Antithrombotic therapy in children

Spontaneous neonatal renal vein thrombosis, a known pathology without clear management guidelines: An overview

Renal vein thrombosis (RVT) was first described in 1837 by Rayer. Although tremendous progress has been achieved in the comprehension of its pathophysiology, its management remains controversial over 20 decades later. Therapeutic modalities vary from supportive measures alone to the utilization of thrombolytic agents whose protocols are derived from adult medicine. This review aims to show how difficult the treatment of RVT still is, especially with regard to the prognosis. The majority of affected neonates end with various renal complications (renal atrophy, dysfunction, hypertension, etc.). Like others, we suggest that simple unilateral RVT be managed conservatively, while thrombolytic agents may be attempted in unilateral RVT with extension to VCI and in bilateral RVT. Further studies are needed to reach appropriate consensual guidelines.

Duration of anticoagulant therapy in pediatric venous thromboembolism: Current approaches and updates from randomized controlled trials

INTRODUCTION

Compared with the incidence of venous thromboembolism in the adult population, pediatric VTE is rare. Yet, recent data suggest that the incidence of VTE in children is increasing, and little is known about the optimal duration of anticoagulation in pediatrics. Areas covered: This review summarizes current evidence-based adult recommendations and associated clinical trials from which current guidelines on the duration of anticoagulation in children have been extrapolated. It also discusses pediatric expert consensus-based guidelines and current pediatric clinical trials on duration of therapy in pediatric VTE. Expert commentary: The vast majority of pediatric VTE are provoked, and evidence on duration of anticoagulation for pediatric VTE is highly limited, but suggests that a maximum duration of 3 months is reasonable for most patients with provoked VTE, whereas longer duration is likely appropriate for unprovoked VTE. Whether shorter duration than 3 months is optimal for pediatric provoked VTE is as yet unclear. Results from the multinational randomized controlled trial studying the duration of anticoagulant therapy for provoked VTE in patients <21 years old (Kids-DOTT) will be critical to inform the future standard of care in pediatric VTE treatment.

Development of a population pharmacokinetic-pharmacodynamic model of a single bolus dose of unfractionated heparin in paediatric patients

BACKGROUND

Unfractionated heparin (UFH) is the anticoagulant of choice in paediatric patients undergoing a variety of cardiac procedures. There are currently no population pharmacokinetic-pharmacodynamic (PKPD) models for UFH in paediatrics.

OBJECTIVE

The aim of the present study was to develop and evaluate a PKPD model of UFH in paediatrics.

METHODS

Data from 64 children who received 75-100 IU kg(-1) of UFH during cardiac angiography were analysed. Five blood samples were collected at baseline and at 15, 30, 45 and 120 min postdose. The UFH concentration was quantified using a protamine titration assay. The UFH effect was quantified using activated partial thromboplastin time (aPTT). A PKPD model was fitted using nonlinear mixed-effects modelling. Patient covariates such as gender, weight (WT) and fat-free mass (FFM) were tested. The final model was evaluated using the likelihood ratio test and visual predictive checks (VPCs).

RESULTS

A one-compartment model with linear elimination provided the best fit for the dose-concentration data. FFM was a significant covariate on clearance. A linear model provided the best fit for the concentration-effect data using aPTT as a biomarker for effect. The models performed well using VPCs. However, when used to simulate UFH infusion (at a much lower dose), the model overpredicted target aPTT responses.

CONCLUSIONS

A PKPD model to describe the time course of the UFH effect was developed in a paediatric population. FFM was shown to describe drug disposition well. However, when applied to smaller UFH infusion doses, the model overpredicted target aPTT responses. This unsuccessful extrapolation may be attributed to a possible nonlinear relationship for heparin PKPD.

Tissue Plasminogen Activator Use in Children: Bleeding Complications and Thrombus Resolution

OBJECTIVE

To review our institutional experience with tissue plasminogen activator (tPA) to determine outcomes related to bleeding complications and thrombus resolution.

STUDY DESIGN

We performed a retrospective review of all patients who received systemic tPA for thrombolysis. Data points included location of thrombus, initial and maximum tPA dose, and duration of tPA. The primary endpoint was bleeding complication.

RESULTS

Between 2005 and 2014, 46 patients received systemic tPA for thrombolysis: 17 (37%) were patients with a primary cardiac diagnosis, there were 17 (37%) hematology/oncology patients, and 12 (26%) patients with noncardiac, nonhematology/oncology diagnoses. The indication for tPA was central venous thrombus (n = 23), pulmonary artery thrombus (n = 9), and cardiac or aortic thrombus (n = 14). Bleeding complications occurred in 15 patients (33%). Median initial tPA dose in the bleeding complication group was 0.10 mg/kg/h vs 0.03 mg/kg/h in the group without bleeding complication group (P = .01). Cardiac patients experienced more bleeding complications (P = .01). Multivariate analysis indicated that dose of tPA (P = .01) and diagnostic category (P < .01) were associated with bleeding complication. Complete thrombus resolution occurred in 21 patients, partial in 10 patients, and no resolution in 15 patients. Complete resolution of thrombus was not associated with diagnosis, thrombus location, tPA dose, or duration.

CONCLUSIONS

Cardiac patients appear to be at highest risk of bleeding complication; bleeding complications were associated with higher doses of tPA, and cardiac patients were the cohort who received the highest doses of tPA. Higher tPA doses are associated with increased risk of bleeding complication but are not associated with successful thrombus resolution.

The Use of Low-Dose Recombinant Tissue Plasminogen Activator to Treat a Preterm Infant with an Intrauterine Spontaneous Arterial Thromboembolism

Neonatal thromboembolic events are rare, and only a few cases of intrauterine spontaneous arterial thromboembolisms have been reported in the literature. Thrombolytic therapy with recombinant tissue plasminogen activator is usually the preferred treatment because it has a short half-life, fewer systemic side effects, and a strong, specific affinity for fibrin. Protocols vary from center to center, but there is still no consensus regarding the proper dosage or treatment duration. Herein, we present the case of an intrauterine spontaneous arterial thromboembolism in a preterm infant that completely resolved after being treated with low-dose recombinant tissue plasminogen activator (0.02 mg/kg/h).

