Antithrombotic benefit of subendothelium-bound urokinase: an experimental study

Comparison of arterial and venous patency in a rat model of subendothelium-stimulated thrombosis

The effectiveness of anticoagulants and platelet aggregation inhibitors was compared using comparable rat models of arterial and venous thrombosis. A mechanical endothelium-denuding injury was created on the lumenal surface of donor Lewis rat carotid arteries. These were cut into 4-5 mm lengths and grafted into femoral veins and arteries of recipient syngeneic rats using microvascular anastomotic techniques. Recipients received either systemic heparin, or aspirin with dipyridamole, or saline (control). In the arteries, the 1-day patency rate was 94% in the heparin-treated rats, but only 50% in the aspirin/dipyridamole group and 44% in the control group. The venous patency rate was 56% in the heparin group, 31% in the aspirin/dipyridamole group, and 0% in the control group. This unique model for comparing thrombosis in arteries and veins shows that anticoagulation is more effective than inhibition of platelet aggregation in the rat arterial system, with less of a differential effect in the venous system.

Effect of low dose aspirin on thrombus formation at arterial and venous microanastomoses and on the tissue microcirculation

In free flap/replantation surgery, failure is usually associated with thrombotic occlusion of a microvascular anastomosis (risk zone I) or, on occasion, flow impairment in the microcirculation of the transferred or replanted tissue (risk zone II). The objective of this study is to describe the effect of low dose aspirin on blood flow at both risk zones in microvascular surgery. Risk zone I: In rat femoral arteries and veins, thrombus formation was measured at the anastomoses using transillumination and videomicroscopy. Forty male Wistar rats were assigned in equal numbers to four groups: either arterial or venous injury with either aspirin (5 mg/kg systemically) or saline treatment. We found that aspirin significantly reduces thrombus formation at the venous anastomosis (p = 0.001). Risk zone II: In the isolated rat cremaster muscle downstream from an arterial anastomosis, we measured capillary perfusion, arteriolar diameters, and the appearance of platelet emboli for 6 hours in the muscle microcirculation. Sixteen male Wistar rats in two equal groups received either aspirin (5 mg/kg systemically) or saline. We found that in aspirin-treated animals, capillary perfusion is significantly (p = 0.002) improved, whereas arteriolar diameters and emboli only slightly increased. In conclusion, low dose aspirin inhibits anastomotic venous thrombosis and improves microcirculatory perfusion in our rat model. These studies provide quantitative data confirming and clarifying the beneficial effects of low dose aspirin in microvascular surgery.

Prevention of microvascular thrombosis by topical application of recombinant tissue factor pathway inhibitor

Tissue factor pathway inhibitor is a naturally occurring protein inhibitor of factor X and the tissue factor-factor VII complex of the extrinsic pathway of coagulation. The potential of tissue factor pathway inhibitor as a topical antithrombotic agent was evaluated in a rabbit model of thrombosis that combined intimal injury, anastomosis, and a twisted pedicle. In 207 rabbit ears, a near-complete amputation was performed, preserving the central ear artery and vein. The central ear artery was transected, the intima was removed mechanically over a 1-cm length, the artery was anastomosed, and the ear was twisted 360 degrees, wrapping the intact vein around the artery. Before recirculation, the lumen was irrigated on a blinded, randomized basis with either hirudin (100 or 500 units/ml), heparin (50 or 100 units/ml), tissue factor pathway inhibitor (10, 40, 125, or 250 microgram/ml), heparin and tissue factor pathway inhibitor together, or vehicle (control). Upon arterial reflow, the ears were observed for 7 days. Patency rates after 7 days were as follows: hirudin, 30 and 55 percent; heparin, 43 and 50 percent; tissue factor pathway inhibitor, 75 and 90 percent; heparin and tissue factor pathway inhibitor, 75 percent; and vehicle, 6 percent. The higher concentrations of tissue factor pathway inhibitor led to significantly higher patency rates than heparin, hirudin, or control solutions. Electron microscopic evaluation of specimens irrigated with gold- labeled tissue factor pathway inhibitor revealed the inhibitor bound to the injured intimal surface for at least 3 days postoperatively. Coagulation studies showed no change in the clotting profile upon intravascular infusion with tissue factor pathway inhibitor even at the highest dose used topically. We conclude that tissue factor pathway inhibitor is a more effective topical antithrombotic agent than either heparin or hirudin.

