TYK3

Which of the following concerning oxygen delivery is true?

A. The P50 value is highest at 10 months of age

B. The P50 value is lowest at 2 months of age

C. Physiologic anemia occurs at 2-3 weeks of age

D. The P50 value reaches adult values at 4-6 weeks of age.

Answer

The answer is A. In the neonate, blood oxygen affinity is extremely high and P50 is low (18 mm Hg). This variation in the oxygen-hemoglobin dissociation curve is due to a low 2,3-DPG level and the presence of fetal hemoglobin (HbF). Fetal hemoglobin reacts poorly with 2,3 DPG and has a higher affinity for oxygen than the adult hemoglobin. Therefore, immediately after birth the oxygen delivery to the tissues is low. Despite having a high oxygen carrying capacity (high hemoglobin), the inability to unload the oxygen at the tissues makes the delivery to the tissues suboptimal. After birth, the amount of total hemoglobin falls. This creates a decrease in total hemoglobin, fetal hemoglobin and an increase in percentage of adult hemoglobin as it is produced after birth. This fall in total hemoglobin reaches its nadir by 2 to 3 months of age (physiologic anemia of infancy). During this time as the amount of fetal hemoglobin decreases, the P50 value increases and exceeds the adult value by 4-6 months and reaches its highest value at 10 months (P50 = 30). This high P50 is most likely due to an increased level of 2,3-DPG. This increased P50 level for the infant may explain why these patients are able to tolerate a lower total hemoglobin (physiologic nadir) without any impairment of oxygen delivery.

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