Critical values of hematocrit and mixed venous oxygen saturation as parameters for a safe cardiopulmonary bypass

Objective: Mixed venous oxygen saturation (SvO2) is high despite a low hematocrit implies that the relationship between oxygen demand and supply is in a safe state. This study was sought to determine the critical values for hematocrit and SvO2 for safe cardiopulmonary bypass. Methods: Study 1: To evaluate the limit of hemodilution without cardiopulmonary bypass, normovolemic hemodilution with Dextran 40 (10%) was performed in 14 rabbits. SvO2 was monitored from the right atrium, and the hemodynamic parameters were recorded continuously. Study 2: To determine the critical values for hematocrit and SvO2 during cardiopulmonary bypass, normothermic and hypothermic cardiopulmonary bypass were performed in 13 rabbits and hemodynamic parameters were corrected. Results: Study 1: The heart rate decreased to unsafe levels abruptly, when the SvO2 was ≦43% or the hematocrit was ≦10%. The lactate concentration increased when the SvO2 was ≦46% or the hematocrit was ≦12%. Study 2: When the hematocrit was ≦12%, the SvO2 decreased gradually. Even when weaning was possible, the animals with a hematocrit ≦12% collapsed hemodynamically within 40 minutes after cardiopulmonary bypass. Most of the animals could not be weaned from cardiopulmonary bypass during either normothermic or hypothermic cardiopulmonary bypass when the SvO2 was ≦46%. Conclusions: Continuous monitoring of hematocrit and SvO2 provides evidence-based guidelines for safe cardiopulmonary bypass. The lower limits of critical range for a safer cardiopulmonary bypass are hematocrit of 12% and SvO2 of 46%.