Summary
Pharmacological thromboprophylaxis is increasingly being used after caesarean section
to prevent venous thromboembolism. Although a variety of low molecular weight heparins
(LMWH) have been used no comparative study exists on their effects on the haemostatic
system in this situation. Furthermore, their antithrombotic effect may be mediated
through effects other than their inhibitory effect on activated factor X. We compared
the plasma anti-factor Xa activity, plasma concentration of tissue factor pathway
inhibitor (TFPI) and the reduction in plasma thrombin-antithrombin (TAT) complex concentration
in 30 women randomised to receive either dalteparin 5,000 IU anti-Xa once daily (n
= 10), enoxaparin 4,000 IU anti-Xa once daily (n = 10) or tinzaparin 50 IU/kg anti-Xa
(average dose 3,650 anti-Xa units) once daily (n = 10) following caesarean section.
Sampling occurred at 0, 1, 3, 6, 12 and 24 h relative to time of dosing. All preparations
produced an increase in mean anti-Xa assay (p <0.0001), a reduction in mean TAT (p
<0.05) and an increase in mean TFPI concentration (p <0.05). Analysis of variance
(ANOVA) revealed a significant difference between the LMWHs in terms of mean anti-factor
Xa activity (p <0.005) and reduction in plasma TAT concentration (p <0.005). Post
hoc analysis indicated that the anti-Xa values of the groups receiving enoxaparin
and dalteparin were significantly higher than those of the group receiving tinzaparin
(p <0.05), but not significantly different from each other. Post hoc analysis of the
reduction in plasma TAT concentration showed the reduction to be significantly less
in the group receiving enoxaparin compared to the dalteparin and tinzaparin groups
(p <0.05), which did not differ significantly from each other. There was no significant
difference between treatment groups with regard to plasma concentration of TFPI. These
findings demonstrate that LMWHs differ in their effects on haemostatic parameters
including thrombin generation as assessed by TAT. The increase in TFPI may be an additional
mediator of LMWH’s antithrombotic effects. Although these findings demonstrate that
LMWHs differ in their haemostatic effects, this does not necessarily infer a clinical
difference between these agents.
Keywords
Low molecular weight heparin (LMWH) - pregnancy - anti-factor Xa - tissue factor pathway
inhibitor - thrombin-antithrombin (TAT) complex