Background: Bleeding in hemophilic neonates has a low incidence. A possible explanation for this could be the peculiarities of the neonatal hemostatic system, especially low levels of the inhibitors tissue factor pathway inhibitor (TFPI) and antithrombin (AT). Objective: We investigated the influence of an elevation of these inhibitors to adult levels oil the thrombin generation (TG) in normal neonatal plasma and factor (F) VIII-depleted neonatal plasma by means of incubation with anti-FVIII-antibodies. Patients/methods: TG was measured after activation with low amounts of tissue factor (TF) by using Calibrated Automated Thrombography. Results: TG in FVIII-depleted neonatal plasma was nearly as high as in normal neonatal plasma. TG decreased after elevation of AT in both neonatal plasmas. After elevation of TFPI TG decreased much more in FVIII-depleted neonatal plasma than in normal neonatal plasma. After elevation of both inhibitors their synergistic effect led to a stronger decrease of TG in FVIII-depleted neonatal plasma. TG measured in plasma of one hemophilic newborn showed the same pattern as in FVIII-depleted neonatal plasma. Conclusion: Our observation provides a biochemical basis for the rare bleeding in hemophilic neonates and shows the important role of the natural inhibitors in the hemostatic system of hemophilic patients.

The aim of our study was to investigate the combined in vitro effects of melagatran and eptifibatide on platelet aggregation and thrombin generation under low and high coagulant challenge in tissue-factor-activated, platelet-rich plasma. Increasing amounts of melagatran dose-dependently decreased prothrombin fragment 1.2 and activated factor X values, and dose-dependently prolonged the lag phase until the onset of platelet aggregation. Eptifibatide exerted a dose-dependent anti-aggregating effect under both high and low coagulant challenge. The combination of melagatran and eptifibatide resulted in significant additive prolongation of the lag phase until the onset of platelet aggregation, which was more pronounced under low coagulant challenge. Under low, but not under high, coagulant challenge, the combination of melagatran and eptifibatide had a significant additive inhibitory effect on platelet aggregation. No additive effects on decreasing prothrombin fragment 1.2 and activated factor X values were observed with combined administration of the drugs. The present study demonstrates the additive effect of melagatran and eptifibatide on platelet aggregation inhibition and on prolongation of the lag phase until the onset of platelet aggregation.

Although relatively rare,thromboembolic complications do occur in pediatric patients [1] and cause significant morbidity with occasional mortality.Venous thrombosis is a multicausal disease involving acquired and genetic factors. The pediatric patient with thrombosis has an average of two and some of four or more predisposing and triggering prothrombotic factors [2]. Genetic abnormalities of antithrombin, protein C, protein S, factor V G1691A, prothrombin G20210A and elevated lipoprotein(a) concentration have been reported in children with venous thrombosis [3, 4, 5].

Platelets of newborns aggregate poorly in-vitro [1, 2]. However, newborns have efficient hemostasis, as illustrated by their short skin bleeding time. It has been demonstrated that elevated von-Willebrand-factor (vWF) concentrations and unusually large vWF multimers, not present in normal adult plasma [3], allow sufficient vWF-collagen binding, probably contributing to the clinically observed effective primary hemostasis of neonates.

INTRODUCTION:Thrombosis is one of the most frequent adverse events after cardiac catheterization, which can be reduced by anticoagulation with unfractionated heparin (UFH) in both children and adults. Low molecular weight heparin (LMWH) might possibly offer advantages. Laboratory signs of thrombin generation during pediatric cardiac catheterization, with unfractionated heparin (UFH) bolus or subcutaneous LMWH for thrombosis prophylaxis, were determined in a first step to investigate the potential of LMWH for antithrombotic cover.MATERIALS AND METHODS:Signs of thrombin generation (D-dimer and F1+2), anti-Xa activity and activated clotting time (ACT) were measured in 65 patients with congenital heart disease. A total of 40 patients were treated with a UFH bolus of 100 IU/kg bodyweight and, in 25 children, enoxaparin was subcutaneously administered at a dosage of 1/1.6 mg/kg bodyweight.RESULTS:The dose to plasma activity of enoxaparin was more consistent than in the UFH group. Only a slight elevation of F1+2 was found in some patients, which was a little higher in the enoxaparin group, but no difference of incidence of increased F1+2 generation was detected between the two groups. D-dimer was elevated in three children after UFH bolus application, but no such effect was observed in any child after LMWH administration.CONCLUSIONS:Application of LMWH was equally efficacious during pediatric cardiac catheterization than UFH bolus administration, as determined by plasma levels and markers of clotting activation. In contrast to UFH bolus, no further monitoring was necessary after the application of LMWH during cardiac catheterization due to a consistent dose to plasma activity.

