Amelioration of Cognitive and Behavioral Deficits after Traumatic Brain Injury in Coagulation Factor XII Deficient Mice.

Based on recent findings that show that depletion of factor XII (FXII) leads to better posttraumatic neurological recovery, we studied the effect of FXII-deficiency on post-traumatic cognitive and behavioral outcomes in female and male mice. In agreement with our previous findings, neurological deficits on day 7 after weight-drop traumatic brain injury (TBI) were significantly reduced in FXII-/- mice compared to wild type (WT) mice. Also, glycoprotein Ib (GPIb)-positive platelet aggregates were more frequent in brain microvasculature of WT than FXII-/- mice 3 months after TBI. Six weeks after TBI, memory for novel object was significantly reduced in both female and male WT but not in FXII-/- mice compared to sham-operated mice. In the setting of automated home-cage monitoring of socially housed mice in IntelliCages, female WT mice but not FXII-/- mice showed decreased exploration and reacted negatively to reward extinction one month after TBI. Since neuroendocrine stress after TBI might contribute to trauma-induced cognitive dysfunction and negative emotional contrast reactions, we measured peripheral corticosterone levels and the ration of heart, lung, and spleen weight to bodyweight. Three months after TBI, plasma corticosterone levels were significantly suppressed in both female and male WT but not in FXII-/- mice, while the relative heart weight increased in males but not in females of both phenotypes when compared to sham-operated mice. Our results indicate that FXII deficiency is associated with efficient post-traumatic behavioral and neuroendocrine recovery.

Heterozygous FXII deficiency is not associated with an increased incidence of thrombotic events: Results of a long term study.

OBJECTIVE: To investigate the incidence of thrombotic events in patients heterozygous for FXII deficiency during a long observation period. PATIENTS AND METHODS: 103 heterozygotes for FXII deficiency, 49 female and 54 male were followed for 19.6 years (range 5-32 years). As controls 103 unaffected family members of same sex and similar age (±5 years) were enrolled. The thrombotic end points were: myocardial infarction, deep vein thrombosis and ischemic stroke. The mean Factor XII level in the heterozygotes was 48.5%: range (35-60%) that of control was 96.5% (range 70-155%). The heterozygotes showed one myocardial infarction, two deep vein thromboses and no ischemic stroke. The unaffected family members observed 2 myocardial infarctions, one deep vein thrombosis and one ischemic stroke. There were seven deliveries (five women) among the heterozygotes and six (five women) among the controls. Furthermore, four and five surgical procedures were carried out in the patient and in the control group, respectively. Immobilization times for surgical procedures or pregnancies were 50 days and 57 days for the heterozygotes and the unaffected family members, respectively. Heterozygotes for FXII deficiency did not show an increased incidence of thrombotic events as compared with unaffected family members during a long follow up.

Influence of factor XII deficiency on activated partial thromboplastin time (aPTT) in critically ill patients.

FXII deficiency results in spontaneous prolongation of activated partial thromboplastin time (aPTT), which is widely used to monitor thromboprophylaxis. Misinterpretation of spontaneously prolonged aPTT may result in omission of thromboembolic treatment or even unnecessary transfusion of blood products. This retrospective analysis was performed to calculate a threshold level of FXII resulting in aPTT prolongation. 79 critically ill patients with spontaneous prolongation of aPTT were included. A correlation analysis and a ROC curve for aPTT prolongation predicted by FXII level were created to find the FXII threshold level. Prolongation of aPTT was associated with disease severity. A significant inverse proportionality between FXII and aPTT was seen. A ROC curve for aPTT prolongation, predicted by FXII level (AUC 0.85; CI 0.76-0.93), revealed a FXII threshold level of 42.5%. Of our patients 50.6% experienced a FXII deficiency, in 80.0% of whom we found aPTT to be prolonged without a significantly higher bleeding rate. The FXII deficiency was more common in patients with higher SAPS3 scores, septic shock, transfusion of red blood cells and platelet concentrates as well as in patients receiving renal replacement therapy. Patients with a FXII deficiency and prolonged aPTT less often received anticoagulatory therapy although they were more severely ill. The rate of thromboembolic events was higher in these patients although the difference was not statistically significant. Of all patients with spontaneous aPTT prolongation 50.6% had a FXII level of 42.5% or less. Those patients received insufficient thromboembolic prophylaxis.

The plasma contact system, a protease cascade at the nexus of inflammation, coagulation and immunity.

