- Email: [email protected]
Neonatal hematology
Seminars in Pediatric Surgery 22 (2013) 199–204
Contents lists available at ScienceDirect
Seminars in Pediatric Surgery journal homepage: www.elsevier.com/locate/sempedsurg
Neonatal hematology Jose Diaz-Miron, MD1, Jacob Miller, MD1, Adam M. Vogel, MDn Division of Pediatric Surgery, Washington University in St. Louis School of Medicine, St. Louis, Missouri
a r t i c l e in fo
Keywords: Neonatal hematology Hematopoiesis Hemostasis TEG
a b s t r a c t Neonatal hematology is a complex and dynamic process in the pediatric population. Surgeons frequently encounter hematologic issues regarding hemostasis, inflammation, and wound healing. This publication provides a surgeon-directed review of hematopoiesis in the newborn, as well as an overview of the current understanding of their hemostatic profile under normal and pathologic conditions. & 2013 Elsevier Inc. All rights reserved.
Hematology is the study of blood and blood-forming organs, as well as their diseases. Adult and pediatric surgeons frequently encounter hematologic issues with respect to wound healing, inflammation, and hemostasis. Because of the complexity and broad scope of hematology, this review is limited to an overview of hematopoiesis and hemostasis in the neonate, comparing and contrasting the similarities and differences noted between the neonatal and adult populations.
Hematopoiesis In the developing embryo, hematopoiesis begins in the ventral aorta, with colonization of secondary centers of hematopoiesis in the liver, spleen, and bone marrow over the ensuing weeks.1,2 At birth, the primary hematopoietic center is the bone marrow; however, under conditions of stress, extramedullary hematopoiesis can be seen most commonly in the liver, spleen, lymph nodes, and paravertebral regions.3 The intricate and self-renewing process of hematopoiesis allows for the reproduction and differentiation of cell lineages into erythrocytes, granulocytes, and megakaryocytes. This differentiation is tightly regulated by a milieu of cytokines, growth factors, and an intact microenvironment in the centers of hematopoiesis.
Erythropoiesis In the developing fetus, erythropoiesis begins on gestational day 20 as two distinct entities. The first is termed primitive erythropoiesis and results in nucleated red blood cells (RBC). The second phase is termed definitive erythropoiesis and, unlike n Correspondence to: St. Louis Children's Hospital, One Children's Place, Suite 5S40, St. Louis, Missouri 63110. E-mail address: [email protected] (A.M. Vogel). 1 Authors contributed equally to this work.
1055-8586/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1053/j.sempedsurg.2013.10.009
primitive erythropoiesis, is erythropoietin (Epo) dependent. This latter phase produces cells that are enucleated, CD34 þ , and comprised of fetal and adult hemoglobin (Hgb).4 Anemia is commonly encountered in the first few weeks of life and is largely considered physiologic with few clinical implications. However, the RBC life span in a premature neonate (35–50 days) is significantly shorter than that of a term neonate (60–70 days), which contributes to an earlier onset and higher degree of anemia.4,5 This anemia is more profound in critically ill neonates, resulting from acute blood loss from frequent phlebotomy and impaired RBC production secondary to inflammation. Finally, iron deficiency anemia is more common in premature infants as the bulk of placental transfer of maternal iron occurs in the late third trimester.4 Table 1 compares the factors contributing to physiologic and pathologic anemia in both term and preterm neonates. The standard treatment for neonatal anemia is blood transfusions and minimization of phlebotomy. Supplemental Epo has not been shown to decrease transfusion requirements.6 Since multiple transfusions are often necessary, repeat transfusions should be from the same unit of adult blood to minimize antigen exposure.6,7 Indications for transfusion in neonates depend on the degree of anemia as reflected in the hemoglobin and hematocrit levels in conjunction with clinical and physiologic parameters. The use of liberal vs restrictive RBC transfusions in neonates is an often debated topic currently under investigation. Two large clinical trials have been performed, with differing results. In a prospective randomized controlled trial of 100 preterm infants assigned into a liberal or restrictive-transfusion group, a higher incidence of adverse outcomes, specifically IVH, periventricular leukomalacia, and apnea was noted in the restrictive group.8 These findings however, were not reproduced in a larger study of the similar design consisting of 451 extremely low-birth weight infants, where no increase in the incidence adverse events was noted.9 These differing results complicate the timing and indications for RBC transfusions and stress the need for larger multicenter trials for guiding clinical practice.
200
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
Table 1 Factors contributing to physiologic and pathologic anemia in infants. (Adapted from Juul.4) Physiologic and pathologic factors contributing to anemia
Term infant
Preterm infant
Time of onset Nadir Hct Epo
Week 10–12 30% Decreased after transitioning from hypoxic placental environment to oxygen-rich circulation Liver before 3–4 mo after birth; kidney thereafter High/high 0.21 g/dL
Week 4–6 21–28% Decreased after transitioning from hypoxic placental environment to oxygen-rich circulation Liver before 3–4 mo after reaching term age; kidney thereafter High/high 0.21 g/dL
Production location for Epo Epo clearance/volume of distribution Percent Hct increase per gestational week RBC life span5 Phlebotomy Iron deficiency Blood loss Hemolysis Hereditary causes Decreased RBC production
60–70 d 35–50 d Likelihood of repeated phlebotomy lower Latrogenic anemia common Lower likelihood Higher likelihood Obstetric complications and twin–twin transfusions ABO, Rh incompatibility, autoimmune hemolytic disease, sepsis, DIC, vitamin E deficiency, and iron deficiency G6PD deficiency, RBC membrane disorders, and hemoglobinopathies Infection, drugs, and genetic
Granulopoiesis Granulocyte production begins at around the 8th gestational week with the production of macrophages. These cells are thought to create the cavity within the bone where bone marrow stem cells will eventually reside.10 Lymphocytes produced in the bone marrow subsequently migrate to the thymus at the 8th week postconception, when this organ develops. Wide variability between reported lymphocyte counts has been identified in the neonate when accounting for gestational age, as well as days of life. A drop in white blood cell (WBC) count is physiologic during the first 4 h after birth and has been potentially attributed to the corticosteroid surge during delivery, as corticosteroids have been shown to cause a decrease in circulating lymphocyte levels.11 Lymphocyte variations in the first 28 days are common.11 Interestingly, lymphocyte counts in the upper and lower 5th percentiles have been associated with increased frequency of early-onset sepsis (EOS) and intraventricular hemorrhage (IVH). Additionally, neonates with lymphocyte counts in the lower 5th percentile were found to have higher rates of retinopathy of prematurity (ROP).11 These variations in lymphocyte counts and associated comorbidities have been attributed to corticosteroid levels, as well as perinatal catecholamines, cytokines, and growth factors.11 Neonates with EOS may experience lymphocytosis from a presumed catecholamine or lymphopenia from presumed corticosteroid release from the stress of neonatal infections. Clinical correlation, as well as a high index of suspicion, must therefore accompany the assessment of the neonate in the extremes of WBC counts.
Megakaryopoiesis Megakaryopoiesis is first noted at about the 5th gestational week, with production of functional platelets depending on the production of thrombopoietic factors, megakaryocyte (MK) proliferation, differentiation and maturation of MK, and the efficient release of platelets into circulation.12 Although the embryo traditionally has been cited to reach platelet levels of 150 109 per liter by the end of the first trimester and adult range levels by 22 weeks' gestation, more recent studies have challenged this notion when including sick children into studies. Among infants admitted to the neonatal intensive care unit (NICU), reference ranges that excluded the upper and lower 5th percentile were 104 109 per liter for infants younger than 32 weeks and 123 109 per liter for those older than this same gestational age.12
Thrombopoiesis differences between adults and neonates are evident in growth factor levels as well as functional and structural MK differences. Levels for the most potent stimulus for thrombopoiesis, thrombopoietin (Tpo), are higher in neonates. MK progenitors in neonates are more sensitive to lower Tpo concentrations. They are also ten times more proliferative than adult counterparts, smaller in size, and with lower ploidy levels. Additionally, MK progenitors in neonates, although expressing lower ploidy levels, are found to be more cytoplasmically mature than adult MK cells, as evident by higher expression of CD42b, a marker of mature MKs.12 Neonatal platelets, although structurally similar to adult counterparts, exhibit different levels of surface glycoproteins and react differently to agonists. For the first 2–4 weeks after birth, neonatal platelets have decreased granule secretion, fibrinogenbinding site expression, and response to agonists.13 They also have fewer pseudopods, smaller glycogen deposits, and fewer visible microtubular structures.14 These “hyporeactive” platelets are responsible for reduced clot strength as seen on laboratory tests that assess the global measures of hemostatic function such as thromboelastography (TEG) and rotational thromboelastometry (ROTEM). While a healthy-term neonate may have a normal adult platelet count, there are many pathologic conditions that can lead to thrombocytopenia. Onset of neonatal thrombocytopenia can be seen early (first 72 h of life) or late (after 72 h of life). Early onset is attributed most commonly to fetal–maternal interaction with intrauterine growth restriction secondary to placental insufficiency, leading to decreased platelet production being the most common cause. Late-onset thrombocytopenia is due to platelet consumption, commonly seen in diffuse bacterial infections, necrotizing enterocolitis, and sepsis.15 Transfusion practices for platelets are discussed further in subsequent paragraphs.
