Anemia of prematurity

Anemia of prematurity is a normochromic, normocytic anemia commonly seen in premature infants cared for in the neonatal intensive care unit.

Pathophysiology
The total volume of blood in premature infants is approximately 100ml/kg of body weight--approximately 5 ounces for a 1.5kg infant. Blood sampling done for laboratory testing in the first days of life can easily remove enough blood to produce anemia. As anemia develops, the amount of oxygen delivered by the hemoglobin in the blood to the body organs declines. Normally this stimulates increased production of erythropoietin (EPO), but this response is diminished in premature infants. While the reason for this decreased response is not fully understood, it is theorized that there is a genetically timed switch from hepatic production of EPO, which occurs in-utero, to renal production. Since hepatic production is stimulated by lower levels of oxygen delivery (reflecting the lower levels present in the fetus) and since the red blood cells are carrying higher amounts of oxygen after birth, the level of red blood cells itself must drop significantly before EPO production will begin in premature infants who have not yet switched from hepatic to renal EPO production. This level may be as low as a hemoglobin of 6.5g/dL, corresponding to an hematocrit of approximately 19.

Treatment
The best treatment is prevention by minimizing the amount of blood drawn from the infant. Modern technology, including non-invasive pulse-oximetry, development of laboratory equipment that can use very small blood volume samples, and in-dwelling monitors that can measure blood-gas parameters without removing any blood have helped delay or even eliminate anemia of prematurity in some infants. Recombinant EPO may be given to premature infants to stimulate red blood cell production. Premature infants are equally responsive to EPO as term infants. However, the response to EPO typically takes up to 2 weeks. To date, studies of EPO use in premature infants have had mixed results. It is likely that only a carefully selected subpopulation of infants may benefit from its use.

Outcome
Anemia of prematurity may be tolerated in the otherwise healthy premature infant without complications and will eventually spontaneously resolve when EPO production resumes. However, hemoglobin levels < 10g/dL may be associated with decreased growth, evidence of compensatory increased cardio-respiratory response (tachycardia and tachypnea), and an increase in apnea. Treatment of symptomatic anemia of prematurity is with blood transfusion.