Platelet function in term and preterm neonates

Thrombotic Alterations under Perinatal Hypoxic Conditions: HIF and Other Hypoxic Markers

Hypoxia is considered to be a stressful physiological condition, which may occur during labor and the later stages of pregnancy as a result of, among other reasons, an aged placenta. Therefore, when gestation or labor is prolonged, low oxygen supply to the tissues may last for minutes, and newborns may present breathing problems and may require resuscitation maneuvers. As a result, poor oxygen supply to tissues and to circulating cells may last for longer periods of time, leading to life-threatening conditions. In contrast to the well-known platelet activation that occurs after reperfusion of the tissues due to an ischemia/reperfusion episode, platelet alterations in response to reduced oxygen exposition following labor have been less frequently investigated. Newborns overcome temporal hypoxic conditions by changing their organ functions or by adaptation of the intracellular molecular pathways. In the present review, we aim to analyze the main platelet modifications that appear at the protein level during hypoxia in order to highlight new platelet markers linked to complications arising from temporal hypoxic conditions during labor. Thus, we demonstrate that hypoxia modifies the expression and activity of hypoxic-response proteins (HRPs), including hypoxia-induced factor (HIF-1), endoplasmic reticulum oxidase 1 (Ero1), and carbonic anhydrase (CIX). Finally, we provide updates on research related to the regulation of platelet function due to HRP activation, as well as the role of HRPs in intracellular Ca2+ homeostasis.

Gestational Diabetes Melitus and Cord Blood Platelet Function Studied via the PFA-100 System

Neonatal platelet hemostasis, although it has been well described over the recent years, remains elusive in specific patient populations, including neonates from high-risk pregnancies, such as those complicated with gestational diabetes mellitus (GDM). We aimed at evaluating the platelet function of neonates born to mothers with GDM using the platelet function analyzer (PFA-100). Cord blood samples were drawn from each subject and tested with two different agonists to provide two closure time (CT) values (collagen with epinephrine (COL/EPI) and collagen with adenosine diphosphate (COL/ADP)). A total of 84 and 118 neonates formed the GDM and the control group (neonates from uncomplicated pregnancies), respectively. COL/EPI CTs were prolonged in neonates from the GDM group compared to neonates from the control group, while no statistically significant difference of COL/ADP CTs was noted between the two groups, GDM and the control. Higher COL/ADP CTs were demonstrated in neonates born via cesarean section and in neonates with blood group O. A negative correlation between COL/ADP CT and gestational age, white blood cells (WBCs) and von Willebrand factor (VWF) activity was noted in neonates from the GDM group. In conclusion, neonates from the GDM group demonstrate a more hyporesponsive phenotype of their platelets, in comparison to the control neonates.

Platelets in the neonate: Not just a small adult

Neonates, particularly those born preterm, have a high incidence of thrombocytopenia and bleeding, most commonly in the brain. Because of this, it has historically been accepted that neonates should be transfused at higher platelet counts than older children or adults, to decrease their bleeding risk. However, a number of observational studies and a recent large, randomized trial found a higher incidence of bleeding and mortality in neonates who received more platelet transfusions. The mechanisms underlying the deleterious effects of platelet transfusions in neonates are unknown, but it has been hypothesized that transfusing adult platelets into the very different physiological environment of a neonate may result in a “developmental mismatch” with potential negative consequences. Specifically, neonatal platelets are hyporeactive in response to multiple agonists and upon activation express less surface P‐selectin than adult platelets. However, this hyporeactivity is well balanced by factors in neonatal blood that promote clotting, such as the elevated hematocrit, elevated von Willebrand factor (VWF) levels, and a predominance of ultra‐long VWF polymers, with the net result of normal neonatal primary hemostasis. So far, most studies on the developmental differences between neonatal and adult platelets have focused on their hemostatic functions. However, it is now clear that platelets have important nonhemostatic functions, particularly in angiogenesis, immune responses, and inflammation. Whether equally important developmental differences exist with regard to those nonhemostatic platelet functions and how platelet transfusions perturb those processes in neonates remain unanswered questions.

