Anemia is defined as a reduction in RBC mass or Hb concentration that results in a decrease in the oxygen-carrying capacity of the blood. The definition of anemia is a Hb concentration two standard deviations below the mean Hb level for a child's age. In many cases, anemia is not a primary hematologic disorder and a final diagnosis itself, but is rather a sign of underlying disorders. As such, the goal of a diagnostic evaluation of the anemic child is to determine the causes of anemia. The initial approach should include taking a detailed history and a physical examination, along with a panel of essential laboratory tests. The history of a child with anemia must include data that demonstrate the acuteness and severity of the condition and suggest a cause of the anemia. Children with acute anemia often present dramatically with clinical findings that include pallor, tachycardia, tachypnea, delayed capillary relief, icterus, hematuria and/or evidence of congestive heart failure. Children with chronic anemia are typically well-compensated and only have tachycardia. The initial laboratory evaluation includes a complete blood count with measurement of RBC indices, a white blood cell differential count and reticulocyte count, and the preparation and examination of a peripheral blood smear. Additional laboratory tests that may be indicated in the diagnosis and treatment of children with acute anemia include: testing of the bilirubin level, lactate dehydrogenase tests (to test for hemolytic anemia), the direct antiglobulin or Coombs test (for autoimmune hemolytic anemia), blood typing and crossmatching to assess possible isoimmune anemia in a neonate and to prepare for transfusion, tests for iron, the total iron-binding capacity (TIBC) and ferritin levels (for iron-deficiency anemia), and bone marrow aspiration and biopsy (to test for bone marrow failure or malignancy). If the reason for anemia is not known, it is very important to ensure that blood samples for investigation are taken prior to transfusion.[5,6] Extensive evaluation, if needed, should be based on the results of initial screening and carried out through appropriate hematologic consultations. It may include the following: Hb electrophoresis (hemoglobinopathies), red cell enzyme studies (e.g., G-6-PD and pyruvate kinase), osmotic fragility tests (spherocytosis), analysis of folate and vitamin B-12 levels (to test for macrocytic/megaloblastic anemia), viral titers (e.g., Epstein–Barr virus and cytomegalovirus [CMV]), testing blood urea nitrogen and creatinine levels to assess renal function, and the analysis of thyroxine (T4)/thyroid-stimulating hormone to rule out hypothyroidism.
The physiological response to anemia varies according to a child's age, acuity and the type of insult. The primary function of RBCs is to bind oxygen as they circulate through the pulmonary bed, and to release oxygen in the capillaries at a pressure high enough for tissue diffusion. In acute anemia, the heart responds to tissue hypoxia by increasing cardiac output. The increased output is matched by decreased peripheral vascular resistance and decreased blood viscosity, so that cardiac output can rise without an increase in blood pressure. Neonatal cardiac output is more dependent on the heart rate than the stroke volume; therefore, tachycardia is likely to compromise cardiac output if prolonged, although not all infants with subnormal heart rates have impaired perfusion. In acute anemia, selective vasoconstriction of the blood vessels subserving certain nonvital areas allows for more blood to flow into critical areas. Increased extraction of oxygen from anemic blood by the tissues produces an increased concentration of deoxyhemoglobin in the RBCs, which stimulates the production of 2,3-diphosphoglycerate. 2,3-diphosphoglycerate shifts the oxyhemoglobin dissociation curve to the right, which decreases oxygen's affinity for Hb. This decreased affinity translates to better oxygen delivery to the target tissues. In cases of a more gradual onset and with a chronic basis, blood volume, the size of the vascular bed and the rate of production of RBCs increases. These compensatory mechanisms permit states of mild-to-moderate anemia without significant symptoms.
Anemia is a common problem in children admitted to the PICU, and up to 50% of children who are hospitalized in a multidisciplinary PICU receive transfusions. Besides, acquired anemia often complicates the course of critically ill children during their stay in the emergency department. Critically ill patients utilize a significant proportion of the overall RBC supply in any given area. The small body size and small total blood volume make children vulnerable to anemia secondary to blood draws, despite the use of microsampling techniques. Recent studies clearly demonstrate that anemia in cases of critical illness is multifactorial in origin, including owing to pre-existing conditions (e.g., chronic anemia, blood loss owing to surgery or injury and chronic diseases such as cancer or treatments being received [e.g., chemotherapy or irradiation]), hemodilution during resuscitation with crystalloid or colloid solutions, hemorrhage and reduced RBC production. This appears to result from a combination of inappropriately low levels of circulating erythropoietin and hyporeactive bone marrow. In critically ill patients, the body's erythropoietic response to anemia is blunted as a consequence of diminished iron availability and the direct inhibitory effects of inflammatory cytokines.[7–9]
The impact of anemia on the outcome of critically ill children is not well understood. Some data suggest that severe anemia may be detrimental to critically ill children with septic shock or hemodynamic compromise. RBC transfusions are a key to the successful management of many premature infants, children with oncologic and hematologic diseases, and those undergoing many surgical procedures or following resuscitation from trauma. Although RBC transfusions can be lifesaving, they are associated with multiple risks, including transfusion-transmitted infections, hemodynamic instability and intravascular volume overload, acute hemolysis, transfusion-related acute lung injury and various immunologic consequences including immunosuppression and graft-versus-host disease. Accordingly, RBC transfusions should only be given when true benefits are likely. Children generally have a greater ability to compensate for anemia and to safely tolerate lower Hb levels than adults. Thus, the transfusion strategy in critically ill children remains controversial and has generated much research and debate.[10–13]
Pediatr Health. 2010;4(2):201-208. © 2010 Future Medicine Ltd.
Cite this: Transfusions in the Critically Ill Pediatric Patient - Medscape - Apr 01, 2010.