Blood volumes in cardiac surgery with cardiopulmonary bypass

Total blood volume (TBV) estimation potentially impacts various aspects of cardiac surgical care, including pharmacological and transfusion interventions, hemodynamic and volume management and perfusion equipment selection. TBV is commonly computed during cardiopulmonary bypass (CPB), using standardized formulae. We hypothesized that these equations fail to accurately predict individual blood volume variability. The aim of this study was to determine TBV with a dilution technique and compare the results to commonly utilized TBV calculations.

After institutional review board approval, data was prospectively collected and analyzed for 101 patients undergoing open-heart surgery. Hematocrits (Hct) just prior to and immediately after the initiation of CPB were used to calculate the TBV. Results were compared to (1) the Allen formula and (2) weight-based standards (70 ml/kg for males (SM); 65 ml/kg for females (SF)).

The average dilution TBV (male: 4684 ± 1641 ml; female: 3027 ± 1067 ml; total: 4175 ± 1617 ml) was significantly smaller (p<0.05) than TBV estimated by Allen's formula (male: 6328 ± 973 ml; female: 4167 ± 643 ml; total: 5665 ± 1134 ml) and weight-based standards (male: 6278 ± 1256 ml; female: 4924 ± 1064 ml; total: 5862 ± 1350 ml). Allen's formula and the weight-based standards correlated strongly (R(2) = 0.821, p<0.001), suggesting similar estimates of TBV when using these methods. In contrast, hemodilution correlated poorly with the estimates by Allen (R(2) = 0.221, p<0.001) and weight-based formulae (R(2) = 0.122, p<0.001), suggesting different TBV computation.

The dilution method during CPB for TBV estimation is applicable and reproducible in the cardiac surgical arena and can be utilized to calculate TBV. Our results suggest that traditional TBV assessment in cardiac surgical patients by Allen's and weight-based formulae lacks the desired accuracy in estimating true TBV.