Fibrin Clot Lysis by Thrombolytic Agents Is Impaired in Newborns due to a Low Plasminogen Concentration

Although thrombolytic drugs have been extensively used in adults, there is sparse information on their effectiveness in newborns whose fibrinolytic system differs significantly from adults. The purpose of this study was to determine if low plasma levels of plasminogen in cord plasma limited the therapeutic effectiveness of thrombolytic agents. Urokinase (UK), streptokinase (SK) and tissue plasminogen activator (TPA) were compared for their ability to lyse washed 125I-labelled adult or cord fibrin clots suspended in cord or adult plasma. 125I-labelled fibrin clots were prepared by recalcifying cord or adult plasma spiked with labelled fibrinogen and then placed into cord or adult plasma which contained either saline or differing amounts of a specific thrombolytic agent. After a 60 min incubation, the remaining 125I-fibrin in clots released 125I-fibrin fragments, and concentrations of fibrinogen, α2-antiplasmin, and plasminogen in the bathing plasma were measured and compared to starting values. Cord fibrin clots were more resistant than adult fibrin clots to all thrombolytic drugs tested (p <0.001). On average, the cord system retained 27% more 125I-fibrin in clots, and released 32% less 125I-fibrin fragments into plasma. Fibrinogenolysis was also decreased in cord plasmas compared to adult plasmas. The degree of fibrinolysis and fibrinogenolysis in cord plasma increased to adult values when plasminogen concentrations were increased in the bathing plasma. Thus, cord fibrin clots have an impaired response to thrombolytic agents secondary to low levels of plasminogen. We speculate that the clinical response of newborns to thrombolytic agents is also impaired and will not be enhanced by increasing doses of thrombolytic agents but may be enhanced by increasing the concentration of plasminogen.

Antithrombotic Properties of Heparin in a Neonatal Piglet Model of Thrombin-Induced Thrombosis

The relative deficiency of antithrombin III (AT III) in neonatal plasma results in lower recovery of heparin in some assay systems. It is uncertain whether low AT III levels also limit the antithrombotic effects of heparin in this age group. We therefore compared the antithrombotic properties of heparin in mature pigs and newborn piglets, whose coagulation and inhibitor system closely resembles that of the human neonate. Animals were pretreated with saline, 10 or 25 U/kg heparin (n ≥16 per age group and dose). Following an injection of 100 U/kg thrombin, systemic 125I-fibrinogen consumption and local 125I-fibrinogen incorporation into jugular venous stasis thrombi were measured. Significantly more 125I-fibrinogen was consumed in piglets than in pigs systemically (p <0.0001), as well as within the occluded vein segment (p = 0.0112), largely because heparin was less effective in piglets than in pigs. This neonatal resistance to heparin could not be explained by lower heparin levels in the newborn animals. However, pretreatment with AT III concentrate significantly improved the antithrombotic properties of heparin in this age group (p <0.0001). We conclude that physiologically low AT III levels reduce the efficacy of heparin in neutralizing thrombin activity in newborn piglets. We speculate that AT III deficiency may also limit the antithrombotic properties of heparin in newborn infants with thrombotic disease.

Human external ophthalmomyiasis occurring in Barbados

Human infection with the sheep nasal botfly Oestrus ovis occurs sporadically. In most cases, there is a history of a strike in the eye by the adult fly. Human O. ovis has been reported rarely from the Americas. We report the first case of O. ovis infection in the Caribbean region, which occurred in an urban area of Barbados. The patient responded to removal of the larvae from the conjunctiva and symptomatic treatment.

Intensive exposure to factor VIII is a risk factor for inhibitor development in mild hemophilia A

BACKGROUND

Inhibitors are rare in boys with mild hemophilia A (MHA; factor (F)VIII:C > 5%) but may arise following intense FVIII exposure, e.g. continuous infusion (CI).

OBJECTIVES

To determine the impact of intense FVIII exposure in inhibitor formation in MHA at our institution and to compare this with previous reports.

PATIENTS AND METHODS

We reviewed FVIII exposure and inhibitor development in boys (ages 0-18 years) with MHA followed at our institution from 1996 to 2001 and conducted a Medline search (1966-2002) on the experience of inhibitor development following intensive/CI exposure to FVIII.