Venous or arterial thrombosis after cardiac catheterization is one of the Most frequent adverse events. Anticoagulation with heparin reduces this risk of thromboembolic complications during cardiac catheterization in both children and adults. Different dosages and Modalities of heparin administration, from heparin bolus therapy to low-dose flush heparin, have been recommended for the pediatric cardiac catheterization laboratories.

In neonates, despite poor platelet function in various in vitro tests, closure times (CTs) in PFA‐100 measurements are shorter than in adults. Neonates have a higher polymeric von Willebrand factor (vWF). They also have a higher haematocrit and higher white blood cell count than adults, which may interfere with the evaluation of platelet and vWF function by means of the PFA‐100 in neonates. To assess the role of different blood constituents on neonatal CTs, red blood cell, platelet and white blood cell counts in cord blood were modified. These modifications did not provide any evidence that the difference in number between adult and neonatal blood cells was responsible for shorter neonatal CTs. In further experiments, platelets and/or vWF were inhibited by means of abciximab and anti‐vWF antibody, and mixing experiments with neonatal platelet‐rich and platelet‐poor plasma were performed. The results showed that short cord blood PFA‐100 CTs were caused by a constituent of neonatal platelet‐poor plasma, probably the neonatal high multimeric vWF. Conclusion: This study demonstrates that CTs in neonates are dependent on the same components, platelets and vWF, as in adults, making it likely that the PFA‐100 can be used in neonates in the same way as in adults to investigate platelet and vWF function.

Arterial thrombosis is the most frequent complication of percutaneous catheterization in children [1]. Several mechanisms of arterial thrombosis following cardiac catheterization are described. These include formation and propagation of a thrombus initiated by intimai injury at the site of introduction of the arterial needle, guide wire and catheter, sometimes in association with subintimal dissection and intimai flap formation [2]. Platelets may become deposited on the catheter and may result in partial or total occlusion of an artery when the catheter is withdrawn [3]. Also, artery spasm may occur when the catheter is introduced into the artery and contributes to thrombus formation [2, 4]. However, few data, especially from children, address appropriate dosages of heparin during cardiac catheterization.

In vitro experiments have shown that antibodies to the GP IIb/IIIa receptor have a diminishing effect on free thrombin generation, and these agents have been suggested as anticoagulant drugs [1, 2]. These experiments were performed after activation of platelet-rich plasma (PRP) with low concentrations of tissue thromboplastin, which did not result in fibrinogen polymerization in the absence of platelets. It has been shown that platelet activation plays a major role in the development of thrombosis when the thrombogenic stimulus is mild [3]. On the other hand, the thrombogenic challenge in vivo is difficult to quantify and atherosclerotic plaques might express tissue thromboplastin concentrations that may overcome the anticoagulant effect of direct platelet inhibition. Thus, under high thrombogenic challenge, GP IIb/IIIa inhibition alone might be ineffective in preventing thrombin formation.

The present study was performed to investigate the combined effects of the platelet glycoprotein IIb/IIIa receptor antagonist c7E3 Fab (abciximab) and the anticoagulants unfractionated heparin (UH), low molecular weight heparin (LMWH), and recombinant hirudin (rH) on platelet aggregation and thrombin generation under high coagulant challenge by extrinsic activation of platelet-rich plasma. Platelet aggregation and thrombin generation were assessed simultaneously in the presence of different concentrations of abciximab and anticoagulants. Increasing concentrations of abciximab resulted in a dose-dependent anti-aggregating effect with a maximum at 20 microg/ml. Doses of 5, 10, and 20 microg/ml abciximab prolonged the lag phase until the onset of platelet aggregation, but this effect was independent of the dosage used. Abciximab had no influence on the thrombin potential under our high coagulant challenge. UH, LMWH, and rH showed a dose-dependent prolongation of the lag phase until the onset of platelet aggregation and decreased the thrombin potential. Addition of anticoagulants did not contribute to further inhibition of platelet aggregation in the presence of abciximab, but the combination of abciximab and anticoagulants exhibited an additive effect on prolongation of the lag phase until the onset of platelet aggregation. Addition of abciximab to anticoagulants did not result in further decrease of the thrombin potential. Our study demonstrates the respective specific effects of abciximab and anticoagulants on platelet aggregation and thrombin potential under high coagulant challenge, and also an additive effect of abciximab and the anticoagulants UH, LMWH, and rH on the lag phase until the onset of platelet aggregation.