The contact system is a potent procoagulant and proinflammatory plasma protease cascade that is initiated by binding ("contact")-induced, auto-activation of factor XII zymogen. Formed active serine protease FXIIa then cleaves plasma prekallikrein to kallikrein that in turn liberates the mediator bradykinin from its precursor high molecular weight kininogen. Bradykinin induces inflammation with implications for host defense and innate immunity. FXIIa also triggers the intrinsic pathway of coagulation that has been shown to critically contribute to thrombosis. Vice versa, FXII deficiency impairs thrombosis in animal models without inducing abnormal excessive bleeding. Recent work has established the FXIIa-driven contact system as promising target for anticoagulant and anti-inflammatory drugs. This review focuses on the biochemistry of the contact system, its regulation by endogenous and exogenous inhibitors, and roles in disease states. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

Acquired factor XII deficiency following transanal excision of rectal lesion by transanal minimally invasive surgery (TAMIS): a case report and literature review.

BACKGROUND: Local excision (LE) is currently one of the most effective methods used in cases of large benign polyps, not suitable for endoscopic treatment, or early-stage neoplasms. LE is also alternative to anterior rectal resection in selected patients suffering from major comorbidities and limits for major abdominal procedure. Furthermore, LE results in less pain, reduced impact on bowel function, shorter duration of hospital stay, and lower rates of morbidity, mortality and stoma creation. In particular, early data on transanal minimally invasive surgery (TAMIS) are promising, but they come from single centre case series related to small groups of patients and more data are needed to draw a final conclusion on the safety of this novel approach for transanal resection. CASE PRESENTATION: A 62-year-old woman, following a positive faecal occult blood test and with unremarkable medical history, was admitted to hospital for excision of a large flat neoplastic lesion. Endoscopic biopsy demonstrated a tubular adenoma with high-grade dysplasia and was decided to proceed with surgical excision by TAMIS. After surgery, short-term outcomes revealed prolonged activated partial thromboplastin time, undetectable factor XII activity, fever, and partial dehiscence of rectal wall defect suture. Cross-mixing studies of patient plasma show no correction in either the immediate or incubated activated partial thromboplastin time, indicating the presence of an acquired factor XII inhibitor. Activated partial thromboplastin time and factor XII improved in the following weeks without any specific therapy in addition to antibiotic therapy. CONCLUSION: This is the first report in which acquired inhibitor of coagulation factor XII is associated with a specific surgical procedure. This case has shown how trans-anal excision of rectal lesions, even when performed by minimally invasive means such as in case of TAMIS, is not free of complications. We consider the acute infection, resulting from early dehiscence of the suture, the trigger in an abnormal immune response, and inhibitor development.

Factor XII regulates the pathological process of thrombus formation on ruptured plaques.

OBJECTIVE: Atherothrombosis is the main cause of myocardial infarction and ischemic stroke. Although the extrinsic (tissue factor-factor VIIa [FVIIa]) pathway is considered as a major trigger of coagulation in atherothrombosis, the role of the intrinsic coagulation pathway via coagulation FXII herein is unknown. Here, we studied the roles of the extrinsic and intrinsic coagulation pathways in thrombus formation on atherosclerotic plaques both in vivo and ex vivo. APPROACH AND RESULTS: Plaque rupture after ultrasound treatment evoked immediate formation of subocclusive thrombi in the carotid arteries of Apoe(-/-) mice, which became unstable in the presence of structurally different FXIIa inhibitors. In contrast, inhibition of FVIIa reduced thrombus size at a more initial stage without affecting embolization. Genetic deficiency in FXII (human and mouse) or FXI (mouse) reduced ex vivo whole-blood thrombus and fibrin formation on immobilized plaque homogenates. Localization studies by confocal microscopy indicated that FXIIa bound to thrombi and fibrin particularly in luminal-exposed thrombus areas. CONCLUSIONS: The FVIIa- and FXIIa-triggered coagulation pathways have distinct but complementary roles in atherothrombus formation. The tissue factor-FVIIa pathway contributes to initial thrombus buildup, whereas FXIIa bound to thrombi ensures thrombus stability.

[Are we to pay attention to factor XII deficiency?]. / Máme venovat' pozornost' defektu koagulacného faktora XII?