Hemostasis Hemostasis is a dynamic process that begins in utero and evolves into the adult system throughout childhood. Andrew et al. coined the term “developmental hemostasis” in the 1980s to describe this maturation process. Although distinct quantitative and qualitative differences (described below) between infants and adults were noted in the index studies of developmental hemostasis, the hemostatic system in the healthy neonate is physiologically fully functional and rarely has negative clinical implications. There are two major approaches to describing “normal” coagulation and hemostasis. The traditional cascade model, first
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
Fig. 1. The cascade model of hemostasis.17
reported in 1964, has been critical to our understanding of the various proteins necessary for hemostasis, anticoagulation, and many disease processes.16 The classic cascade model (Figure 1) describes a linear series of enzymatic reactions involving coagulation factors, proteins, platelets, and platelet products that interact through two major pathways (intrinsic/contact and extrinsic/ tissue factor) to culminate in thrombin formation.17 However, the cascade model is unable to accurately describe certain coagulopathic conditions or predict selected treatments. For example, hemophiliacs, with an intact extrinsic pathway, would be expected to have normal thrombin formation. Additionally, exogenous factor VII therapy as an adequate treatment for hemophiliacs is equally unexplained by this model.18 The cell-based model of hemostasis, originally proposed by Mann et al.,16 is a more recent and developing model of hemostasis. This model assigns an important role to cells, especially platelets, in providing the anionic phospholipid membrane necessary for the assembly of the coagulation complex.18 The cell-based model is better able to explain what the cascade model, with its separate intrinsic and extrinsic pathways, cannot. The cell-based model of hemostasis is thought to transpire in three overlapping phases. Initiation occurs on tissue factor (TF)-bearing cells, often a subendothelial cell, when there is an endothelial injury and it is exposed to the small amount of circulating, constitutively active factor VII. A sufficiently strong procoagulant stimulus forms enough thrombin and factors Xa and IXa, starting the coagulation process. Amplification ensues after TF-bearing cell activity moves to the platelet surface. Here, the procoagulant stimulus is amplified through further platelet adhesion, activation, and accumulation of surface cofactors. During the propagation phase, platelet surface cofactors combine with active proteases which ultimately leads to the generation of thrombin and fibrin polymerization18 (Figure 2). The cell-based model of hemostasis is able to explain the above observations in hemophiliacs that are not justified by the cascade model. While hemophiliacs are able to form thrombin efficiently via the extrinsic pathway, they do not generate activated factor X. Without activated factor X, the transition from the TF-bearing cell to the platelet is severely impaired. It has been proposed that giving the factor VII to hemophiliacs will enable the extrinsic pathway on the TF-bearing cell to generate an increased amount of thrombin capable of diffusing and activating platelets.18
201
decreased levels of factors II, VII, IX, XI, and XII but increased levels of thrombomodulin, tPA, and plasminogen activator inhibitor-1, predisposing to bleeding. However, they also have larger and more abundant vWF and lower levels of antithrombin, heparin cofactor II, α-2-macroglobulin, protein C, protein S, and plasminogen that balance this bleeding tendency.20,21 Neonates thus have prolongation of prothrombin time (PT) and activated partial thromboplastin time (aPTT) but a shortened bleeding time. Additionally, functional differences in coagulation factors, like fibrinogen, further affect the newborn's ability to maintain hemostasis. Fibrinogen in the neonate undergoes post-translational modification, attaining higher sialic acid levels and a decreased ability to cross-link.22 Interestingly, in pediatric patients who receive an adult liver transplant, coagulation protein levels remain the same, indicating that it is not under direct control from the liver itself.23 One theory to explain the altered coagulation profile in neonates revolves around the concept that many of the proteins responsible for coagulation have multiple functions outside of hemostasis.21 One such protein is antithrombin, which is decreased in the neonate. Antithrombin is required not only for anticoagulation but is also known to possess anti-angiogenic properties. The protein is likely decreased in the neonate to avoid the anti-angiogenic properties; however, it is also critical for anticoagulation. Other protein levels must be altered in order to maintain a physiologic hemostatic profile. This theory not only helps to explain the difference in coagulation protein levels and platelet activity but also highlights the dangers of transfusing patients with platelets, fresh frozen plasma (FFP), or cryoprecipitate.22 When transfusing neonates with adult blood components, clinicians must consider the differences in protein levels, functionality, and activity of the components, as adult platelets—at least
Developmental hemostasis Several coagulation factors in the neonate have values that are outside of the normal adult range (Table 2).19 Newborns have
Fig. 2. Cell-based model of hemostasis: the three phases of coagulation— (A) initiation, (B) amplification, and (C) propagation.17
202
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
Table 2 Developmental hemostasis: age-related differences in hemostatic factors19 Preterm neonates vs Neonates vs older term neonates children/adults
Primary hemostasis Platelet count Decreased (o32 wk) Platelet function Decreased Higher Percentage of reticulated platelets vWF level NA vWF large NA multimers Coagulation factors FII, FVII, FIX, and FX FV FVIII FXI FXII Fibrinogen level Fibrinogen function
Same Decreasedb Higher
2–4 weeks NA
Higher Higher
3 mo 3 mo
Lower
Lower
16 yr
Lower Higher Lower Lower Same NA
Same or lower Same or lower Lower Lower Same Decreased
16 yr 1 moc 1 yr 16 yr 5 yr
Lower Lower Lower Higher Reduced Lower
3 mo 16 yr 1 mo NA NA Adult
Lower
Lower
6 mo
NA
Decreased
NA
Higher Lower Higher Same or higher
5d 5d Adult 5d
Regulation of coagulation Antithrombin Lower Protein C Lower Total protein S Lower Free protein S NA APCR generation NA Free TFPI NA Fibrinolysis Plasminogen level Plasminogen function tPA α2 antiplasmin α2 M PAI
Approximate age of adult valuesa
d
Same Lower Same Samed
NA, indicates not available; APCR, activated protein C resistance; and TFPI, tissue factor pathway inhibitor. a
Maximum age reported. Decreased response was reported to agonists such as thrombin, collagen, epinephrine, and thrombin activation peptide as tested by flow cytometry. c Lower levels compared with adults are reported from 1 mo to 16 yr of age. d Higher levels in extremely preterm neonates on day 10 of life compared with older preterm or term neonates. b
in vitro—have been shown to cause a hypercoagulable state when they are transfused to neonatal plasma.24 Interpreting and assessing the coagulation system of the critically ill neonate is an ongoing challenge. The concept of developmental hemostasis, with the age-related changes in hemostatic components, complicates the interpretation of common laboratory tests such as PT, PTT, platelet count, and bleeding time. Additionally, the tests commonly used to assess the coagulation system require relatively large volumes of blood for adequate interpretation and are not exempt from intrinsic sample problems such as component activation and heparin contamination, to name a few. Because of these limitations, point-of-care (POC) sample collections are optimal, but scarce. In recent years, thromboelastography or TEG has been evaluated as a potentially suitable examination that can provide an individualized evaluation for each patient's bleeding risk and their need for blood component transfusion.25 TEG is a POC test that uses a small amount of blood and provides a graphical representation (Figure 3) on the dynamics of clot development, stabilization, and dissolution (fibrinolysis).26 This test records viscoelastic
