Age-related P50

Generic Clinical Sciences

Oxygen travels in the blood as a soluble gas or bound to hemoglobin. This is most clearly demonstrated with the equation for O2 content: 

(1.34 x Hgb x SaO2) + (Pa02 x 0.003)

The P-50 of oxygen is the oxygen tension at which 50% of the hemoglobin is bound to O2. The P-50 of adults at a temperature of 37 degrees and a pH of 7.4 is 27 mmHg. In the newborn, fetal hemoglobin (HgbF) has extremely high oxygen affinity and the P50 is low (18-19 mmHg,) because 2,3-DPG is low and fetal hemoglobin (HbF) reacts poorly with 2,3-DPG. After birth, total hemoglobin, including HgbF decreases rapidly reaching a nadir at 2-3 months. At the same time, the P-50 rises, exceeding the normal adult value by 4-6 months (30 mm Hg) and reaching the highest at 10 months, and remaining high until the first decade. Children therefore have a lower 02 affinity for hemoglobin than adults and higher rate of O2 unloading. This translates to more efficient 02 unloading with lower levels of hemoglobin in infants and children.


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Jacquelyn Francis, MD