Overview of venous thrombogenesis

Update on the lower extremity venous ultrasonography examination

Lower extremity venous ultrasonography is an accurate method to diagnose acute deep venous thrombosis (DVT). Recurrent DVT is often difficult diagnosis. The decision to order ultrasonography can be based on pretest risk assessment. If the ultrasonography study is negative, the report may recommend follow-up for patients whose clinical condition changes or for patients with specific risks. Lower extremity venous ultrasonography is the gold standard for diagnosis of DVT. It is accurate and objective, and because the clinical assessment of patients is limited and its potential complication, pulmonary embolism, is significant, the impact of a positive and negative test is high.

Hypercoagulable state, pathophysiology, classification and epidemiology

Hypercoagulable state is not a uniform disease. It is a complex condition with an abnormal propensity for thrombosis that may or may not lead to thrombosis, depending on complex gene-gene and gene-environment interactions. The prevalence of the hypercoagulable state depends on the ethnicity and clinical history of the population being studied. The consequences of a hypercoagulable state due to thrombosis of veins and arteries are the most important cause of sickness and death in developed countries at present. Primary hypercoagulable state is an inherited condition caused by the reduced level of natural anticoagulants due to a qualitative defect or quantitative deficiency of an antithrombotic protein, or increased concentrations or function of coagulation factors. Most of the inherited abnormalities recognized to date have little or no effect on arterial thrombosis and are associated primarily with venous thromboembolism. Arterial thrombosis usually develops as a complication of atherosclerosis and patients usually have more than one traditional risk factor. Secondary hypercoagulable states generally occur as a result of a large number of transient or permanent acquired conditions that increase the tendency for formation of blood clots. New epidemiological data and clinical trials suggest that many acquired risk factors in the pathophysiology of arterial and venous thrombosis overlap and coexist for both disorders.

Vascular bed-specific thrombosis

Hemostasis represents a finely tuned balance between procoagulant and anticoagulant forces. An imbalance of these forces may lead to clinically significant disease, including arterial, venous and/or microvascular thrombosis. The vast majority of hypercoagulable states are associated with local thrombus formation. The goal of this review is to discuss the mechanisms underlying site-specific thrombosis.

Laparoscopic ligation of the infrarenal vena cava in combination with transfemoral thrombin infusion: a new animal model of chronic deep venous thrombosis

OBJECTIVE

We describe a new technique to create an animal model of chronic venous thrombosis. The morphological and histological properties of the resulting thrombi are described.

METHODS

Thirteen pigs underwent laparoscopic ligation of the infrarenal vena cava in combination with transfemoral thrombin infusion. After 1, 3, 6, 9, 12 and 15 days, respectively, two animals were killed and the thrombosed vein segments were explanted. After recording their weight and dimensions, the thrombi underwent histological examination by light microscopy.

RESULTS

In all 13 cases, the procedure was completed laparoscopically and all 13 animals survived the procedure. While 12 pigs (92%) had an uneventful postoperative course, one animal died on the first postoperative night of an unknown cause. Autopsy revealed correct placing of the ligature with occlusive thrombosis of the inferior vena cava and the iliac veins in 12 animals, in one animal the ligature had been incorrectly placed around the origin of the right iliac vein with thrombosis limited to that vessel. Histological evaluation demonstrated mixed thrombi that showed increasing signs of organisation with advancing age.

CONCLUSIONS

Laparoscopic ligation of the infrarenal vena cava in combination with transfemoral thrombin infusion is a safe and reliable way to produce chronic venous thrombosis in an animal model. The resulting thrombi are comparable to human deep venous thrombosis in terms of extent, size and organisation process.

