Bioprosthetic valve thrombosis and degeneration following transcatheter aortic valve implantation (TAVI).

Bioprosthetic valve thrombosis (BPVT) is a recognised complication of prosthetic aortic valves and can be found in up to 13% of patients after transcatheter implantation. The mechanism of BPVT is not well known, abnormal flow conditions in the new and native sinuses and lack of functional endothelialisation are suspected causes. BPVT may result in valve dysfunction, possibly related to degeneration, and recurrence of patient symptoms, or remain subclinical. BPVT is best diagnosed at multiphase gated computed tomography (CT) angiography as the presence of reduced leaflet motion (RELM) and hypoattenuating aortic leaflet thickening (HALT). Although CT is used to exclude BPVT in symptomatic patients and those with increased valve gradients, the value of screening and prophylactic anticoagulation is debatable.

Global Thrombosis Test (GTT) can detect major determinants of haemostasis including platelet reactivity, endogenous fibrinolytic and thrombin generating potential.

BACKGROUND: Detection of both thrombosis and bleeding risk are essential in clinical cardiology. Thrombin generated by activated platelets and from the extrinsic coagulation pathway is the major determinant of thrombogenesis and hemostasis. Although novel oral anticoagulants further increase the bleeding risk of antiplatelet drugs, platelet function tests do not reliably predict hemorrhagic complications. It seems that in addition to platelet aggregation, true assessment of bleeding risks requires the measurement of both platelet and plasma derived thrombin activity. OBJECTIVE: To adapt a novel, near-patient test for the assessment of both antithrombotic and anticoagulant effects of oral thrombin inhibitors. METHODS: The point-of-care Global Thrombosis Test (GTT), which measures platelet reactivity to shear-activation in native blood, was used. Thrombin, generated from activated platelets (procoagulant activity) plays a pivotal role in GTT measurement. In order to assess endogenous thrombin potential, in a separate blood sample thrombin generation was induced by microparticles formed during hypotonic hemolysis. Thus two blood samples were tested to measure simultaneously platelet reactivity (occlusion time, OT) and hemolysis (microparticles)-induced endogenous thrombin potential (OT-H). RESULTS: In healthy subjects (n=32), OT measured in native blood was reduced in hemolysed blood (100% vs. 43 ± 4%; OT vs. OT-H respectively). Shortening of OT in hemolysed blood (OT-H) was dose-dependently inhibited by the in vitro added thrombin inhibitor argatroban. In patients receiving dabigatran (n=27), OT and, to a lesser extent, OT-H was prolonged, compared to healthy volunteers. Intra-assay variation of OT-H was low (4.5%), but interindividual variation was great, both in healthy subjects (61%) and in patients on dabigatran (65%). Thrombin inhibitors argatroban, heparin (in vitro) and dabigatran (in vivo) all prolonged both OT and OT-H. There was no correlation between the measured OT and OT-H data. CONCLUSIONS: Microparticles shed from erythrocytes during hypotonic lysis of native blood considerably shortened OT. In a direct proportion to the applied concentrations, various thrombin inhibitors prolonged both OT (antithrombotic effect) and to a lesser extent, OT-H (anticoagulant effect). Further large studies are required to evaluate the usefulness of this technique in a clinical setting, in assessing the anticoagulant and antithrombotic effects of medication and relating GTT results with observed thrombotic and bleeding events.

PAR-1 antagonist vorapaxar favorably improves global thrombotic status in patients with coronary disease.

To assess the effect of vorapaxar on global thrombotic and thrombolytic status. The propensity for thrombus formation is determined by the balance between prothrombotic factors and endogenous thrombolysis. Impaired thrombolytic status increases cardiovascular risk. Vorapaxar is a novel, oral, protease-activated receptor-1 antagonist that inhibits thrombin-induced platelet activation. In the TRACER and TRA 2°P-TIMI 50 studies, patients with acute coronary syndromes and established atherosclerosis were randomized to vorapaxar 2.5 mg daily or placebo, in addition to standard care. In 57 patients enrolled in a single center, blood was tested with the point-of-care global thrombosis test, on and off treatment. This automated test employs non-anticoagulated blood to assess thrombotic and thrombolytic status, measuring the time required to form a shear-induced thrombus under physiological conditions (occlusion time, OT), and subsequently, the time to achieve endogenous lysis of the thrombus (lysis time, LT). Patients on vorapaxar exhibited longer OT on vs. off treatment [median 561 s (interquartile range 422-654) vs. 372 s(338-454), P = 0.003] and shorter LT on treatment than off [1,158 s(746-1,492) vs. 1,733 s(1,388-2,230), P = 0.016]. Patients on placebo showed no difference in OT [419 s(343-514) vs. 411 s(346-535), P = 0.658] or LT [1,236 s(985-1,594) vs. 1,400 s(1,092-1,686), P = 0.524] on and off treatment. During treatment, OT was longer in patients taking vorapaxar [561 s(422-654) vs. 419 s(343-514), P = 0.009], but LT was similar in vorapaxar and placebo arms [1,158 s(746-1,492) vs. 1,236 s(985-1,594), P = 0.277]. Vorapaxar prolongs OT and shortens LT, with favorable effects on thrombotic and thrombolytic status. In addition to its antiplatelet effect, vorapaxar may enhance endogenous thrombolysis, which is frequently impaired in coronary disease.