changes that occur during coagulation in whole blood and has previously been shown to have largely comparable results between neonates and adults.22 TEG provides values for the entirety of the coagulation and fibrinolytic processes, allowing for assessment of all aspects of the coagulation system (Table 3). Briefly, the r-value is defined as the time between the initiation of the test and fibrin formation and is a representative of clotting factors. The k-time is the time needed for the tracing to reach 20 mm from 2 mm and is increased with hypofibrinogenemia or platelet deficiency. The α-angle is the slope of the tracing that represents the rate of clot formation and decreases with hypofibrinogenemia or platelet deficiency. The maximal amplitude (MA) is the greatest amplitude of the tracing and represents the platelet contribution to clot strength. Finally, LY30 is the percent amplitude reduction at 30 min after achievement of MA and represents fibrinolysis.27 Reference ranges for these parameters have been described in neonates, infants, and children.24–26,28 Goal-directed hemostatic resuscitation employs TEG-based transfusion of fresh frozen plasma, cryoprecipitate, platelets, and adjunctive hemostatic agents such as activated factor VII to correct shock and coagulopathy in critically ill patients. The resuscitation strategy may have a role in treating critically ill, coagulopathic neonates.29
Clinical implications of the neonatal coagulation system Coagulation advantage The term neonate, as previously noted, has a shorter bleeding time and shorter closing time on platelet function testing (PFA), indicating faster formation of the initial platelet plug. These observations are likely due to the increase in hematocrit at birth and the amount and size of vWF.13,24 It is, in fact, thought that neonates have a small coagulation advantage, for reasons that are poorly understood. Vitamin K deficiency bleeding Vitamin K deficiency bleeding, formerly known as hemorrhagic disease of the newborn, is a coagulopathy that presents early (within 24 h), classically (24 h to 7 days), or late (2–12 weeks) after birth.30 Etiologies according to timing of onset include maternal intake of drugs that inhibit vitamin K, delayed or insufficient feeding, and an infant diet consisting exclusively of breast milk.30 Bleeding is caused by a lack of the vitamin K-dependent coagulation factors II, VII, IX, and X. Premature neonates, are prone to low levels of vitamin K due to low levels of vitamin K-dependent factors, low levels of vitamin K in breast milk, and the absence of bacterial gut colonization that produces vitamin K.10 This nutritional deficiency is rarely seen in the US because of prophylactic vitamin K supplementation. However, if this diagnosis is suspected and severe clinical bleeding is present, transfusing FFP will temporarily supply the missing factors. Intraventricular hemorrhage (IVH) IVH is a highly morbid condition in infants of extreme low-birth weight (ELBW) and prematurity. This complication is multifactorial, with immature vessels, a friable germinal matrix, fluctuations in cerebral blood flow, and respiratory failure requiring mechanical ventilation causing large variations in intrathoracic and venous pressures cited as contributors.31 The delicate balance between the relative bleeding propensity from lower levels of vitamin K-dependent coagulation factors and the procoagulant tendency from lower anticoagulant, antithrombin, and tissue
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
203
Fig. 3. Clot formation from TEG test.
factor pathway inhibitor is easily disrupted in neonates experiencing acquired or congenital defects in hemostasis.31 However, heritable coagulopathies are infrequently present, and occasionally undiagnosable, in the neonatal period. Acquired coagulopathies, like thrombocytopenia, are common in premature and small-forgestational-age infants. These same infants are similarly at risk for sepsis and other conditions that further worsen the degree of thrombocytopenia. Additional coagulation derangements have been shown to coincide with high-grade IVH in ELBW infants, with lower levels of factor VII serving as the most sensitive marker for the development of IVH.32 Interventions for IVH prevention have traditionally included correction of thrombocytopenia with platelet transfusion. Thresholds for platelets transfusion vary widely between countries and institutions due to the shortage of randomized controlled trials. In the United States and Canada, transfusions are administered to nonbleeding neonates with counts of more than 50 109 per liter,33 while in the United Kingdom, thresholds for healthy-term ( o25 109 per liter) and preterm ( o30 109 per liter)34 neonates are much more conservative. Further interventions for the prevention of IVH include pharmacologic agents that target the hemostatic system. Ethamsylate is a nonsteroidal agent that increases capillary endothelial resistance and promotes platelet adhesion and has been associated with reduction in IVH rates, but with no effect on overall mortality.31 Vitamin K, administered prophylactically to newborns, increases the concentration of vitamin K-dependent factors and reduces both mortality and occurrence of IVH.31 Both FFP and prothrombin complex concentrate (PCC) increase levels of some circulating coagulation factors. However, conflicting outcomes have been implicated with the use of both FFP and PCC.31 Finally, recombinant activated factor VIIa (rFVIIa) is a pharmacologic hemostatic adjunct that anecdotally and in small series has been used for IVH; however, no randomized controlled data supports the use of this agent in preventing IVH.31
Disseminated intravascular coagulation Disseminated intravascular coagulation (DIC) is a thrombohemorrhagic disorder that is seen in characteristic clinical situations with laboratory evidence of procoagulant activation, fibrinolytic activation, inhibitor consumption, and evidence of end-organ damage or failure.35 Derangements in the delicate balance of the neonatal coagulation system from low levels of hemostatic components, hypoxia, intravascular volume contraction after birth, and sepsis predispose this population to a higher risk for the development of DIC.36 Diagnosis includes prolongation of all coagulation parameters, thrombocytopenia, and microangiopathic hemolytic anemia. Paramount treatment is prompt management of the underlying process. Aggressive resuscitation, frequently with blood components, and inotropic support may be necessary to correct circulatory compromise and shock. Transfusions with fresh frozen plasma (FFP) and platelets are recommended to keep the fibrinogen 41.0 g/L and the platelets 450 109 per liter. Coagulation-based therapeutic approaches, when used, should focus on preventing or reversing microthrombi formation and preventing or stopping hemorrhage.36 Recombinant activated factor VII (rFVIIa) is another therapeutic intervention that has been used in select cases of catastrophic acute life-threatening hemorrhage, where conventional methods to achieve hemostasis have failed.36 Caution must be exercised when using rFVIIa for fear of exacerbating the microvascular thrombus formation. Extracorporeal life support Extracorporeal life support (ECLS) is an invasive therapy used for select neonates with reversible cardiorespiratory failure, often as a result of meconium aspiration syndrome, persistent pulmonary hypertension of the newborn, sepsis, and congenital diaphragmatic hernia.37 Once therapy is initiated, the nonbiologic surface of ECLS components predisposes the neonate to
Table 3 Description of TEG values. (Adapted with permission from Alexander et al.27) Value
Quantifies
Represents
Factors affecting the value
R K α-angle MA LY30
Reaction time Coagulation time Clotting rate Maximum amplitudes Percentage lysis 30 min after MA
Time to original fibrinogen plug Clot-formation time Rate of clot formation Strength of clot Rate of fibrinolysis
Clotting factor deficiency, pharmaceutical anticoagulation, and thrombocytopenia Decreased fibrinogen or platelets Clotting factor deficiency, platelet dysfunction, thrombocytopenia, or low fibrinogen Quantity and function of platelets Anticoagulation proteins quantity and function
204
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
thrombotic complications, making systemic anticoagulation necessary. The risk of thrombosis and hemorrhage both increase morbidity and mortality. With rare exception, anticoagulation is maintained with unfractionated heparin (UNFH). Heparin acts as a cofactor for antithrombin, deactivating thrombin and other coagulation factors. Monitoring of heparin-based systemic anticoagulation is typically performed using the activated clotting time (ACT) test, with goal targets of 180–220 seconds. Heparin concentrations though, have been shown to have only a moderate correlation with ACT values.38 If an abnormally high dose of UNFH is required, administration of antithrombin, via recombinant antithrombin or paradoxically FFP, can be used to increase the level of anticoagulation while decreasing the necessary dose of UNFH.39 Antifactor Xa assay is an additional test that may be advantageous due to its ability to monitor the anticoagulation effects of heparin directly.39 TEG is an alternate test increasing in popularity that is used to measure the adequacy of anticoagulation, especially in patients who require prolonged ECLS or will require additional surgical interventions. Additional trials are warranted that evaluate the optimal anticoagulant for neonates, as well as the ideal testing assays to evaluate the effectiveness of these anticoagulants.