A practical approach to the use of low molecular weight heparins in VTE treatment and prophylaxis in children and newborns

Low-molecular weight heparins are currently the most commonly used anticoagulants in children and newborns. However, since thrombotic complications rarely occur outside large children's hospitals, physicians often encounter some practical problems in managing these treatments when a pediatric thrombosis specialist is not available. The drug of choice is enoxaparin, due to its favorable FXa/FIIa ratio and the availability of pharmacokinetic and pharmacodynamic data. The treatment of acute thrombosis should be started with two daily injections but when compliance is an issue, a single daily administration schedule could be chosen for secondary prophylaxis ensuring careful measurement of the post 24-hour anti-FXa activity. Furthermore, a subcutaneous device may be a useful tool and a topical dermal anesthetic could be effective in controlling pain without affecting anti-FXa levels. In neonate and toddlers, where mini doses are frequently needed, the dead space of syringes and needles could represent an issue and therefore the use of insulin syringes without dead space is advisable, while a dilution of the drug is useful with other syringes. This article derives from a nonsystematic review of the available literature, with special attention to recent international guidelines and expert recommendations, combined to authors' clinical practice in large tertiary pediatric hospitals and will provide concise and practical information for the use of low-molecular weight heparin in childhood and infancy in a sort of "answering frequently asked questions."

Anticoagulant therapy in pediatrics

Thromboembolic episodes are disorders encountered in both children and adults, but relatively more common in adults. However, the occurrence of venous thromboembolism and use of anticoagulants in pediatrics are increasing. Unfractionated Heparin (UH) is used as a treatment and prevention of thrombosis in adults and critically ill children. Heparin utilization in pediatric is limited by many factors and the most important ones are Heparin Induced Thrombocytopenia (HIT) and anaphylaxis. However, Low Molecular Weight Heparin (LMWH) appears to be an effective and safe alternative treatment. Hence, it is preferred over than UH due to favorable pharmacokinetic and side effect profile. Direct Thrombin Inhibitors (DTI) is a promising class over the other anticoagulants since it offers potential advantages. The aim of this review is to discuss the differences between adult and pediatric thromboembolism and to review the current anticoagulants in terms of pharmacological action, doses, drug reactions, pharmacokinetics, interactions, and parameters. This review also highlights the differences between old and new anticoagulant therapy in pediatrics.

Fibrinolytic therapy of thrombosis in 27 newborns followed-up in neonatal intensive care unit

BACKGROUND AND AIM

We aimed to report newborns with thrombosis and needed fibrinolytic treatment.

PATIENTS AND METHODS

This was a retrospective study conducted on 27 newborns with thrombosis followed-up in a neonatal intensive care unit between December 2007 and December 2012. The patients were divided into two groups according to treatment protocol; Group 1 (n = 15): recombinant tissue-type plasminogen activator (r-tPA) and Group 2 (n = 12): streptokinase (STK). The groups were compared in terms of the efficacy and complications of the treatments.

RESULTS

In Group 1, flow restoration was complete in nine (60%) patients, partial in two (13.3%), and absent in four (26.7%). In Group 2, flow restoration was complete in seven (58.3%) patients, partial in three (25%), and absent in two (16.0%). The incidence of complete/partial recovery was similar in the groups. There was no difference between the two groups with regard to the duration of thrombus resolution. Fibrinolytic treatment was terminated in seven patients (46.7%) in Group 1, while in three patients (25%) in Group 2 due to the complications. The most common complications were minor or major bleedings. There was no statistically significant difference with regard to all bleedings complications between the two groups (P = .08), although the incidence of skin hemorrhages was higher in Group 1 (P = .007). There was no significant difference between the mortality rates.

CONCLUSION

Even though the use of STK is not further recommended because of its potential undesirable side effects in newborns, its efficacy and safety appears to be similar to those of r-tPA.

Therapy in pediatric stroke

The aim of this review is to summarize the existing literature on therapy and management of cerebrovascular insults in children and adolescents. As data sources, studies were identified by MEDLINE, PubMed, Cochrane Library, and relevant bibliographies for the topic "pediatric stroke." We also reviewed guidelines for "stroke in adults." As a result, pediatric stroke is underestimated. The annual incidence for all stroke entities (cerebral venous thrombosis and hemorrhagic and arterial ischemic stroke) is as high as for pediatric brain tumors, 3-15/100.000 children per year. A distinct etiology can be determined only in a minority of them. Underlying risk factors are multiple, mainly vasculopathies, congential heart diseases, coagulopathies, lipometabolic disorders, and sickle cell anemia. Current recommendations for therapy are based on adult studies, are preliminary, and discussed controversially. Antithrombotic therapy is uniformly recommended for the acute stage of pediatric stroke; no consensus exists on antiplatelet therapy with acetylsalicylic acid (ASA, aspirin) (5 mg/d), with ultra-fractionated or low-molecular-weight heparin. Thrombolysis using recombinant tissue plasminogen activator is not advised, despite the fact that current practice takes a different approach. None of the guidelines specify the duration of ASA for secondary prevention. Additional supportive therapy measures are osmotherapy and decompressive craniectomy. Oxygen in the absence of hypoxemia, intensive insulin therapy, antiepileptic drugs in the absence of clinical or electrographic seizures, corticosteroids, and GP-IIb/IIIa-receptor antagonists should not be used outside clinical trials. In conclusions, current therapeutic guidelines for pediatric stoke are still based on consensus and expert and society opinions and differ between countries. Consensus prevails on the need for acute anticoagulation using either antiplatelets or heparin. Long-term treatment with acetylsalicylic acid in all or only high-risk patients and for how long remains the subject of debate. Lifelong secondary prevention has never been investigated in children or adults. All guidelines agree that there is no indication for thrombolysis in children outside clinical trials, although clinical practice in large centers differs.

Chylopericardium and chylothorax: unusual mechanical complications of central venous catheters

Obstruction and thrombosis of major systemic veins can occur due to indwelling central venous catheters. If obstruction of the innominate vein or superior vena cava occurs, lymphatic drainage can be impaired due to an increase in pressure in the thoracic duct and lymphatics. We describe a case where superior vena cava syndrome, chylopericardium and chylothorax occurred in a 16-year-old girl due to an indwelling central venous catheter. This was successfully treated with removal of the line, anticoagulation and angioplasty of the innominate vein and superior vena cava.

Updates in thrombosis in pediatrics: where are we after 20 years?

