The hypoxic intrauterine environment predisposes the fetus to adaptive changes to support intrauterine growth which includes high hemoglobin concentration. After birth the abundance of oxygen results in removal of this hypoxic stimulation to the bone marrow. All infants experience a decrease in hemoglobin concentrations after birth, as the infant transitions from a relatively hypoxic state in utero to a relatively hyperoxic state in room air<xref ref-type="bibr" rid="bib001">1</xref>. Erythropoiesis decreases after birth as a result of increased tissue oxygenation and an associated decrease in production of erythropoietin(EPO). Immediately after birth placental transfusion determines the hemoglobin in the first few hours. This is also influenced by the fluid (oral or intravenous) intake of the baby soon after birth. The hemoglobin level plateaus out by the end of 12 hours and remains fairly steady except for iatrogenic losses till about the first week of life. Thereafter there is a gradual fall in hemoglobin due to decreased red cell production, the shortened lifespan of the red blood cell, plasma dilution and the increase in blood volume of the infant consequent to growth. These contribute to the ‘physiological anemia of the newborn’. By the end of 8 to 12 weeks of life, the hemoglobin reaches a nadir in a term infant and erythropoiesis starts and the hemoglobin rises.</sec><sec id="sec002"><title>Definition

Journal of Neonatology

0971-2179

National Neonatology Forum

jn-21-2-008

REVIEW ARTICLE

Anemia of Prematurity - Do we know enough

Reddy

Sameera

Shenoi

Arvind

Neonatal Services, Department of Pediatrics, Manipal Hospital, Bangalore 560017.

*E-mail: arvind.shenoi@gmail.com

4–62007

212113116

The hypoxic intrauterine environment predisposes the fetus to adaptive changes to support intrauterine growth which includes high hemoglobin concentration. After birth the abundance of oxygen results in removal of this hypoxic stimulation to the bone marrow. All infants experience a decrease in hemoglobin concentrations after birth, as the infant transitions from a relatively hypoxic state in utero to a relatively hyperoxic state in room air<xref ref-type="bibr" rid="bib001">1</xref>. Erythropoiesis decreases after birth as a result of increased tissue oxygenation and an associated decrease in production of erythropoietin(EPO). Immediately after birth placental transfusion determines the hemoglobin in the first few hours. This is also influenced by the fluid (oral or intravenous) intake of the baby soon after birth. The hemoglobin level plateaus out by the end of 12 hours and remains fairly steady except for iatrogenic losses till about the first week of life. Thereafter there is a gradual fall in hemoglobin due to decreased red cell production, the shortened lifespan of the red blood cell, plasma dilution and the increase in blood volume of the infant consequent to growth. These contribute to the ‘physiological anemia of the newborn’. By the end of 8 to 12 weeks of life, the hemoglobin reaches a nadir in a term infant and erythropoiesis starts and the hemoglobin rises.</sec><sec id="sec002"><title>Definition

Anemia of prematurity (AOP) is an exaggerated and pathologic response of the preterm infant to this transition. AOP is a normocytic, normochromic, hypo regenerative anemia that is characterized by the existence of a low serum EPO level in an infant who has what may be a remarkably reduced hemoglobin concentration. While the physiology and pathophysiology of AOP are well studied, controversy exists regarding the timing, method, and effectiveness of therapeutic interventions for AOP.

Pathophysiology

The basic mechanisms for the development of AOP include inadequate red blood cell (RBC) production for a growing premature infant, shortened RBC life span or hemolysis, and blood loss. The major physiologic impact of anemia is decreased oxygen delivery to tissue, resulting in both compensatory cardio-vascular responses and chronic consequences like poor growth, decreased activity and limited cardiovascular reserve.

Inadequate red blood cell production

The first mechanism of anemia is inadequate RBC production for the growing premature infant. The location of EPO and RBC production changes during gestation of the fetus. EPO does not cross placenta but is produced by the fetus. Levels rise with increasing gestation and are high in cord blood at birth. High EPO levels are thought to be physiologic response to low oxygen tension in-utero and are responsible for high Hb at birth. EPO levels fall soon after birth and remain inappropriately low for the level of anemia. This blunted response is thought to be partly due to the fact that at this age the switch from liver to kidney as the site for EPO production is incomplete and the liver may be less sensitive to hypoxia than kidney.

