Central venous-to-arterial CO2-gap may increase in severe isovolemic anemia

Despite blood transfusions are administered to restore adequate tissue oxygenation, transfusion guidelines consider only hemoglobin as trigger value, which gives little information about the balance between oxygen delivery and consumption. Central venous oxygen saturation is an alternative, however its changes reflect systemic metabolism and fail to detect regional hypoxia. A complementary parameter to ScvO2 may be central venous-to-arterial carbon dioxide difference (CO2-gap). Our aim was to investigate the change of alternative transfusion trigger values in experimental isovolemic anemia. After splenectomy, anesthetized Vietnamese mini pigs (n = 13, weight range: 18-30 kg) underwent controlled bleeding in five stages (T1-T5). During each stage approximately 10% of the estimated starting total blood volume was removed and immediately replaced with an equal volume of colloid. Hemodynamic measurements and blood gas analysis were then performed. Each stage of bleeding resulted in a significant fall in hemoglobin, the O2-extraction increased significantly from T3 and ScvO2 showed a similar pattern and dropped below the physiological threshold of 70% at T4. By T4 CO2-gap increased significantly and well correlated with VO2/DO2 and ScvO2. To our knowledge, this is the first study to show that anemia caused altered oxygen extraction may have an effect on CO2-gap.