The effect of different antithrombotic regimens on platelet aggregation after myocardial infarction

Aspirin resistance in patients undergoing hemodialysis and effect of hemodialysis on aspirin resistance

The aim of this study was to evaluate the prevalence of aspirin resistance (AR) in patients undergoing hemodialysis (HD) and to assess the effect of HD on the results of the Multiplate test. A total of 54 patients undergoing HD were included in this study. Blood samples were taken just before and after the HD session. To determine AR, we used Multiplate test. Platelet aggregation values of the study population were 363.01 ± 225.69 aggregation unit (AU) × minutes before and 375.33 ± 254.05 AU × minutes after the HD (P = .597). There was strong correlation between the values before and after HD (R = .755, P < .0001). The AR status was changed in 9 (16.6%) patients after HD. Agreement of AR status before and after HD was substantial (κ coefficient = .667, P < .0001). The prevalence of AR in patients undergoing HD seems higher than in most of the studied populations, and this study has shown that the AR statuses of a significant number of patients undergoing HD change after an HD session.

Aspirin Resistance in Chinese Stroke Patients Increased the Rate of Recurrent Stroke and other Vascular Events

Introduction

To investigate the prevalent of aspirin resistance (AR) in Chinese stroke patients and its association with recurrent stroke and other vascular events, including cardiovascular disease and death.

Methods

We prospectively enrolled 634 Chinese stroke patients. Aspirin was administrated to every patient from the first day of admission. Whole blood samples were collected for platelet aggregation testing after aspirin was administered for 7–10 days. A follow-up period of 12–24 months was performed to record vascular events and hemorrhagic side effects.

Results

Aspirin resistance (AR) was detected in 129 patients (20·4%), aspirin semi-resistance (ASR) in 28 patients (4·4%) and aspirin sensitivity (AS) in 477 patients (75·2%). Logistic regression analysis found that diabetes and high levels of low density lipoprotein cholesterol (LDL) were independent risk factors for ASR and AR. During a median follow-up period of 19·4 months, the prevalence of recurrent stroke, death from all causes, myocardial infarction and vascular events overall were higher in patients with AR + ASR than in patients with AS. Cox regression analysis found that diabetes and AR were independent risk factors for vascular events.

Conclusion

Aspirin resistance is common in Chinese patient taking antiplatelet medications. Diabetes and high LDL may induce platelet activation and thrombosis and increase the occurrence of aspirin resistance. Patients who are detected to be aspirin resistant are at a greater risk of clinically important vascular events.

Antiplatelet therapy: aspirin resistance and all that jazz!

Platelets play a crucial role in the pathogenesis of atherosclerosis, thrombosis, and stroke. Aspirin used alone or in combination with other antiplatelet drugs has been shown to offer significant benefit to patients at high risk of vascular events. Resistance to the action of aspirin may decrease this benefit. Aspirin resistance has been defined by clinical and/or laboratory criteria; however, detection by laboratory methods prior to experiencing a clinical event will likely provide the greatest opportunity for intervention. Numerous laboratory methods with different cutoff points have been used to evaluate the resistance. Noncompliance with aspirin treatment has also confounded studies. A single assay is currently insufficient to establish resistance. Combinations of results to confirm compliance and platelet inhibition may identify "at-risk" individuals who truly have aspirin resistance. The most effective strategy for managing patients with aspirin resistance is unknown; however, studies are currently underway to address this issue.

Aspirin prophylaxis for the prevention of thrombosis: expectations and limitations

Platelets play a very important role in the pathogenesis of acute vascular events leading to thrombosis of the coronary and cerebral arteries. Blockage of these arteries leading to regional ischemia of heart and brain tissues precipitate heart attacks and stroke. Acetyl salicylic acid (Aspirin) has been the drug of choice for over half a century for the primary and secondary prophylaxis of thrombotic events. In spite of its extensive use as an antiplatelet drug for the prevention of vascular thrombosis, there is considerable concern about the degree of protection it offers, to patients under aspirin therapy. In this paper, we explain the phenomenon of aspirin resistance, discuss the limitations of aspirin therapy, and suggest methods to monitor "at-risk" individuals. Ability to monitor and determine at risk patients will provide opportunities for the clinicians to customize antiplatelet therapies.

Pharmacologic inhibition of platelet function: correlation between in vitro tests of platelet function and clinical outcomes

Atherothrombosis continues to be a leading cause of death and disability despite advances in pharmacologic and procedural therapies. Antiplatelet agents have been extensively studied and validated to improve outcomes in multiple settings of cardiovascular disease. The emergence of the phenomenon of resistance to antiplatelet therapy resulted in the availability of platelet function tests to assess the effectiveness of these agents. Subsequent evaluations have shown considerable inter-individual variability in platelet inhibition in patients receiving antiplatelet agents. Several small studies showed that patients who were deemed 'resistant' to antiplatelet therapy by platelet function testing had adverse clinical outcomes. It is essential that ongoing investigations help delineate a standard definition of 'resistance' to antiplatelet therapy, which test of platelet function should be the gold standard, and what therapy, once identified, can help overcome resistance to the currently available antiplatelet agents. Data are needed to determine if outcomes can be improved by changes to existing antiplatelet therapy based on the results of platelet function tests. Alternatively, newer antiplatelet agents may prove effective in overcoming resistance; however, these agents also await validation in large-scale clinical trials.

Aspirin resistance or variable response or both?