The incidence of venous thromboembolism (VTE) in the pediatric population is increasing. Technological advances in medicine and imaging techniques, improved awareness of the disease, and longer survival of life-threatening or chronic medical conditions all contribute to the increase in VTE rates. There is a paucity of data on management of VTE based on properly designed clinical trials, but there is significant advancement in the last 2 decades. This review summarizes the progress made in pediatric thrombosis, including epidemiological changes, advances in anticoagulant agents, and outcomes of VTE.

Antithrombotic therapy in neonates and children: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

BACKGROUND

Neonates and children differ from adults in physiology, pharmacologic responses to drugs, epidemiology, and long-term consequences of thrombosis. This guideline addresses optimal strategies for the management of thrombosis in neonates and children.

METHODS

The methods of this guideline follow those described in the Methodology for the Development of Antithrombotic Therapy and Prevention of Thrombosis Guidelines: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines.

RESULTS

We suggest that where possible, pediatric hematologists with experience in thromboembolism manage pediatric patients with thromboembolism (Grade 2C). When this is not possible, we suggest a combination of a neonatologist/pediatrician and adult hematologist supported by consultation with an experienced pediatric hematologist (Grade 2C). We suggest that therapeutic unfractionated heparin in children is titrated to achieve a target anti-Xa range of 0.35 to 0.7 units/mL or an activated partial thromboplastin time range that correlates to this anti-Xa range or to a protamine titration range of 0.2 to 0.4 units/mL (Grade 2C). For neonates and children receiving either daily or bid therapeutic low-molecular-weight heparin, we suggest that the drug be monitored to a target range of 0.5 to 1.0 units/mL in a sample taken 4 to 6 h after subcutaneous injection or, alternatively, 0.5 to 0.8 units/mL in a sample taken 2 to 6 h after subcutaneous injection (Grade 2C).

CONCLUSIONS

The evidence supporting most recommendations for antithrombotic therapy in neonates and children remains weak. Studies addressing appropriate drug target ranges and monitoring requirements are urgently required in addition to site- and clinical situation-specific thrombosis management strategies.

Spinal cord injuries in children and adolescents

This chapter provides an overview of spinal cord injuries (SCI) in children and adolescents, including epidemiology, medical and musculoskeletal complications, rehabilitation and psychosocial aspects. Males are more commonly affected than females during adolescence; however, as the age at injury decreases, the preponderance of males becomes less marked, and by 3 years of age the number of females with SCIs equals that of males. The neurologic level and degree of completeness varies with age; among children injured prior to 12 years of age approximately two-thirds are paraplegic and approximately two-thirds have complete lesions. Among adolescents, approximately 50% have paraplegia and 55% have complete lesions. Management of pediatric-onset SCI should be family centered and developmentally based, responsive to the dynamic changes that occur during growth and development. Distinctive anatomical and physiological features of children and adolescents, along with growth and development, are responsible for unique manifestations and complications of pediatric SCI. SCI without radiological abnormalities (SCIWORA), birth injuries, lap-belt injuries, upper cervical injuries, and the delayed onset of neurological deficits are relatively unique to pediatric SCI. Children who sustain their SCI before puberty experience a higher incidence of musculoskeletal complications, such as scoliosis and hip dislocation.

Venous thrombosis in children: an emerging issue

More and more cases of venous thrombosis are diagnosed in children thanks to newer imaging modalities. Central venous catheters have become commonplace in the care of critically ill children and have contributed to the increased rate of thrombotic events. Lastly, children who develop life-threatening or chronic medical conditions are surviving longer because of advanced medical therapies; these intensive therapies can be complicated by events such as thrombosis. Over the last 10 years, specific guidelines for treating thrombosis in children have become available. Nevertheless, in many situations anticoagulant treatment is specially tailored to each individual patient's needs. Some new antithrombotic drugs which have undergone clinical testing in adults might be beneficial to paediatric patients with thromboembolic disorders; unfortunately, clinical data and reports on the use of these drugs in children, when available, are extremely limited. The aim of this review is to provide physicians with enough background information to be able to manage thrombosis in children. First, by helping them detect a thrombotic event in a child. Upon confirmation of the diagnosis, the physician will request the appropriate tests and will choose the best treatment on the basis of the guidelines and recommendations. Moreover, the paediatrician will have the information he or she needs to identify which children are at highest risk of acute thrombotic events and relevant long-term sequelae and, therefore, to decide on the appropriate prophylactic or pharmacologic strategy. Lastly, we would like to provide the paediatrician with information on future drugs with regard to the treatment and prophylaxis of thrombosis.

Oral anticoagulants in pediatric cardiac practice: A systematic review of the literature

Recent advances in the pediatric heart surgery, especially the Fontan procedure, has necessitated an increased use of oral anticoagulants in pediatric cardiac patients. Warfarin is the standard agent used for most pediatric indications, though there are very few randomized control studies in children regarding its use. This review summarizes the current indications and evidence base regarding the use of oral anticoagulants in the pediatric age group.

Evaluation and Treatment of Neonatal Thrombus Formation in 17 Patients

Introduction: Thrombosis is a rare but serious event in neonates. Perinatal risk factors associated with the developing hemostatic system increase the risk of thromboembolism. Treatment protocols vary between different centers. In this study, thrombosis and subsequent treatment were evaluated in 17 neonates hospitalized in a neonatal intensive care unit. This is the largest series reported to date. Method: From January 2007 to December 2009, thrombosis was diagnosed in 17 newborns hospitalized in a neonatal intensive care unit. Most were treated with recombinant tissue plasminogen activator (r-tPA) and enoxaparin as anticoagulant therapy. Results: Eleven courses of r-tPA therapy were administered to 10 patients. Sixteen patients received low-molecular-weight heparin (LMWH) treatment. None of the patients had hemorrhagic complications due to therapy. Recurrence was observed in 1 patient 1.5 months after the first course of r-tPA and retreatment was performed. A complete decrease in thrombus size was achieved in 9 patients and a partial decrease in 5 patients. One patient died before treatment was initiated; another died at the end of the first day of treatment and thus could not be evaluated. One patient who had a homozygous mutation for factor V Leiden did not respond to treatment and the extremity involved was amputated. Conclusions: Thrombosis in neonates is a multifactorial disorder. Treatment consisting of r-tPA and an anticoagulant was shown to be a safe and effective approach to clot dissolution in neonates.