RESULTS

We identified 54 boys with MHA. Twenty-nine (54%) had been exposed to FVIII. Seven had received FVIII by CI. Four developed inhibitors; three high titer (at ages 10 years, 16 years and 17 years) and one low titer (at 1 month old). All four had received a CI of recombinant (r) FVIII of at least 6 days within 6 weeks of developing inhibitors. Baseline FVIII levels fell to < 1% in all cases and the three with high-titer inhibitors developed severe bleeding. Immune tolerance therapy (ITT) was attempted in two boys and was successful in one. Our literature search identified 35 cases (only four children) with MHA developing inhibitors following intense FVIII exposure often in the context of surgery.

CONCLUSIONS

The incidence of inhibitors in our MHA population was 7.4%. If expressed according to exposure the incidence was significantly higher: 14% (4/29) for any exposure to FVIII and 57% (4/7) for exposure by CI. A prospective study to address whether CI is associated with an increased incidence of inhibitor development in MHA is warranted.

The influence of developmental haemostasis on the laboratory diagnosis and management of haemostatic disorders during infancy and childhood

The physiology of the hemostatic system in children is different from that in adults. Reference ranges for components of the hemostatic system are age dependent. During early childhood, physiologic values may overlap with values observed in congenital deficiencies of acquired pathological conditions. Physiologically decreased plasma concentrations of hemostatic components also influence the response to antithrombotic therapy. The relevance of developmental hemostasis for the laboratory diagnosis and management of hemorrhagic and thrombotic disorders during childhood is discussed.

Acquired antithrombin III deficiency secondary to asparaginase therapy in childhood acute lymphoblastic leukaemia

As improved treatment regimens for acute lymphoblastic leukaemia (ALL) continue to improve survival future, therapy must also take into consideration the many secondary problems. Most of these are the direct result of combination chemotherapy and L-asparaginase (ASP), is an example of a highly effective chemotherapeutic agent with serious side-effects such as thromboembolic events. ASP interferes with protein synthesis resulting in an acquired deficiency of antithrombin III. This review explores the effects of ALL and ASP on haemostasis, and the link between ASP and thromboembolic events in childhood ALL.

Fibrin clot lysis by thrombolytic agents is impaired in newborns due to a low plasminogen concentration

Although thrombolytic drugs have been extensively used in adults, there is sparse information on their effectiveness in newborns whose fibrinolytic system differs significantly from adults. The purpose of this study was to determine if low plasma levels of plasminogen in cord plasma limited the therapeutic effectiveness of thrombolytic agents. Urokinase (UK), streptokinase (SK) and tissue plasminogen activator (TPA) were compared for their ability to lyse washed 125I-labelled adult or cord fibrin clots suspended in cord or adult plasma. 125I-labelled fibrin clots were prepared by recalcifying cord or adult plasma spiked with labelled fibrinogen and then placed into cord or adult plasma which contained either saline or differing amounts of a specific thrombolytic agent. After a 60 min incubation, the remaining 125I-fibrin in clots released 125I-fibrin fragments, and concentrations of fibrinogen, alpha 2-antiplasmin, and plasminogen in the bathing plasma were measured and compared to starting values. Cord fibrin clots were more resistant than adult fibrin clots to all thrombolytic drugs tested (p less than 0.001). On average, the cord system retained 27% more 125I-fibrin in clots, and released 32% less 125I-fibrin fragments into plasma. Fibrinogenolysis was also decreased in cord plasmas compared to adult plasmas. The degree of fibrinolysis and fibrinogenolysis in cord plasma increased to adult values when plasminogen concentrations were increased in the bathing plasma. Thus, cord fibrin clots have an impaired response to thrombolytic agents secondary to low levels of plasminogen.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for Individual Female Host Specificity in 2 Australian Bronze-Cuckoos (Chrysococcyx Spp)

Australian evidence suggests that the host segregation of cuckoos has evolved as a result of interspecific competition between cuckoo species. However, this does not explain how female cuckoos manage to deposit their eggs in nests of the correct host. Data are presented on the laying patterns of the cuckoos Chrysococcyx basalis and C. lucidus from an 8-year study of these species in Western Australia and three scenarios for the mechanisms of host selection are discussed.