Severe coagulation factor XII (FXII) deficiency is a very rare, mysterious and not well known inherited condition. Unlike other coagulation factor deficiencies, it is totally asymptomatic. Surprisingly, it does not lead to abnormal bleeding, even with major surgical procedures. The explanation for the lack of bleeding manifestations is unknown. It is suggested, but unproven, that patients are not sufficiently protected from thrombosis. FXII deficiency is usually discovered by accident through a routine coagulation testing done prior to surgery. Since FXII plays an important role in clot formation during in vitro measurements, its deficiency causes a marked prolongation of the activated partial thromboplastin time in the laboratory examination. The main concern related to FXII deficiency is the unnecessary testing, delay in health care and worry of surgical interventions that may be prompted by the abnormal laboratory result.

Selective depletion of plasma prekallikrein or coagulation factor XII inhibits thrombosis in mice without increased risk of bleeding.

Recent studies indicate that the plasma contact system plays an important role in thrombosis, despite being dispensable for hemostasis. For example, mice deficient in coagulation factor XII (fXII) are protected from arterial thrombosis and cerebral ischemia-reperfusion injury. We demonstrate that selective reduction of prekallikrein (PKK), another member of the contact system, using antisense oligonucleotide (ASO) technology results in an antithrombotic phenotype in mice. The effects of PKK deficiency were compared with those of fXII deficiency produced by specific ASO-mediated reduction of fXII. Mice with reduced PKK had ∼ 3-fold higher plasma levels of fXII, and reduced levels of fXIIa-serpin complexes, consistent with fXII being a substrate for activated PKK in vivo. PKK or fXII deficiency reduced thrombus formation in both arterial and venous thrombosis models, without an apparent effect on hemostasis. The amount of reduction of PKK and fXII required to produce an antithrombotic effect differed between venous and arterial models, suggesting that these factors may regulate thrombus formation by distinct mechanisms. Our results support the concept that fXII and PKK play important and perhaps nonredundant roles in pathogenic thrombus propagation, and highlight a novel, specific and safe pharmaceutical approach to target these contact system proteases.

A role for factor XIIa-mediated factor XI activation in thrombus formation in vivo.

Mice lacking factor XII (fXII) or factor XI (fXI) are resistant to experimentally-induced thrombosis, suggesting fXIIa activation of fXI contributes to thrombus formation in vivo. It is not clear whether this reaction has relevance for thrombosis in pri mates. In 2 carotid artery injury models (FeCl(3) and Rose Bengal/laser), fXII-deficient mice are more resistant to thrombosis than fXI- or factor IX (fIX)-deficient mice, raising the possibility that fXII and fXI function in distinct pathways. Antibody 14E11 binds fXI from a variety of mammals and interferes with fXI activation by fXIIa in vitro. In mice, 14E11 prevented arterial occlusion induced by FeCl(3) to a similar degree to total fXI deficiency. 14E11 also had a modest beneficial effect in a tissue factor-induced pulmonary embolism model, indicating fXI and fXII contribute to thrombus formation even when factor VIIa/tissue factor initiates thrombosis. In baboons, 14E11 reduced platelet-rich thrombus growth in collagen-coated grafts inserted into an arteriovenous shunt. These data support the hypothesis that fXIIa-mediated fXI activation contributes to thrombus formation in rodents and primates. Since fXII deficiency does not impair hemostasis, targeted inhibition of fXI activation by fXIIa may be a useful antithrombotic strategy associated with a low risk of bleeding complications.

Characterization of the thrombin generation potential of leukemic and solid tumor cells by calibrated automated thrombography.

BACKGROUND: Thrombin, the final enzyme of blood coagulation, is a multifunctional serine protease also involved in the progression of cancer. Tumor cells may activate blood coagulation proteases through the expression of procoagulant activities. However, specific information about the thrombin generation potential of malignant tissues is lacking. In this study we applied a single global coagulation test, the calibrated automated thrombogram assay, to characterize the specific procoagulant phenotypes of different tumor cells. DESIGN AND METHODS: Malignant hematologic cells (i.e. NB4, HEL, and K562) or solid tumor cells (i.e. MCF-7 breast cancer and H69 small cell lung cells) were selected for the study. The calibrated automated thrombo-gram assay was performed in normal plasma and in plasma samples selectively deficient in factor VII, XII, IX or X, in the absence or presence of a specific anti-tissue factor antibody. Furthermore, cell tissue factor levels were characterized by measuring antigen, activity and mRNA expression. RESULTS: In normal plasma, NB4 induced the highest thrombin generation, followed by MCF-7, H69, HEL, and K562 cells. The anti-tissue factor antibody, as well as deficiencies of factors VII, IX and XII affected the thrombin generation potential of malignant cells to different degrees, allowing differentiation of the two different pathways of blood clotting activation - by tissue factor or contact activation. The thrombin generation capacity of NB4 and MCF-7 cells was tissue factor-dependent, as it was highly sensitive to inhibition by anti-tissue factor antibody and factor VII deficiency, while the thrombin generation capacity of H69, HEL and K562 was contact activation-dependent, as no thrombin was generated by these cells in factor XII-deficient plasma. CONCLUSIONS: This study demonstrates that the calibrated automated thrombogram assay is capable of quantifying, characterizing, and comparing the thrombin generation capacity of different tumor cells. This provides a useful tool for understanding the key factors determining the global pro-coagulant profile of tumors, which is important for addressing specific targeted therapy for the prevention of thrombosis and for cancer.