Dramatic increase in venous thromboembolism in children's hospitals in the United States from 2001 to 2007

OBJECTIVES

The goals were to determine whether there has been an increase in the rate of venous thromboembolism (VTE) in pediatric tertiary care hospitals and to evaluate the use of anticoagulants in the treatment of hospitalized pediatric patients with VTE.

METHODS

A retrospective cohort study of patients <18 years of age who were discharged from 35 to 40 children's hospitals (depending on the year) across the United States in 2001-2007 was performed. By using the Pediatric Health Information System administrative database, cases were assessed for discharge diagnosis codes for VTE; the use of anticoagulants was assessed by using patient-specific pharmacy files.

RESULTS

During the 7-year study period, in which 11 337 hospitalized patients were diagnosed with VTE, the annual rate of VTE increased by 70%, from 34 to 58 cases per 10,000 hospital admissions (P < .001). This increase was observed in neonates, infants, children, and adolescents. The majority (63%) of children with VTE had > or =1 coexisting chronic complex medical condition. Pediatric malignancy was the medical comorbid condition associated most strongly with recurrent VTE (P < .001). The proportion of children with VTE who were treated with enoxaparin increased from 29% to 49% during this time period (P < .001); the use of warfarin decreased slightly from 11.4% to 9.6% (P= .02). Increasing age was associated with increased likelihood of patients with VTE being treated with either enoxaparin or warfarin.

CONCLUSION

This multicenter study demonstrates a dramatic increase in the diagnosis of VTE at children's hospitals from 2001 to 2007.

Antithrombotic therapy in neonates and children: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition)

This chapter about antithrombotic therapy in neonates and children is part of the Antithrombotic and Thrombolytic Therapy: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Grade 1 recommendations are strong and indicate that the benefits do, or do not, outweigh risks, burden, and costs, and Grade 2 suggests that individual patient values may lead to different choices (for a full understanding of the grading, see Guyatt et al in this supplement, pages 123S-131S). In this chapter, many recommendations are based on extrapolation of adult data, and the reader is referred to the appropriate chapters relating to guidelines for adult populations. Within this chapter, the majority of recommendations are separate for neonates and children, reflecting the significant differences in epidemiology of thrombosis and safety and efficacy of therapy in these two populations. Among the key recommendations in this chapter are the following: In children with first episode of venous thromboembolism (VTE), we recommend anticoagulant therapy with either unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) [Grade 1B]. Dosing of IV UFH should prolong the activated partial thromboplastin time (aPTT) to a range that corresponds to an anti-factor Xa assay (anti-FXa) level of 0.35 to 0.7 U/mL, whereas LMWH should achieve an anti-FXa level of 0.5 to 1.0 U/mL 4 h after an injection for twice-daily dosing. In neonates with first VTE, we suggest either anticoagulation or supportive care with radiologic monitoring and subsequent anticoagulation if extension of the thrombosis occurs during supportive care (Grade 2C). We recommend against the use of routine systemic thromboprophylaxis for children with central venous lines (Grade 1B). For children with cerebral sinovenous thrombosis (CSVT) without significant intracranial hemorrhage (ICH), we recommend anticoagulation initially with UFH, or LMWH and subsequently with LMWH or vitamin K antagonists (VKAs) for a minimum of 3 months (Grade 1B). For children with non-sickle-cell disease-related acute arterial ischemic stroke (AIS), we recommend UFH or LMWH or aspirin (1 to 5 mg/kg/d) as initial therapy until dissection and embolic causes have been excluded (Grade 1B). For neonates with a first AIS, in the absence of a documented ongoing cardioembolic source, we recommend against anticoagulation or aspirin therapy (Grade 1B).

Management of oral anticoagulation in a population of children with cardiac disease using a computerised system to support decision-making

OBJECTIVE

To assess the impact of a computerised system to support decision-making concerning the management of warfarin used in maintenance of anti-coagulation.

DESIGN

Retrospective case series study comparing manual and computerised records of prescribing.

SETTING

A tertiary paediatric cardiology department in a teaching hospital.

PARTICIPANTS

The 26 children receiving warfarin to maintain anticoagulation at the time of introduction of a computerised system to support decision-making.

INTERVENTIONS

A rules-based computerised system to support decisions, based on existing departmental guidelines, for management of anticoagulation using warfarin was introduced to aid prescribing physicians.

MAIN OUTCOMES

We assessed the stability of the International Normalised Ratio, along with the number of checks made of the ratio, and the adjustments of dosage. Dosages, and recheck interval prescriptions, were compared to the guidelines established by our department.

RESULTS

We compared 274 prescriptions made manually, and 608 made using the computerised system to support decision-making, covering periods of 4, and 11, months respectively. The mean proportion of time spent by the patients within their target range for the International ratio was maintained during the period studied, at 76 percent versus 79 percent (p = 0.79). The median number of checks of the ratio made for each patient over a period of 28 days was unchanged, at 1.9 versus 2.1 (p = 0.58). There was a significant change in prescribing practices, which more closely followed the departmental guidelines.

CONCLUSION

The introduction of a computerised system to support decision-making maintained the stability of the International ratio using warfarin, without increasing the number of checks or adjustments of dosages, in a point-of-care service for anticoagulation in children.

Home INR monitoring of oral anticoagulant therapy in children using the CoaguChek™ S point-of-care monitor and a robust education program

INTRODUCTION

Management strategies such as self-monitoring of anticoagulant therapy have been reported with increased frequency. Whilst patient education is frequently mentioned, details regarding the educational interventions employed are scarce. This study aimed to improve the outcomes of home monitoring of warfarin therapy in children through the development and implementation of a robust intervention, based upon the PRECEDE model of education.

MATERIALS AND METHODS

Participating parents had to complete an intensive education and training program. After demonstrating practical and theoretical competency, parents commenced home monitoring. Every second scheduled home INR (H-INR) required a paired INR on the same day, obtained by a trained pathology collector (C-INR). Demographic and statistical outcome data was collected.

RESULTS

Parental understanding of warfarin therapy improved significantly following the educational intervention (p<0.0001). 65.5% of H-INRs and 64.4% of C-INRs were within the target range (ns). Lin's correlation coefficient between H-INRs and C-INRs was 0.949. There were no warfarin-related adverse events.

CONCLUSION

This study demonstrated a significant improvement in parental knowledge following participation in a robust educational intervention. Furthermore, compared to previous reports in children, a greater level of correlation between home and hospital-based INRs was achieved by participating parents. The use of similar educational interventions may serve to improve the outcomes of similar management strategies.