The number of erythroid progenitor cells is shown to be relatively low for the degree of anemia presumably secondary to the lack of erythropoietin drive but burst forming unit-erythroid(BFU-E) colonies retain their responsiveness to EPO. ² In the first few weeks of embryogenesis, fetal erythrocytes are produced in the yolk sac.^3. This site is succeeded by the fetal liver, which, by the end of the first trimester, has become the primary site of erythropoiesis. Bone marrow then begins to take on a more active role in producing erythrocytes. By approximately 32 weeks’ gestation, the burden of erythrocyte production in the fetus is shared evenly between the liver and bone marrow. By 40 weeks’ gestation, the marrow is the sole erythroid organ. Premature delivery does not accelerate the ontogeny of these processes.

In addition, EPO, whether endogenously produced or exogenously administered, has a larger volume of distribution and is eliminated more rapidly by neonates, resulting in a curtailed time for bone marrow stimulation. Erythroid progenitors of premature infants are quite responsive to EPO when that growth factor finally is produced or administered, but the response may be blunted if iron stores are insufficient. While the infant's response may produce increased EPO concentrations and reticulocyte counts, the infant's rapid growth may prevent the appropriate increase in hemoglobin concentration.

Shortened red blood cell life span or hemolysis

The average life span of a neonatal RBC is only one-half to two thirds that of the RBC life span in an adult. Cells of the most immature infants may survive only 35–50 days. The shortened RBC life span of the neonate is a result of multiple factors, including diminished levels of intracellular ATP, carnitine, and enzyme activity, increased susceptibility to lipid peroxidation; and increased susceptibility of the cell membrane to fragmentation.

Blood loss

The total volume of blood in premature infants is approximately 100ml/kg of body weight. 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, but this response is diminished in premature infants.⁴ Other causes of blood loss like placental abruption, placenta previa, feto-maternal transfusion, twin-twin transfusion, cephalhematoma also play a role.

Nutritional deficiencies

Serum vitamin E levels are reduced in newborn infants. It has been reported that this deficiency is responsible, in part, for the development of anemia in premature infants during the first 6 weeks of life. However there is no evidence to support a policy of administering vitamin E to premature infants to prevent the anemia of prematurity.⁵

Nutritional deficiencies of iron, vitamin B₁₂, and folate may exaggerate the degree of anemia but supplementation of the same does not prevent AOP.

Age

More immature an infant, the more likely the development of AOP. This typically is not a significant issue for infants born beyond 32 weeks of gestation.

The nadir of the hemoglobin level typically is observed when the tiniest infants are aged 4–10 weeks, with concentrations of 8–10 g/dl if birth weight was 1200–1400 grams or 6–9 g/dl if birth weight was less than 1200 grams.

Symptoms

Few symptoms are universally accepted as attributable to AOP; however, the following are among the symptoms that clinicians attribute to anemia of prematurity. Debate regarding the presence or absence of physical findings in the infant with AOP is ongoing. Clinical trials designed to determine the efficacy of blood transfusions in relieving these findings have produced conflicting results.

Poor growth despite adequate caloric intake

Apnea if severe enough, anemia may result in respiratory depression manifested by increased periodic breathing and apnea.

Decreased activity: Lethargy frequently is attributed to anemia, with subjective improvement subsequent to transfusion.

Metabolic acidosis: Significant anemia can result in decreased oxygen-carrying capacity less than the needs of the tissue, resulting in increased anaerobic metabolism with production of lactic acid. Blood transfusions have been documented to decrease lactic acid levels in otherwise healthy infants who are anemic and premature. So it is suggested to use lactate levels as an aid in determining the need for transfusion.

Tachycardia

Tachypnea

Flow murmurs

ROP: Anemia did not affect severity of ROP as an independent risk factor. However, the number of blood transfusions did affect the highest stage of ROP in this group of premature infants. Infants who remained severely anemic (Hgb<8 g/dl or Hct<25%) for longer periods of time developed milder ROP than less anemic infants.

Laboratory evaluation

CBC: Normal white cell and platelet counts, low hemoglobin with normal RBC indices.

Peripheral blood Smear shows Normocytic, normochromic anemia

Reticulocyte count is low when the degree of anemia is considered as a result of the low levels of EPO. A rising reticulocyte count may not predict recovery from anemia of prematurity.

Direct Coombs test: This test may be coincidentally positive; however, with such a finding, ensure that an immune-mediated hemolytic process is not ongoing.

