A Novel Calculation to Estimate Blood Volume and Hematocrit During Bypass

Abstract

Patient blood volume impacts most facets of perfusion care, including volume management, transfusion practices, and pharmacologic interventions Unfortunately, there is a wide variability in individual blood volumes, and experimental measurement is not practical in the clinical environment. The purpose of this study was to evaluate a mathematical algorithm for estimating individual blood volume. After institutional review board approval, volumetric and transfusion data were prospectively collected for 165 patients and applied to a series of calculations. The resultant blood volume estimate (BVE) was used to predict the first and last bypass hematocrit. The estimated hematocrits using both BVE and 65 mL/kg were compared with measured hematocrits using the Pearson moment correlation coefficient and the Bland Altman measures of accuracy and precision. There was a wide range of BVE (minimum, 35 mL/kg; mean ± SD, 64 ± 22 mL/kg; maximum, 129 mL/kg). Using BVE, the estimated hematocrit was similar to the measured first (24.7 ± 6.4% vs. 24.5 ± 6.2%, r = 0.9884, p > .05) and last (24.5 ± 5.9% vs. 25.1 ± 5.7%, r = 0.9001, p > .05) bypass hematocrit. Using 65 mL/kg resulted in a larger difference between estimated and measured hematocrits for the first (25.6 ± 4.5% vs. 24.5 ± 6.2%, r = 0.6885, p = .030) and last (23.8 ± 3.6% vs. 25.1 ± 5.7%, r = 0.5990, p = .001) bypass hematocrits. Compared with using 65 mL/kg for blood volume, the BVE allowed for a more precise estimated hematocrit during CPB.