Conclusion Implications of the complex and partly understood neonatal coagulation system can be troublesome to the surgical team. An understanding of how the unique neonatal hematologic and hemostatic physiology differs from term infants, children, and adults is essential for understanding the clinical implication of these systems, particularly during critical illness. Management decisions made daily and almost reflexively in the adult population pose a greater dilemma in the neonatal population. The lack of established guidelines for transfusion and anticoagulation further complicate clinical decision-making. Pediatric surgeons and neonatologists must balance the risks and benefits associated with blood component transfusions that address the anemic, leukopenic, and thrombocytopenic neonate. Additional studies are warranted to optimize transfusion guidelines and anticoagulation practices for children with derangements of their hemostatic system.
Acknowledgment The authors wish to acknowledge Mrs. Susan Phillips for her critical appraisal of the manuscript. References 1. Bollerot K, Pouget C, Jaffredo T. The embryonic origins of hematopoietic stem cells: a tale of hemangioblast and hemogenic endothelium. APMIS. 2005; 113(11–12):790–803. 2. Dieterlen-Lievre F, Godin I, Pardanaud L. Where do hematopoietic stem cells come from? Int Arch Allergy Immunol. 1997;112(1):3–8. 3. Sohawon D, Lau KK, Lau T, et al. Extra-medullary haematopoiesis: a pictorial review of its typical and atypical locations. J Med Imaging Radiat Oncol. 2012; 56(5):538–544. 4. Juul S. Erythropoiesis and the approach to anemia in premature infants. J Matern Fetal Neonatal Med. 2012;25(suppl 5):97–99. 5. Bard H, Widness JA. The life span of erythrocytes transfused to preterm infants. Pediatr Res. 1997;42(1):9–11. 6. Strauss RG. Anaemia of prematurity: pathophysiology and treatment. Blood Rev. 2010;24(6):221–225. 7. Luban NL. Management of anemia in the newborn. Early Hum Dev. 2008; 84(8):493–498.
8. Bell EF, Strauss RG, Widness JA, et al. Randomized trial of liberal versus restrictive guidelines for red blood cell transfusion in preterm infants. Pediatrics. 2005;115(6):1685–1691. 9. Kirpalani H, Whyte RK, Andersen C, et al. 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(3):301–307. 10. Gleason CA, Devaskar SU, Avery ME. In: Gleason Christine A, Devaskar Sherin U, editors. Avery's Diseases of the Newborn, 9th ed. Philadelphia, PA: Elsevier/ Saunders; 2012. 11. Christensen RD, Baer VL, Gordon PV, et al. Reference ranges for lymphocyte counts of neonates: associations between abnormal counts and outcomes. Pediatrics. 2012;129(5):e1165–e1172. 12. Sola-Visner M. Platelets in the neonatal period: developmental differences in platelet production, function, and hemostasis and the potential impact of therapies. Hematology Am Soc Hematol Educ Program. 2012(1):506–511. 13. Israels SJ. Diagnostic evaluation of platelet function disorders in neonates and children: an update. Semin Thromb Hemost. 2009;35(2):181–188. 14. Strauss T, Sidlik-Muskatel R, Kenet G. Developmental hemostasis: primary hemostasis and evaluation of platelet function in neonates. Semin Fetal Neonatal Med. 2011;16(6):301–304. 15. Ulusoy E, Tufekci O, Duman N, et al. Thrombocytopenia in neonates: causes and outcomes. Ann Hematol. 2013;92(7):961–967. 16. Mann KG, Krishnaswamy S, Lawson JH. Surface-dependent hemostasis. Semin Hematol. 1992;29(3):213–226. 17. Smith SA. The cell-based model of coagulation. J Vet Emerg Crit Care (San Antonio). 2009;19(1):3–10. 18. Hoffman M, Monroe DM 3rd. A cell-based model of hemostasis. Thromb Haemost. 2001;85(1):958–965. 19. Revel-Vilk S. The conundrum of neonatal coagulopathy. Hematology Am Soc Hematol Educ Program. 2012;2012:450–454. 20. Ignjatovic V, Lai C, Summerhayes R, et al. Age-related differences in plasma proteins: how plasma proteins change from neonates to adults. PLoS One. 2011;6(2):e17213. 21. Monagle P, Massicotte P. Developmental haemostasis: secondary haemostasis. Semin Fetal Neonatal Med. 2011;16(6):294–300. 22. Monagle P, Ignjatovic V, Savoia H. Hemostasis in neonates and children: pitfalls and dilemmas. Blood Rev. 2010;24(2):63–68. 23. Lisman T, Platto M, Meijers JC, et al. The hemostatic status of pediatric recipients of adult liver grafts suggests that plasma levels of hemostatic proteins are not regulated by the liver. Blood. 2011;117(6):2070–2072. 24. Ferrer-Marin F, Chavda C, Lampa M, et al. Effects of in vitro adult platelet transfusions on neonatal hemostasis. J Thromb Haemost. 2011;9(5):1020–1028. 25. Radicioni M, Mezzetti D, Del Vecchio A, et al. Thromboelastography: might work in neonatology too? J Matern Fetal Neonatal Med. 2012;25(suppl 4):18–21. 26. Strauss T, Levy-Shraga Y, Ravid B, et al. Clot formation of neonates tested by thromboelastography correlates with gestational age. Thromb Haemost. 2010;103(2):344–350. 27. Alexander DC, Butt WW, Best JD, et al. Correlation of thromboelastography with standard tests of anticoagulation in paediatric patients receiving extracorporeal life support. Thromb Res. 2010;125(5):387–392. 28. Vogel AM, Radwan ZA, Cox CS CS Jr, et al. Admission rapid thrombelastography delivers real-time “actionable” data in pediatric trauma. J Pediatr Surg. 2013; 48(6):1371–1376. 29. Afshari A, Wikkelso A, Brok J, et al. Thrombelastography (TEG) or thromboelastometry (ROTEM) to monitor haemotherapy versus usual care in patients with massive transfusion. Cochrane Database Syst Rev. 2011;(3):CD007871. 30. Van Winckel M, De Bruyne R, Van De Velde S, et al. Vitamin K, an update for the paediatrician. Eur J Pediatr. 2009;168(2):127–134. 31. Kuperman AA, Kenet G, Papadakis E, et al. Intraventricular hemorrhage in preterm infants: coagulation perspectives. Semin Thromb Hemost. 2011;37(7): 730–736. 32. Piotrowski A, Dabrowska-Wojciak I, Mikinka M, et al. Coagulation abnormalities and severe intraventricular hemorrhage in extremely low birth weight infants. J Matern Fetal Neonatal Med. 2010;23(7):601–606. 33. Josephson CD, Su LL, Christensen RD, et al. Platelet transfusion practices among neonatologists in the United States and Canada: results of a survey. Pediatrics. 2009;123(1):278–285. 34. Chaudhary R, Clarke P. Current transfusion practices for platelets and fresh, frozen plasma in UK tertiary level neonatal units. Acta Paediatr. 2008;97(1):135. 35. Bick RL. Disseminated intravascular coagulation: a review of etiology, pathophysiology, diagnosis, and management: guidelines for care. Clin Appl Thromb Hemost. 2002;8(1):1–31. 36. Veldman A, Fischer D, Nold MF, et al. Disseminated intravascular coagulation in term and preterm neonates. Semin Thromb Hemost. 2010;36(4):419–428. 37. Dalton HJ, Butt WW. Extracorporeal life support: an update of Rogers' Textbook of Pediatric Intensive Care. Pediatr Crit Care Med. 2012;13(4):461–471. 38. Baird CW, Zurakowski D, Robinson B, et al. Anticoagulation and pediatric extracorporeal membrane oxygenation: impact of activated clotting time and heparin dose on survival. Ann Thorac Surg. 2007;83(3):912–919 [discussion 919920]. 39. Annich G, Adachi I. Anticoagulation for pediatric mechanical circulatory support. Pediatr Crit Care Med. 2013;14(suppl 1):S37–S42.
