to cardiovascular decompensation (CRI = 0). The hypothesis was that Hem and LBNP will elicit similar CRI values and that CRI would have higher specificity than stroke volume (SV) in predicting decompensation. Blood was removed in four steps: 6.25%, 12.5%, 18.75%, and 25% of total blood volume. Four weeks after Hem, the same animals were subjected to four levels of LBNP that was matched on the basis of their central venous pressure. Data (mean ± 95% confidence interval) indicate that CRI decreased (P < 0.001) from baseline during Hem (0.69 ± 0.10, 0.57 ± 0.09, 0.36 ± 0.10, 0.16 ± 0.08, and 0.08 ± 0.03) and LBNP (0.76 ± 0.05, 0.66 ± 0.08, 0.36 ± 0.13, 0.23 ± 0.11, and 0.14 ± 0.09). CRI was not different between Hem and LBNP (P = 0.20). Linear regression analysis between Hem CRI and LBNP CRI revealed a slope of 1.03 and a correlation coefficient of 0.96. CRI exhibited greater specificity than SV in both Hem (92.3 vs. 82.1) and LBNP (94.8 vs. 83.1) and greater ROC AUC in Hem (0.94 vs. 0.84) and LBNP (0.94 vs. 0.92). These data support the hypothesis that Hem and LBNP elicited the same CRI response, suggesting that measurement of compensatory reserve is superior to SV as a predictor of cardiovascular decompensation.

Keywords: blood loss; blood pressure; central hypovolemia; compensatory mechanisms; stroke volume.

Figures

Fig. 1.

Illustration of the different characteristics of an arterial pressure waveform during normovolemia (A) and hypovolemia (B). The features of the arterial pressure waveform (gray outline) are dependent on systolic and diastolic pressure, as well as and the ejected and reflected waves of the pressure pulse. This illustration has been previously published (11).

Fig. 2.

Compensatory reserve index (CRI) during baseline and four steps of hemorrhage (●) and lower-body negative pressure (LBNP; ○) corresponding to 6.25%, 12.5%, 18.75%, and 25% total blood volume loss. CRI values during hemorrhage or LBNP were not statistically different from one another. Data are expressed as means ± 95% confidence interval. P values from two-way repeated ANOVA are shown.

Fig. 3.

Correlation between compensatory reserve index (CRI) during hemorrhage and LBNP for baseline, 6.25%, 12.5%, 18.75%, and 25% total blood volume loss. Linear regression analysis revealed an amalgamated correlation coefficient between CRI during hemorrhage and LBNP to be 0.96 ± 0.04. Data are expressed as means ± 95% confidence interval.

Fig. 4.

Changes in stroke volume (A) and compensatory reserve (B) during baseline, 6.25%, 12.5%, 18.75%, and 25% total blood volume loss in animals with low tolerance (○, broken lines) and high tolerance (●, solid lines) to hemorrhage. Data are expressed as means ± 95% confidence interval. P values are reported for tests of difference in slopes between low and high tolerance for stroke volume and compensatory reserve index.