Platelets in pediatric and neonatal sepsis: novel mediators of the inflammatory cascade

Sepsis, a dysregulated host response to infection, has been difficult to accurately define in children. Despite a higher incidence, especially in neonates, a non-specific clinical presentation alongside a lack of verified biomarkers has prevented a common understanding of this condition. Platelets, traditionally regarded as mediators of haemostasis and thrombosis, are increasingly associated with functions in the immune system with involvement across the spectrum of innate and adaptive immunity. The large number of circulating platelets (approx. 150,000 cells per microlitre) mean they outnumber traditional immune cells and are often the first to encounter a pathogen at a site of injury. There are also well-described physiological differences between platelets in children and adults. The purpose of this review is to place into context the platelet and its role in immunology and examine the evidence where available for its role as an immune cell in childhood sepsis. It will examine how the platelet interacts with both humoral and cellular components of the immune system and finally discuss the role the platelet proteome, releasate and extracellular vesicles may play in childhood sepsis. This review also examines how platelet transfusions may interfere with the complex relationships between immune cells in infection. IMPACT: Platelets are increasingly being recognised as important "first responders" to immune threats. Differences in adult and paediatric platelets may contribute to differing immune response to infections. Adult platelet transfusions may affect infant immune responses to inflammatory/infectious stimuli.

TEG® and ROTEM® Traces: Clinical Applications of Viscoelastic Coagulation Monitoring in Neonatal Intensive Care Unit

The concentration of the majority of hemostatic proteins differs considerably in early life, especially in neonates compared to adulthood. Knowledge of the concept of developmental hemostasis is an essential prerequisite for the proper interpretation of conventional coagulation tests (CCT) and is critical to ensure the optimal diagnosis and treatment of hemorrhagic and thrombotic diseases in neonatal age. Viscoelastic tests (VETs) provide a point-of-care, real-time, global, and dynamic assessment of the mechanical properties of the coagulation system with the examination of both cellular and plasma protein contributions to the initiation, formation, and lysis of clots. In this work, we provide a narrative review of the basic principles of VETs and summarize current evidence regarding the two most studied point-of-care VETs, thromboelastography (TEG^®) and rotational thromboelastometry (ROTEM^®), in the field of neonatal care. A literature analysis shows that viscoelastic hemostatic monitoring appears to be a useful additive technique to CCT, allowing targeted therapy to be delivered quickly. These tools may allow researchers to determine the neonatal coagulation profile and detect neonatal patients at risk for postoperative bleeding, coagulation abnormalities in neonatal sepsis, and other bleeding events in a timely manner, guiding transfusion therapies using the goal-oriented transfusion algorithm. However, diagnosis and treatment algorithms incorporating VETs for neonatal patients in a variety of clinical situations should be developed and applied to improve clinical outcomes. Further studies should be performed to make routinary diagnostic and therapeutic application possible for the neonatal population.

A Proposed Role of Surfactant in Platelet Function and Treatment of Pulmonary Hemorrhage in Preterm and Term Infants

BACKGROUND

We evaluated the effect of surfactant on platelet function as a potential contributing mechanism to the pathogenesis of pulmonary hemorrhage (PHEM) in term and preterm infants.

METHODS

Cord blood samples were collected from neonates following delivery. Complete blood count and platelet function were measured using a cone and platelet analyzer (CPA). Increasing surfactant concentrations were added to platelets in vitro, and the adhesion molecule P-selectin and the monoclonal antibody PAC-1 were evaluated following platelet activation by flow cytometry.

RESULTS

Forty-one infants (11 preterm and 30 term) were studied. CPA revealed a significant decrease in the average size of the aggregates and in platelet adhesion when surfactant was added. In term infants, the addition of surfactant to native platelets yielded an increased binding capacity of PAC-1 but did not affect P-selectin expression. In preterm infants, platelet activation with adenosine diphosphate in the presence of a high surfactant concentration (0.5 mg/mL) resulted in increased PAC-1 binding and no change in P-selectin expression.

CONCLUSIONS

The platelets of preterm infants are less active (hyporesponsive) than those of term infants, both in their native state as well as after stimulation with various agonists. Surfactant may play an important role in treating PHEM in preterm infants.