Contents lists available at ScienceDirect
Seminars in Pediatric Surgery journal homepage: www.elsevier.com/locate/sempedsurg
Neonatal hematology Jose Diaz-Miron, MD1, Jacob Miller, MD1, Adam M. Vogel, MDn Division of Pediatric Surgery, Washington University in St. Louis School of Medicine, St. Louis, Missouri
a r t i c l e in fo
Keywords: Neonatal hematology Hematopoiesis Hemostasis TEG
a b s t r a c t Neonatal hematology is a complex and dynamic process in the pediatric population. Surgeons frequently encounter hematologic issues regarding hemostasis, inflammation, and wound healing. This publication provides a surgeon-directed review of hematopoiesis in the newborn, as well as an overview of the current understanding of their hemostatic profile under normal and pathologic conditions. & 2013 Elsevier Inc. All rights reserved.
Hematology is the study of blood and blood-forming organs, as well as their diseases. Adult and pediatric surgeons frequently encounter hematologic issues with respect to wound healing, inflammation, and hemostasis. Because of the complexity and broad scope of hematology, this review is limited to an overview of hematopoiesis and hemostasis in the neonate, comparing and contrasting the similarities and differences noted between the neonatal and adult populations.
Hematopoiesis In the developing embryo, hematopoiesis begins in the ventral aorta, with colonization of secondary centers of hematopoiesis in the liver, spleen, and bone marrow over the ensuing weeks.1,2 At birth, the primary hematopoietic center is the bone marrow; however, under conditions of stress, extramedullary hematopoiesis can be seen most commonly in the liver, spleen, lymph nodes, and paravertebral regions.3 The intricate and self-renewing process of hematopoiesis allows for the reproduction and differentiation of cell lineages into erythrocytes, granulocytes, and megakaryocytes. This differentiation is tightly regulated by a milieu of cytokines, growth factors, and an intact microenvironment in the centers of hematopoiesis.
Erythropoiesis In the developing fetus, erythropoiesis begins on gestational day 20 as two distinct entities. The first is termed primitive erythropoiesis and results in nucleated red blood cells (RBC). The second phase is termed definitive erythropoiesis and, unlike n Correspondence to: St. Louis Children's Hospital, One Children's Place, Suite 5S40, St. Louis, Missouri 63110. E-mail address: [email protected] (A.M. Vogel). 1 Authors contributed equally to this work.
1055-8586/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1053/j.sempedsurg.2013.10.009
primitive erythropoiesis, is erythropoietin (Epo) dependent. This latter phase produces cells that are enucleated, CD34 þ , and comprised of fetal and adult hemoglobin (Hgb).4 Anemia is commonly encountered in the first few weeks of life and is largely considered physiologic with few clinical implications. However, the RBC life span in a premature neonate (35–50 days) is significantly shorter than that of a term neonate (60–70 days), which contributes to an earlier onset and higher degree of anemia.4,5 This anemia is more profound in critically ill neonates, resulting from acute blood loss from frequent phlebotomy and impaired RBC production secondary to inflammation. Finally, iron deficiency anemia is more common in premature infants as the bulk of placental transfer of maternal iron occurs in the late third trimester.4 Table 1 compares the factors contributing to physiologic and pathologic anemia in both term and preterm neonates. The standard treatment for neonatal anemia is blood transfusions and minimization of phlebotomy. Supplemental Epo has not been shown to decrease transfusion requirements.6 Since multiple transfusions are often necessary, repeat transfusions should be from the same unit of adult blood to minimize antigen exposure.6,7 Indications for transfusion in neonates depend on the degree of anemia as reflected in the hemoglobin and hematocrit levels in conjunction with clinical and physiologic parameters. The use of liberal vs restrictive RBC transfusions in neonates is an often debated topic currently under investigation. Two large clinical trials have been performed, with differing results. In a prospective randomized controlled trial of 100 preterm infants assigned into a liberal or restrictive-transfusion group, a higher incidence of adverse outcomes, specifically IVH, periventricular leukomalacia, and apnea was noted in the restrictive group.8 These findings however, were not reproduced in a larger study of the similar design consisting of 451 extremely low-birth weight infants, where no increase in the incidence adverse events was noted.9 These differing results complicate the timing and indications for RBC transfusions and stress the need for larger multicenter trials for guiding clinical practice.
200
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
Table 1 Factors contributing to physiologic and pathologic anemia in infants. (Adapted from Juul.4) Physiologic and pathologic factors contributing to anemia
Term infant
Preterm infant
Time of onset Nadir Hct Epo
Week 10–12 30% Decreased after transitioning from hypoxic placental environment to oxygen-rich circulation Liver before 3–4 mo after birth; kidney thereafter High/high 0.21 g/dL
Week 4–6 21–28% Decreased after transitioning from hypoxic placental environment to oxygen-rich circulation Liver before 3–4 mo after reaching term age; kidney thereafter High/high 0.21 g/dL
Production location for Epo Epo clearance/volume of distribution Percent Hct increase per gestational week RBC life span5 Phlebotomy Iron deficiency Blood loss Hemolysis Hereditary causes Decreased RBC production
60–70 d 35–50 d Likelihood of repeated phlebotomy lower Latrogenic anemia common Lower likelihood Higher likelihood Obstetric complications and twin–twin transfusions ABO, Rh incompatibility, autoimmune hemolytic disease, sepsis, DIC, vitamin E deficiency, and iron deficiency G6PD deficiency, RBC membrane disorders, and hemoglobinopathies Infection, drugs, and genetic
Granulopoiesis Granulocyte production begins at around the 8th gestational week with the production of macrophages. These cells are thought to create the cavity within the bone where bone marrow stem cells will eventually reside.10 Lymphocytes produced in the bone marrow subsequently migrate to the thymus at the 8th week postconception, when this organ develops. Wide variability between reported lymphocyte counts has been identified in the neonate when accounting for gestational age, as well as days of life. A drop in white blood cell (WBC) count is physiologic during the first 4 h after birth and has been potentially attributed to the corticosteroid surge during delivery, as corticosteroids have been shown to cause a decrease in circulating lymphocyte levels.11 Lymphocyte variations in the first 28 days are common.11 Interestingly, lymphocyte counts in the upper and lower 5th percentiles have been associated with increased frequency of early-onset sepsis (EOS) and intraventricular hemorrhage (IVH). Additionally, neonates with lymphocyte counts in the lower 5th percentile were found to have higher rates of retinopathy of prematurity (ROP).11 These variations in lymphocyte counts and associated comorbidities have been attributed to corticosteroid levels, as well as perinatal catecholamines, cytokines, and growth factors.11 Neonates with EOS may experience lymphocytosis from a presumed catecholamine or lymphopenia from presumed corticosteroid release from the stress of neonatal infections. Clinical correlation, as well as a high index of suspicion, must therefore accompany the assessment of the neonate in the extremes of WBC counts.
Megakaryopoiesis Megakaryopoiesis is first noted at about the 5th gestational week, with production of functional platelets depending on the production of thrombopoietic factors, megakaryocyte (MK) proliferation, differentiation and maturation of MK, and the efficient release of platelets into circulation.12 Although the embryo traditionally has been cited to reach platelet levels of 150 109 per liter by the end of the first trimester and adult range levels by 22 weeks' gestation, more recent studies have challenged this notion when including sick children into studies. Among infants admitted to the neonatal intensive care unit (NICU), reference ranges that excluded the upper and lower 5th percentile were 104 109 per liter for infants younger than 32 weeks and 123 109 per liter for those older than this same gestational age.12
Thrombopoiesis differences between adults and neonates are evident in growth factor levels as well as functional and structural MK differences. Levels for the most potent stimulus for thrombopoiesis, thrombopoietin (Tpo), are higher in neonates. MK progenitors in neonates are more sensitive to lower Tpo concentrations. They are also ten times more proliferative than adult counterparts, smaller in size, and with lower ploidy levels. Additionally, MK progenitors in neonates, although expressing lower ploidy levels, are found to be more cytoplasmically mature than adult MK cells, as evident by higher expression of CD42b, a marker of mature MKs.12 Neonatal platelets, although structurally similar to adult counterparts, exhibit different levels of surface glycoproteins and react differently to agonists. For the first 2–4 weeks after birth, neonatal platelets have decreased granule secretion, fibrinogenbinding site expression, and response to agonists.13 They also have fewer pseudopods, smaller glycogen deposits, and fewer visible microtubular structures.14 These “hyporeactive” platelets are responsible for reduced clot strength as seen on laboratory tests that assess the global measures of hemostatic function such as thromboelastography (TEG) and rotational thromboelastometry (ROTEM). While a healthy-term neonate may have a normal adult platelet count, there are many pathologic conditions that can lead to thrombocytopenia. Onset of neonatal thrombocytopenia can be seen early (first 72 h of life) or late (after 72 h of life). Early onset is attributed most commonly to fetal–maternal interaction with intrauterine growth restriction secondary to placental insufficiency, leading to decreased platelet production being the most common cause. Late-onset thrombocytopenia is due to platelet consumption, commonly seen in diffuse bacterial infections, necrotizing enterocolitis, and sepsis.15 Transfusion practices for platelets are discussed further in subsequent paragraphs.