A comparison between prophylaxis and on demand treatment for severe haemophilia

Patients with severe haemophilia can be treated for bleeding either prophylactically or on demand. Each treatment modality has advantages and disadvantages from both a medical and economic point of view. This study aims to find which modality requires more units of clotting factors per body weight per year and to compare the number of bleeds between the two. The study sample consisted of 133 patients with severe haemophilia A and B treated in the Katharine Dormandy Haemophilia Centre at the Royal Free Hampstead NHS Trust in London. The average number of clotting factors used per body weight per year was 2181.7 units for prophylaxis vs. 711 units for on demand treatment (P = 0.000). Although more units used means more money spent, and although prophylaxis has additional complications, namely venipunctures and increased risk of viral contamination, other criteria must be considered including the total number of bleeds and health-related quality of life. The total number of bleeds per year was significantly (P = 0.021) less for prophylactically (7.4) vs. on-demand treated patients (11.4). This suggests that prophylaxis reduces the risk of arthropathies, the number of future hospital visits and orthopaedic surgeries, and is thus the optimal modality of treatment for patients with severe haemophilia.

Long-term anticoagulation therapy and thromboembolic complications after the Fontan procedure

BACKGROUND

The necessity for chronic anticoagulation of Fontan patients remains controversial. We determined the prevalence of thromboembolic complications after the Fontan procedure in relation to different long-term anticoagulation strategies.

METHODS

The clinical outcomes, postoperative anticoagulation strategies and occurrence of thromboembolic complications in 102 ethnic Chinese patients who had undergone Fontan procedure between 1980 and 2002 were reviewed.

RESULTS

The early and late surgical mortalities, all unrelated to thromboembolism, were 10.8% (11/102) and 5.8% (6/104), respectively. Of the 85 survivors, 46 (54%) were maintained on long-term warfarin therapy, 8 (9%) on aspirin prophylaxis while 31 (37%) were not on chronic anticoagulation. Four (4.5%) patients, two with and two without warfarin prophylaxis, developed thromboembolic complications at 0.14 to 7.7 years after the Fontan procedure (0.74%/patient-year). Three had a grossly dilated right atrium after atriopulmonary connection, two of whom had atrial fenestrations. The other had atrial tachycardia. Freedom from development of thromboembolic complications (mean+/-S.E.) at 1, 5 and 10 years after surgery was 97+/-19%, 96+/-2.5% and 92+/-4.2%, respectively. When compared with those on long-term warfarin therapy, patients without chronic anticoagulation were followed-up longer (p=0.001), more likely to have undergone atriopulmonary connection (p<0.001), less likely to have fenestrations (p=0.02) and cardiac arrhythmias (p=0.02) but not predisposed to increased risk of thromboembolism (p=1.00).

CONCLUSION

The study supports the contention that chronic anticoagulation may not be required for majority of ethnic Chinese Fontan patients. Nonetheless, it may perhaps be considered in those with grossly dilated right atrium, cardiac arrhythmias and residual right-to-left shunts.

Antithrombotic Therapy in Children*

This article about antithrombotic therapy in children is part of the 7th American College of Chest Physicians Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. Grade 1 recommendations are strong and indicate that the benefits do, or do not, outweigh the risks, burden, and costs. Grade 2 suggests that individual patients' values may lead to different choices (for a full understanding of the grading see Guyatt et al, CHEST 2004; 126:179S-187S). Among the key recommendations in this article are the following. In neonates with venous thromboembolism (VTE), we suggest treatment with either unfractionated heparin or low-molecular-weight heparin (LMWH), or radiographic monitoring and anticoagulation therapy if extension occurs (Grade 2C). We suggest that clinicians not use thrombolytic therapy for treating VTE in neonates, unless there is major vessel occlusion that is causing the critical compromise of organs or limbs (Grade 2C). For children (ie, > 2 months of age) with an initial VTE, we recommend treatment with i.v. heparin or LMWH (Grade 1C+). We suggest continuing anticoagulant therapy for idiopathic thromboembolic events (TEs) for at least 6 months using vitamin K antagonists (target international normalized ratio [INR], 2.5; INR range, 2.0 to 3.0) or alternatively LMWH (Grade 2C). We suggest that clinicians not use thrombolytic therapy routinely for VTE in children (Grade 2C). For neonates and children requiring cardiac catheterization (CC) via an artery, we recommend i.v. heparin prophylaxis (Grade 1A). We suggest the use of heparin doses of 100 to 150 U/kg as a bolus and that further doses may be required in prolonged procedures (both Grade 2 B). For prophylaxis for CC, we recommend against aspirin therapy (Grade 1B). For neonates and children with peripheral arterial catheters in situ, we recommend the administration of low-dose heparin through a catheter, preferably by continuous infusion to prolong the catheter patency (Grade 1A). For children with a peripheral arterial catheter-related TE, we suggest the immediate removal of the catheter (Grade 2C). For prevention of aortic thrombosis secondary to the use of umbilical artery catheters in neonates, we suggest low-dose heparin infusion (1 to 5 U/h) (Grade 2A). In children with Kawasaki disease, we recommend therapy with aspirin in high doses initially (80 to 100 mg/kg/d during the acute phase, for up to 14 days) and then in lower doses (3 to 5 mg/kg/d for > or = 7 weeks) [Grade 1C+], as well as therapy with i.v. gammaglobulin within 10 days of the onset of symptoms (Grade 1A).

Oral Anticoagulation Therapy in Children: Successfully Controlled by Self-Management

BACKGROUND

Children with congenital heart disease and who are on oral anticoagulation therapy present special challenges due to, for example, rapid fluctuations in international normalized ratio (INR) values, interruption in daily life due to frequent hospital/doctor visits, and difficulties and pain to the child in the performance of venipuncture. We hypothesize that oral anticoagulation therapy can be successfully controlled by self-management for this subset of patients. The aim of this study was to assess the treatment quality of self-managed oral anticoagulation therapy as the proportion of time within the therapeutic INR target range in children with congenital heart disease.

METHODS

Children (N = 22) with a mean age of 10.6 years (range, 1.8-18.6 years) and their parents were trained in home blood analysis of INR and in coumarin dosage adjustment. After training, the children were monitored by weekly INR measurements. The therapeutic range in target INR values was +/-0.5. The indications for initiating oral anticoagulation therapy were the presence of a mechanical heart valve (n = 16) and total cavopulmonary connection (n = 6). The children had no physical restrictions.

