Acute Lung Injury in the ICU: Focus on Prevention

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.

Central venous oxygen saturation is a good indicator of altered oxygen balance in isovolemic anemia

BACKGROUND

Red blood cell transfusion is done primarily as a means to improve oxygen delivery (DO₂). Current transfusion guidelines are based solely on hemoglobin levels, regardless of actual DO₂ need. As central venous oxygen saturation (ScvO₂) may reflect imbalances in DO₂ and consumption (VO₂) the aim of this study was to investigate the value of ScvO₂ as an indicator of oxygen balance in isovolemic anemia.

METHODS

After splenectomy, anesthetized Vietnamese mini pigs (n = 13, weight range: 18-30 kg) underwent controlled bleeding in five stages (T₀-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.

RESULTS

Each stage of bleeding resulted in a significant fall in hemoglobin, T₀ : 125 (113-134) to T(5) : 49 (43-55) g/l [T₀ : 7.7 (6.9-8.2) to T₅ : 3.0 (2.6-3.4) mmol/l]. The O₂-extraction (VO₂/DO₂) increased significantly only from T₃ : 35 (21-40) %, P < 0.05. The change of ScvO₂ showed a similar pattern and dropped below the physiological threshold of 70% at T₄ : 68 (61-76) %. At this point, hemoglobin was below the recommended transfusion trigger value, 59 (53-67) g/l [3.6 (3.3-4.1) mmol/l]. There was a strong significant association between ScvO₂< 70%) and VO₂/DO₂ > 30%): r = -0.71, r²  = 0.50, P < 0.001.

CONCLUSION

The results of this study show that ScvO₂ reflects changes of VO₂/DO₂ in isovolemic anemia better than Hb alone, therefore it may be used as an additional indicator of blood transfusion in clinical practice.