In vitro fibrinolysis after adding low doses of plasminogen activators and plasmin generation with and without oxidative inactivation of plasmin inhibitors in newborns and adults

Reference ranges of thromboelastometry in healthy full-term and pre-term neonates

Background:

Rotational thromboelastometry (ROTEM) is an attractive method for rapid evaluation of hemostasis in neonates. Currently, no reference values exist for ROTEM assays in full-term and pre-term neonates. Our aim was to establish reference ranges for standard extrinsically activated ROTEM assay (EXTEM) in arterial blood samples of healthy full-term and pre-term neonates.

Methods:

In the present study, EXTEM assay was performed in 198 full-term (≥37 weeks’ gestation) and 84 pre-term infants (<37 weeks’ gestation) using peripheral arterial whole blood samples.

Results:

Median values and reference ranges (2.5th and 97.5th percentiles) for the following main parameters of EXTEM assay were determined in full-term infants: clotting time (seconds), 41 (range, 25.9–78); clot formation time (seconds), 70 (range, 40–165.2); maximum clot firmness (mm), 66 (range, 41–84.1); lysis index at 60 min (LI60, %), 97 (range, 85–100). The only parameter with a statistically significant difference between full-term and pre-term neonates was LI60 (p=0.006). Furthermore, it was inversely correlated with gestational age (p=0.002) and birth weight (p=0.016) in pre-term neonates.

Conclusions:

In conclusion, an enhanced fibrinolytic activity in pre-term neonates was noted. For most EXTEM assay parameters, reference ranges obtained from arterial newborn blood samples were comparable with the respective values from studies using cord blood. Modified reagents, small size samples, timing of sampling, and different kind of samples might account for any discrepancies among similar studies. Reference values hereby provided can be used in future studies.

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.

Neonatal thromboembolic emergencies

Thrombosis risk is multifactorial, with interaction of hereditary risk factors and acquired environmental and clinical conditions. Newborns are at particular risk for thrombotic emergencies secondary to the unique properties of their hemostatic system, influences of the maternal-fetal environment, and perinatal complications and interventions. Thrombotic complications range from arterial and venous catheter thrombosis to purpura fulminans. Prompt identification and appropriate management of thrombotic emergencies is critical in avoiding limb-, organ-, and life-threatening complications. Treatment strategies have been extrapolated from adult literature but clinical experience from small-scale neonatal studies has resulted in therapeutic guidelines, which should be individualized for each neonate, taking into consideration age and clinical status.

Thrombolytic therapy in children

Thrombolysis is increasingly considered a treatment option in newborns and children with arterial and venous thromboembolic events, or occluded central venous lines. However, no uniform recommendations are available with regard to indications, drug of choice, route of administration, and dosing regimen. Thus, several protocols are used for the different thrombolytic agents, leading to differing outcome with respect to the effectiveness of therapy and bleeding complications. This article will summarize the available information on the use of thrombolytic agents in newborns and children, focussing on the potential indications, efficacy and safety profiles, and evidence supporting dosing schedules.

Arterial thromboembolic complications in critically ill children

PURPOSE

To evaluate incidence and characteristics of arterial thromboembolic complications in critically ill children.

MATERIALS AND METHODS

Hospital records of all consecutive patients with arterial thromboembolic events (ATEs) occurring in the pediatric intensive care unit (PICU) from January 1997 to August 2001 were reviewed. Data collected included demographics and location, treatment modalities and outcome of ATEs.

RESULTS

Fifty-four ATEs in 51 children (median age, 14 days) were identified, reflecting an incidence of 1.2% of all PICU patients. Arterial thromboembolic events were located in peripheral arteries in 52 (96%) cases and were associated with indwelling arterial catheters (n=26) or cardiac catheterization (n=26). The remaining 2 ATEs were located in the left ventricle and cerebral arteries, respectively. Therapy consisted of heparin (n=51), thrombolysis (n=22), oral anticoagulation (n=12), and aspirin (n=34). Complete resolution was noted in 33 (70%), partial resolution in 10 (21%), and no resolution in 4 (8.5%) cases. Bleeding complications occurred in 1 patient treated with heparin and in 12 (54%) of the 22 patients receiving thrombolytic therapy.

