Laboratory Evaluation of Platelet Dysfunction

Platelets as Predictors of Vascular Risk: Is There a Practical Index of Platelet Activity?

Activated platelets play a role in the pathogenesis of coronary heart disease (CHD). Following activation, platelets change shape, aggregate, and release several bioactive substances. The aim of this review is to identify if there is a simple and cost-effective method that indicates platelet activation and predicts the risk of CHD and vascular events. The rationale for identifying high-risk patients is to reduce their risk of vascular events by administering appropriate and effective antiplatelet treatment, like aspirin, clopidogrel, or combination regimens. Many laboratory tests estimating platelet activity have been described. Some are relatively simple, such as spontaneous or agonist-induced platelet aggregation. Other tests include measuring the mean platelet volume (MPV) or plasma soluble P-selectin levels. Some more complex tests include flow cytometry to determine platelet GP Ilb/Illa receptors, platelet surface P-selectin, plateletmonocyte aggregates, and microparticles. Only few prospective studies assessed the predictive value of platelet activation in healthy individuals. Although the MPV seems an 'easy method, there are insufficient data supporting its ability to predict the risk of a vascular event in healthy adults. Platelet aggregation, in whole blood or in platelet-rich plasma was not consistently predictive of vascular risk. Soluble P-selectin measurement is a promising method but it needs further evaluation. Flow cytometry methods are costly, time-consuming, and need specialized equipment. Thus, they are unlikely to be useful in estimating the risk in large numbers of patients. There is as yet no ideal test for the detection of platelet activation. Each currently available test has merits and disadvantages. Simple methods such as the MPV and the determination of platelet release products need further evaluation.

In vivo models for the evaluation of antithrombotics and thrombolytics

The development and application of animal models of thrombosis have played a crucial role in the discovery and validation of novel drug targets and the selection of new agents for clinical evaluation, and have informed dosing and safety information for clinical trials. These models also provide valuable information about the mechanisms of action/interaction of new antithrombotic agents. Small and large animal models of thrombosis and their role in the discovery and development of novel agents are described. Methods and major issues regarding the use of animal models of thrombosis, such as positive controls, appropriate pharmacodynamic markers of activity, safety evaluation, species specificity, and pharmacokinetics, are highlighted. Finally, the use of genetic models of thrombosis/hemostasis and how these models have aided in the development of therapies that are presently being evaluated clinically are presented.

Hemorheology in patients with liver cirrhosis: special emphasis on its relation to severity of esophageal variceal bleeding

BACKGROUND

Patients with decreased blood viscosity are supposed to have a higher risk of bleeding and increased severity of bleeding (severity of bleeding proportional to transmural pressure x area of variceal tear/blood viscosity). However, the hemorheological factors have never been assessed in patients with esophageal variceal bleeding. Therefore, the purpose of the present study was to examine the hemorheological factors in liver cirrhotic patients with special emphasis on the outcome of variceal bleeding.

METHODS

Forty-two liver cirrhosis patients with variceal bleeding and another 44 matched patients without bleeding were enrolled. The hemorheological and hemostatic factors of their peripheral blood were examined. The clinical course was under careful surveillance.

RESULTS

Patients with poor hepatic reserve (Child B + C vs A) had lower whole blood viscosity (4.34 +/- 0.56 mPa.s vs 5.06 +/- 1.35 mPa.s, P < 0.05), lower hematocrit levels (32.86 +/- 5.97% vs 36.62 +/- 5.44%, P < 0.05), lower platelet counts (79.7 +/- 47.6 x 10(3)/mL vs 108.0 +/- 71.2 x 10(3)/mL, P < 0.05) and prolonged prothrombin time (2.88 +/- 2.33 s vs 1.27 +/- 1.37 s, P < 0.05). Patients with bleeding (vs non-bleeding group) had lower hematocrit levels (31.44 +/- 5.75% vs 36.57 +/- 5.19%, P < 0.01) and lower fibrinogen levels (226.7 +/- 92.7 mg/dL vs 286.4 +/- 111.8 mg/dL, P < 0.05). Patients with bleeding with shock had worse liver cirrhosis (Child A/B/C = 0/5/4 vs 11/18/4, P < 0.05), lower whole blood viscosity (4.01 +/- 0.17 mPa.s vs 4.57 +/- 0.76 mPa.s, P < 0.05), reduced erythrocyte aggregability (2.94 +/- 0.41 vs 3.54 +/- 0.61, P < 0.001), and lower platelet counts (56.22 +/- 17.05 x 10(3)/mL vs 88.87 +/- 38.12 x 10(3)/mL, P < 0.001). The Child-Pugh grade and erythrocyte aggregability were two independent factors associated with bleeding shock.

CONCLUSIONS

Whole blood viscosity, hematocrit levels and platelet counts were lower in patients with advanced liver cirrhosis. Advanced liver cirrhosis and reduced erythrocyte aggregability were independent factors for hypovolemic shock in cirrhotic patients with esophageal variceal bleeding. However, the causal relationship between hemorheology and bleeding needs to be clarified in further studies.