Rheological analyses of coagulation of blood from different individuals with special reference to procoagulant activity of erythrocytes

In our previous papers, we reported that factor IX (FIX), when activated by erythrocyte membranes, causes coagulation. We have identified and characterized the FIX-activating enzyme located in normal human erythrocyte membranes. In this paper, to examine physiological and pathological significances of procoagulant activity of erythrocytes, coagulation of blood obtained from different individuals was analyzed by means of a rheological technique. In more than 65% of subjects including normals and patients, the initiation of coagulation seemed to be governed by erythrocytes. Coagulation of whole blood and platelet-free plasma supplemented with erythrocytes had a tendency to occur rapidly in the elderly. It was suggested that the concentration of FIX-activating enzyme on erythrocyte membranes for aged donors was somewhat higher than that for young ages. Propagation reactions on erythrocyte membranes (i.e. factor X activation leading to thrombin generation after FIX activation) was slower than that on platelet membranes. Moreover, the propagation reaction on erythrocyte membranes was greatly dependent on individuals, whereas that on platelet membranes was not so much. Our study demonstrates that the activation of FIX by erythrocytes and subsequent propagation reaction on platelet membranes may be important for initiating and controlling blood coagulation reactions.

Activation of factor IX by erythrocyte membranes causes intrinsic coagulation

As an extension of our earlier work, procoagulant activity of erythrocyte [red blood cell (RBC)] membrane was examined using biochemical and rheological techniques. Western blot analysis of coagulation factors incubated with erythrocytes (RBCs) showed that only factor IX (FIX) was activated by RBC membranes in the presence of calcium ions. A fluorogenic assay suggested that activated FIX is capable of activating factor X. A preliminary crude extraction of the substance from RBC membranes suggested that a FIX-activating enzyme may be located on the RBC membrane. The initiation of FIX activation by RBCs was enhanced by an elevation in hematocrit. Moreover, the rate of FIX activation by RBCs from normal pregnant women and diabetic patients was much faster than that from normal subjects. In addition, glucose treatment of normal RBCs resulted in the increase in procoagulant activity. It is suggested that FIX activation by RBC membranes may serve as a triggering mechanism for blood coagulation, although further study will be required to clarify the putative FIX activating enzyme on the RBC membrane and to permit more extensive physiological experiments to be performed.

Prototype percutaneous thrombolytic device: preclinical testing in subacute inferior vena caval thrombosis in a pig model

PURPOSE

To develop an animal model of subacute inferior vena caval (IVC) thrombosis and apply this model in evaluating the safety and efficacy of a prototype percutaneous thrombolytic device for restoring patency.

MATERIALS AND METHODS

In 11 pigs, a stent with a ligature in the middle was placed in the IVC. Thrombin was injected to induce thrombosis. Hemostasis was achieved by using an occlusion balloon. The stent was ligated to prevent thrombus migration. Five to 8 days after thrombus induction, the ligature was broken and the stent fully deployed. In 10 animals, thrombectomy was performed by using the percutaneous thrombolytic device. A vena caval filter was inserted at the beginning of each declotting procedure. Thrombus removal percentage was estimated and pulmonary angiograms obtained to detect embolism before and after thrombectomy. The IVC was analyzed histologically. To determine thrombus composition, one animal was sacrificed without thrombectomy. Concerning procedural safety, failure of the stent delivery system, stent migration, and venous perforation due to balloon inflation and the stent placement or thrombectomy procedure were evaluated.

RESULTS

Thrombus creation was successful in all animals. Fragmentation led to 75%--100% thrombus removal with flow restoration in all cases. There were no episodes of stent delivery failure, stent migration, or venous perforation. No significant pulmonary embolism was observed. In one case, a vessel dissection was identified at histologic examination.

CONCLUSION

In this animal model of IVC thrombosis, the percutaneous thrombolytic device is effective and safe for clot removal.

Thrombosis and the pharmacology of antithrombotic agents

OBJECTIVE

To provide an overview of the current state of knowledge regarding the physiology of hemostasis, the pathophysiology of thrombosis, and the pharmacology of antithrombotic agents.

DATA SOURCES

A MEDLINE search was conducted to identify pertinent literature published since 1984. Recently published textbooks devoted to the subjects of hematology, hemostasis, and thrombosis also were reviewed, particularly their bibliographies. The bibliographies of selected review articles also were reviewed.

STUDY SELECTION

As the amount of literature was vast, only the most significant and noteworthy published studies were reviewed. Review articles and book chapters authored by researchers of international reputation also were reviewed.