CONCLUSIONS

Arterial thromboembolic events are frequent complications of PICU, particularly affecting neonates, and mostly associated with catheters. Studies to determine safe and effective prophylactic and treatment modalities of ATEs in children are required.

Maternal thrombophilia and neonatal thrombosis

In neonates and infants, numerous clinical and environmental conditions lead to elevated thrombin generation and subsequent thrombus formation. Genetic prothrombotic defects (protein C, protein S and antithrombin deficiency, mutations of coagulation factor V and factor II, elevated lipoprotein (a)) have been established as risk factors of thromboembolic events in neonates and infants. The interpretation of the laboratory evaluation relies on age-dependent normal reference values. Because appropriate clinical trials are missing in these age groups, treatment recommendations are adapted from small-scale studies in neonates and infants and from guidelines relating to adult patient protocols. Secondary long-term anticoagulation should be administered on an individual basis.

Management of preterm infants with intracardiac thrombi: use of thrombolytic agents

Improvement in neonatal care has led to improvements in survival and patient outcome in preterm infants; however, this improved survival has been associated with the development of secondary complications, such as catheter-associated intravascular and intracardiac thrombus formation with a non-negligible morbidity and mortality. The sick preterm infant is at high risk of catheter-related thrombus formation because of the combination of a high prothrombotic activity, low levels of natural anticoagulants, and various imbalances in the fibrinolytic systems. Based on clinical experience in adults and children, and several neonatal case reports demonstrating the efficacy and tolerability of specific thrombolytic treatment, this approach should be recommended as a first choice treatment in the premature infant with intracardiac or intravascular thrombosis. The thrombolytic agents of choice are urokinase or tissue plasminogen activator (tPA); however, none of them have proven to be superior to the other in terms of efficacy or tolerability, either in adult patients or premature infants. In the past, it has been suggested that newborn infants may require higher doses of thrombolytic agents than adults for effective systemic thrombolysis; however, based on more recent in vitro studies, it seems unlikely that this is true. Nevertheless, systemic (high dose) fibrinolysis is of concern as premature neonates present an increased risk of cerebral haemorrhage during the first weeks of life; therefore, low dose treatment has been proposed with, if possible, direct infusion of the fibrinolytic agent into, or close to, the thrombus. This approach has proven to be efficient and well tolerated in several small case series of newborn and preterm infants. Recommended doses are 1000 to 3000 U/kg/h for urokinase or 0.01 to 0.05 mg/kg/h for tPA. A systemic proteolytic state will not be induced by this low dose; however, specific monitoring of fibrinogen plasma levels has to be recommended. Fibrinogen levels should remain above 100 mg/dL during low dose treatment. Lower levels of fibrinogen will indicate the presence of an unwanted systemic fibrinolytic state. After successful thrombolysis, a follow-up treatment, preferentially with low-molecular-weight heparin for neonates at adjusted doses, should be instituted for at least 6 weeks in the absence of any persisting thrombophilic factor. A longer course (3 to 6 months) of anticoagulation therapy is recommended when thrombophilic factors (i.e. hereditary thrombophilia or central venous catheter still in place) are present. Furthermore, it is recommended that any neonate with thrombosis should be evaluated for hereditary thrombophilia later in life.

Differences between neonates and adults in carbohydrate sequences and reaction kinetics of plasmin and alpha(2)-antiplasmin

