RESUMO
Acute normovolemic hemodilution (ANH) is a blood-conservation method in which whole blood (WB(ANH)) is sequestered into blood collection bags before the commencement of surgery. However, for cardiac surgery, extracorporeal circuit (ECC) priming techniques limit the amount of blood that may collected prior to the actual initiation of cardiopulmonary bypass (CPB). In this study, computational modeling was used to examine the effect of reducing extracorporeal "pump" prime volume (PPV) on WB(ANH) prior to the CPB. Increments of estimated blood volume (EBV), precardiopulmonary bypass hemoglobin concentration [Hb(Pre-CPB)], and PPV volume were manipulated to assess effects on predicted hemoglobin concentration during cardiopulmonary bypass [Hb(CPB)]. Similarly, increments of EBV and preanesthetic hemoglobin concentration [Hb(Pre-Anes)] were manipulated to examine the change in WB(ANH) volume. The impact of PPV reduction on the minimum acceptable precardiopulmonary bypass hemoglobin concentration [Hb(Pre-CPB-MA)] was then measured by computing PPV, EBV, and the minimum acceptable cardiopulmonary bypass hemoglobin [Hb(CPB-MA)]. Finally, by manipulating EBV and target hemoglobin concentration [Hb(Target)], the change in [Hb(Pre-CPB)] produced by PPV reduction was used to quantify the effect on WB(ANH) volume. The net increase in the [Hb(CPB)] produced by PPV reduction is inversely proportional to EBV. Higher [Hb(Pre-Anes)] or lower [Hb(Target)] facilitates sequestration of larger WB(ANH) volume. Although PPV and [Hb(Pre-CPB-MA)] bear a direct relationship, as EBV decreases, proportionally greater increases in [Hb(Pre-CPB-MA)] occur. The impact of PPV reduction on precardiopulmonary bypass hemoglobin concentration [Hb(Pre-CPB)] is reflected by the "excess hemoglobin" over the minimum hemoglobin threshold (designated as 7 g/dL in this study). For each 100-mL decrement in PPV, "excess hemoglobin" increases from 1% (EBV = 8000 mL) to 2% (EBV = 4000 mL). In turn, increases in "excess hemoglobin" are associated with expansion of WB(ANH) volume. In conclusion, sequential PPV reduction from 2000 mL increases the volume of WB(ANH) that potentially may be sequestered prior to initiation of CPB. Therefore, the combination of PPV reduction with ANH may represent a useful blood conservation approach in adult patients undergoing cardiac surgery with CPB.