Factor XI and XII as antithrombotic targets.

PURPOSE OF REVIEW: Arterial and venous thrombosis are major causes of morbidity and mortality, and the incidence of thromboembolic diseases increases as a population ages. Thrombi are formed by activated platelets and fibrin. The latter is a product of the plasma coagulation system. Currently available anticoagulants such as heparins, vitamin K antagonists and inhibitors of thrombin or factor Xa target enzymes of the coagulation cascade that are critical for fibrin formation. However, fibrin is also necessary for terminating blood loss at sites of vascular injury. As a result, anticoagulants currently in clinical use increase the risk of bleeding, partially offsetting the benefits of reduced thrombosis. This review focuses on new targets for anticoagulation that are associated with minimal or no therapy-associated increased bleeding. RECENT FINDINGS: Data from experimental models using mice and clinical studies of patients with hereditary deficiencies of coagulation factors XI or XII have shown that both of these clotting factors are important for thrombosis, while having minor or no apparent roles in processes that terminate blood loss (hemostasis). SUMMARY: Hereditary deficiency of factor XII (Hageman factor) or factor XI, plasma proteases that initiate the intrinsic pathway of coagulation, impairs thrombus formation and provides protection from vascular occlusive events, while having a minimal impact on hemostasis. As the factor XII-factor XI pathway contributes to thrombus formation to a greater extent than to normal hemostasis, pharmacological inhibition of these coagulation factors may offer the exciting possibility of anticoagulation therapies with minimal or no bleeding risk.

Factor XII deficiency evaluated by thrombin generation assay.

INTRODUCTION: Coagulation factor XII (FXII) plays a role in thrombin generation, fibrinolysis, inflammation, angiogenesis, chemotaxis and diapedesis. FXII deficiency is not associated with bleeding risk unlike other coagulation factors. MATERIALS/METHODS: We investigated thrombin generation assay (TGA) profile modification in FXII deficiency and the correlation with TGA and deficiency severity. TGA was performed in platelet poor plasma (PPP) with tissue factor (1 pmol/L) and phospholipid (4 µmol/L) standardized concentration. Thrombin generation profiles were compared in 54 patients with FXII deficiency, 25 healthy controls and 23 patients with hemophilia A (factor VIII (FVIII) deficiency. Patients with FXII deficiency were classified in three groups based on FXII activity (30-50%, 10-29%, <10%). FVIII deficiency was included as a bleeding control group. RESULTS: As expected, we found a correlation between FXII deficiency and activated partial thromboplastin time (aPTT). A decrease of thrombin generation was observed in healthy controls and all FXII deficiency groups. A decrease of endogenous thrombin potential (ETP), peak and velocity was observed in patients with FVIII deficiency compared to FXII deficiency. A decrease of thrombin generation was noted in patients with FXII deficiency and bleeding history compared to patients with FXII deficiency and thrombosis history. CONCLUSION: In this study, thrombin generation profiles were not sensitive to FXII deficiency. TGA could distinguish bleeding and thrombotic tendency in FXII deficiency. Our results should therefore be considered as exploratory and deserve confirmation.

A novel mutation in the coagulation factor 12 gene in subjects with hereditary angioedema and normal C1-inhibitor.

In hereditary angioedema with normal C1-inhibitor two different missense mutations of codon p.Thr328* in the coagulation factor 12 gene have been reported in some families. In this study a novel factor 12 gene mutation, the deletion of 72 base pairs (bp) (c.971_1018+24del72*), was identified in a family of Turkish origin, in two sisters with recurrent skin swellings and abdominal pain attacks and in their symptom-free father. This deletion caused a loss of 48 bp of exon 9 (coding amino acids 324* to 340*) in addition to 24 bp of intron 9, including the authentic donor splice site of exon 9. The large deletion of 72 bp was located in the same F12 gene region as the missense mutations p.Thr328Lys* and p.Thr328Arg* reported previously. Our findings confirm the association between F12 gene mutations modifying the proline-rich region of the FXII protein and hereditary angioedema with normal C1-inhibitor.