Serum bilirubin: With an elevated serum bilirubin level, consider other possible explanations for the anemia

Management

The treatment options available to the clinician treating an infant with anemia of prematurity are prevention, blood transfusion, and recombinant EPO treatment.

Blood transfusion

Packed red blood cell (PRBC) transfusions: Despite disagreement regarding timing and efficacy, PRBC transfusions continue to be the mainstay of therapy for the individual with AOP. The frequency of blood transfusions varies with gestational age, degree of illness, and, interestingly, the hospital evaluated. The decision to give a transfusion should not be made lightly, as significant infectious, hematologic, immunologic, and metabolic complications exist. Transfusions also transiently decrease erythropoiesis and EPO levels, but this effect is not sustained.

Recently, the Premature Infant in Need of Transfusion (PINT) study demonstrated that transfusing infants to maintain a high hemoglobin level (8.5–13.5 g/dL) confers no benefit in terms of mortality, severe morbidity, or apnea intervention compared with infants transfused to maintain a lower hemoglobin level (7.5–11.5 g/dL). This differs from a study from Iowa, which found fewer brain injuries and less apnea in infants whose transfusion criteria was not restricted and whose hemoglobin level was higher. Clearly, no set guidelines for transfusion in infants with anemia of prematurity are prescribed.⁶

Using various audit criteria and indications for transfusions suggested by Canadian, American, and British authorities, the Medical University of South Carolina has instituted the following transfusion guidelines:

Do not transfuse for phlebotomy losses alone.

Do not transfuse for hematocrit alone, unless the hematocrit level is less than 21% with a reticulocyte count less than 1%.

Transfuse for shock associated with acute blood loss,

For an infant with cyanotic heart disease, maintain a hemoglobin level that provides an equivalent fully saturated level of 11–12 g.

Transfuse for hematocrit levels less than 35–40% in the following situations:

Infant with severe pulmonary disease (defined as requiring >35% supplemental hood oxygen or continuous positive airway pressure [CPAP] or mechanical ventilation with a mean airway pressure of >6 cm water)

Infant in whom anemia may be contributing to congestive heart failure

In the following situations, transfuse for a hematocrit level that is 25–30% or less:

The patient requires nasal CPAP of 6 cm water or less (supplemental hood oxygen of <35% by hood or nasal cannulae).

The patient has significant apnea and bradycardia (defined as >9 episodes in 12 h or 2 episodes in 24 h, requiring bag-mask ventilation while receiving therapeutic doses of methylxanthines).

The patient has persistent tachycardia or tachypnea without other explanation for 24 hours.

Weight gain of patient is deemed unacceptable in light of adequate caloric intake without other explanation, such as known increases in metabolic demands or known losses in metabolic demands (malabsorption).

The patient is scheduled for surgery; transfuse in consultation with the surgery team.

Preservative for storage of blood

In the past, increases in extracellular potassium, a fall in pH levels and a decline in RBC 2, 3-diphosphoglycerate during RBC storage were of concern to physicians caring for sick newborns. The duration of storage is also a concern because RBCs stored for extended periods may worsen the toxicity by increasing further the extracellular potassium.

In 1993, the preservative used for the storage of RBCs was changed from citrate, phosphate, dextrose and adenine to adenine-saline (AS-3) anticoagulant and/or preservative. AS-3 solutions may be used safely for small-volume RBC transfusion in high risk newborn infants. The increase in the storage period allows an increase in the availability of blood, which limits donor exposure. After five days, the serum potassium is substantially elevated, and the units should be used only for slow, small-volume, 'top-up' transfusions. Rapid and large-volume transfusions should use blood that is less than five days old.

Irradiated blood

If the blood is obtained from first- or second-degree relatives, it should be irradiated to prevent graft-versus-host disease, which may otherwise occur. Leukocyte-reduction filters do not safely prevent graft-versus-host disease and are not an alternative method. Unless an infant had a previous in utero transfusion or a directed blood donation has been used from a relative, small volume RBC transfusions do not need to be irradiated. Because of the risk of potassium leakage, RBCs should be transfused within 24 h of irradiation if hyperkalemia represents a clinical risk extra washing may be necessary.^7–9

Infectious screening of donated blood

Ideally each unit of donated blood should be analyzed for the presence of antibody to HIV-1 and HIV-2, hepatitis C virus, human T-lymphotrophic virus (HTLV)-I and HTLV-II, presence of hepatitis B surface antigen and HIV-1 p24 antigen. Screening for syphilis and cytomegalovirus (CMV) antibodies is also desirable. In spite of the low incidence of morbidity due to transfusion-transmitted CMV infection in the neonatal population, most guidelines recommend the provision of CMV-negative blood components for low birth weight infants. It is uncertain whether leuko-depletion is adequate for the prevention of CMV, such that CMV antibody positive units could be given to preterm infants.¹⁰.