Significant Hypo-Responsiveness to GPVI and CLEC-2 Agonists in Pre-Term and Full-Term Neonatal Platelets and following Immune Thrombocytopenia

Neonatal platelets are hypo-reactive to the tyrosine kinase-linked receptor agonist collagen. Here, we have investigated whether the hypo-responsiveness is related to altered levels of glycoprotein VI (GPVI) and integrin α2β1, or to defects in downstream signalling events by comparison to platelet activation by C-type lectin-like receptor 2 (CLEC-2). GPVI and CLEC-2 activate a Src- and Syk-dependent signalling pathway upstream of phospholipase C (PLC) γ2. Phosphorylation of a conserved YxxL sequence known as a (hemi) immunotyrosine-based-activation-motif (ITAM) in both receptors is critical for Syk activation. Platelets from human pre-term and full-term neonates display mildly reduced expression of GPVI and CLEC-2, as well as integrin αIIbβ3, accounted for at the transcriptional level. They are also hypo-responsive to the two ITAM receptors, as shown by measurement of integrin αIIbβ3 activation, P-selectin expression and Syk and PLCγ2 phosphorylation. Mouse platelets are also hypo-responsive to GPVI and CLEC-2 from late gestation to 2 weeks of age, as determined by measurement of integrin αIIbβ3 activation. In contrast, the response to G protein-coupled receptor agonists was only mildly reduced and in some cases not altered in neonatal platelets of both species. A reduction in response to GPVI and CLEC-2, but not protease-activated receptor 4 (PAR-4) peptide, was also observed in adult mouse platelets following immune thrombocytopenia, whereas receptor expression was not impaired. Our results demonstrate developmental differences in platelet responsiveness to GPVI and CLEC-2, and also following immune platelet depletion leading to reduced Syk activation. The rapid generation of platelets during development or following platelet depletion is achieved at the expense of signalling by ITAM-coupled receptors.

Influence of platelet count, platelet mass index, and platelet function on the spontaneous closure of ductus arteriosus in the prematurity

BACKGROUND

This study aims at evaluating the influence of platelet count, platelet mass index, and platelet function on the spontaneous closure of ductus arteriosus in prematurity.

METHODS

All preterm babies were divided into two groups, including Group 1 with "open PDA" and Group 2 with "closed PDA". The variables of platelet count, mean platelet volume, platelet mass index, and platelet function were analyzed and compared between two groups of patients to identify the factors that significantly influenced spontaneous closure of ductus arteriosus.

RESULTS

Twenty-four patients were in the "open PDA" group, whereas 36 patients were in the "closed PDA" group. Mean GA and BW were 27.6 ± 1.8 (23.1-30.4) and 28 ± 1.6 (23.4-30.6) weeks and 1009 ± 270 (585-1480) g and 1035 ± 298 (505-1500) g in "open PDA" and "closed PDA" groups, respectively (p > 0.05). The incidence of "Collagen-ADP > 130 s" was significantly higher in the "open PDA" group, and the levels of hemoglobin and hematocrit were significantly lower in the "open PDA" group (p < 0.05). Multivariate logistic regression analysis showed that respiratory distress syndrome (OR: 9, CI: 1.5-51.8) and collagen-ADP > 130 s (OR: 5.7 CI: 1.55-21.3) are two independent factors associated with ductal patency.

CONCLUSION

This is the first study in the English literature providing evidence of the influence of platelet dysfunction on the spontaneous closure of ductus arteriosus in prematurity. Longer collagen-ADP duration is identified as a risk factor of ductal closure.

Developmental hemostasis: A lifespan from neonates and pregnancy to the young and elderly adult in a European white population

Absolute values of reference ranges for coagulation assays in humans vary within the entire lifespan and confirm the concept of developmental hemostasis. It is known that physiologic concentrations of coagulation factors (F) gradually increase over age: they are lower in premature infants as compared to full-term babies, healthy children or adults. Here we demonstrate in a cohort of 1011 blood donors and in a group of 193 healthy pregnant women, that the process of developmental hemostasis proceeds in adults. During the course of pregnancy F and activation markers steadily increase until delivery with a parallel decrease noticed for protein S. From adolescents, young adults to the elderly there is a further increase of F, reaching significance starting between 35 and 50years of age compared to younger subjects. Covering the entire lifespan FVIII and von-Willebrand-factor showed the lowest values in carriers of blood group "O". Apart from pregnancy differences related to gender, pill users, smoking habits or the presence of thrombophilic variants were reported. Laboratory test results should be compared to age-related reference intervals when hemostatic defects are suspected to avoid misclassifications as being "healthy", prone to "bleeding" or vice versa to "thrombosis".