Hemostasis Hemostasis is a dynamic process that begins in utero and evolves into the adult system throughout childhood. Andrew et al. coined the term “developmental hemostasis” in the 1980s to describe this maturation process. Although distinct quantitative and qualitative differences (described below) between infants and adults were noted in the index studies of developmental hemostasis, the hemostatic system in the healthy neonate is physiologically fully functional and rarely has negative clinical implications. There are two major approaches to describing “normal” coagulation and hemostasis. The traditional cascade model, first
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
Fig. 1. The cascade model of hemostasis.17
reported in 1964, has been critical to our understanding of the various proteins necessary for hemostasis, anticoagulation, and many disease processes.16 The classic cascade model (Figure 1) describes a linear series of enzymatic reactions involving coagulation factors, proteins, platelets, and platelet products that interact through two major pathways (intrinsic/contact and extrinsic/ tissue factor) to culminate in thrombin formation.17 However, the cascade model is unable to accurately describe certain coagulopathic conditions or predict selected treatments. For example, hemophiliacs, with an intact extrinsic pathway, would be expected to have normal thrombin formation. Additionally, exogenous factor VII therapy as an adequate treatment for hemophiliacs is equally unexplained by this model.18 The cell-based model of hemostasis, originally proposed by Mann et al.,16 is a more recent and developing model of hemostasis. This model assigns an important role to cells, especially platelets, in providing the anionic phospholipid membrane necessary for the assembly of the coagulation complex.18 The cell-based model is better able to explain what the cascade model, with its separate intrinsic and extrinsic pathways, cannot. The cell-based model of hemostasis is thought to transpire in three overlapping phases. Initiation occurs on tissue factor (TF)-bearing cells, often a subendothelial cell, when there is an endothelial injury and it is exposed to the small amount of circulating, constitutively active factor VII. A sufficiently strong procoagulant stimulus forms enough thrombin and factors Xa and IXa, starting the coagulation process. Amplification ensues after TF-bearing cell activity moves to the platelet surface. Here, the procoagulant stimulus is amplified through further platelet adhesion, activation, and accumulation of surface cofactors. During the propagation phase, platelet surface cofactors combine with active proteases which ultimately leads to the generation of thrombin and fibrin polymerization18 (Figure 2). The cell-based model of hemostasis is able to explain the above observations in hemophiliacs that are not justified by the cascade model. While hemophiliacs are able to form thrombin efficiently via the extrinsic pathway, they do not generate activated factor X. Without activated factor X, the transition from the TF-bearing cell to the platelet is severely impaired. It has been proposed that giving the factor VII to hemophiliacs will enable the extrinsic pathway on the TF-bearing cell to generate an increased amount of thrombin capable of diffusing and activating platelets.18
201
decreased levels of factors II, VII, IX, XI, and XII but increased levels of thrombomodulin, tPA, and plasminogen activator inhibitor-1, predisposing to bleeding. However, they also have larger and more abundant vWF and lower levels of antithrombin, heparin cofactor II, α-2-macroglobulin, protein C, protein S, and plasminogen that balance this bleeding tendency.20,21 Neonates thus have prolongation of prothrombin time (PT) and activated partial thromboplastin time (aPTT) but a shortened bleeding time. Additionally, functional differences in coagulation factors, like fibrinogen, further affect the newborn's ability to maintain hemostasis. Fibrinogen in the neonate undergoes post-translational modification, attaining higher sialic acid levels and a decreased ability to cross-link.22 Interestingly, in pediatric patients who receive an adult liver transplant, coagulation protein levels remain the same, indicating that it is not under direct control from the liver itself.23 One theory to explain the altered coagulation profile in neonates revolves around the concept that many of the proteins responsible for coagulation have multiple functions outside of hemostasis.21 One such protein is antithrombin, which is decreased in the neonate. Antithrombin is required not only for anticoagulation but is also known to possess anti-angiogenic properties. The protein is likely decreased in the neonate to avoid the anti-angiogenic properties; however, it is also critical for anticoagulation. Other protein levels must be altered in order to maintain a physiologic hemostatic profile. This theory not only helps to explain the difference in coagulation protein levels and platelet activity but also highlights the dangers of transfusing patients with platelets, fresh frozen plasma (FFP), or cryoprecipitate.22 When transfusing neonates with adult blood components, clinicians must consider the differences in protein levels, functionality, and activity of the components, as adult platelets—at least
Developmental hemostasis Several coagulation factors in the neonate have values that are outside of the normal adult range (Table 2).19 Newborns have
Fig. 2. Cell-based model of hemostasis: the three phases of coagulation— (A) initiation, (B) amplification, and (C) propagation.17
202
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
Table 2 Developmental hemostasis: age-related differences in hemostatic factors19 Preterm neonates vs Neonates vs older term neonates children/adults
Primary hemostasis Platelet count Decreased (o32 wk) Platelet function Decreased Higher Percentage of reticulated platelets vWF level NA vWF large NA multimers Coagulation factors FII, FVII, FIX, and FX FV FVIII FXI FXII Fibrinogen level Fibrinogen function
Same Decreasedb Higher
2–4 weeks NA
Higher Higher
3 mo 3 mo
Lower
Lower
16 yr
Lower Higher Lower Lower Same NA
Same or lower Same or lower Lower Lower Same Decreased
16 yr 1 moc 1 yr 16 yr 5 yr
Lower Lower Lower Higher Reduced Lower
3 mo 16 yr 1 mo NA NA Adult
Lower
Lower
6 mo
NA
Decreased
NA
Higher Lower Higher Same or higher
5d 5d Adult 5d
Regulation of coagulation Antithrombin Lower Protein C Lower Total protein S Lower Free protein S NA APCR generation NA Free TFPI NA Fibrinolysis Plasminogen level Plasminogen function tPA α2 antiplasmin α2 M PAI
Approximate age of adult valuesa
d
Same Lower Same Samed
NA, indicates not available; APCR, activated protein C resistance; and TFPI, tissue factor pathway inhibitor. a
Maximum age reported. Decreased response was reported to agonists such as thrombin, collagen, epinephrine, and thrombin activation peptide as tested by flow cytometry. c Lower levels compared with adults are reported from 1 mo to 16 yr of age. d Higher levels in extremely preterm neonates on day 10 of life compared with older preterm or term neonates. b
in vitro—have been shown to cause a hypercoagulable state when they are transfused to neonatal plasma.24 Interpreting and assessing the coagulation system of the critically ill neonate is an ongoing challenge. The concept of developmental hemostasis, with the age-related changes in hemostatic components, complicates the interpretation of common laboratory tests such as PT, PTT, platelet count, and bleeding time. Additionally, the tests commonly used to assess the coagulation system require relatively large volumes of blood for adequate interpretation and are not exempt from intrinsic sample problems such as component activation and heparin contamination, to name a few. Because of these limitations, point-of-care (POC) sample collections are optimal, but scarce. In recent years, thromboelastography or TEG has been evaluated as a potentially suitable examination that can provide an individualized evaluation for each patient's bleeding risk and their need for blood component transfusion.25 TEG is a POC test that uses a small amount of blood and provides a graphical representation (Figure 3) on the dynamics of clot development, stabilization, and dissolution (fibrinolysis).26 This test records viscoelastic
