e1 13 Ramaswamy K, Hsieh L, Leven E, Thompson MV, Nugent D, Bussel JB Thrombopoietic agents for the treatment of persistent and chronic immune thrombocytopenia in children J Pediatr 2014;165 600 605 e[.]
e1 References Hughes H, Kahl L, eds Harriet Lane Handbook [electronic resource]: A Manual for Pediatric House Officers Philadelphia, PA: Elsevier; 2018 Iosub S, Naik M, Bhalani K, Gromisch DS Leukocyte and neutrophil counts in healthy Puerto Rican children and children with acute appendicitis Clin Pediatr (Phila) 1986;25:366-368 Orkin SH, Zon LI Hematopoiesis: an evolving paradigm for stem cell biology Cell 2008;132:631-644 Sieff CA, Daley GQ, Zon LI Anatomy and physiology of hematopoiesis In: Orkin S, Fisher D, Ginsburg D, et al., eds Nathan and Oski’s Hematology of Infancy and Childhood 8th ed Philadelphia, PA: Saunders Elsevier; 2015 Weiss L The hematopoietic microenvironment of the bone marrow: an ultrastructural study of the stroma in rats Anat Rec 1976;186:161-184 Morrison SJ, Scadden DT The bone marrow niche for haematopoietic stem cells Nature 2014;505:327-334 Lo Celso C, Fleming HE, Wu JW, et al Live-animal tracking of individual haematopoietic stem/progenitor cells in their niche Nature 2009;457:92-96 Fiorito BA, Mirza F, Doran TM, et al Intraosseous access in the setting of pediatric critical care transport Pediatr Crit Care Med 2005;6:50-53 Orlowski JP, Julius CJ, Petras RE, Porembka DT, Gallagher JM The safety of intraosseous infusions: risks of fat and bone marrow emboli to the lungs Ann Emerg Med 1989;18:1062-1067 10 Craig FE, Foon KA Flow cytometric immunophenotyping for hematologic neoplasms Blood 2006;111:3941-3967 11 Weissman IL, Shizuru JA The origins of the identification and isolation of hematopoietic stem cells, and their capability to induce donorspecific transplantation tolerance and treat autoimmune diseases Blood 2008;112:3543-3553 12 Goodell MA Stem-cell “plasticity”: befuddled by the muddle Curr Opin Hematol 2003;10:208-213 13 Ramaswamy K, Hsieh L, Leven E, Thompson MV, Nugent D, Bussel JB Thrombopoietic agents for the treatment of persistent and chronic immune thrombocytopenia in children J Pediatr 2014;165:600-605.e4 14 Steinberg MH, Benz EJ, Adewoye HA, et al Pathobiology of the human erythrocyte and its hemoglobins In: Hoffman R, Benz EJ, Shattil SJ, eds Hematology, Basic Principles and Practice 4th ed Philadelphia, PA: Elsevier; 2005 15 Delivoria-Papadopoulos M, Miller LD, Forster RE II, Oski FA The role of exchange transfusion in the management of low-birth-weight infants with and without severe respiratory distress syndrome I Initial observations J Pediatr 1976;89:273-278 16 McKoy JM, Stonecash RE, Cournoyer D, et al Epoetin-associated pure red cell aplasia: past, present, and future considerations Transfusion 2008;48:1754-1762 17 Dinauer MC, Newburger PE The phagocyte system and disorders of granulopoiesis and granulocyte function In: Orkin S, Fisher D, Look AT, et al., eds Nathan and Oski’s Hematology of Infancy and Childhood 7th ed Philadelphia, PA: Saunders Elsevier; 2009 18 Christensen RD, Rothstein G Exhaustion of mature marrow neutrophils in neonates with sepsis J Pediatr 1980; 96:316-318 19 Rodriguez S, Chora A, Goumnerov B, et al Dysfunctional expansion of hematopoietic stem cells and block of myeloid differentiation in lethal sepsis Blood 2009;114: 4064-4076 20 Vose JM, Armitage JO Clinical applications of hematopoietic growth factors J Clin Oncol 1995;13:1023-1035 21 Rizzo JD, Somerfield MR, Hagerty KL, et al Use of epoetin and darbepoetin in patients with cancer: 2007 American Society of Clinical Oncology/American Society of Hematology clinical practice guideline update J Clin Oncol 2008;26:132-149 22 Villalba A, Osorio J, Freiria C, et al Copper deficiency: a cause of misdiagnosis of myelodysplastic syndrome Ann Hematol 2018; 97:1737-1738 23 Kaushansky K Thrombopoietin N Engl J Med 1998; 339:746-754 e2 Abstract: The hematopoietic system includes the bone marrow and, at different stages of fetal and postnatal development, the liver, spleen, lymph nodes, and thymus It maintains a balance between production and destruction of erythrocytes, platelets, and