Antithrombotic and Bleeding Effects of a New Synthetic Direct Thrombin Inhibitor and of Standard Heparin in the Rabbit

Venous thrombosis produced in the vena cava of rabbits by vascular damage and stasis

The main objective of these experiments was to develop and characterize a new experimental model of venous thrombosis, and determine whether a combination of vascular wall damage (crushing with hemostat clamps) and prolonged stasis produced more reproducible clots than prolonged stasis per se. Rabbits were laparotomized, and a segment of the vena cava was dissected free and looped with two silk ligatures, 2.5 cm apart. The proximal tie was ligated 5 min after release of the clamps; the distal tie applied shortly thereafter, trapping a volume of blood in the isolated segment. At 2 hr after ligation, the isolated venous sac was excised and examined for the presence of a clot. Large, well-formed clots, which could be readily transferred and weighed, were invariably observed in the "clamp" and "no clamp" groups, the latter being a sham control. Mean clot weights did not differ in the two groups (23.1 +/- 1.6 versus 30.8 +/- 5.4 mg dry weight, clamped versus no clamp, respectively, p greater than 0.05). However, the precision of the method was improved significantly (p less than 0.005) by clamping as determined by homogeneity of variance testing. Time-course studies showed that a considerable lag period (about 60 min) preceded development of a detectable clot, and that the thrombus evolved rapidly during the interval of 60-90 min postligation. The location of the small clots at 60 min in clamped segments, as well as the failure of prolonged (120 min) stasis without caval isolation to cause substantial thrombi, strongly suggests that clot formation attributed to "stasis per se" is in fact due to focal vascular lesions created at the tie-down points. The present study is also the first report of blockade of localized venous thrombosis by recombinant tissue factor pathway inhibitor (rTFPI). When given as an i.v. bolus 20 min prior to ligation, rTFPI at 400 and 800 micrograms/kg completely blocked formation of the thrombus or greatly reduced its size in five of the six animals tested.