RESULTS

The mean observation time was 3.6 years (range, 0.9-5.8 years), and the total number of patient-years was 75.4. The patients were within the therapeutic INR target range for a median of 73.1% (range, 30.3%-91.0%) of the observation time. Two children died for reasons not related to the oral anticoagulation therapy. None of the patients experienced thromboembolic or bleeding complications requiring doctor intervention.

CONCLUSION

Self-management of oral anticoagulation therapy is safe and provides a good quality of treatment for selected children with congenital heart disease.

Selective COX-2 inhibition and cardiovascular effects: a review of the rofecoxib development program

See related Editorials on pages 561 and 563. Cyclo-oxygenase-2 (COX-2) inhibitors appear to alter the balance of vasoactive eicosanoids (prostacyclin and thromboxane) and to suppress the inflammatory mediators implicated in the progression of atherogenesis and ischemic myocardial injury. Neutral, harmful, and beneficial cardiovascular (CV) effects have all been postulated to result from these changes. Investigations conducted with rofecoxib, a selective COX-2 inhibitor, have substantially contributed to our understanding of this scientific area. Rofecoxib had little or no effect on platelet aggregation or platelet-derived thromboxane synthesis but reduced systemic prostacyclin synthesis by 50% to 60%. These findings prompted extensive analyses of CV thrombotic events within the rofecoxib development program. Among 5435 osteoarthritis trial participants, similar rates of CV thrombotic events were reported with rofecoxib, placebo, and comparator, nonselective NSAIDs (ibuprofen, diclofenac, and nabumetone). In the VIGOR gastrointestinal outcomes trial of >8000 patients, naproxen (an NSAID with aspirin-like sustained antiplatelet effects throughout its dosing interval) was associated with a significantly lower risk of CV events than was rofecoxib. A subsequent pooled analysis from 23 studies (including VIGOR) encompassing multiple disease states and including more than 14,000 patient-years at risk also demonstrated that rofecoxib was not associated with excess CV thrombotic events compared with either placebo or nonnaproxen NSAIDs. Again, naproxen appeared to be the outlier, suggesting a cardioprotective benefit of naproxen. Finally, among the predominantly elderly, male population participating in Alzheimer trials, both rofecoxib- and placebo-treated patients had similar rates of CV thrombotic events. The totality of data is not consistent with an increased CV risk among patients taking rofecoxib.

[Acenocoumarol (Sintrom) and fluinidione (Previscan) in pediatrics after cardiac surgical procedures]

PATIENTS AND METHODS

Between 1997 and 2001, 150 children (one month to 16 years of age) were treated with oral anticoagulants after cardiac surgery (Fontan's operations and congenital heart diseases without valvulopathy: 62%, valvular prosthesis: 20%, arrhythmia: 4.6%, thrombosis: 4%, other: 9.4%). They were first treated by either unfractionated heparin (49%) or nadroparin (51%), then by acenocoumarol (n1 = 114) or fluindione (n2 = 36) until steady state.

RESULTS

The retrospective analysis of data (age, body weight, international normalized ratio, loading and maintenance doses, time to achieve the steady state) led to the building of a dosage nomogram usable in pediatrics.

CONCLUSION

We demonstrated that the mean maintenance dose depended on age and weight. After three years, that dose (mg/kg) was getting close to adult values; it was higher before three years of age, especially before 12 months (p < 0.01), and very variable from a child to another. The recommended loading dose should be as close as possible to the effective maintenance dose: within that cohort, about 0.14 and 0.05 (acenocoumarol) or 1.1 and 0.40 mg kg-1 day-1 (fluindione), before 12 months and after three years respectively.

Safety and outcomes of thrombolysis with tissue plasminogen activator for treatment of intravascular thrombosis in children

OBJECTIVES

In this study, we tried to determine the safety and outcomes of thrombolysis with tissue plasminogen activator of intravascular thrombus.

STUDY DESIGN

Eighty consecutive children were treated between 1985 and 1999 in a tertiary care setting in a retrospective case series. There were 65 arterial thrombi (56 after cardiac catheterization) and 15 venous thrombi treated with tPA at an average dose of tPA of 0.5 mg/kg/hour for a median duration of 6 hours.

RESULTS

Clot resolution was complete in 65% of children, partial in 20%, and there was no effect in 15%. There were major complications in 40%, minor complications in 30%, and no complications in 30%. Two patients had cerebral ischemia secondary to hypotension because of profound bleeding, with intracranial hemorrhage in 2 additional patients. Clot resolution was not related to patient age or weight, dose, and duration of tPA therapy and fibrinogen levels. However, complications were more likely in patients who weighed less, had a longer duration of therapy, a greater decrease in fibrinogen levels, and who failed to have resolution of their clot.

CONCLUSIONS

tPA therapy can be effective in the thrombolysis of intravascular thrombus in children, but is associated with a low margin of safety and an unknown risk-benefit ratio.

Treatment of neonatal thrombus formation with recombinant tissue plasminogen activator: six years experience and review of the literature

BACKGROUND

Thrombosis is a relatively rare event in children. However, many conditions in the neonatal period result in an increased risk of thrombus formation. The major risk factor is the indwelling intravascular catheter. Numerous small studies have reported experience of thrombolytic treatment for neonatal thrombotic disease with a wide range of different thrombolytic agents in various forms of administration, dosage, and duration, but no conclusions on the most effective treatment for neonates has been reached.

OBJECTIVE

To assess the efficacy and safety of thrombolytic treatment of neonatal catheter related thrombus (CRT) formation with recombinant tissue plasminogen activator (rt-PA).

METHOD

Over a six year period, 14 neonates with CRT were treated with the same rt-PA protocol (an initial bolus of 0.7 mg/kg over 30-60 minutes followed by infusion of 0.2 mg/kg/h).

RESULTS

Complete clot dissolution was documented in 11 patients, and partial clot lysis in two patients, leading to a patency rate of 94%. In two cases, local bleeding occurred, resulting in treatment failure in one case. Finally, antithrombin III substitution was required in one case. No other complications such as severe bleeding were recognised.

CONCLUSION

With the use of close clinical and haematological monitoring on a neonatal intensive care unit combined with serial two dimensional colour echocardiography, the present rt-PA protocol was shown to be a safe and effective method of clot dissolution in neonates.

