Improved Estimation of Total Blood Volume Can Provide a Reliable Prediction of Dilutional Hematocrit and Oxygen Delivery during Cardiopulmonary Bypass

Abstract

Optimal oxygen delivery during cardiopulmonary bypass (CPB) for open heart surgery is crucial to protect organ function, improve outcomes, and reduce hospital stay. Effective preparation for optimal oxygen delivery during CPB is dependent on a reliable estimation of total blood volume (TBV) to accurately predict dilutional hematocrit (Hct) and calculate indexed oxygen delivery (iDO2). Unreliable estimation of TBV is a major reason for inaccurate prediction of dilutional Hct and iDO2. We performed a retrospective study of 113 patients who underwent cardiac surgery to compare the accuracy of predicted versus measured Hct and iDO2. We used four conventional methods of TBV estimation: 1) weight based A; 70 mL/kg for all patients, 2) weight based B; 70 mL/kg for males and 65 mL/kg for females, 3) Allen’s formula, and 4) Nadler’s formula. We found that TBV estimated by Allen’s formula predicted dilutional Hct better than the other three. However, all four methods overestimate TBV when Hct is low and underestimate when Hct is high, suggesting that TBV is dependent on Hct in addition to body size and gender. Our analysis indicates that two individuals with the same body size and gender can have different TBV depending on Hct. We revised Allen’s formula by adding a component that adjusts TBV depending on Hct to provide a more accurate prediction of dilutional Hct and iDO2.