Assessment of Functional Fibrinolysis in Cord Blood Using Modified Thromboelastography

BACKGROUND

The fibrinolytic system in newborns is immature and probably impaired. The aim of this study was to prospectively evaluate functional fibrinolytic capacity of newborn's cord blood using a new thromboelastometry (rotational thromboelastogram, ROTEM®) test.

METHODS

Infants born at Sheba Medical Center were studied prospectively. Cord blood was obtained immediately after clumping, and ROTEM parameters were assessed applying non-activated TEM (NATEM) assay with increasing concentration of tissue plasminogen activator (tPA, 0-200 U/ml). Baseline clotting time (CT), clot formation time (CFT), alpha angle, and maximum clot firmness (MCF) were compared among infants versus adults. Each infant's demographic information was prospectively followed up until discharge.

RESULTS

One hundred one newborns were tested. CT and CFT values were lower and alpha angles were higher among neonate's cord blood compared to adults (n = 23; P = 0.001, 0.03, and 0.02, respectively). The addition of tPA significantly shortened CT and CFT, and reduced alpha angles and MCF in both groups. The lysis index at 30 min (LI30) and lysis onset time (LOT) decreased significantly, and fibrinolysis was more rapid in the newborns. Hematocrit and platelet counts in neonates correlated with LI30 (P = 0.035 and 0.037, respectively) and LOT (P = 0.02) when higher tPA concentrations were used. ROTEM values were unrelated to the occurrence of postnatal complications.

CONCLUSIONS

This first report of functional fibrinolysis in cord blood demonstrated that neonatal fibrinolysis may be augmented as compared to adult values. Further studies are required to validate this test and assess its predictive value and clinical relevance.

Understanding platelet generation from megakaryocytes: implications for in vitro-derived platelets

Platelets are anucleate cytoplasmic discs derived from megakaryocytes that circulate in the blood and have major roles in hemostasis, thrombosis, inflammation, and vascular biology. Platelet transfusions are required to prevent the potentially life-threatening complications of severe thrombocytopenia seen in a variety of medical settings including cancer therapy, trauma, and sepsis. Platelets used in the clinic are currently donor-derived which is associated with concerns over sufficient availability, quality, and complications due to immunologic and/or infectious issues. To overcome our dependence on donor-derived platelets for transfusion, efforts have been made to generate in vitro-based platelets. Work in this area has advanced our understanding of the complex processes that megakaryocytes must undergo to generate platelets both in vivo and in vitro. This knowledge has also defined the challenges that must be overcome to bring in vitro-based platelet manufacturing to a clinical reality. This review will focus on our understanding of committed megakaryocytes and platelet release in vivo and in vitro, and how this knowledge can guide the development of in vitro-derived platelets for clinical application.

Neonatal Platelet Function

Similarly to the development of the plasma coagulation system, which matures during the early weeks and months of life, age-dependent mechanisms and developmental changes influence platelet production and function in neonates. Platelet function testing on cord blood and peripheral blood demonstrates a generalized platelet hyporeactivity, during the first days of life. This reactivity reaches normal adult levels between the fifth and ninth day of life. The persistence of hyporeactivity after the tenth day of life might indeed suggest a platelet disorder.

Developmental hemostasis: primary hemostasis and evaluation of platelet function in neonates

Hemostasis is a dynamic process and physiologic concentrations of coagulation proteins gradually increase with gestational age. Nevertheless, the risk for bleeding in term neonates is counterbalanced by the protective effects of physiologic deficiencies of the inhibitors of coagulation. Although laboratory diagnosis of coagulation disorders in infants may be difficult to establish, due to the need to adapt all assays for small amounts of blood and the age-related interpretation required for test results - evaluation of infants with secondary hemostatic defects is quite feasible, whereas laboratory assessment of primary hemostasis in neonates remains a challenge. While platelet number and volume are similar in neonates as compared to adult values, neonatal platelets certainly exhibit hyporesponsiveness. Analysis of platelet function may include aggregation studies or flow cytometry assays, using fluorescence-stained monoclonal antibodies against platelet membranes and cellular antigens. Data on platelet function in correlation with gestational age are scarce and the duration of platelet hyporeactivity and its clinical significance have not yet been completely elucidated. Whole-blood-based platelet function assays have shown in neonates as well as in premature infants progressive improvement of clot formation with gestational age. This article reviews platelet function, assessed by various techniques, and its development in the premature as well as healthy term neonate.