DATA EXTRACTION

Identified studies from the primary literature and selected reviews were analyzed carefully. Information regarding hemostasis, thrombosis, and antithrombotic drugs was extracted. Particular attention was given to data regarding drugs currently available or soon to be available on the US market.

DATA SYNTHESIS

Knowledge regarding the regulation of blood coagulation has expanded substantially in recent years. Hemostasis involves the dynamic interplay of numerous intravascular constituents, including the vessel wall, circulating procoagulants and anticoagulants, platelets, and fibrinolytic proteins. Thrombosis is the abnormal formation of a clot within the vascular system. When sufficiently large, thrombi can prevent the flow of blood and nutrients to vital tissues. Thrombosis is associated with many common diseases and is among the leading causes of death in developed countries. Many drugs are now available to prevent the formation and propagation of thrombi. These agents work by different pharmacologic mechanisms and are useful in different clinical situations.

CONCLUSIONS

Thrombosis research has increased our understanding of the pharmacology of antithrombotic drugs and promoted the discovery of new agents targeted more specifically toward the critical steps in pathologic clot formation. New agents have the potential for greater efficacy and fewer adverse effects. An increased understanding of hemostasis, thrombosis, and the pharmacology of antithrombotic drugs should enable the clinician to use these agents appropriately.

Mesothelial cell adherence to vascular prostheses and their subsequent growth in vitro

Cell seeding may decrease the thrombogenicity of implanted vascular grafts, but its application is hampered by the limited availability of autologous endothelial cells. Human peritoneal mesothelial cells have blood flow supporting qualities and are readily available. This study investigated the adherence of mesothelial cells to vascular prostheses and their subsequent growth in vitro. Circular pieces of various vascular prosthetic materials were seeded with 51Chromium-labeled mesothelial and endothelial cells and left for either 5, 15, 30, 60, and 120 minutes. The unattached cells were removed and the degree of cell attachment was measured. The number of mesothelial cells to Dacron increased during the first 60 min up to 35.2% of the seeded inoculum whereafter a plateau was reached. Scanning electron microscopy showed spread mesothelial cells adherent to the Dacron fibers. A significant increase in adherence was observed after preincubation of Dacron with 10 micrograms/mL fibronectin, but no improvement was found after preincubation with human serum albumin or gelatin. Mesothelial cells adhered better to Gel-coated than to Gel-sealed or plain Dacron. The adherence of mesothelial cells to ePTFE (Teflon) was significantly poorer. No significant differences in adherence were found between mesothelial and endothelial cells. Mesothelial cell growth on Dacron resulted in a modest increase in the number of viable cells during 27 days, which implies biocompatibility of Dacron and mesothelial cells in vitro.

Antithrombotic activity of recombinant tick anticoagulant peptide and heparin in a rabbit model of venous thrombosis

An in vivo rabbit model of venous thrombosis which includes physiological blood flow was used to compare the efficacy of the potent and specific factor Xa inhibitor recombinant tick anticoagulant peptide (rTAP) with standard heparin in the prevention of venous thrombus formation. In anesthetized rabbits, an autologous thrombus was induced with thrombin in a jugular vein and the increase in thrombus size was determined by measuring the accretion of intravenously injected [125I]fibrin(ogen) onto the developing thrombus. The effects of rTAP on hemostasis were monitored by changes in APTT values and template bleeding times. Inhibition of thrombus formation by an intravenous bolus followed by infusion of either rTAP or heparin exhibited a dose-response relationship with an IC50 of 0.9 micrograms/kg/min and 0.12 units/kg/min, respectively. At the IC50 doses, both rTAP and heparin inhibited fibrin(ogen) deposition without any significant effect on APTT or bleeding times. Bleeding times were modestly elevated at the fully efficacious doses of rTAP and heparin. Significant changes in APTT (1.9 +/- 0.3 fold over baseline) were only evident at the highest dose of rTAP while heparin caused a significant dose-dependent increase from 1.3 +/- 0.2 to greater than 4.2 +/- 0.6 fold over baseline. Therefore, in this rabbit model of venous thrombosis, specific inhibition of factor Xa by rTAP is an effective antithrombotic mechanism that does not require changes in systemic hemostatic parameters.

