Fetal Hb: Oxygen transport

Oxygen travels in the blood as a soluble gas or bound to hemoglobin. A significantly larger portion of oxygen is bound to hemoglobin and a relatively smaller portion is dissolved in blood. This is characterized best by examining the formula for oxygen content in arterial blood [CaO₂=(1.34 x hb x O₂ sat) + 0.003 PaO₂]. Fetal hemoglobin (Hb F) more avidly binds oxygen than adult hemoglobin (Hb A). This binding of oxygen shifts the oxygen –hemoglobin disassociation curve to the left and less oxygen is unloaded. The P50 (partial pressure of oxygen when the oxygen saturation is 50%) for hb F is 19.7 mmhg compared to 27 mmgh for adults and 30 mmhg for 10 month old infants. The presence of Hb F is the primary reason P50 is lowest for the neonate. By 2-3 months of age Hb F is gone and the P50 steadily increase until it reaches its peak at 10-12 months of age. Oxygen affinity is lower in infants than adults. The P50 then decreases to the adult value at approximately 10-12 years of age as 2-3 DPG falls. As hb F decreases so does oxygen affinity. More oxygen is unloaded for any given partial pressure of oxygen. Other variables that shift the oxygen-hemoglobin disassociation curve include pH, temperature, and 2-3 DPG. An increase in 2-3 DPG facilitates unloading of oxygen and shifts the curve to the right. An increase in pH increases oxygen affinity and shifts the curve to the left. Likewise, a decrease in temperature increases oxygen affinity and shifts the curve to the left. Dr Motoyama described a hemoglobin requirement for equivalent tissue oxygen delivery for neonates, infants and adults based on the oxygen affinity of hemoglobin. The hemoglobin required to transport an equivalent amount of oxygen is 14-15 g/dl for the neonate, 8 g/dl for the infant, and 10 g/dl for the adult.