Platelet indices in SGA newborns

PURPOSE

The current study objective was to compare blood platelet indices in full-term small-for-gestational-age newborns (SGA) and full-term appropriate-for-gestational-age newborns (AGA).

MATERIALS/METHODS

We introduced to our study 61 SGA newborns (31 females and 30 males) and 70 eutrophic infants (32 females and 38 males). The SGA newborns were divided into two groups: those weighing less than the 5th centile: 35 infants (16 females and 19 males) and those between the 5th and 10th centiles: 26 infants (15 females and 11 males). Platelet indices were estimated in blood samples collected from the umbilical artery.

RESULTS

SGA demonstrated a decreased count of blood platelets (238×103/μ) as compared with AGA (286×103/μL), p=0.0001. Platelet hematocrit (PTC) also showed differences in both groups (SGA=0.19% vs. AGA=0.22%; p=0.0005). Mean platelet volume (MPV) was higher in SGA (8.25fl) as compared with AGA (7.84fl); p=0.008. Large platelet count (LPLT) was higher in AGA 6.26% vs. SGA=4.75%; p=0.01. Platelet distribution width (PDW) was found to be nearly the same (SGA=47%, AGA=46%). PDW was higher in SGA newborns < 5th centile (43%) as compared with SGA infants between the 5th and 10th centiles (52%); p=0.008.

CONCLUSIONS

A decreased blood platelet count, platelet hematocrit and large metabolically active platelet count, which in addition to reduced synthesis and excessive consumption of coagulation factors in states of hiperclotting is characteristic of IUGR, enhances the possibility of bleeding complications and increases the risk of infections. From a clinical point of view, it is important to take into consideration the degree of intrauterine hypotrophy during the evaluation of hemostatic disorders.

Effects of in vitro adult platelet transfusions on neonatal hemostasis

BACKGROUND

Thrombocytopenia is frequent among neonates, and 20-25% of affected infants are treated with platelet transfusions. These are frequently given for mild thrombocytopenia (platelets: 50-100 × 10(9) L(-1)), largely because of the known hyporeactivity of neonatal platelets. In tests of primary hemostasis, however, neonates have shorter bleeding and closure times (CTs) than adults. This has been attributed to their higher hematocrits, higher von Willebrand factor (VWF) concentrations, and predominance of longer VWF polymers.

OBJECTIVE

To determine whether the 'transfusion' of adult (relatively hyperreactive) platelets into neonatal blood results in a hypercoagulable profile.

METHODS

Cord blood (CB) and adult peripheral blood (PB) were separated (with a modified buffy coat method) to generate miniaturized platelet concentrates (PCs) and thrombocytopenic blood. PB-derived and CB-derived PCs (n = 7 per group) were then 'transfused'in vitro into thrombocytopenic CB and PB. The effects of autologous vs. allogeneic (developmentally mismatched) 'transfusions' were evaluated with whole blood aggregometry, a platelet function analyzer (PFA-100), and thromboelastography (TEG).

RESULTS

Adult platelets aggregated significantly better than neonatal platelets in response to thrombin receptor-activating peptide, ADP, and collagen, regardless of the blood into which they were transfused. The 'transfusion' of adult platelets into thrombocytopenic CB resulted in shorter CTs-EPI (PFA-100) and higher clot strength and firmness (TEG) than 'transfusion' of neonatal autologous platelets.

CONCLUSIONS

In vitro'transfusion' of adult platelets into neonatal blood results in shorter CTs than 'transfusion' with neonatal platelets. Our findings should raise awareness of the differences between the neonatal and adult hemostatic system and the potential 'developmental mismatch' associated with platelet transfusions for neonatal hemostasis.