Local drug delivery for percutaneous coronary intervention.

Since the first successful coronary angioplasty by Andreas Grüntzig in 1977, the field of percutaneous coronary intervention (PCI) has expanded rapidly. Rapid technological refinement has seen equipment and complementary pharmacotherapy to improve the outcome of PCI evolve dramatically, driven by clinical need and enormous market forces. The ideal intervention should expand the vessel lumen without inflicting endothelial injury, and provide local drug delivery to prevent subsequent acute thrombosis and neointimal hyperplasia. Drug eluting stents, once regarded as the "gold standard" in PCI, and established as the treatment of choice for nearly a decade, remain limited in their performance by important risks of in-stent restenosis and late stent thrombosis. In this review, we discuss need for local drug therapy as an adjunct to angioplasty and present exciting new technological advances to deliver local pharmacotherapy to the coronary artery, which will hopefully overcome some of the limitations of DES and may represent the way forward in coronary intervention.

Shear-induced global thrombosis test of native blood: pivotal role of ADP allows monitoring of P2Y12 antagonist therapy.

BACKGROUND: It is claimed that in shear-induced platelet function tests, shear-stress is the sole agonist causing platelet activation and resultant thrombosis. However, the fact that red blood cells (RBC) are essential to achieve platelet aggregation in these tests supports recent evidence that ADP makes an important contribution to shear-induced platelet reaction. AIM: To establish the role of ADP in shear-induced thrombosis, and investigate whether a shear-induced thrombosis test can assess ADP-receptor (P2Y12) antagonist medication. METHODS: Blood from healthy volunteers was tested using the Global Thrombosis Test (GTT), before and after clopidogrel. To investigate the importance of contact of blood with plastic, the reactive part of the tube was primed with saline. We also investigated the effect of priming the tube with water, to cause localised haemolysis and ADP release. RESULTS: Saline-priming prolonged occlusion times (OT) by 25% (p<0.01) confirming ADP release from platelets and RBC as a result of contact. Water-priming shortened OT, accelerating the thrombotic reaction (accelerated GTT; aGTT) (OT 379 vs. 177s, p<0.01). Clopidogrel increased OT (379 vs. 477s, p<0.01), preventing the shortening of aGTT-OT (177 vs. 362s, pre- and post-clopidogrel; p<0.01). CONCLUSION: In addition to thrombin formation, ADP released from platelets and RBC in native blood subjected to high shear-stress makes an important contribution to the resultant thrombotic occlusion. The described aGTT sensitively detected the effect of clopidogrel and thus seems suitable for monitoring and individualizing ADP-receptor antagonist therapy. Parallel measurement of GTT and aGTT would allow assessment of both global thrombotic status and response to P2Y12 antagonist therapy.

Antiplatelet Resistance-Does it Exist and How to Measure it?

Aspirin and clopidogrel are the most commonly used antiplatelet agents in patients with coronary artery disease. The existence of resistance to these agents has been a controversial issue and new drugs are being developed to overcome this problem. Laboratory tests, which can identify resistance and correlate this with clinical outcome, are being studied in order to identify patients at risk of future thrombotic events. We discuss the evidence for the existence of antiplatelet resistance-both in the laboratory and in the clinical setting. So far, platelet aggregometry has been considered the gold standard test, but is very operator dependant, time consuming, and has shown little correlation with other available tests of antiplatelet resistance. We discuss the available tests of platelet function, their limitations, and evidence for their use. A simple, rapid, near-patient test, which is affordable and useful in the clinical (not just laboratory) setting, could allow risk stratification of patients and individualization of antiplatelet medication to improve outcome.

Enhanced spontaneous thrombolysis: a new therapeutic challenge.