Platelet function disorders

Platelet function disorders are a rare cause of bleeding in hematological practice. The inherited variety includes defects in platelet adhesion, aggregation, secretion and platelet procoagulant activities. The clinical presentation is usually mild with mainly mucocutaneous bleeds. Milder phenotypes may pass off unrecognized in childhood and present later in life. Testing includes a coagulation screen, which may show a prolonged bleeding time, followed by platelet aggregation tests with agonists. Flow cytometry for platelet surface markers, membrane glycoprotein analysis and facilities for identification of the genetic defect are usually available in research laboratories. Recent advances have opened vistas for antenatal testing. Bone marrow transplantation is the only curative therapeutic option available.

Effect of carprofen on hemostatic variables in dogs

OBJECTIVE

To evaluate the effect of carprofen on hemostatic variables in clinically normal dogs.

ANIMALS

12 clinically normal Labrador Retrievers.

PROCEDURE

10 dogs (6 females, 4 males) received carprofen (2.2 mg/kg of body weight, PO, q 12 h) for 5 days. Two dogs (untreated control group; 1 female, 1 male) did not receive carprofen. Hemostatic variables (platelet count, activated partial thromboplastin time, prothrombin time, fibrinogen, platelet aggregation, and bleeding time) were assessed for all dogs prior to treatment, on day 5 of treatment, and 2 and 7 days after discontinuation of the drug (days 7 and 12). Serum biochemical variables and Hct were assessed prior to treatment and on days 5 and 12.

RESULTS

In dogs receiving carprofen, platelet aggregation was significantly decreased, and onset of aggregation was significantly delayed on days 5, 7, and 12, compared with pretreatment values. Activated partial thromboplastin time was significantly increased on days 5, 7, and 12 over pretreatment values in treated dogs, but values remained within reference ranges. Significant differences were not detected in buccal mucosal bleeding time, other serum biochemical and hemostatic variables, or Hct, compared with pretreatment values and the internal control group.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of carprofen for 5 days causes minor but not clinically important alterations in hemostatic and serum biochemical variables in clinically normal Labrador Retrievers. Carprofen is commonly used to treat osteoarthritis and chronic pain in dogs, but prior to this study, its effect on platelet aggregation and hemostatic variables was unknown.

Regional anesthesia and anticoagulation

The perioperative use of neuraxial techniques in the presence of anticoagulation is a controversial issue. There are significant pharmacokinetic differences between anticoagulants that will affect the timing of neuraxial needle insertion or catheter removal. The pharmacologic profiles of commonly used anticoagulants in the perioperative period are reviewed. Studies examining the use of neuraxial techniques in the presence of various anticoagulants are reviewed and evaluated in the context of the American Society of Regional Anesthesia consensus statements.

Platelet function disorders

Platelet function defects comprise a large and heterogeneous group of bleeding disorders that range in severity from mild to severe. Patients may be asymptomatic; however, the majority who are diagnosed present with easy bruising and mucocutaneous bleeding, or with excessive haemorrhage following injury or surgery. As the complex internal biochemical and signal transduction pathways are further understood, and as structural analysis of platelets advances, more of the mechanisms leading to platelet function defects will be uncovered. Despite advances in the understanding of the etiology of these defects in function, treatment remains fairly rudimentary. For platelet function disorders associated with a defect in a plasma coagulation factor, such as vWD and afibrinogenemia, treatment consists of replacement of the deficient coagulation factor. Adjunctive therapies (such as antifibrinolytics, microfibular collagen, fibrin glue, etc.), DDAVP/Stimate, and platelet transfusions remain the mainstay of therapy available at this time. Corticosteroids may play a role in the management of some of these disorders and may lead to a shortening of bleeding time, especially in storage pool defects or release defects failing to respond to DDAVP/Stimate.

Contribution of in vivo models of thrombosis to the discovery and development of novel antithrombotic agents

Cardiovascular and cerebrovascular diseases continue to be the leading cause of death throughout the world. Over the past two decades, great advances have been made in the pharmacological treatment and prevention of thrombotic disorders (e.g., tissue plasminogen activators, platelet GPIIb/IIIa antagonists, ADP receptor antagonists such as clopidogrel, low-molecular weight heparins, and direct thrombin inhibitors). New research is leading to the next generation of antithrombotic compounds such as direct coagulation FVIIa inhibitors, tissue factor pathway inhibitors, gene therapy, and orally active direct thrombin inhibitors and coagulation Factor Xa (FXa) inhibitors. Animal models of thrombosis have played a crucial role in discovering and validiting novel drug targets, selecting new agents for clinical evaluation, and providing dosing and safety information for clinical trials. In addition, these models have provided valuable information regarding the mechanisms of these new agents and the interactions between antithrombotic agents that work by different mechanisms. This review briefly presents the pivitol preclinical studies that led to the development of drugs that have proven to be effective clinicallly. The role that animal models of thrombosis are playing in the discovery and development of novel antithrombotic agents is also described, with specific emphasis on FXa inhibitors. The major issues regarding the use of animal models of thrombosis, such as the use of positive controls, appropriate pharmacodynamic markers of activity, safety evaluation, species-specificity, and pharmacokinetics, are highlighted. Finally, the use of genetic models in thrombosis/hemostasis research and pharmacology is presented using gene-therapy for hemophilia as an example of how animal models have aided in the development of these therapies that are now being evaluated clinically. In summary, animal models have contributed greatly to the discovery of currently available antithrombotic agents and will play a primary role in the discovery and characterization of the novel antithrombotic agents that will provide safe and effective pharmacological treatment for life-threatening thrombotic diseases.