changes that occur during coagulation in whole blood and has previously been shown to have largely comparable results between neonates and adults.22 TEG provides values for the entirety of the coagulation and fibrinolytic processes, allowing for assessment of all aspects of the coagulation system (Table 3). Briefly, the r-value is defined as the time between the initiation of the test and fibrin formation and is a representative of clotting factors. The k-time is the time needed for the tracing to reach 20 mm from 2 mm and is increased with hypofibrinogenemia or platelet deficiency. The α-angle is the slope of the tracing that represents the rate of clot formation and decreases with hypofibrinogenemia or platelet deficiency. The maximal amplitude (MA) is the greatest amplitude of the tracing and represents the platelet contribution to clot strength. Finally, LY30 is the percent amplitude reduction at 30 min after achievement of MA and represents fibrinolysis.27 Reference ranges for these parameters have been described in neonates, infants, and children.24–26,28 Goal-directed hemostatic resuscitation employs TEG-based transfusion of fresh frozen plasma, cryoprecipitate, platelets, and adjunctive hemostatic agents such as activated factor VII to correct shock and coagulopathy in critically ill patients. The resuscitation strategy may have a role in treating critically ill, coagulopathic neonates.29
Clinical implications of the neonatal coagulation system Coagulation advantage The term neonate, as previously noted, has a shorter bleeding time and shorter closing time on platelet function testing (PFA), indicating faster formation of the initial platelet plug. These observations are likely due to the increase in hematocrit at birth and the amount and size of vWF.13,24 It is, in fact, thought that neonates have a small coagulation advantage, for reasons that are poorly understood. Vitamin K deficiency bleeding Vitamin K deficiency bleeding, formerly known as hemorrhagic disease of the newborn, is a coagulopathy that presents early (within 24 h), classically (24 h to 7 days), or late (2–12 weeks) after birth.30 Etiologies according to timing of onset include maternal intake of drugs that inhibit vitamin K, delayed or insufficient feeding, and an infant diet consisting exclusively of breast milk.30 Bleeding is caused by a lack of the vitamin K-dependent coagulation factors II, VII, IX, and X. Premature neonates, are prone to low levels of vitamin K due to low levels of vitamin K-dependent factors, low levels of vitamin K in breast milk, and the absence of bacterial gut colonization that produces vitamin K.10 This nutritional deficiency is rarely seen in the US because of prophylactic vitamin K supplementation. However, if this diagnosis is suspected and severe clinical bleeding is present, transfusing FFP will temporarily supply the missing factors. Intraventricular hemorrhage (IVH) IVH is a highly morbid condition in infants of extreme low-birth weight (ELBW) and prematurity. This complication is multifactorial, with immature vessels, a friable germinal matrix, fluctuations in cerebral blood flow, and respiratory failure requiring mechanical ventilation causing large variations in intrathoracic and venous pressures cited as contributors.31 The delicate balance between the relative bleeding propensity from lower levels of vitamin K-dependent coagulation factors and the procoagulant tendency from lower anticoagulant, antithrombin, and tissue
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
203
Fig. 3. Clot formation from TEG test.
factor pathway inhibitor is easily disrupted in neonates experiencing acquired or congenital defects in hemostasis.31 However, heritable coagulopathies are infrequently present, and occasionally undiagnosable, in the neonatal period. Acquired coagulopathies, like thrombocytopenia, are common in premature and small-forgestational-age infants. These same infants are similarly at risk for sepsis and other conditions that further worsen the degree of thrombocytopenia. Additional coagulation derangements have been shown to coincide with high-grade IVH in ELBW infants, with lower levels of factor VII serving as the most sensitive marker for the development of IVH.32 Interventions for IVH prevention have traditionally included correction of thrombocytopenia with platelet transfusion. Thresholds for platelets transfusion vary widely between countries and institutions due to the shortage of randomized controlled trials. In the United States and Canada, transfusions are administered to nonbleeding neonates with counts of more than 50 109 per liter,33 while in the United Kingdom, thresholds for healthy-term ( o25 109 per liter) and preterm ( o30 109 per liter)34 neonates are much more conservative. Further interventions for the prevention of IVH include pharmacologic agents that target the hemostatic system. Ethamsylate is a nonsteroidal agent that increases capillary endothelial resistance and promotes platelet adhesion and has been associated with reduction in IVH rates, but with no effect on overall mortality.31 Vitamin K, administered prophylactically to newborns, increases the concentration of vitamin K-dependent factors and reduces both mortality and occurrence of IVH.31 Both FFP and prothrombin complex concentrate (PCC) increase levels of some circulating coagulation factors. However, conflicting outcomes have been implicated with the use of both FFP and PCC.31 Finally, recombinant activated factor VIIa (rFVIIa) is a pharmacologic hemostatic adjunct that anecdotally and in small series has been used for IVH; however, no randomized controlled data supports the use of this agent in preventing IVH.31
Disseminated intravascular coagulation Disseminated intravascular coagulation (DIC) is a thrombohemorrhagic disorder that is seen in characteristic clinical situations with laboratory evidence of procoagulant activation, fibrinolytic activation, inhibitor consumption, and evidence of end-organ damage or failure.35 Derangements in the delicate balance of the neonatal coagulation system from low levels of hemostatic components, hypoxia, intravascular volume contraction after birth, and sepsis predispose this population to a higher risk for the development of DIC.36 Diagnosis includes prolongation of all coagulation parameters, thrombocytopenia, and microangiopathic hemolytic anemia. Paramount treatment is prompt management of the underlying process. Aggressive resuscitation, frequently with blood components, and inotropic support may be necessary to correct circulatory compromise and shock. Transfusions with fresh frozen plasma (FFP) and platelets are recommended to keep the fibrinogen 41.0 g/L and the platelets 450 109 per liter. Coagulation-based therapeutic approaches, when used, should focus on preventing or reversing microthrombi formation and preventing or stopping hemorrhage.36 Recombinant activated factor VII (rFVIIa) is another therapeutic intervention that has been used in select cases of catastrophic acute life-threatening hemorrhage, where conventional methods to achieve hemostasis have failed.36 Caution must be exercised when using rFVIIa for fear of exacerbating the microvascular thrombus formation. Extracorporeal life support Extracorporeal life support (ECLS) is an invasive therapy used for select neonates with reversible cardiorespiratory failure, often as a result of meconium aspiration syndrome, persistent pulmonary hypertension of the newborn, sepsis, and congenital diaphragmatic hernia.37 Once therapy is initiated, the nonbiologic surface of ECLS components predisposes the neonate to
Table 3 Description of TEG values. (Adapted with permission from Alexander et al.27) Value
Quantifies
Represents
Factors affecting the value
R K α-angle MA LY30
Reaction time Coagulation time Clotting rate Maximum amplitudes Percentage lysis 30 min after MA
Time to original fibrinogen plug Clot-formation time Rate of clot formation Strength of clot Rate of fibrinolysis
Clotting factor deficiency, pharmaceutical anticoagulation, and thrombocytopenia Decreased fibrinogen or platelets Clotting factor deficiency, platelet dysfunction, thrombocytopenia, or low fibrinogen Quantity and function of platelets Anticoagulation proteins quantity and function
204
J. Diaz-Miron et al. / Seminars in Pediatric Surgery 22 (2013) 199–204
thrombotic complications, making systemic anticoagulation necessary. The risk of thrombosis and hemorrhage both increase morbidity and mortality. With rare exception, anticoagulation is maintained with unfractionated heparin (UNFH). Heparin acts as a cofactor for antithrombin, deactivating thrombin and other coagulation factors. Monitoring of heparin-based systemic anticoagulation is typically performed using the activated clotting time (ACT) test, with goal targets of 180–220 seconds. Heparin concentrations though, have been shown to have only a moderate correlation with ACT values.38 If an abnormally high dose of UNFH is required, administration of antithrombin, via recombinant antithrombin or paradoxically FFP, can be used to increase the level of anticoagulation while decreasing the necessary dose of UNFH.39 Antifactor Xa assay is an additional test that may be advantageous due to its ability to monitor the anticoagulation effects of heparin directly.39 TEG is an alternate test increasing in popularity that is used to measure the adequacy of anticoagulation, especially in patients who require prolonged ECLS or will require additional surgical interventions. Additional trials are warranted that evaluate the optimal anticoagulant for neonates, as well as the ideal testing assays to evaluate the effectiveness of these anticoagulants.
