Summary
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.
Keywords
Heparin - Newborn infant - Piglets - Antithrombotic properties
