Fig. 1.

Oxyhemoglobin dissociation curve (CaO₂ versus PaO₂). The higher oxygen affinity of fetal hemoglobin (blue curve) shifts the oxygen dissociation curve to the left, which results in a greater release in oxygen at a lower partial pressure of oxygen (PO₂) compared with adult hemoglobin (red curve). Only a small change in PO₂ (≈10 mm Hg) from the higher oxygenated fetal venous blood (30–40 mm Hg) to the arterial blood (20–30 mm Hg) results in a release of ≈25% of oxygen to the tissues. In the pregnant woman, however, a similar release in oxygen from adult hemoglobin requires a drop from the arterial PO₂ (97 mm Hg) to the venous PO₂ (40 mm Hg) of 57 mm Hg. In a neonate, anemia (hyphenated orange curve) is associated with lower CaO₂ without any change in P50 (partial pressure of oxygen resulting in 50% oxygen saturation or 50% peak CaO₂). P50 is shown as an open circle along the oxygen–hemoglobin dissociation curve. Transfusion with packed red cells from an adult source (with hemoglobin (Hgb) A) results in increased CaO₂ and also an increase in P50 towards adult values. MAPO₂, maternal arterial PO₂; MVPO₂, maternal venous PO₂; UAPO₂, umbilical artery PO₂; UVPO₂, umbilical vein PO₂. Data from Rudolph (2009). Note that the oxyhemoglobin dissociation curve is often presented with SaO₂ on the y-axis; we opted to present these data with CaO₂ on the y-axis to more clearly demonstrate the impact of blood transfusion.