Coagulation of flowing native blood: Advantages over stagnant (tube) clotting tests

The anti-thrombotic active constituents from Centella asiatica

The in vitro effects of a methanol extract from the aerial parts of Centella asiatica on shear-induced platelet activation and coagulation were assessed after oral administration to rats, by subjecting non-anticoagulated blood to haemostatometry. 3,5-Di-O-caffeoyl quinic acid, 1,5-di-O-caffeoyl quinic acid, 3,4-di-O-caffeoyl quinic acid, 4,5-di-O-caffeoyl quinic acid, and chlorogenic acid, together with asiaticoside, kaempferol, quercetine, kaempferol-3-O-beta-D-glucoside and quercetin-3-O-beta-D-glucoside were all isolated from the methanol extract. Amongst these, only 3,5-di-O-caffeoylquinic acid showed significant inhibition of shear-induced platelet activation and dynamic coagulation. The reactive curve of the inhibitory effect on the platelet reaction and the dynamic coagulation showed a bell-shape.

The effects of heparin, protamine, and heparin/protamine reversal on platelet function under conditions of arterial shear stress

Platelet dysfunction contributes to blood loss after cardiopulmonary bypass. This study examined the antiplatelet effects of heparin, protamine, and varying heparin/protamine ratios in an in vitro physiologic model and further elucidated the mechanism of the antiplatelet and anticoagulant effects of protamine. We used the Clot Signature Analyzer (CSA(TM)), a system that analyzes coagulation in flowing whole blood, to test two aspects of platelet function, with different concentrations of heparin and protamine, under conditions simulating arterial flow: collagen-induced thrombus formation (CITF) under moderate shear and high shear platelet activation, platelet hemostasis time (PHT). In addition, platelet aggregometry, celite activated clotting time (Hepcon(TM) ACT), prothrombin time (PT), and partial thromboplastin time (PTT) were measured. Both PHT and the CITF were prolonged by heparin at 20 microg/mL, protamine at 20 and 40 microg/mL, and heparin/protamine ratios of 1:1 and 1:2, but not at 1:1.5. The Hepcon ACT was prolonged by heparin 20 microg/mL and protamine alone at 20 and 40 microg/mL, was normal at a ratio of 1:1, and was prolonged at 1:1.5 and 1:2. Protamine 80 microg/mL prolonged the PT and PTT. Dependency on thrombin, protein kinase C activation, and nonspecific charge effects were examined. The direct thrombin inhibitor D-phenylalanyl-L-prolyl-L-arginyl-chloromethyl ketone prolonged the PHT and ACT, but not the CITF, whereas the polycationic molecules polyarginine and polylysine prolonged the CITF, but not the PHT. The effect of protamine on the PTT, but not PT, could be shortened by the addition of excess phospholipid. Therefore, heparin inhibits both high shear collagen-independent and moderate shear collagen-dependent platelet activation; however, the latter is not mediated by its antithrombin activity. Protamine's antithrombin effect may explain its inhibition of platelet activation at high shear stress. Protamine's nonspecific charge effects are more important for inhibiting moderate shear collagen-induced platelet activation.

IMPLICATIONS

This study suggests that protamine reversal of heparin's antiplatelet effect occurs within a narrow window because of the direct antiplatelet effects of protamine. Antithrombin effects may explain the inhibition of shear activation of platelets by both heparin and protamine. Nonspecific charge effects of protamine may explain the inhibition of collagen platelet activation in the presence of medium shear.

Coagulation and Bleeding Disorders: Review and Update

Hemostasis is initiated by injury to the vascular wall, leading to the deposition of platelets adhering to components of the subendothelium. Platelet adhesion requires the presence of von Willebrand factor and platelet receptors (IIb/IIIa and Ib/IX). Additional platelets are recruited to the site of injury by release of platelet granular contents, including ADP. The “platelet plug” is stabilized by interaction with fibrinogen. In this review, I consider laboratory tests used to evaluate coagulation, including prothrombin time, activated partial thromboplastin time, thrombin time, and platelet count. I discuss hereditary disorders of platelets and/or coagulation proteins that lead to clinical bleeding as well as acquired disorders, including disseminated intravascular coagulation and acquired circulating anticoagulants.

Evaluation of the Clot Signature Analyzer as a hemostasis test in healthy volunteers exposed to low doses of aspirin