Conclusion Implications of the complex and partly understood neonatal coagulation system can be troublesome to the surgical team. An understanding of how the unique neonatal hematologic and hemostatic physiology differs from term infants, children, and adults is essential for understanding the clinical implication of these systems, particularly during critical illness. Management decisions made daily and almost reflexively in the adult population pose a greater dilemma in the neonatal population. The lack of established guidelines for transfusion and anticoagulation further complicate clinical decision-making. Pediatric surgeons and neonatologists must balance the risks and benefits associated with blood component transfusions that address the anemic, leukopenic, and thrombocytopenic neonate. Additional studies are warranted to optimize transfusion guidelines and anticoagulation practices for children with derangements of their hemostatic system.
Acknowledgment The authors wish to acknowledge Mrs. Susan Phillips for her critical appraisal of the manuscript. References 1. Bollerot K, Pouget C, Jaffredo T. The embryonic origins of hematopoietic stem cells: a tale of hemangioblast and hemogenic endothelium. APMIS. 2005; 113(11–12):790–803. 2. Dieterlen-Lievre F, Godin I, Pardanaud L. Where do hematopoietic stem cells come from? Int Arch Allergy Immunol. 1997;112(1):3–8. 3. Sohawon D, Lau KK, Lau T, et al. Extra-medullary haematopoiesis: a pictorial review of its typical and atypical locations. J Med Imaging Radiat Oncol. 2012; 56(5):538–544. 4. Juul S. Erythropoiesis and the approach to anemia in premature infants. J Matern Fetal Neonatal Med. 2012;25(suppl 5):97–99. 5. Bard H, Widness JA. The life span of erythrocytes transfused to preterm infants. Pediatr Res. 1997;42(1):9–11. 6. Strauss RG. Anaemia of prematurity: pathophysiology and treatment. Blood Rev. 2010;24(6):221–225. 7. Luban NL. Management of anemia in the newborn. Early Hum Dev. 2008; 84(8):493–498.
8. Bell EF, Strauss RG, Widness JA, et al. Randomized trial of liberal versus restrictive guidelines for red blood cell transfusion in preterm infants. Pediatrics. 2005;115(6):1685–1691. 9. Kirpalani H, Whyte RK, Andersen C, et al. 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(3):301–307. 10. Gleason CA, Devaskar SU, Avery ME. In: Gleason Christine A, Devaskar Sherin U, editors. Avery's Diseases of the Newborn, 9th ed. Philadelphia, PA: Elsevier/ Saunders; 2012. 11. Christensen RD, Baer VL, Gordon PV, et al. Reference ranges for lymphocyte counts of neonates: associations between abnormal counts and outcomes. Pediatrics. 2012;129(5):e1165–e1172. 12. Sola-Visner M. Platelets in the neonatal period: developmental differences in platelet production, function, and hemostasis and the potential impact of therapies. Hematology Am Soc Hematol Educ Program. 2012(1):506–511. 13. Israels SJ. Diagnostic evaluation of platelet function disorders in neonates and children: an update. Semin Thromb Hemost. 2009;35(2):181–188. 14. Strauss T, Sidlik-Muskatel R, Kenet G. Developmental hemostasis: primary hemostasis and evaluation of platelet function in neonates. Semin Fetal Neonatal Med. 2011;16(6):301–304. 15. Ulusoy E, Tufekci O, Duman N, et al. Thrombocytopenia in neonates: causes and outcomes. Ann Hematol. 2013;92(7):961–967. 16. Mann KG, Krishnaswamy S, Lawson JH. Surface-dependent hemostasis. Semin Hematol. 1992;29(3):213–226. 17. Smith SA. The cell-based model of coagulation. J Vet Emerg Crit Care (San Antonio). 2009;19(1):3–10. 18. Hoffman M, Monroe DM 3rd. A cell-based model of hemostasis. Thromb Haemost. 2001;85(1):958–965. 19. Revel-Vilk S. The conundrum of neonatal coagulopathy. Hematology Am Soc Hematol Educ Program. 2012;2012:450–454. 20. Ignjatovic V, Lai C, Summerhayes R, et al. Age-related differences in plasma proteins: how plasma proteins change from neonates to adults. PLoS One. 2011;6(2):e17213. 21. Monagle P, Massicotte P. Developmental haemostasis: secondary haemostasis. Semin Fetal Neonatal Med. 2011;16(6):294–300. 22. Monagle P, Ignjatovic V, Savoia H. Hemostasis in neonates and children: pitfalls and dilemmas. Blood Rev. 2010;24(2):63–68. 23. Lisman T, Platto M, Meijers JC, et al. The hemostatic status of pediatric recipients of adult liver grafts suggests that plasma levels of hemostatic proteins are not regulated by the liver. Blood. 2011;117(6):2070–2072. 24. Ferrer-Marin F, Chavda C, Lampa M, et al. Effects of in vitro adult platelet transfusions on neonatal hemostasis. J Thromb Haemost. 2011;9(5):1020–1028. 25. Radicioni M, Mezzetti D, Del Vecchio A, et al. Thromboelastography: might work in neonatology too? J Matern Fetal Neonatal Med. 2012;25(suppl 4):18–21. 26. Strauss T, Levy-Shraga Y, Ravid B, et al. Clot formation of neonates tested by thromboelastography correlates with gestational age. Thromb Haemost. 2010;103(2):344–350. 27. Alexander DC, Butt WW, Best JD, et al. Correlation of thromboelastography with standard tests of anticoagulation in paediatric patients receiving extracorporeal life support. Thromb Res. 2010;125(5):387–392. 28. Vogel AM, Radwan ZA, Cox CS CS Jr, et al. Admission rapid thrombelastography delivers real-time “actionable” data in pediatric trauma. J Pediatr Surg. 2013; 48(6):1371–1376. 29. Afshari A, Wikkelso A, Brok J, et al. Thrombelastography (TEG) or thromboelastometry (ROTEM) to monitor haemotherapy versus usual care in patients with massive transfusion. Cochrane Database Syst Rev. 2011;(3):CD007871. 30. Van Winckel M, De Bruyne R, Van De Velde S, et al. Vitamin K, an update for the paediatrician. Eur J Pediatr. 2009;168(2):127–134. 31. Kuperman AA, Kenet G, Papadakis E, et al. Intraventricular hemorrhage in preterm infants: coagulation perspectives. Semin Thromb Hemost. 2011;37(7): 730–736. 32. Piotrowski A, Dabrowska-Wojciak I, Mikinka M, et al. Coagulation abnormalities and severe intraventricular hemorrhage in extremely low birth weight infants. J Matern Fetal Neonatal Med. 2010;23(7):601–606. 33. Josephson CD, Su LL, Christensen RD, et al. Platelet transfusion practices among neonatologists in the United States and Canada: results of a survey. Pediatrics. 2009;123(1):278–285. 34. Chaudhary R, Clarke P. Current transfusion practices for platelets and fresh, frozen plasma in UK tertiary level neonatal units. Acta Paediatr. 2008;97(1):135. 35. Bick RL. Disseminated intravascular coagulation: a review of etiology, pathophysiology, diagnosis, and management: guidelines for care. Clin Appl Thromb Hemost. 2002;8(1):1–31. 36. Veldman A, Fischer D, Nold MF, et al. Disseminated intravascular coagulation in term and preterm neonates. Semin Thromb Hemost. 2010;36(4):419–428. 37. Dalton HJ, Butt WW. Extracorporeal life support: an update of Rogers' Textbook of Pediatric Intensive Care. Pediatr Crit Care Med. 2012;13(4):461–471. 38. Baird CW, Zurakowski D, Robinson B, et al. Anticoagulation and pediatric extracorporeal membrane oxygenation: impact of activated clotting time and heparin dose on survival. Ann Thorac Surg. 2007;83(3):912–919 [discussion 919920]. 39. Annich G, Adachi I. Anticoagulation for pediatric mechanical circulatory support. Pediatr Crit Care Med. 2013;14(suppl 1):S37–S42.