Physiologic Anemia of Infancy
Chapter 447 Physiologic Anemia of Infancy
At birth, normal full-term infants have higher hemoglobin and hematocrit levels and larger red blood cells (RBCs) than do older children and adults. However, within the 1st wk of life, a progressive decline in hemoglobin level begins and then persists for 6-8 wk. The resulting anemia is known as the physiologic anemia of infancy. Several factors appear to be involved.
With the onset of respiration at birth, considerably more oxygen becomes available for binding to hemoglobin, and, as a consequence, the hemoglobin-oxygen saturation increases from 50% to 95% or more. There is also a gradual, normal developmental switch from fetal to adult hemoglobin synthesis after birth that results in the replacement of high-oxygen-affinity fetal hemoglobin with lower-affinity adult hemoglobin, capable of delivering more oxygen to tissues. The increase in blood oxygen content and delivery results in the downregulation of erythropoietin (EPO) production, leading to suppression of erythropoiesis. Because there is no erythropoiesis, aged RBCs that are removed from the circulation are not replaced and the hemoglobin level decreases. The hemoglobin concentration continues to decline until tissue oxygen needs become greater than oxygen delivery. Normally, this point is reached between 8 and 12 wk of age, when the hemoglobin concentration is about 11 g/dL. At this juncture, EPO production increases and erythropoiesis resumes. The supply of stored reticuloendothelial iron, derived from previously degraded RBCs, remains sufficient for this renewed hemoglobin synthesis, even in the absence of dietary iron intake, until approximately 20 wk of age. In all, this “anemia” should be viewed as a physiologic adaptation to extrauterine life, reflecting the excess oxygen delivery relative to tissue oxygen requirements. There is no hematologic problem, and no therapy is required.
Premature infants also develop a physiologic anemia, known as physiologic anemia of prematurity. The hemoglobin decline is both more extreme and more rapid. Minimal hemoglobin levels of 7-9 g/dL commonly are reached by 3-6 wk of age, and levels may be even lower in very small premature infants (Chapter 97
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