Several variables affect bleeding time that make it difficult to obtain consistent measurements. The Clot Signature Analyzer (CSA) has been developed to assess in vitro hemostasis using well-controlled flow chambers. In this study, the equivalencies in the CSA parameters with the conventional bleeding time or platelet aggregation methods were evaluated in subjects exposed to aspirin. The CSA parameters, platelet hemostasis time (PHT) and collagen-induced thrombus formation (CITF), were compared to bleeding time (Surgicutt2) and collagen-induced platelet aggregation, respectively. Fifty-three healthy volunteers were given two doses of aspirin (81 and 243 mg) in one day. Following the baseline period, the volunteers took 81 mg of aspirin and then took 243 mg 2 hours later. The changes in each value from the baseline to that at either aspirin dose (2 hours after dosing) were evaluated. Platelet hemostasis time and CITF correlated well with bleeding time and aggregation, respectively, but PHT was not significantly increased after 81 mg of aspirin, whereas bleeding time was significantly increased. The variation in PHT was slightly higher than that of bleeding time. At 81 mg, CITF was significantly increased but aggregation was not, even though the variation was comparable. This suggests that PHT and CITF can simulate the changes in bleeding time and aggregation, respectively, but the sensitivity of PHT for detecting the changes in bleeding time was no better than the conventional method. Also, CITF was more sensitive than aggregation in detecting platelet response to collagen. In conclusion, the proposed CSA is not always suitable for detecting hemostatic abnormalities.

Norepinephrine, but not epinephrine, enhances platelet reactivity and coagulation after exercise in humans

The effects of exercise and catecholamines on platelet reactivity or coagulation and fibrinolysis appear to be inconsistent. This may be partly due to the methods employed in previous studies. In the present study, we investigated the effects of acute aerobic exercise and catecholamines on the thrombotic status by a novel in vitro method, shear-induced hemostatic plug formation (hemostatometry), using nonanticoagulated (native) blood. Aerobic exercise (60% maximal O2 consumption) was performed by healthy male volunteers for 20 min, and the effect on platelet reactivity and coagulation was assessed by performing hemostatometry before and immediately after exercise. Exercise significantly increased shear-induced platelet reactivity, coagulation, and catecholamine levels. The effect of catecholamines on platelet reactivity and coagulation was assessed in vitro by adding catecholamines to blood collected in the resting state. The main findings of the present study are that elevation of circulating norepinephrine at levels that are attained during exercise causes platelet hyperreactivity and a platelet-mediated enhanced coagulation. This may be a mechanism of an association of aerobic exercise with thrombotic risk.

Variable influence of heparin and contrast agents on platelet function as assessed by the in vitro bleeding time

Both heparin and contrast agents have anticoagulant effects which are well-documented but their effects on platelets are not well-characterized. The purpose of the present study was to evaluate the sequential effects of heparin and then a contrast agent on platelet function during an angiographic procedure. Blood samples from 54 patients were obtained at baseline, after a 5000 unit bolus of heparin and after administration of a contrast agent (iohexol, n = 30: diatrizoate, n = 24) during angiography. The in vitro bleeding time (IVBT) was determined on nonanticoagulated whole blood using a hollow fiber device under physiological flow conditions. Mean IVBT at baseline was 3.6 +/- 2.7 minutes and increased to 17.0 +/- 12.3 minutes after heparin (p < 0.01). After heparin, 44.5% of the patients still had a normal IVBT (< 9.0 minutes), 11% of the patients had a moderately increased IVBT and the remaining patients had a large increase in their IVBT. When contrast was given (167 +/- 52 mls) following heparin, mean IVBT was higher in those who received diatrizoate (23.3 +/- 9.4 minutes) compared with iohexol (15.0 +/- 10.9 minutes, p < 0.05). However, 15 patients (28%) continued to have a normal IVBT after contrast and of these 80% had received iohexol.

Effect of iodixanol on in vitro bleeding time

RATIONALE AND OBJECTIVES

Both monomeric and dimeric ionic radio contrast media (CM) have been shown to be more anticoagulant than nonionic monomeric CM. Iodixanol is a relatively new nonionic dimeric contrast medium. We investigated whether the dimeric structure of iodixanol would produce the same level of anticoagulation that has been observed using nonionic monomeric CM.

METHODS

We used a global screening device that operates under physiologic flow conditions to monitor the effects of iodixanol on in vitro bleeding time (IVBT). This flow dynamic technique perfuses nonanticoagulated whole blood through a hollow fiber device. A leak in the fiber is created by a precision needle, and the resulting pressure fluctuations within the fiber are monitored to examine the ability of the hemostatic system to close the leak by forming a stable platelet plug.

RESULTS

In 20 donors, iodixanol (25% CM/blood, v/v) was shown to lengthen the mean IVBT (18.74 min) compared with the normal blood mean IVBT (4.24 min).

CONCLUSION

The addition of dimeric iodixanol to normal blood affects the IVBT in the same manner as nonionic monomeric CM (ioversol, iopamidol, and iohexol).

Evidence against hypercoagulability in coronary artery disease

The strong epidemiological association between elevated plasma clotting factors and coronary artery disease is generally interpreted as evidence that patients with coronary atherosclerosis are in a procoagulant (hypercoagulable) state. A dynamic global test was used to assess the overall coagulation status of 761 patients with coronary artery disease scheduled for coronary artery bypass grafting and compared to healthy matched controls (n = 100). Platelet reactivity to shear-stress was simultaneously measured from identical, non-anticoagulated blood samples. Contrary to expectation, the overall coagulation in cardiac patients did not differ significantly from that of controls. Furthermore, the coagulation status of patients bore no relationship to the severity of coronary atherosclerosis. The latter is in contrast with platelet reactivities, which were significantly increased in patients with > or = 2 vessel disease as compared with single vessel disease. The present results do not necessarily conflict with the finding of elevated plasma clotting factors in cardiac patients. However, they do not support the claim that these markers are a reflection of a hypercoagulable state. Indeed, this study confirms that such patients are in a prothrombotic state, which is related to enhanced platelet reactivities, and not to a prothrombotic imbalance of the coagulation mechanism.