Blood transfusions depress erythropoiesis in infants with AOP and stimulate HbF synthesis but this effect is not sustained. Reticulocyte counts and erythropoietin levels are unhelpful in predicting recovery from AOP.¹¹

Recombinant erythropoietin treatment

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. Variability in the success of rEpo-therapy in numerous studies of neonates is related to multiple factors including the wide range of dosage regimens (75 to 1500 Units/kg/week), differences in timing of rEpo with early (age <3 wks) vs. late (>3 wks) administration, inadequate iron supplementation in some studies, and severity of illness of the study population. It seems clear that the major clinical goal for neonates with anemia is to reduce RBC transfusions, so as to minimize multiple donor exposures, infection, and immune risks. EPO alone cannot achieve this goal reliably. Rather, a combination of strategies is best, including reduction of phlebotomy losses, strict adherence to a conservative transfusion protocol, selected use of rEpo with sufficient dosing, and optimizing nutrition to promote growth and hematopoiesis. We need to be prepared to accept lower Hgb/Hct levels in asymptomatic neonates, recognizing that overall growth and stability are more important than any specific Hgb number for an individual infant.

It is likely that only a carefully selected subpopulation of infants may benefit from its use.

Multiple investigations have established that premature infants respond to exogenously administered recombinant human EPO and supplemental iron with a brisk reticulocytosis. While EPO cannot prevent early transfusions, modest decreases in the frequency of late PRBC transfusions have been documented.^12,13

Trials have evaluated the impact of EPO treatment in populations of the most immature neonates. These studies likewise have demonstrated that infants with VLBW are capable of responding to EPO with a reticulocytosis and that the drug appears to be safe. EPO with iron does not adversely affect growth or developmental outcomes, but the impact on the number of transfusions a premature infant receives ranges from nonexistent to small.¹⁴

No agreement regarding timing, dosing, route, or duration of therapy exists. Meta-analysis of controlled clinical trials show some benefit to EPO, but they cannot give firm guidelines on its use or recommend its routine use to prevent AOP. In short, the cost-benefit ratio for EPO has yet to be clearly established, and this medication is not accepted universally as a standard therapy for the individual with AOP. ¹⁵

Anemia of prematurity may be tolerated in an 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. Hence treatment of symptomatic anemia is essential with available modalities to enhance the growth and development of preterm neonates.

Prevention

To prevent and reduce the severity of anemia, and to reduce donor exposure or the risks associated with RBC transfusions, strategies to consider may include:

delayed clamping of the umbilical cord;

restricting blood sampling; using micro methods to analyse blood

using iron supplementation or vitamins to minimize the severity of anemia;

using appropriately collected and stored multipack RBC units;

using appropriately screened and handled RBCs from regular or designated donors

using recombinant human erythropoietin to stimulate erythropoiesis;

Vitamin and mineral supplementation

Several studies have reported the problem of inadequate iron intake with or without erythropoietin treatment. Although the optimal dose of iron is not known, based on a previous statement by the Canadian Pediatric Society's Nutrition Committee preterm infants need a supplement of 4.0 to 4.5 mg/kg of dietary iron to prevent late anemia.

It is generally accepted that 25 IU of vitamin E be administered to the preterm infant to prevent hemolytic anemia Appropriate intake of folate is 50 micrograms/day or 500 micrograms/week and vitamin B₁₂ must also be part of the nutritional support of high risk preterm newborn infants.

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.

Clinical practice algorithm

Practice blood conservation practices for all VLBW neonates

Is the VLBW neonate showing signs of anemia? If yes -

Check hematocrit, rule out differential diagnosis - e.g. sepsis, UTI, etc.

Check guidelines for transfusion

Transfuse packed RBC if indicated and inform blood bank in order to reduce donor exposure

Check iron and vitamin E supplementation

Research issues

The main research issues in AOP are

Defining those neonates who are at high risk of AOP

Better methods of prevention of AOP

Role of EPO in management of AOP

While it is well known that the smallest and sickest neonates are at risk of AOP what is unclear why some neonates have a unresponsive bone marrow and need multiple PRBC transfusions. This subgroup would probably benefit from EPO therapy but when and and what dose is yet to be answered.