white blood cells Inadequate hematopoiesis can result in anemia, thrombocytopenia, and/or neutropenia Excessive hematopoiesis may cause erythrocytosis, thrombocytosis, or leukocytosis Poorly controlled hematopoiesis may contribute to autoimmunity, macrophage activation syndromes, or chronic inflammation This chapter reviews the anatomy and physiology of the hematopoietic system with an emphasis on the bone marrow to provide an understanding of abnormalities in the pediatric intensive care unit setting Key words: Bone marrow, hematopoiesis, hematopoietic growth factors, intraosseous infusion, splenic function 87 The Erythron AL LAN DOCTOR, AHMED SAID, AND STEPHEN ROGERS PEARLS • • • Together, red blood cells (RBCs) at each stage of development may be considered an organ (termed the erythron) now appreciated to participate in active regulation of regional blood flow distribution as well as oxygen (O2) and carbon dioxide (CO2) transport RBCs are subject to intense biochemical, biomechanical, and physiologic stress during repeated circulatory transit As such, they possess unique properties and robust energetic and antioxidant systems to maintain functionality for a 3- to 4-month lifetime RBCs actively regulate blood flow volume and distribution to maintain coupling between O2 delivery and demand The The erythron includes red blood cells (RBCs) at all stages of development, from progenitors to senescent forms, and is the organ (composed of anucleated cells in suspension) responsible for oxygen (O2) transport from lungs to tissue.1 This role is newly appreciated to include active vasoregulation (by RBCs) that links regional blood flow to O2 availability in the lung and to consumption in the periphery.2 Considerable energy is devoted to maintaining a robust RBC population (20–30 trillion cells circulate in the average adult—approximately 25% of the cells in the body are RBCs); 1.4 million RBCs are released into the circulation per second, replacing 1% of the circulating mass per day Mature RBCs have a life span of 4 months, the majority of which is spent traversing the microcirculation It is estimated that an RBC travels approximately 400 km during this interval, having made 170,000 circuits through the vascular tree Circulating RBCs demonstrate unique physiology and are adapted to withstand significant biomechanical and biochemical stress As RBCs age, energy and antioxidant systems fail Key proteins, including hemoglobin (Hb) and lipids, suffer oxidative injury, negatively impacting performance (rheology, adhesion, gas transport, vascular signaling) Such cells acquire marks of senescence and are cleared by the spleen or undergo eryptosis (a process unique to RBCs, similar to apoptosis) Of importance, this process may be accelerated in the course of critical illness and, by limiting O2 delivery, influence organ failure progression and outcome Moreover, it is essential to note that in the setting of insufficient O2 delivery, blood flow (rather than content) is the focus of O2 delivery regulation: O2 content is relatively fixed, whereas flow • trapping, processing, and delivery of nitric oxide (NO) by RBCs has emerged as a conserved mechanism through which regional blood flow is linked to biochemical cues of perfusion sufficiency A new paradigm for O2 delivery homeostasis has emerged based on coordinated gas transport and vascular signaling by RBCs By coordinating vascular signaling in a fashion that links O2 and NO flux, RBCs couple vessel caliber (and thus blood flow) to O2 need in tissue Malfunction of this signaling system is implicated in a wide array of pathophysiologies and may explain, in part, the dysoxia frequently encountered in the critical care setting can be increased by several orders of magnitude Thus, blood flow volume and distribution are actively regulated to maintain coupling between O2 delivery and demand The trapping, processing, and delivery of nitric oxide (NO) by RBCs has emerged as a conserved mechanism through which regional blood flow is linked to biochemical cues of perfusion sufficiency This chapter reviews conventional RBC physiology influencing O2 