Effect of warfarin on arterial thrombogenesis: problems of monitoring

Platelet reactivity to shear stress and collagen and dynamic overall coagulation were measured in vitro from nonanticoagulated blood of 137 patients on warfarin. One hundred five matched, healthy subjects served as controls. Platelet reactivity to both stimuli and contribution of platelets to plasmatic coagulation were significantly inhibited in patients on warfarin. No correlation was found between platelet reactivity and the coagulation status assessed by the international normalized prothrombin time ratio (INR). Despite similar INR, platelet reactivity showed great individual variation. In 98 patients who were followed up for 3 months, measurement of platelet reactivity to shear stress could discriminate between those who had either bleeding or thromboembolic episodes. These findings suggest that monitored platelet function would help in individualizing oral anticoagulant regimens and hence would increase the benefit of therapy without the risk of bleeding complications.

Enhanced platelet reactivity and hypercoagulability in the steady state of sickle cell anaemia

A prospective controlled study was undertaken to investigate the haemostatic and coagulation status of 18 adult subjects in the steady state of sickle cell anaemia (SCA), using a relatively new in vitro technique. Shear induced haemostasis, whole blood dynamic coagulation, and spontaneous thrombolysis were measured using nonanticoagulated blood. As expected, the haemoglobin levels were significantly lower and platelet counts significantly higher in subjects with SCA compared with controls. Haemostasis and coagulation were significantly enhanced in SCA. No correlation was found between the raised platelet count and enhanced haemostasis or the reduced haemoglobin and hypercoagulation, respectively. Hyperactivity of the haemostatic system may have a pathogenic role in vaso-occlusive microthrombotic events and in the leg ulcers, both of which occur frequently in SCA.

Significance of plasma fibrinogen in coronary arterial disease: marker or causative risk factor for arterial thrombosis?

The relationship between fibrinogen and severity of disease was measured in patients with coronary arterial disease (n = 301) prior to surgical coronary revascularisation. Platelet reactivity (shear-induced haemostasis) was measured from non-anticoagulated blood, in vitro. Coagulation was assessed by the clotting time of flowing native blood (dynamic) and by the conventional (stagnant) tube tests. Significantly enhanced platelet reactivity to shear-stress was observed when patients with one-vessel disease were compared to those with two- or three-vessel disease (P = 0.003). Neither coagulation nor fibrinogen were significantly related to the severity of disease. Furthermore, patients who had myocardial infarction (n = 144) showed enhanced platelet reactivity (P = 0.02) as compared to those who had not (n = 157). Again, neither coagulation nor fibrinogen discriminated between these groups of patients. Relationship between plasma fibrinogen and platelet reactivity was also investigated in vitro. Identical blood samples with normal (220-280 mg/dl) and elevated plasma fibrinogen (approximately 500 mg/dl) were compared by measuring platelet reactivity and coagulation from native blood and platelet aggregation in whole blood. The in vitro studies suggested that plasma fibrinogen and platelet reactivity are inversely associated. Furthermore, increased fibrinogen prolonged dynamic coagulation. These findings do not support the assertion that elevated plasma fibrinogen is a true causative factor for coronary arterial disease and arterial thrombosis.

High-dose aspirin inhibits shear-induced platelet reaction involving thrombin generation

BACKGROUND

A unifying concept of explaining all pharmacological actions of aspirin by the irreversible blockage of the enzyme cyclooxygenase and therefore the inhibition of prostaglandin biosynthesis has left many unanswered questions.

METHODS AND RESULTS

Two hundred ninety-four patients taking 75 mg/day aspirin were tested 3 months after coronary artery bypass surgery. Platelet thromboxane formation (whole blood aggregation to arachidonate) was completely prevented in 80% of patients. Compared with matched healthy controls (n = 95), a significant platelet hyperreactivity was observed in patients (p less than 0.0001 versus less than 0.002). Ninety patients were advised to increase their daily dose of aspirin from 75 mg to 300 mg. Platelet reactivity retested 1 month after increasing the dose has significantly decreased (p = 0.0008; less than 0.0001), whereas it remained unchanged in those patients (n = 84) who continued with the same dose regimens. In normal subjects, ingestion of a single 600-mg aspirin significantly inhibited shear-induced platelet reaction.

CONCLUSIONS

It is concluded that aspirin does affect the platelet response to shear forces, but this requires higher dosage (greater than 300 mg/day), suggesting a mechanism probably different from that of interference with thromboxane formation.