Spontaneous thrombolysis is an endogenous protective mechanism against lasting arterial thrombotic occlusion, which is implicated in the pathogenesis of myocardial infarction and acute coronary events. Novel therapies for coronary heart disease (CHD) targeting atherosclerosis and thrombosis, together with cardiovascular prevention programs targeting risk-factors and lifestyle provide evidence that CHD is preventable. Although reduced fibrinolytic activity is a recognized risk-factor for ischemic cardiovascular events, it has so far been neglected. Our knowledge of the fibrinolytic effect of drugs commonly used for CHD such as antiplatelet agents (aspirin, ticlopidine, clopidogrel), anti-diabetic biguanides (phenformin, metformin) or anti-hypertensive drugs is scanty and conflicting. This is mainly due to the lack of a global test of spontaneous thrombolysis, as opposed to fibrinolysis of plasma or whole blood, i.e. the assessment of various activators and inhibitors of the fibrinolytic system. A recently described technique allows the measurement of spontaneous thrombolysis, that is, lysis of an autologous platelet-rich thrombus in the absence of added plasminogen activators. Early results suggest that this test may have significant clinical potential both in identifying those at risk of fatal cardiac events and in finding new therapeutic avenues or lifestyles to improve spontaneous thrombolytic activity.

Treatment of iatrogenic aortic dissection by percutaneous stent placement.

During elective cardiac catheterization via the right femoral artery, difficulty was encountered advancing the guidewire beyond the aortic arch and the procedure abandoned. The patient later developed back pain and hypotension. A contrast-enhanced spiral computed tomography scan confirmed aortic dissection, extending from an entry point in the external iliac artery to the arch. We hypothesized that the entry site was responsible for filling the dissection in the thoraco-abdominal component. A stent was placed in the iliac artery, occluding the entry point. This is the first report of aortic dissection, a rare complication of angiography, managed by percutaneous stenting of the entry point.

Platelet membrane glycoprotein Ibalpha gene -5T/C Kozak sequence polymorphism as an independent risk factor for the occurrence of coronary thrombosis.

OBJECTIVE: To explore the potential of the GPIbalpha gene variable number tandem repeat (VNTR) and -5T/C Kozak polymorphisms to act as independent risk factors for myocardial infarction. METHODS: 256 patients aged 33-80 years (180 caucasian, 76 Indian Asian) were recruited at cardiac catheterisation for any diagnostic indication, and divided into two groups: group A, with confirmed previous myocardial infarction evident on ECG or ventriculogram (88 patients, 79 men, 9 women) and group B, with no evidence of myocardial infarction (168 patients, 101 men, 67 women). RESULTS: There was no significant difference in race, age, hypertension, smoking status, or family history between the infarct and non-infarct groups, though there was a significant difference in sex (89.8% male in group A, 60.1% male in group B, p < 0.001). Genotype analysis showed a strong association between the GPIbalpha Kozak homozygous TT genotype and the occurrence of myocardial infarction (group A: TT 85.2%, TC 12.5%, CC 2.3%; group B: TT 67.3%, TC 32.7%, p = 0.001). No significant association was found between myocardial infarction and the GPIbalpha VNTR, although analysis of the CC VNTR genotype against all other GPIbalpha VNTR genotypes showed a marginal association with myocardial infarction (p = 0.059). There was no association between the Kozak sequence polymorphism (p = 0.797) or GPIbalpha VNTR (p = 0.714) and the degree of vessel disease. CONCLUSIONS: The homozygous TT Kozak genotype may be a significant factor in the outcome of coronary artery disease completed by myocardial infarction. Conversely, the Kozak C allele in the heterozygous state TC may confer some protection against myocardial infarction.

A potential role for endothelin-1 in peripheral vascular disease.

The distribution of endothelin-1 (ET-1) and its receptors (ET(A)/ET(B)) has been studied in segments of femoral artery obtained from patients undergoing operation for peripheral vascular disease (PVD) using a combination of immunohistochemistry and autoradiography. Both receptor subtypes were located on the tunica media of vessel segments, with ET(A)-receptors predominating. Densitometric analysis showed that there was no difference in receptor binding to proximal/distal arterial segments from PVD patients. High-resolution autoradiography identified ET(B) binding to vascular nerves and vasa vasorum, mainly in distal portions of femoral arteries. Immunoreactive ET-1 was also identified that was associated with the vasa vasorum.

Platelet reactivity and streptokinase resistance following antecedent streptokinase therapy for myocardial infarction.

AIMS: To assess the efficacy of second-time administration of streptokinase (SK). First-time thrombolysis with SK in myocardial infarction (MI) is established but the efficacy of subsequent SK is unknown. METHODS AND RESULTS: Platelet reactivity to shear stress, spontaneous and SK-induced thrombolysis were measured in vitro in 28 patients who had received SK for MI and compared to 15 controls. SK antibody (Ab) titres were inversely related to time from MI. Platelet reactivity was greatly enhanced in patients (p < 0.0001). Spontaneous thrombolysis in patients was poor and in 17 failed to occur. In contrast, thrombolysis occurred in all but 1 control. In patients platelet reactivity was strongly related to thrombolytic activity (r = -0.516; p = 0.0029). SK in vitro was at least 4 times more effective in controls than in patients. CONCLUSION: The chances of achieving patency with second administration of SK are poor. Ab titre is not a reliable predictor of resistance.