Bleeding disorders in neonates

SUMMARY

Bleeding disorders may present during the neonatal period, however, absent patient history along with unique physical signs, physiologically decreased levels of plasma proteins and laboratory variations of platelet function tests may render any diagnosis difficult to establish. Intra cranial haemorrhage (ICH) may be the clinical presenting symptom of a severe coagulation factor deficiency. Haemophilia in the newborn period poses unique challenges in diagnosis and management, Data presented from the UDC and similar surveillance systems world-wide can be used to further clinical research and improve management strategies. Development haemostasis should be considered as well as laboratory variations of coagulation tests while evaluating and diagnosis neonates suspected of bleeding disorders. Therapy of bleeding episodes in the neonate relies upon proper replacement and repeated haemostatic evaluation of patients' status, while dealing with underlying etiological causes. This manuscript discusses the unique aspects of clinical presentation, laboratory assessment, and treatment of various bleeding disorders in neonates.

[Specificities of neonatal hemostasis and implications in pathologic situations]

The haemostasis of healthy newborn differs from those of normal adult but remains well balanced without bleeding or thrombosis. However, this equilibrium is unstable, and the neonate is exposed to acquired or inherited haemostasis disorders that necessitate to be early diagnosed in order to be appropriately treated. Several studies provided reference ranges for haemostatic components in the foetus, the newborn and throughout childhood. The particularities of neonatal haemostasis are therefore better defined and contribute to further understand the pathophysiology and characteristics of hemorrhagic and thrombotic disorders that occur in newborns. Some examples of the impact of age on haemostasis are: the risk of neonatal alloimmune thrombocytopenia is high in the first newborn of a woman at risk since the involved antigens are fully expressed by foetal platelets; the newborn is at risk for vitamin K deficiency with bleeding due to poor transport of vitamin K across the placenta and low levels of coagulation factors II, VII, IX, X; the diagnosis of some inherited coagulation deficiencies can be difficult in the newborn due to physiologically low levels of coagulation factors; thrombotic events are rare in the healthy neonate, despite physiologically very low levels of several coagulation inhibitors; the pharmacokinetic and effects of antithrombotic agents are influenced by the specificities of haemostasis in neonates. This review will discuss about the foetal development of haemostasis until birth, and some implications regarding the pathophysiology, the diagnosis and the treatment of bleeding disorders in the human neonate.

Advances and controversies in neonatal ICU platelet transfusion practice

Some of the platelet transfusions currently given to NICU patients are unnecessary and convey no benefits. Although ordered with good intentions, unnecessary platelet transfusions carry known and unknown risks. Identifying and eliminating any unnecessary platelet transfusions in NICUs would be a step toward better care, lower costs, and more careful preservation of blood component resources. A renewed interest in platelet transfusion studies is needed, if essential data is to be gathered to improve NICU platelet transfusion practice. Retrospective studies can be of value: for instance, seeking associations between bleeding events and platelet counts can suggest the possibility of cause and effect relationships. Such studies might identify approximate platelet count levels that convey high hemorrhagic risk and might help focus future prospective trials. Prospective indirect studies also can be of value, for instance, measuring the template bleeding time and the PFA-100 closure time as a function of platelet count and perhaps as a function of circulating platelet mass, and would provide new information with relevance to platelet transfusion benefits. Such studies might give a better awareness of how low the platelet count can fall before platelet plug formation is impaired. It seems inescapable, however, that new, multicentered, randomized, prospective studies are needed, where NICU patients are assigned different platelet transfusion triggers and then carefully tracked for bleeding events and long-term neurodevelopmental outcomes. Only that type of study is likely to generate the evidence base needed for widespread implementation of improvements in NICU platelet transfusion practice.

Prevention of intraventricular haemorrhage: a role for recombinant activated factor VII?

As the survival from extreme prematurity continues to improve, focus on the quality of this survival becomes increasingly important. Prevention of intraventricular haemorrhage (IVH) and its potential long-term sequelae remains one of the major challenges in the early management of these infants. Recombinant activated factor VII (rVIIa), a novel haemostatic agent with an ever-expanding list of potential applications, warrants consideration for use in this setting. This review examines the pathogenesis and prevention of IVH, current concepts of haemostasis both in adults and neonates, and the postulated mechanism of action and various uses of rVIIa. Published data specifically relating to use of rVIIa in neonates is summarised. The hypothesis that early (prophylactic) administration of rVIIa to extremely preterm infants (<28 weeks) would reduce the incidence of severe IVH is explored.