Effect of ticlopidine and other antithrombotics on the venous thrombosis induced by endothelial damage of jugular vein in rats

A venous thrombosis model was produced by vessel wall damage generated by freezing a small segment of the rat jugular vein. The process of thrombus formation was investigated by electron microscopic study. Ultrastructural studies demonstrated that the initiation of thrombus formation could be deendothelialization caused by freezing of vessel. When blood flow was reestablished, platelets adhered to subendothelium within 1 min. Then platelets aggregated on the adhering platelets, and fibrin net was formed. Finally, thrombi composed predominantly of fibrin and red blood cells with platelet aggregates and leukocytes were generated. An anti-platelet agent, ticlopidine, revealed a potent antithrombotic effect in this model. Because ticlopidine decreased the number of platelet aggregates, reduced the size of aggregates, and inhibited platelet degranulation, it is conceivable that platelet aggregation in early phase of thrombus formation plays a crucial role even in venous thrombosis model. A synthetic thrombin inhibitor, argatroban, also showed a potent antithrombotic effect, but a thrombolytic agent, urokinase, was less effective. In conclusion, this model is both platelet- and coagulation-dependent.

Selective inhibition by a synthetic hirudin peptide of fibrin-dependent thrombosis in baboons

To determine the importance of the thrombin substrate recognition exosite for fibrinogen binding in the formation of both arterial and venous thrombi, we evaluated the antithrombotic effects of the tyrosine-sulfated dodecapeptide from residues 53-64 of hirudin (H peptide) in a nonhuman primate model. This peptide was studied because it inhibits thrombin cleavages of fibrinogen by simple competition without blocking enzyme catalytic-site function. When an exteriorized arteriovenous access shunt model was used in baboons (Papio anubis), thrombus formation was induced by placing a thrombogenic device made of (i) a segment of tubing coated covalently with type I collagen, which generated platelet-rich thrombi under arterial flow conditions, and (ii) two subsequent annular regions of flow expansion that produced fibrin-rich thrombi typically associated with venous valves and veins. Thrombus formation was quantified by measurements of 111In-labeled platelet and 125I-labeled fibrinogen deposition in both arterial-flow and venous-flow portions of the device. Continuous infusion of H peptide (0.5, 15, and 75 mg/kg) proximal to the device for 40 min interrupted, in a dose-response fashion, formation of fibrin-rich thrombus in the regions of disturbed flow and generation of fibrinopeptide A. In contrast, H peptide did not inhibit the capacity of platelets to deposit on the collagen surface (P greater than 0.2 at all doses) or to form hemostatic plugs (as assessed by measurements of bleeding time; P greater than 0.1 at all doses). These findings suggest that, by competitive inhibition of fibrinogen binding to thrombin, fibrin-rich venous-type thrombus formation may be selectively prevented. This strategy may be therapeutically attractive for preserving normal platelet function when conventional anticoagulant therapy is contraindicated.

Acute phase reaction, fibrinogen level and thrombus size

The influence of an acute phase reaction on the size, weight, and fibrin content of experimental venous thrombi was examined in 10 pairs of rabbits. Jugular vein thrombosis was provoked by venous stasis plus mechanical injury 36 hours after one member of each pair received 0.5 ml/kg sterile turpentine in olive oil by subcutaneous injection. Fibrinogen level, factor VIII activity, and antithrombin III activity were significantly higher at the time of thrombus formation in turpentine treated rabbits, as were thrombus size (assessed by visual scoring), dry weight of thrombi, and their fibrin content (derived by measuring 125I-fibrinogen incorporation). In addition, the fibrinogen concentration correlated significantly with size, weight, and fibrin content of thrombi when results from turpentine treated and control animals were pooled, suggesting that plasma fibrinogen concentration at the time of thrombus formation may strongly influence the extent of thrombosis. This effect could help explain observations of a "hypercoagulable state" after injury.