Key Messages

AOP is a symptomatic nomochromic normocytic anemia seen largely in very low birth weight neonates characterized by low erythropoietin levels.

The symptoms are suggestive of tissue hypoxia and cardiovascular response to the same.

The emphasis is prevention with minimal sampling, and using micro-analytical methods.

Should the neonate need to be transfused the emphasis is on using screened blood, with minimal donor exposure.

Adequate supplementation of Vitamin E and iron need to be made, as nutritional deficiencies may exaggerate AOP.

References1

Brown Phibbs

Garcia

Dallman

Decreased response of plasma immunoreactive erythropoietin to “available oxygen” in anemia of prematurity. J Pediatr, 1984; 105: 793–798.2Smith Owen

Ian Malcolm

Hann

Robert Areci

, Pediatric Hematology, 3rd Edition; 144–146.3

Ohls

Developmental erythropoiesis. In: Polin

Fox

eds. Fetal and Neonatal Physiology. Vol 2. 2nd ed. Philadelphia, Pa: WB Saunders Co: 1762–1786.4Spitzer

Alan

(2005) intensive care of the fetus & neonate. Elsevier, 1286. ISBN 1-56053-512-1.5Pathak

Roth

Piscitelli Johnson

. Effects of vitamin E supplementation during erythropoietin treatment of the anaemia of prematurity. Arch. Dis. Child. Fetal Neonatal Ed., 2003; 88: F324–328.6Kirpalani

Whyte

Andersen

: The Premature Infants in Need of Transfusion (PINT) study: a randomized, controlled trial of a restrictive (low) versus liberal (high) transfusion threshold for extremely low birth weight infants. J Pediatr, 2006; 149: 301–307.7Voak

Chapman

Finney

. BCSH Blood Transfusion Task Force: Guidelines on gamma irradiation of blood components for the prevention of transfusion-associated graft-versus-host disease. Transfusion Med, 1996; 6: 261–271.8Akahoshi

Takanashi

Masuda

. A case of transfusion-associated graft-versus-host disease not prevented by white cell-reduction filters. Transfusion, 1992; 32: 169–172.9

Linden

Pisciotto

Transfusion-associated graft-versus-host disease and blood irradiation. Transfusion Med Rev, 1992; 6: 116–123.10Bowden

Slichter

Sayers

: A comparison of filtered leukocyte-reduced and cytomegalovirus (CMV) seronegative blood products for the prevention of transfusion-associated CMV infection after marrow transplant. Blood, 1995; 86: 3598–3603.11Schwarz

Dear

Gill

: Effects of transfusion in anemia of prematurity. Pediatr Hematol Oncol, 2005; 22: 551–559.12

Nazir

Peverini

R. L.

Deming

D. D.

Hopper

A. O.

Vyhmeister

N. R.

Comparison of 2 Iron Doses in Infants Receiving Recombinant Human Erythropoietin Therapy. Arch Pediatr Adolesc Med, 2002; 156: 540–544.13

Vamvakas

Strauss

Meta-analysis of controlled clinical trials studying the efficacy of rHuEPO in reducing blood transfusions in the anemia of prematurity. Transfusion, 2001; 41: 406–415.14

Ohls

R. K.

Ehrenkranz

R. A.

Wright

L. L.

Lemons

J. A.

Korones

S. B.

Stoll

B. J.

Stark

A. R.

Shankaran

Donovan

E. F.

N. C.

Das

for the National Institute of Child Health and Hum. Effects of Early Erythropoietin Therapy on the Transfusion Requirements of Preterm Infants Below 1250 Grams Birth Weight: A Multicenter, Randomized, Controlled Trial. Pediatrics, 2001; 108: 934–942.15

Donato

Vain

Rendo

Vivas

Prudent

Larguía

Digregorio

Vecchiarelli

Valverde

García

Subotovsky

Solana

Gorenstein

for the Private Hospitals Neonatal Network. Effect of Early Versus Late Administration of Human Recombinant Erythropoietin on Transfusion Requirements in Premature Infants: Results of a Randomized, Placebo-Controlled, Multicenter Trial. Pediatrics, 200; 105: 1066–1072.