Effect of phosphate concentration in buffered irrigating solutions on arterial thrombosis

The concentration of phosphate in buffered saline irrigation solutions was evaluated for its effect upon the incidence of arterial thrombosis. A rabbit thrombosis model of combined intimal abrasion and twisted pedicle in the rabbit central ear artery was used. Higher concentrations of phosphate (20 mM) in normal saline yielded a lower thrombosis rate at 7 days postoperatively (50%) in comparison to 5 mM phosphate (89% thrombosis rate; P < 0.05). A standard phosphate-buffered salt solution (Dulbecco's; 9.5 mM phosphate with potassium, calcium, and magnesium salts) also had a high thrombosis rate (91%). This study demonstrates that subtle variations in the irrigation solution can have profound effects upon arterial thrombosis.

[Amputation injuries of the upper extremity--early complications after replantation and revascularization]

Within a time of ten years 144 patients with difficult injuries on their hands were operated. These were 95 replantations and 102 revascularizations. 26 patients had to be operated again because of arterial and venous thrombosis. 46% of our patients had early vascular complications already within 24 hours. The frequency of thrombosis was increasing with the accompanying soft tissue injury and therefore with the gravity of the trauma. Avulsion injuries had of course the worst prognosis. After diagnosis of vascular occlusion within the replanted finger there should be a quick surgical revision of the anastomosis. With the help of application of urokinase into the artery we could create the circulation without any new production of the anastomosis in two cases. With a subtile technique of the disposition of the anastomosis and two venous connections to one artery with the replanted fingers we could reduce the rate of thrombosis and improve the results.

Maintenance of patency after vascular trauma by topical irrigation with a peptide homologous to the carboxy-terminus of the fibrinogen gamma chain

Synthetic peptides with amino acid sequences homologous to the carboxy-terminal sequence of the gamma chain of human fibrinogen were evaluated for their capacity to inhibit thrombus development. A 21-residue peptide effectively inhibited surface-mediated fibrin clot propagation in vitro. Since this effect was localized to the surface, we proceeded with in vivo evaluation, using site-specific topical application only. In a rat model of arterial trauma and microvascular repair, topical application of the 21-residue peptide significantly reduced the rate of thrombosis to 17%, as compared with a control rate of 83%; in comparison, a 12-residue homologue reduced the thrombosis rate only modestly to 61%, which was not significant compared with the control rate. These results indicate the feasibility of antithrombotic therapy with topically applied agents, an approach that may obviate the use of systemic anticoagulation in extremity-replantation surgery.

The de-endothelialized rat carotid arterial graft: a versatile experimental model for the investigation of arterial thrombosis

A novel model of arterial thrombosis was developed. A mechanical endothelium-denuding injury was created (using a scalpel blade) on harvested, freezer-stored rat carotid arteries. Vessel length of 5 mm. were grafted into the femoral arteries of recipient Sprague-Dawley rats using microvascular anastomotic technique. Patency rates in untreated animals were compared with those in animals receiving systemic aspirin or heparin. The control group patency after 2 hours of flow was 15%, while grafts in aspirin- and heparin-treated animals achieved 35% and 95% patency rates, respectively. Uninjured non-frozen carotid grafts in untreated animals yielded a 95% patency rate, while frozen grafts achieved an 80% patency. Therapeutic levels of aspirin, heparin, and urokinase were confirmed through tail bleeding and whole blood clotting tests, as well as platelet aggregation studies and scanning electron microscopy of the graft lumenal surfaces. A long-term series using syngeneic grafts placed in recipients (Lewis-to-Lewis) and employing systemic heparinization demonstrated maintenance of patency for 4 weeks. Scanning electron microscopy revealed good re-endothelialization, well advanced by one week. Histology confirmed the regrowth of endothelial cells, but showed sparse cellular repopulation of medial and adventitial layers. The mechanical injury model was compared to enzymatic de-endothelialization (using trypsin or collagenase), for which patency rates were similar (10% and 0%, respectively). Trypsin de-endothelialized vessels were tested in vitro for the amount of active trypsin remaining bound to the lumenal surface; no detectable activity was found when trypsin inhibitor was applied following trypsin treatment. The versatility of allowing both in vitro evaluation and in vivo patency assessment demonstrates the uniqueness and value of this new model, offering an avenue toward more direct investigations of surface-mediated thrombotic processes.