This study investigates reaction kinetics by slow-binding kinetics methods of both adult and fetal plasmin (Types 1 and 2) with adult and fetal alpha(2)-antiplasmin. In addition, carbohydrate sequences of Fetal and Adult Plasminogen Types 1 and 2, as well as fetal and adult alpha(2)-antiplasmin, were determined by mass spectrometric analysis. All curves of plasmin-alpha(2)-antiplasmin interaction followed the same pattern, indicating reversible slow-binding inhibition with an initial loose complex and a following tight complex. Differences between fetal and adult plasmin reactions with alpha(2)-antiplasmin were predominantly due to the initial loose complex. Values for K(i initial) in the reaction with adult alpha(2)-antiplasmin were 1.5 and 1.6 nM for Fetal Plasmin Types 1 and 2, respectively; compared to 0.3 and 0.7 nM for the corresponding adult types. Increasing concentrations of tranexamic acid resulted in a continuous increase of K(i initial) until a plateau was reached which was similar for all plasmin types. Almost identical values could be obtained when fetal alpha(2)-antiplasmin was used instead of adult alpha(2)-antiplasmin. Mass spectrometric analyses of the glycans present on plasminogen revealed a higher level of truncated N-glycans on the fetal material compared to the adult. The O-glycans of fetal and adult plasminogen were closely similar and only minor differences were observed between N-glycans of fetal and adult alpha(2)-antiplasmin. In conclusion, both fetal plasmin isoforms are less inhibited by alpha(2)-antiplasmin compared to the adult plasmin variants. These findings are important for the understanding of the physiology of the fibrinolytic system in neonates and provide further evidence that differences in glycosylation could be associated with marked effects on protein function.

Differences between neonates and adults in the urokinase-plasminogen activator (u-PA) pathway of the fibrinolytic system

This study deals with plasminogen activation kinetics of fetal and adult Glu-plasminogen types 1 and 2 as well as fetal and adult Lys-plasminogen by urokinase in the presence and absence of the lysine analogues epsilon-amino-n-caproic acid (EACA) and tranexamic acid. In addition, activation kinetics of single-chain urokinase-plasminogen activator (scu-PA) by adult and fetal plasmin types were investigated in the absence and presence of soluble fibrin. All Lys-plasminogen isoforms were more readily activated by urokinase than their corresponding Glu-plasminogen types. No substantial differences of the catalytic constants of urokinase-catalyzed plasminogen activation could be obtained when all fetal plasminogen types were compared to the respective adult types. In the case of all Glu-plasminogen isoforms, EACA as well as tranexamic acid first stimulated the activation process and, at higher concentrations, showed inhibitory properties. Again, the relative ability of all fetal plasminogen types to interact with lysine analogues revealed no differences compared to the respective adult glycoforms. In the absence of soluble fibrin, the catalytic efficiency of scu-PA activation by plasmin was significantly lower for both fetal plasmin isoforms. However, there were no differences in catalytic efficiency between fetal and adult plasmin types in the presence of 4 microM soluble fibrin. In conclusion, no substantial differences exist in urokinase-catalyzed plasminogen activation between neonates and adults, which is in contrast to reported data on plasminogen activation by tissue-type plasminogen activator. In the absence of soluble fibrin, scu-PA activation by fetal plasmin is markedly slower than by adult plasmin. However, this is compensated when fibrin is added at a concentration that is close to the physiological fibrinogen concentration in plasma. It can be summarized that the differences in carbohydrate structures of fetal and adult plasminogen are not associated with major differences in the global function of this part of fibrinolysis, despite functional alterations of scu-PA activation.

Successful systemic low-dose lysis of a caval thrombus by rt-PA in a neonate with congenital nephrotic syndrome

PURPOSE

Thrombotic complications in nephrotic syndrome due to renal loss of antithrombin III (AT III) are well known. With this case report, we want to demonstrate the possibility of achieving the lysis of such a thrombosis in the neonatal period with low-dose rt-PA.

PATIENTS AND METHODS

We treated a 10-day-old newborn who had congenital nephrotic syndrome, who developed a caval thrombosis during the first days of his life. After a trial of heparin (up to 20 IU/kg/hour) over a period of 24 hours and treatment with AT III (2 x 250 IU/day) proved to be ineffective, we started systemic thrombolytic therapy with rt-PA. An initial bolus of 0.4 mg/kg during 1 hour was followed by an infusion of 0.5 mg/kg/d rt-PA over a period of 36 hours. Low-dose heparin (5 IU/kg/hour) was given simultaneously. Complete clot dissolution could be achieved this way. No adverse effects were observed, including no clinical signs of bleeding.

CONCLUSION

It seems that low-dose rt-PA treatment is safe and effective in dissoluting large caval thromboses in neonates.