delivery (O2 affinity and rheology) and introduces a new paradigm for O2 delivery homeostasis based on coordinated gas transport and vascular signaling by RBCs By coordinating vascular signaling in a fashion that links O2 and NO flux, RBCs couple vessel caliber (and thus blood flow) to O2 need in tissue Malfunction of this signaling system is implicated in a wide array of pathophysiology and may explain, in part, the dysoxia frequently encountered in the critical care setting Oxygen Transport Hb is formed by two a-globin and two b-globin chains, each carrying a heme prosthetic group, composed of a porphyrin ring bearing a ferrous atom that can reversibly bind an O2 molecule In the deoxygenated state, the Hb tetramer is electrostatically held in a tense (T) conformation Binding of the first O2 molecule leads to mechanical disruption of these bonds, an increase in free energy, and transition to the relaxed (R) conformation Each successive O2 captured by T-state Hb shifts the Hb tetramer closer to the R state, which has an estimated 500-fold increase in O2 affinity This concept of thermodynamically coupled “cooperativity” in 1033 1034 S E C T I O N I X Pediatric Critical Care: Hematology and Oncology O2 binding was first described by Perutz and explains the sigmoidal appearance of the O2-Hb binding curve, also known as the oxyhemoglobin dissociation curve (ODC) Moreover, understanding of allosteric influence of protein function by “heterotropic effectors” is essential For example, for Hb, O2, which binds to the heme “active” site, is the homotropic ligand; all other molecules influencing the Hb-O2 binding relationship are termed heterotropic effectors In addition to the homotropic effects of ligand binding on quaternary conformational changes (e.g., cooperativity), primary ligand binding affinity (O2) is also affected by multiple heterotropic effectors of significant physiologic relevance The major heterotropic effectors that influence Hb-O2 affinity are hydrogen ion (H1), chloride ion (Cl2), carbon dioxide (CO2), and 2,3-diphosphoglycerate (DPG) P50, the oxygen tension at which 50% of Hb binding sites are saturated, is the standard metric employed to quantify change in Hb-O2 affinity and is inversely related to the binding affinity of Hb for O2.3,4 Elevated levels of H1, Cl2, and CO2 reduce O2 binding affinity (e.g., raise P50) This allosteric shift in O2 affinity, called the Bohr effect, arises from the interactions among the heterotropic effectors mentioned earlier bound to different sites on Hb, all of which serve to stabilize the low-energy, low-affinity, Tstate Hb conformation This effect is achieved by complex interactions between carbonic anhydrase (CA) and the anion exchange protein (AE1—also known as the band [B3] membrane protein; Fig 87.1) Specifically, CA generates H1 and HCO32 from CO2 encountered in the microcirculation; HCO32 then exchanges for Cl2 across the RBC membrane through AE1 As a consequence, extra erythrocytic CO2 is converted into intraerythrocytic HCl by the CA-AE1 complex, acidifying RBC cytoplasm and raising P50 (lowering affinity, also termed right shifting the ODC) Additionally, through the Haldane effect, CO2 more directly lowers O2 affinity (by binding to the N-terminus of the globin chains to form a carbamino, further stabilizing T-state Hb); carbamino formation also releases another hydrogen ion (further reinforcing the right shift in the ODC; see Fig 87.1) This set of reactions is reversed in the alkaline (and low CO2) milieu in the pulmonary circulation, leading to increased Hb-O2 binding affinity (lower P50) This physiology vastly improves O2 transport efficiency by enhancing gas capture in the lung and release to tissue and does so in proportion to perfusion sufficiency (impaired perfusion, acidosis, and hypercapnea improve O2 release) In the setting of impaired O2 delivery that results in anaerobic glycolysis, lactate diffusion into capillary blood (by lowering pH) further increases O2 dissociation and facilitates O2 delivery Of note, this tightly regulated modulation of O2 affinity requires coordinated interaction of a complex intraerythrocytic protein