PEDIATRIC ANESTHESIA: Research Report
The hemostatic system of neonates is quantitatively deficient at birth, and many coagulation variables constantly change over the first months of life (1–3). Furthermore, reference values for coagulation tests in preterm neonates with a gestational age (GA) younger than 30 wk are limited (2). Thrombelastography (TEG®) appears to provide valuable additional information on the coagulation status of patients. TEG® enables a rapid global assessment of hemostatic function (4) and also assesses platelet function and fibrinolysis, which are not generally available from routine coagulation tests. The aim of this study was to use TEG® to assess the coagulation system of preterm and term neonates.
With local ethics committee approval, residual blood from routine coagulation testing of neonates admitted to the neonatal intensive care unit of the Vienna University Children's Hospital from June 1999 to July 2000 was used to perform TEG® measurements. Neonates were classified as clinically stable and eligible for inclusion into the study group after exclusion of the following conditions: asphyxia (activity, pulse, grimace, appearance, and respiration score <7 at 5 min and umbilical cord pH <7.10); evidence of sepsis (C-reactive protein >1.1 mg/dL and immature/total neutrophil ratio >0.15) 72 h before and after testing; laboratory evidence of disseminated intravascular coagulation (platelet count <140,000/μL and fibrinogen level <150 g/L); mechanical ventilation or supplemental oxygen ≥30%; known bleeding disorder, acute hemorrhage, or intraventricular hemorrhage higher than Grade II 72 h before and after testing; surgical interventions <7 days before testing; and transfusion of blood or plasma components before testing, except for former preterm infants, who did not receive transfusions less than 3 days before testing. All neonates received 1 mg of IV vitamin K daily during the first 3 days of life and received 2 mg once orally at 4 wk. All infants with a birth weight less than 1300 g received erythropoietin 300 IU/kg body weight per day. Infusion fluids contained unfractionated heparin 0.5 IU/mL.
TEG® measurements were performed with a computerized Coagulation Analyzer (Hemoscope, Skokie, IL) within 30 min after sampling. Samples of 300 μL of citrated whole blood were incubated with 20 μL (1 IU) of heparinase I (Sigma-Aldrich, Taufkirchen, Germany) for 60 s and were then recalcified with 40 μL of 0.645% CaCl2. Reaction time (r), coagulation time, angle α, maximum amplitude (MA), elastic shear modulus, and fibrinolysis after 60 min were assessed. Platelet and red blood cell counts, plasma fibrinogen, and prothrombin time were also determined.
Infants were subdivided into 4 groups by age: severely preterm (GA, 27–31 wk; postnatal age, ≤48 h), moderately preterm (GA, 32–36 wk; postnatal age, ≤48 h), term (GA, 36–40 wk; postnatal age, ≤48 h), and former preterm (corrected GA, 34–40 wk; post-natal age, 31–90 days). Fourteen healthy adult volunteers served as a control group.
Differences among the groups were assessed with analysis of variance, followed by Bonferroni's post hoc test. P < 0.05 was considered significant; data are expressed as mean ± SD.
Two hundred seventeen TEGs® were performed during the study period, of which 40 were analyzed for the study. The remaining 177 infants either were not clinically stable or could not be assigned to either group. Demographic data are given in Table 1, and coagulation variables are summarized in Table 2.
When compared with the adult group, all infants except for the moderately preterm infants showed significant shortenings of r. In the group of former preterm infants, all TEG® variables except for MA and fibrinolysis after 60 min indicated an accelerated coagulation. The prothrombin time was decreased in both the preterm and term groups compared with the adult and former preterm groups. The clot strength, as assessed with elastic shear modulus, was increased in former preterm infants compared with all other groups.
In this study, the coagulation of preterm, term, and former preterm neonates was assessed with TEG® and compared with that of healthy adults. Our data indicated no defects in coagulation in any group of infants with heparinase-modified TEG®.
The discrepancy between the known reduced activity of most coagulation factors and the functional integrity of the coagulation system in neonates may be explained by two circumstances (1–3). First, plasma levels of inhibitors (antithrombin and proteins C and S) are reduced in neonates, whereas major procoagulant factors such as fibrinogen, factor V, factor VIII, and von Willebrand factor (vWF) reach 70% to 140% of adult values in neonates (1,2). This unusual balance between procoagulants and inhibitors might be reflected in a shorter r on TEG® in neonates compared with adults. Second, plasmatic coagulation tests do not assess the procoagulant effect of platelets. Preterm and term neonates exhibit high plasma levels of vWF (1,2), which mediate the procoagulant activity of platelets. Moreover, neonatal vWF is more multimerized and has a higher functional (collagen-binding) activity than adult vWF (5). Similar to TEG®, other whole-blood coagulation tests, such as bleeding time and platelet function analyzer, showed a more coagulable state in neonates compared with adults (6–8).
Increased levels of vWF may also contribute to an MA on TEG® equal to that of adults, despite the known hyporeactivity of platelets during the first two weeks of life (9–11). The increased MA in former preterm compared with preterm and term infants might reflect the recovery of intrinsic hyporeactivity of neonatal platelets.
The group of former preterm infants showed accelerated clotting compared with all other groups. The cause may be that the thrombin-generation capacity in infants with coexisting diseases is more than expected (12). Furthermore, the administration of erythropoietin may have increased platelet reactivity in the former preterm infants (13,14).
The study is limited by the small number of patients recruited for the reference groups. To minimize the influence of confounding variables, strict exclusion criteria were defined. These led to the exclusion of all infants younger than 27 weeks of GA, thereby illustrating the difficulties of establishing reference ranges for coagulation tests in preterm neonates.
In conclusion, clinically stable preterm and term neonates have a functionally intact coagulation system when assessed by TEG®. TEG® appears to be a promising point-of-care test in neonates because it offers the advantages of a small sample volume and gives global information on the coagulation system, including platelet function and fibrinolysis.
We thank Viki Valenta and Mascha Kwatschewski for their enthusiasm and cooperation in this trial.
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© 2004 International Anesthesia Research Society
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