The comparison of the pharmacokinetics of a low molecular weight heparin in the newborn and adult pig

Standard heparin (SH) is frequently used in the sick neonate to prevent catheter related thrombosis. SH can cause significant bleeding complications in the adult and its use in the neonate is linked to an increased incidence of intraventricular hemorrhage. Recently available low molecular weight heparins (LMWH) offer potential advantages over SH in the adult by exhibiting a longer half life and decreased bleeding side effects compared to SH. Whether LMWHs would offer similar therapeutic advantages to the sick neonate is unknown. Using the porcine model of neonatal hemostasis we measured the pharmacokinetics of a LMWH (Choay 222) in the pig (ATIII level: 100%), in the piglet (ATIII level:50% of adult) and in the piglet given exogenous ATIII. All pigs were bolused with 125I-LMWH (5, 25 or 100 anti-factor Xa units/kg) and blood samples collected for the measurement of 125I-radioactivity, and anti-factor Xa activity. The half life of LMWH, measured as either 125I-radioactivity or as anti-factor Xa activity, was not dose dependent and was similar in pigs and piglets; however, the volume of distribution was greater in the piglet resulting in an increased total clearance compared to the pig. As well, the supplementation of the piglet with exogenous ATIII did not influence the pharmacokinetics of LMWH. The half life of the LMWH in both pigs and piglets was approximately twice as long as previously reported values for SH in the same animal model. Thus the longer half life of LMWH in the piglet, and the similarity of the half life in piglets and pigs suggest that LMWH may have a therapeutic advantage in the newborn over SH.

Heparin clearance and ex vivo recovery in newborn piglets and adult pigs

The newborn infant requires more heparin per kg body weight than the adult to achieve similar heparin plasma levels. Possible mechanisms include altered heparin pharmacokinetics and/or a decreased expression of anticoagulant activity of heparin in new-born plasma because of low levels of antithrombin III (AT-III). We measured the pharmacokinetics and the anticoagulant activity of heparin in the pig (AT-III level: 100%), in the piglet (levels of AT-III: 50% of adult) and the piglet given exogenous porcine AT-III. All pigs were bolused with 125I-heparin (25 or 100 units/kg) and blood samples collected for the measurement of 125I-radioactivity, and antifactor Xa activity. The half-life of 125I-heparin was dose-dependent and similar in pigs and piglets; however, the volume of distribution was greater in the newborn resulting in an increased total clearance compared to the pig. The anti-factor Xa activity disappeared earlier in the piglet than in the pig. Both the kinetics and the absolute recovery of anti-factor Xa activity were normalized to pig values (after correction for different volumes of distribution) when the piglets were infused with exogenous AT-III. Thus apparent heparin resistance of the newborn is due to both an increased volume of distribution and the low AT-III level which limits the measurement of the anticoagulant activity of heparin in conventional anti-factor Xa assays.

Antithrombotic properties of heparin in a neonatal piglet model of thrombin-induced thrombosis

The relative deficiency of antithrombin III (AT III) in neonatal plasma results in lower recovery of heparin in some assay systems. It is uncertain whether low AT III levels also limit the antithrombotic effects of heparin in this age group. We therefore compared the antithrombotic properties of heparin in mature pigs and newborn piglets, whose coagulation and inhibitor system closely resembles that of the human neonate. Animals were pretreated with saline, 10 or 25 U/kg heparin (n greater than or equal to 16 per age group and dose). Following an injection of 100 U/kg thrombin, systemic 125I-fibrinogen consumption and local 125I-fibrinogen incorporation into jugular venous stasis thrombi were measured. Significantly more 125I-fibrinogen was consumed in piglets than in pigs systemically (p less than 0.0001), as well as within the occluded vein segment (p = 0.0112), largely because heparin was less effective in piglets than in pigs. This neonatal resistance to heparin could not be explained by lower heparin levels in the newborn animals. However, pretreatment with AT III concentrate significantly improved the antithrombotic properties of heparin in this age group (p less than 0.0001). We conclude that physiologically low AT III levels reduce the efficacy of heparin in neutralizing thrombin activity in newborn piglets. We speculate that AT III deficiency may also limit the antithrombotic properties of heparin in newborn infants with thrombotic disease.

Identification and Development of the Nestling Cuckoos, Chrysoccyx-Basalis and Clucidus-Plagosus, in Western-Australia

Criteria for distinguishing between the nestlings of the cuckoos, Chrysococcyx basalis and C. lucidus plagosus, are presented. Aging parameters based on three body measurements are given. The development of the feather tracts is outlined.