network and may fail consequent to acquired RBC injury in the setting of critical illness.5–8 This partially explains the dysoxia commonly observed in this setting Less acute modulation of P50 is achieved by DPG, a glycolytic intermediate that binds in an electrically charged pocket between the b chains of hemoglobin, which stabilizes the T conformation, decreasing O2 affinity and elevating P50 DPG binding also releases protons, lowering intracellular pH and further reinforcing the Bohr effect DPG in RBCs increases whenever O2 availability is diminished (as in hypoxia or anemia) or when glycolytic flux is stimulated Last, temperature significantly influences Hb-O2 affinity As body temperature increases, affinity lessens (P50 increases, the ODC shifts right); the reverse happens in hypothermia This feature is of physiologic importance during heavy exercise, fever, or induced hypothermia It should be noted that clinical co-oximetry results and blood gas values are reported at 37°C and not at true in vivo temperature and can lead to either underor overestimation of true Hb-O2 saturation (HbSO2) percentage values and blood O2 tension Carbon Dioxide Transport As a lipophilic molecule, CO2 produced from tissue respiration readily diffuses into capillary blood and through the RBC membrane.9 A minority (5%–10%) of diffused CO2 binds to the a-amino terminus of Hb globin chains as carbaminohemoglobin, lowering O2 affinity (raising P50); this P50 shift is termed the Haldane effect.9,10 The majority of CO2 is hydrated by the two intraerythrocytic CA isoforms, CA I (low-turnover rate) and CA II (high-turnover rate) This conversion into bicarbonate (and a proton, see earlier discussion) is essential for efficient CO2 transport and for maintaining the venous-arterial difference in total CO2 The released protons stabilize the T conformation of Hb, facilitating O2 offloading in proportion to CO2 genesis (and thus regional tissue respiration) Carbaminohemoglobin also acts to directly stabilize T-state Hb The process reverses during pulmonary transit (since the lungs act as a CO2 sink), raising intraerythrocytic pH and enhancing O2 affinity in proportion to regional alveolar ventilation (CO2 removal) Biophysical Factors Influencing Gas Transport Blood Rheology Disease-based variation in blood fluidity has been recognized since the early 20th century,11 and there is substantive evidence that this property strongly influences tissue perfusion.12 Plasma is a newtonian fluid (viscosity is independent of shear rate); its viscosity is closely related to protein content and in critical illness, physiologically significant changes in viscosity may vary with concentration of acute phase reactants.13–15 Whole blood, however, is considered a nonnewtonian suspension (fluidity cannot be described by a single viscosity value) Whole blood fluidity is determined by combined rheologic properties of plasma and the cellular components The cellular components of blood, particularly RBCs, influence blood viscosity as a function of both number and deformability RBC concentration in plasma (hematocrit) has an exponential relationship with viscosity and meaningfully diminishing tissue perfusion when Hct exceeds 60 to 65 RBC deformability, or behavior under shear stress, also strongly influences blood fluidity Normal RBCs behave like fluid drops under most conditions, are highly deformable under shear, and orient with flow streamlines However, during inflammatory stress, RBCs tend to aggregate into linear arrays like a stack of coins (rouleaux) Fibrinogen and other acute phase reactants in plasma stabilize such aggregates, significantly increasing blood viscosity Such a change in viscosity most impacts O2 delivery during low flow (e.g., low shear) states (such as in critical illness) in the microcirculation.16 RBC biomechanics and aggregation impact blood viscosity, strongly influencing the volume and distribution of O2 delivery (again, more so in the low-shear microcirculation or when vessel tone is abnormal).17 This hemorheologic physiology is perturbed by oxidative stress (common in critical illness)18,19 and in sepsis.20–25 This