Physiologic Anemia and the Anemia of Prematurity

Physiologic anemia, occurring 6–8 wk after birth, is a normal process in term infants.

Anemia of prematurity is a pathologic anemia occurring in preterm infants. Extent of prematurity and comorbidities correlate with extent of anemia.

Term infants with physiologic anemia tolerate minor surgery well.

Premature infants must be evaluated for symptoms due to anemia that may contribute to increased risk of preop events.

Major surgery occurring at the physiologic nadir of anemia may require blood transfusion.

Preterm infants with anemia undergoing physiologic stress due to surgery are at risk for tachycardia, tachypnea, lactic acidosis, and periop apnea and bradycardia.

Physiologic anemia is normal response to extrauterine life. Nadir at 9th–12th wk of life, Hgb level varies 9–11 g/dL.

In preterm infants, nadir occurs at 4–8 wk of life and may decrease to 8 g/dL.

Anemia of prematurity may be asymptomatic or give rise to nonspecific symptoms such as tachycardia, tachypnea, lethargy, pallor, apnea and bradycardia, poor feeding, poor growth, and lactic acidosis.

Transition to extrauterine life includes requirement for increased oxygen to bind to hemoglobin (HbO ₂ saturation 50% in utero, 95% ex utero). Fetal hemoglobin with high oxygen affinity starts to be replaced with low-oxygen-affinity adult hemoglobin.

Survival of neonatal erythrocytes is shorter than that of adult erythrocytes. Hemoglobin decreases until oxygen needs are greater than supply. Production of EPO is triggered and erythropoiesis increases.

Rapid growth in infants causes a rapid increase in blood volume, resulting in hemodilution. Growth is more rapid in preterm than term infants.

Preterm infants have more severe anemia because the less sensitive hepatic oxygen sensor triggers EPO production until 40 wk PCA. After 40 wk PCA, an extremely sensitive renal oxygen sensor takes over triggering and production of EPO.

Iron storage occurs in the last trimester; therefore, premature infants are relatively iron deficient and have difficulty increasing iron stores by feeding.

Extent of prematurity correlates with the amount of blood loss due to blood sampling.