Microvascular thrombosis in pediatric multiple organ failure: Is it a therapeutic target?

PURPOSE: To discuss the current rationale for the use of specific and nonspecific therapies for thrombotic microangiopathy in thrombocytopenia-associated pediatric multiple organ failure syndromes. Methods: Pertinent PubMed and MEDLINE citations and proceedings of recent critical care meeting presentations were reviewed. RESULTS: Critical care clinicians have reported using antithrombin III concentrate, protein C concentrate, activated protein C, prostacyclin and its analogues, heparin, tissue factor pathway inhibitor concentrate, plasma infusion, plasma exchange, whole blood exchange, pentoxifylline, tissue plasminogen activator, urokinase, and streptokinase with perceived therapeutic benefits in patients with thrombocytopenia-associated multiple organ failure, including those with thrombotic thrombocytopenic purpura/hemolytic uremic syndrome, disseminated intravascular coagulation syndrome, and secondary thrombotic microangiopathy syndrome without prolonged prothrombin time/activated partial thromboplastin time. CONCLUSION: Assuming that underlying disease is remediable, a consensus has developed that thrombotic microangiopathy is a therapeutic target in children with thrombocytopenia-associated multiple organ failure syndromes. Studies are warranted to delineate efficacious use of specific and nonspecific therapies to prevent and reverse thrombotic microangiopathy in these patients.

Thrombolytic therapy in an adolescent ischemic stroke

We report the case of a 16-year-old Caucasian girl who developed acute onset of left hemiplegia, left hemisensory deficit, and dysarthria. After a negative computed tomographic scan of the brain, the patient was given intravenous recombinant tissue plasminogen activator according to established adult guidelines. The patient experienced a marked improvement within 24 hours. Stroke etiology was determined to be a paradoxical embolus via a patent foramen ovale associated with pelvic vein thrombosis. This case illustrates the importance of early recognition of stroke and the utility of thrombolytics in treating ischemic infarcts in the adolescent population.

Stroke in Children

Identification and treatment of the underlying risk factors for stroke reduce the potential for additional strokes; therefore, a thorough search for treatable risk factors is justified. Because some risk factors can have a cumulative effect, even children with known risk factors for stroke sometimes need to be evaluated for other conditions. Cerebral angiography is often helpful; I recommend angiography in any child with an unexplained infarction or hemorrhage. Angiography is especially important in children with intraparenchymal hemorrhage because more than one third of such children will prove to have some type of potentially treatable congenital vascular anomaly such as an arteriovenous malformation (AVM) or aneurysm. The evidence that periodic blood transfusion effectively prevents cerebral infarction due to sickle cell disease is compelling. Transfusions apparently must be continued indefinitely to maintain the reduction of stroke risk, and without iron chelation, chronic transfusion eventually results in severe iron toxicity and, most likely, death, so the decision to begin transfusion is not an easy one. Measurement of the time-averaged mean flow velocity in the large cerebral vessels with transcranial Doppler (TCD) is highly predictive of stroke risk in these children, enough to justify its routine use in screening patients with sickle cell disease for stroke risk. I believe that patients with sickle cell disease should be offered chronic transfusion after an initial large-vessel stroke or when the TCD results suggest a high risk of stroke. The family must be made aware of the serious complications of chronic transfusion and the importance of complying with chelation once it is started. There are no controlled clinical trials to guide the use of anticoagulants, antiplatelet agents, or thrombolytic agents in children, although these drugs are being used more and more often in pediatric patients. For the most part, our approach has been adapted from our experience with adults. Heparin followed by warfarin is often used for sinovenous thrombosis and for arterial dissection. I also suggest long-term anticoagulation for children with coagulopathy or a high risk of embolism due to congenital or acquired cardiac disease. It is reasonable to use a thrombolytic agent in children with an acute infarction; because few children present soon enough after the onset of symptoms, however, thrombolysis is infrequently used. Aspirin is used more than other antiplatelet agents in children, largely because of years of experience with aspirin and the lack of evidence that other agents are more effective. Despite its frequent use, there are no unequivocal indications for the use of aspirin in children. Aspirin is often started empirically in children suspected to be at substantial risk for additional ischemic stroke but whose risk is ill defined, an approach not too dissimilar from that often used in adult patients. Although the risk of Reye's syndrome in a child taking daily aspirin for stroke prevention is a common concern, I know of no published examples of children who developed Reye's syndrome while taking prophylactic aspirin. This apparently low risk must be weighed against the often-considerable risk of ischemic stroke that could be reduced by the use of daily aspirin. In situations such as vasculopathy or infarction of unknown cause, the small risk of Reye's syndrome seems acceptable.

Dose-adjusted thrombosis prophylaxis in trauma surgery according to levels of D-Dimer

In 234 trauma surgery patients, thrombosis prophylaxis with Nadroparin-Calcium low-molecular-weight heparin (LMWH) was adjusted according to levels of D-Dimer. Basic prophylaxis was 2,850 IU per day. If D-Dimer concentrations rose above 2 mg/L after the fourth postoperative (p.o.) day, LMWH was administered twice a day. Color Doppler ultrasound was performed between the fifth and seventh p.o. days. Patients were divided into a high-risk (group 1: hip, femur, or knee replacement surgery, n=102) and a moderate-risk group (group 2: other surgery of the knee, tibia, fibula, or foot, n=132). Group 1 showed significantly higher D-Dimer levels than group 2 (p<0.001). Measurement of D-Dimer on days 2 and 4 p.o. showed a sensitivity of 100% and a specificity of 72.8% in identifying patients at risk (i.e., D-Dimer>2 mg/L after day 4 p.o.). The overall deep vein thrombosis (DVT) rate in group 1 was 3.9%, and the rate of proximal DVT was 1.96%. In group 2, one distal DVT (0.8%) occurred. The results show that D-Dimer is a useful marker to monitor p.o. coagulation activation and to manage LMWH prophylaxis in trauma surgery patients.

Intracardiac thrombus formation after the Fontan operation

OBJECTIVES

Intracardiac thrombus formation is suspected to be a specific sequela after the Fontan operation and is difficult to determine by means of routine transthoracic echocardiography. The aim of our study was to evaluate the occurrence of intracardiac thrombi in the different types of Fontan modifications and to identify predisposing risk factors.

METHODS

We evaluated 52 patients who had undergone a Fontan-type operation and were free of symptoms regarding thrombosis as determined by transesophageal echocardiography.