has been attributed to increased intracellular 2,3-DPG concentration,26 intracellular free Ca21,27 and decreased intraerythrocytic adenosine triphosphate (ATP) with subsequent decreased sialic acid CHAPTER 87 The Erythron NO group NO uptake HbSO2 B Trapping Delivery FeNO Low pO2 Free plasma thiol O2 gradient in tissue Hb B Deoxy Hb Oxy Hb C SNOHb A NO S-nitrosothiols (SNOs) O2 NO O2 O2 NO groups without specific designation S-nitrosothiol SNO (SNO) export High pO2 A 1035 + D + NO AE1 NO SNO export species Hb RSH ? Equilibrium amongst SNOs and thiol pool Plasma thiol pool SNO Circulating NO NO groups Plasma SNOs Avid O2 extraction in hypoxic tissue initiates SNO export from perfusing RBCs O O gr ad ien t in tis su e As desaturating RBCs deliver vasodilator activity, blood flow improves, resolving steep O2 gradients and perfusion insufficiency gr ad ien t in tis su e C • Fig 87.1 Red blood cells (RBCs) transduce regional oxygen (O2) gradients in tissue to control nitric oxide (NO) bioactivity in plasma by trapping or delivering NO groups as a function of hemoglobin (Hb) O2 saturation (A) In this fashion, circulating NO groups are processed by Hb into the highly vasoactive (thiolbased) NO congener, S-nitrosothiol (SNO) By exporting SNOs as a function of Hb deoxygenation, RBCs precisely dispense vasodilator bioactivity in direct proportion to the lack of regional blood flow (B) O2 delivery homeostasis requires biochemical coupling of vessel tone to environmental cues that matches perfusion sufficiency to metabolic demand Because oxy- and deoxy-Hb process NO differently (see text), allosteric transitions in Hb conformation afford context-responsive (O2-coupled) control of NO bioavailability, thereby linking the sensor and effector arms of this system Specifically, Hb conformation governs the equilibria among deoxy-HbFeNO (A; NO sink), SNO-oxy-Hb (B; NO store), and acceptor thiols, including the membrane protein SNO-AE-1 (C; bioactive NO source) Direct SNO export from RBCs or S-transnitrosylation from RBCs to plasma thiols (D) or to endothelial cells directly (not shown) yields vasoactive SNOs, which influence resistance vessel caliber and close this signaling loop Thus, RBCs either trap (A) or export (D) NO groups to optimize blood flow (C) NO processing in RBCs (A and B) couples vessel tone to tissue partial pressure of O2; this system subserves hypoxic vasodilation in the arterial periphery and thereby calibrates blood flow to regional tissue hypoxia content in RBC membranes.28 Both increased direct contact between RBCs and white blood cells (WBCs) and reactive oxygen species (ROS) released during sepsis have also been shown to alter RBC membrane properties.29,30 Red Blood Cell Aggregation and Adhesion As noted earlier, in the absence of shear, RBCs suspended in autologous plasma stack in large aggregates, known as rouleaux Acute-phase reactants—especially fibrinogen, C-reactive protein, serum amyloid A, haptoglobin, and ceruloplasmin—have been shown to increase RBC aggregation.31 Pathophysiologic conditions, such as sepsis and ischemia-reperfusion injury, have been shown to alter RBC surface proteins and increase RBC “aggregability.”19 Activated WBCs are also thought to cause structural changes in the RBC glycocalyx and increase RBC aggregability.32 Under normal conditions, RBC adherence to endothelial cells (ECs) is insignificant, and RBC deformability permits efficient passage through the microcirculation Again, under normal conditions, enhanced EC adherence plays a role in removal of senescent ... FeNO Low pO2 Free plasma thiol O2 gradient in tissue Hb B Deoxy Hb Oxy Hb C SNOHb A NO S-nitrosothiols (SNOs) O2 NO O2 O2 NO groups without specific designation S-nitrosothiol SNO (SNO) export... hemoglobin (Hb) O2 saturation (A) In this fashion, circulating NO groups are processed by Hb into the highly vasoactive (thiolbased) NO congener, S-nitrosothiol (SNO) By exporting SNOs as a function... P50); this P50 shift is termed the Haldane effect.9,10 The majority of CO2 is hydrated by the two intraerythrocytic CA isoforms, CA I (low-turnover rate) and CA II (high-turnover rate) This conversion