[Analysis of an hereditary coagulation factor XII deficiency in a consanguineous pedigree].

OBJECTIVE: To analyze genetic mutation and explore its molecular pathogenesis for an hereditary coagulation factor XII(F XII) deficiency in a pedigree featuring consanguineous marriage. METHODS: Activated partial thromboplastin time (APTT), F XII procoagulant activity (F XII:C), F XII antigen (F XII:Ag) and other coagulant parameters were assayed. For the proband and his family members, exons 1-4, introns including the splice junctions of the F XII gene were amplified with polymerase chain reaction (PCR). The PCR product was purified and sequenced. The mutations were confirmed by sequencing the complimentary strand. RESULTS: The proband has featured prolonged APTT at 157.5 s (reference range, 27.0-41.0 s). The APTT of his son has increased slightly at 48.3 s. The remaining members of the family were in normal range. F XII activity and F XII antigen of the proband were significantly decreased (<1%). The F XII activity of his wife, daughter, son and mother was also dropped to about 51%, 21%, 21% and 50%, respectively, and so was the F XII antigen (42%, 32%, 37% and 48%, respectively). Homozygous missense mutation of G→A transition at position 8699 in exon 14 resulting in Gly542Ser was identified in the proband. His mother, son and daughter were heterozygous for Gly542Ser. In the promoter regions of F XII gene, the genotype of the proband and the other members was 46T/T. CONCLUSION: Homozygous missense mutation Gly542Ser was found in a pedigree of hereditary F XII deficiency. The homozygous missense mutation might have resulted from his parents by consanguineous marriage. Gly542Ser and 46T/T have contributed to the pathogenesis of the hereditary factor XII deficiency pedigree.

Report 2 cases of congenital factor XII deficiency: a rare coagulation disorder.

Factor XII (F.XII, Hageman factor) is one of the contact system factors which initiates an intrinsic coagulation pathway. But its definite role is still unclear, because many cases of severe F.XII deficiency experience thrombotic events instead of a bleeding problem. Moreover most of them are asymptomatic. There have only been a few reports of F.XII deficiency in Thailand. The author reports two cases of congenital F.XII deficiency in Thai children.

Genetic analysis of a pedigree with hereditary coagulation factor XII deficiency.

The aim of this study was to elucidate the molecular defects of a Chinese family with hereditary coagulation factor XII (FXII) deficiency. The FXII activity (FXII:C) and FXII antigen (FXII:Ag) levels were measured by clotting assay and ELISA, respectively. To identify mutations, the F12 gene sequencing was carried out. ClustalX-2.1-win and four online bioinformatics tools were applied to study the conservatism and harm of the mutation. The proband's FXII:C and FXII:Ag were 3 and 4%, respectively. Sequencing analysis revealed compound heterozygous mutations, including the deletion mutation (c.130delG) resulting in p.E26Sfs∗50 and the missense mutation (c.1561G>A) resulting in p.E502K. Bioinformatics indicated that mutations probably disrupt the function of the FXII protein. The c.130delG heterozygous deletion variation and the c.1561G>A heterozygous missense variation were responsible for the reduction of FXII:C in this family, of which c.130delG was first reported in the world.

A novel homozygous missense mutation (Met527Ile) in a consanguineous marriage family with inherited factor XII deficiency.

OBJECTIVE: To identify potential mutations of the FXII gene (F12) in a consanguineous marriage family with hereditary coagulation factor XII (FXII) deficiency, and it will improve the understanding of the pathogenesis involved in the disease. CLINICAL PRESENTATION: The proband was a 58-year-old male who had chronic gastritis. He was found to have a significantly prolonged activated partial thromboplastin time (APTT) at 101.0s (reference range, 29.0-43.0 s) before stomachendoscopy. TECHNIQUES: The coagulation factor XII activity (FXII:C) and FXII antigen (FXII:Ag) were measured by one-stage clotting assay and enzyme-linked immunosorbent assay, respectively. The F12 gene was amplified by polymerase chain reaction and sequenced. Mutation sites were further confirmed by reverse sequencing. The conservatism and possible impact of the amino acid substitution were analyzed by multiple bioinformatics tools, as well as 3D protein model analysis. RESULTS: The proband had a prolonged APTT (101.0 s), whose FXII:C and FXII:Ag were obviously reduced, both at 1.0% (normal range, 72-113%). Gene sequencing revealed that he carried a homozygous missense mutation of Met527Ile. Family study showed that his mother, son and daughter carried a heterozygous Met527Ile. Bioinformatics and model analysis of the mutation indicated that Met527Ile may be detrimental and potentially alters the structure and the function of the protein. CONCLUSION: The novel mutation Met527Ile could potentially account for the reduced activity of FXII in this family.