RESULTS

In 17 (33%) patients thrombus formation could be found without clinical evidence of thromboembolic complications. Neither underlying morphologic disease nor age at operation, type of Fontan operation, sex, follow-up interval, arrhythmias, or laboratory or hemodynamic findings could be identified as predisposing risk factors.

CONCLUSION

In patients having had a Fontan operation with inadequate or without anticoagulation medication, we would recommend routine transesophageal echocardiography to exclude eventual thrombi. Because of the high incidence of thrombi, we suggest oral anticoagulation therapy in all patients.

The use of low molecular weight heparin in pediatric patients: a prospective cohort study

OBJECTIVE

Low molecular weight heparins (LMWHs) offer several advantages over standard anticoagulant therapy (unfractionated heparin/warfarin) including predictable pharmacokinetics, minimal monitoring, and subcutaneous administration. Our objective was to determine the safety and efficacy of LMWHs in children.

METHODS

A prospective cohort of children treated with the LMWH enoxaparin (Rhone Poulenc Rorer) was monitored at the Hospital for Sick Children, Toronto, Canada, from March 1994 until July 1997.

RESULTS

There were 146 courses of LMWH administered for treatment and 31 courses for prophylaxis of thromboembolic events (TEs). Clinical resolution of TEs occurred in 94% of children receiving therapeutic doses of LMWH, and 96% of children receiving prophylactic doses of LMWH had no symptoms of recurrent or new TEs. Major bleeding occurred in 5% of children receiving therapeutic doses. Recurrent or new TEs occurred in 1% and 3% of children receiving therapeutic and prophylactic doses of LMWH, respectively.

CONCLUSION

LMWH appears to be efficacious and safe for both management and prophylaxis of TEs. The results of this cohort study justify a randomized controlled trial comparing LMWH with standard therapy for the management of TEs in children.

Analysis of Warfarin Therapy in Pediatric Patients: A Prospective Cohort Study of 319 Patients

This study details warfarin use in a large pediatric population followed in a central anticoagulation clinic. A prospective, consecutive cohort of nonselected children were studied. Patients were divided into groups by age, target international normalized ratio (INR) range, disease, medications, and vitamin K supplemented enteral nutrition use. Groups were analyzed on multiple aspects of warfarin therapy using multivariate methods. A total of 319 patients received 352 warfarin courses representing 391 treatment years. Age independently influenced all aspects of therapy. When compared with all older children, the ≤1 year of age group required increased warfarin doses, longer overlap with heparin, longer time to achieve target INR ranges, more frequent INR testing and dose adjustments, and fewer INR values in the target range. Although significantly different than children ≤1 year, children 1 to 6 years of age showed the same findings when compared with 7- to 18-year-olds. Fontan patients required 25% decreased dosage as compared with other congenital heart disease patients. Children on corticosteroids had less INRs in the target range and children on phenobarbital/carbamazepine required increased maintenance dosages of warfarin. Also, patients receiving enteral nutrition required increased dosages of warfarin. Serious bleeding occurred in 2 children (0.5% per patient year). Recurrent thromboembolic events (TEs) occurred in 8 children. Two children had recurrences while receiving warfarin (1.3% per patient year). This study outlines the profound effect of age and relative complexity of clinical management of warfarin therapy in children.

Analysis of Warfarin Therapy in Pediatric Patients: A Prospective Cohort Study of 319 Patients

This study details warfarin use in a large pediatric population followed in a central anticoagulation clinic. A prospective, consecutive cohort of nonselected children were studied. Patients were divided into groups by age, target international normalized ratio (INR) range, disease, medications, and vitamin K supplemented enteral nutrition use. Groups were analyzed on multiple aspects of warfarin therapy using multivariate methods. A total of 319 patients received 352 warfarin courses representing 391 treatment years. Age independently influenced all aspects of therapy. When compared with all older children, the ≤1 year of age group required increased warfarin doses, longer overlap with heparin, longer time to achieve target INR ranges, more frequent INR testing and dose adjustments, and fewer INR values in the target range. Although significantly different than children ≤1 year, children 1 to 6 years of age showed the same findings when compared with 7- to 18-year-olds. Fontan patients required 25% decreased dosage as compared with other congenital heart disease patients. Children on corticosteroids had less INRs in the target range and children on phenobarbital/carbamazepine required increased maintenance dosages of warfarin. Also, patients receiving enteral nutrition required increased dosages of warfarin. Serious bleeding occurred in 2 children (0.5% per patient year). Recurrent thromboembolic events (TEs) occurred in 8 children. Two children had recurrences while receiving warfarin (1.3% per patient year). This study outlines the profound effect of age and relative complexity of clinical management of warfarin therapy in children.

Venous thromboembolic disease in the pediatric intensive care unit

Long recognized to be a major source of morbidity in the adult population, venous thromboembolism is being increasingly recognized in the pediatric age group. Pediatric intensive care unit patients are exposed to multiple risk factors for venous thromboembolism. Prothrombotic tendencies may be inherited or acquired, secondary to either the underlying disease or selected therapeutic interventions. In children in whom venous thromboembolism is diagnosed, the most commonly identified risk factor is the presence of a central venous catheter. Many cases are not diagnosed until autopsy. Because current treatment recommendations are extrapolated from adult studies, further investigation is needed to define the optimal treatment and prophylaxis regimens in critically ill children.

Population pharmacokinetic of nadroparin calcium (Fraxiparine) in children hospitalised for open heart surgery

Administration of low molecular weight heparin following heart surgery in paediatric patients in order to prevent thromboembolic events results in a large variation in anti-Xa activities. A population study was undertaken to determine pharmacokinetic parameters after nadroparin calcium (Fraxiparine) administration and the effects of potential covariates; this study included 154 children divided into two groups: a model group (124 patients) and a validation group (30 patients). The 432 anti-Xa activities were analysed using NONMEM on the basis of a one-compartment model with three parameters: apparent clearance, apparent volume of distribution and absorption rate. The influence of body weight, age, sex and dose regimen (once or twice daily) were investigated. The best fit corresponds to the formula: apparent clearance (l/min)=0. 541 x weight1.51/(6.151.51 + weight1.51) and apparent volume (l)=0.355 x weight. The inter-individual variability (expressed in coefficient of variation) of these parameters are high, especially with regard to the apparent volume (92%), but no other available covariate was found to explain this variability.