Raised factor VIII is associated with coronary thrombotic events.

Coagulation is triggered during the onset of myocardial infarction, resulting in vascular occlusion. However, a causal role for individual haemostatic factors in the development of thrombotic occlusion is not established. Three cases (all relatively young women) are reported of raised factor VIII associated with myocardial infarction. Two patients presented acutely with myocardial infarction at a relatively young age with no preceding history of angina. The other patient had had venous thrombosis when young and activated protein C resistance (APCR), without the presence of factor V Leiden. A functional relation exists between APCR and factor VIII; therefore, raised factor VIII may contribute to APCR and the increased thrombotic risk in patients without factor V Leiden. Factor VIII is an important risk factor for atherothrombotic events, including sudden death, in patients with vascular disease. These cases support the association of raised factor VIII with acute thrombotic events, even in patients without significant underlying atheromatous disease.

Thrombotic status analyser. Measurement of platelet-rich thrombus formation and lysis in native blood.

A physiologically relevant global in vitro test is described which allows the overall assessment of both thrombotic and thrombolytic activities of blood. In principle, native blood is drawn in pulses through a capillary tube where haemodynamic forces induce a platelet reaction culminating in vessel occlusion. Dislodgement/disintegration of the stabilised thrombus under pressure is a reflection of thrombolysis. Evidence is presented for the platelet-rich nature of the occlusion and that disruption of the thrombus and re-established patency is the result of thrombolysis, that is fibrinolysis with significant contribution from platelets. This test sensitively detects hypercoagulability (stasis); platelet hyperreactivity (coronary artery disease); anti-platelet effect (aspirin, prostacyclin) and the thrombolytic effect of, thrombin generation by, and resistance to streptokinase. Therefore, this overall assessment of thrombotic status could be of great diagnostic and therapeutic benefit in clinical practice.

Activation of the phagocytic system increases intimal proliferation in hypercholesterolemic rabbits.

The effect of in vivo stimulation of the phagocytic system (neutrophils, monocytes and hepatic Kupffer-cells) by inducing phagocytosis of intravenously administered latex particles on lipid peroxidation and aortic intimal proliferation was tested in cholesterol-fed rabbits. Three weeks after starting the diet, aortic intimal proliferation was measured by the intimal to medial ratios and by the incorporation of [3H]thymidine, infused into the circulation for the preceding 14 days. Intimal to medial ratios were increased (0.473 +/- 0.023 vs. 0.282 +/- 0.011, P < 0.01) and aortic [3H]thymidine contents were higher (66.8 +/- 3.5 vs. 27.8 +/- 49 counts/min per mg, P = 0.0001) in latex bead-treated than in control animals. Injection of beads transiently increased plasma lipid peroxide levels. At the end of the 3 week experiment, plasma lipid peroxide levels were still elevated and lipid peroxide contents of the aortic walls were higher in the latex-treated rabbits (82.8 +/- 5.8 vs. 46.4 +/- 4.9 nmol/mg cholesterol, latex-treated vs. controls, P = 0.004). These data suggest a significant acceleration of atherogenesis by the stimulated phagocytic system, the mechanism of which may involve lipid peroxidation.

Thrombin generation following arterial injury is a critical initiating event in the pathogenesis of the proliferative stages of the atherosclerotic process.

Vascular injury, activation of the coagulation system and thrombosis are common initial events in the accelerated atherosclerotic process. The role of thrombin generated at the site of aortic injury in the subsequent neointimal proliferation was studied in rabbits (n = 16) 3 weeks after balloon catheter injury. In half of these animals, potent thrombin antagonists, r-hirudin and P-PACK, were administered to prevent acute thrombotic events. Compared to aortas with intact endothelium (n = 8), aortas de-endothelialised 21 days earlier showed neointimal hyperplasia as measured by the intimal/medial ratio (0.68 vs. 0.04, injured vs. normal aortas) and an increase in both total cholesterol (4.08 vs. 3.31 mg/g, p < 0.05) and lipid peroxide content (31.3 vs. 1.1 nmol/g; p < 0.001). Neointimal hyperplasia following endothelial denudation was inhibited in rabbits treated with thrombin-antagonists (0.27 vs. 0.68, treated vs. untreated, p = 0.012) and neither total cholesterol (3.48 mg/g) nor lipid peroxide content (1.5 nmol/g) differed significantly from that of intact arteries. By demonstrating a strong relationship between thrombin generation following de-endothelialisation and the progressive intimal proliferation, this study supports the hypothesis that thrombin is an important contributor to restenosis after vascular injury. The highly atherogenic lipid peroxidation seems to be linked to the early, thrombin-mediated events, as it was completely prevented by adequate thrombin antagonism.