Monitoring thrombin generation: is addition of corn trypsin inhibitor needed?

Thrombin generation monitoring has the potential to be used as a clinical diagnostic tool in the near future. However, robust preanalytical conditions may be required, and one factor that has been reported is in-vitro contact activation that might influence in-vitro measurements of thrombin generation and thereby act as an unpredictable pre-analytical variable. The aim of the current study was to investigate the influence of contact activation and the necessity of corn trypsin inhibitor (CTI) to abolish contact activation in thrombin generation measurements at low tissue factor (TF) concentrations. Thrombin generation was performed using the calibrated automated thrombinoscopy (CAT), thereby determining the endogenous thrombin potential (ETP), peak height, and the lag time, in plasma obtained from healthy volunteers. Addition of CTI after plasma preparation had no significant influence on thrombin generation triggered with 0.5 pM TF or higher, as demonstrated by unaltered ETP and lag time values between analyses with and without CTI. Addition of CTI before blood collection reduced thrombin generation triggered with 0.5 pM TF: both the ETP and peak height were significantly reduced compared to no CTI addition. In contrast, thrombin generation remained unaltered at a 1 pM TF trigger or above. This study demonstrates that addition of CTI after plasma separation is not necessary when triggering with TF concentrations of 0.5 pM and higher. Furthermore, it was demonstrated that it is not needed to pre-fill blood collecting tubes with CTI when measuring thrombin generation at TF concentrations of >/=1 pM.

Contact factor deficiencies and cardiopulmonary bypass surgery: detection of the defect and monitoring of heparin.

Contact factor pathway deficiencies do not cause surgical bleeding but make heparin monitoring by the activated partial thromboplastin time (APTT) and activated clotting time (ACT) unreliable. Heparin monitoring during cardiopulmonary bypass (CPB) surgery in these patients is particularly challenging. Here we describe heparin monitoring during CPB using the chromogenic anti Xa assay in two patients with severe factor XII deficiency (FXII < 0.01 U/mL) and one patient with severe prekallikrein (PK) deficiency (PK < 0.01 U/mL). Anti Xa levels of the three patients during CPB varied between 3.8 and 4.8 U/mL in keeping with a control group (mean anti Xa 4.5 U/mL and ACT > 480 s). There were no bleeding or thrombotic complications. We also found that detection of severe PK deficiency by the APTT in the PK deficient patient was dependent on the reagent used and discuss the sensitivity of different APTT reagents for contact factor deficiencies. We conclude that the sensitivity of APTT methods for contact pathway deficiencies is highly variable and although insensitivity is not a clinical problem in terms of bleeding, it can be a cause of discrepancy between different APTT reagents and the ACT. This can lead to confusion about a possible haemorrhagic tendency and delays in surgery. If these patients need to undergo cardiac surgery requiring high dose heparin treatment, monitoring by chromogenic anti Xa assay is a good alternative.

Intrinsic pathway of coagulation and arterial thrombosis.

Formation of a fibrin clot is mediated by a group of tightly regulated plasma proteases and cofactors. While this system is essential for minimizing blood loss from an injured blood vessel (hemostasis), it also contributes to pathologic fibrin formation and platelet activation that may occlude vessels (thrombosis). Many antithrombotic drugs target key elements of the plasma coagulation mechanism such as thrombin and factor Xa, based on the premise that plasma elements contributing to thrombosis are primarily those involved in hemostasis. Recent studies with genetically altered mice raise questions about this paradigm. Deficiencies of the intrinsic pathway proteases factor XII and factor XI are not associated with abnormal hemostasis in mice, but impair formation of occlusive thrombi in arterial injury models, indicating that pathways not essential for hemostasis participate in arterial thrombosis. If factor XII or factor XI make similar contributions to thrombosis in humans, these proteases could be ideal targets for drugs to treat or prevent thromboembolic disease with minimal risk of therapy-associated bleeding.