Numerous clinical trials have demonstrated that aspirin is effective in secondary prevention and in high-risk primary prevention of adverse cardiovascular events. However, a constellation of clinical and laboratory evidence exists that demonstrates diminished or absent response to aspirin in some patients. This has led to the concept of "aspirin resistance," which is a poorly defined, somewhat misleading term. The mechanism for aspirin resistance has not been fully established, but it is almost certainly due to a combination of clinical, biological, and genetic properties affecting platelet function. There are no criteria for distinguishing true resistance from treatment failure, and there is no consensus on whether the definition of aspirin resistance should be based on clinical outcomes, laboratory evidence, or both. Studies in large populations are needed to define antiplatelet resistance using consistent and reproducible assays and correlate the measurements with clinical outcomes. One such prospective randomized trial is completed, and 2 others are under way: the Clopidogrel for High Atherothrombotic Risk and Ischemic Stabilization, Management, and Avoidance (CHARISMA) trial compared clopidogrel and aspirin with placebo and aspirin for high-risk primary or secondary prevention, and the Aspirin Nonresponsiveness and Clopidogrel Endpoint Trial (ASCET) is evaluating whether switching to clopidogrel will be superior to continued aspirin therapy in improving clinical outcomes in aspirin-resistant patients with angiographically documented coronary artery disease. The Research Evaluation to Study Individuals Who Show Thromboxane or P2Y(12) Receptor Resistance (RESISTOR) trial is investigating whether modifying antiplatelet regimens could prevent myonecrosis after percutaneous coronary intervention in patients with aspirin and clopidogrel resistance.

POINT OF CARE TESTING OF ASPIRIN RESISTANCE IN PATIENTS WITH VASCULAR DISEASE

INTRODUCTION

The reported range in rates of aspirin resistance (5.5-60%) may reflect difficulties in studying platelet function and the variety of tests used. This study used a platelet function analyzer (PFA-100) to prospectively document aspirin resistance in a cohort of patients with arterial disease.

METHODS

Patients with internal carotid artery (ICA) stenosis or intermittent claudication (IC) were recruited. Exclusion criteria were contraindications to aspirin, prescription of other medication with known antiplatelet effects or known platelet abnormalities. After prescription of 100 mg aspirin/day for 2 weeks an uncuffed venous blood sample was taken and analysed with the PFA-100. Aspirin resistance was defined as closure time (CT) less than the upper limit of normal (158 s collagen/epinephrine agonist; 118 s collagen/adenosine diphosphate (ADP) agonist).

RESULTS

Thirty-three patients with IC and 12 patients with ICA stenosis were recruited (n = 45). Median (range) age was 74 years (49-85) and the male to female ratio was 1.5:1. The median (range) CT was >300 (85 to >300) s with collagen/epinephrine and 100 (52 to >300) s with collagen/ADP agonist. Twelve patients (27%) in the collagen/epinephrine group had normal CT despite treatment with 100 mg aspirin, indicating resistance. Of the 33 patients with collagen/epinephrine CT prolonged by aspirin, 10 patients also had prolonged collagen/ADP CT, suggesting excessive platelet inhibition.

CONCLUSION

A significant proportion of patients taking aspirin do not show laboratory evidence of platelet inhibition and may not be protected from atherothrombotic events. The PFA-100 appears to be a useful tool to screen for both aspirin resistance and excessive aspirin mediated platelet inhibition.

Antiplatelet resistance with aspirin and clopidogrel: is it real and does it matter?

Platelets play a pivotal role in the pathophysiology of ischemic complications of atherosclerotic cardiovascular disease. Aspirin and clopidogrel are oral antiplatelet drugs that have been shown to reduce adverse clinical events across the wide spectrum of patients with atherothrombotic disease. However, recurrent ischemic events still occur in a significant proportion of patients despite treatment with these antiplatelet drugs. The concept of antiplatelet resistance therefore emerges. Although uniform definitions and standardized assays are not yet available, numerous studies have documented the interindividual variability in platelet responsiveness to oral antiplatelet drugs. Evidence is also accumulating to demonstrate that hyporesponsiveness to antiplatelet drugs in the laboratory (ie, resistance) is associated with adverse clinical events in different patient populations. Clinical application of antiplatelet resistance will require proof from prospective randomized trials that modifications of antiplatelet therapy based on tests of antiplatelet responsiveness will improve the outcomes of patients with antiplatelet resistance.

Antiplatelet therapy in peripheral occlusive arterial disease

BACKGROUND

Antiplatelet therapy (APT) in patients with peripheral occlusive arterial disease (POAD) may reduce cardiovascular (CV) morbidity and mortality by inhibiting atherothrombosis. This article reviews the current evidence for APT in patients with stable POAD and in patients undergoing revascularization procedures for POAD.

METHODS

A Medline and Pubmed literature search (January 1966 to February 2003) was conducted to identify articles relating APT and POAD. Manual cross referencing was also used.

RESULTS AND CONCLUSIONS

Meta-analyses suggest that APT (most commonly aspirin) in patients with stable POAD significantly reduces the incidence of nonfatal stroke, myocardial infarction and CV death. However, this conclusion is based on subset analysis of data predominantly involving patients with coronary and cerebrovascular atherosclerosis. There is a little direct evidence for the use of aspirin in patients with isolated POAD, but in practice, aspirin remains the most commonly used antiplatelet agent as high rates of coronary and cerebrovascular diseases are observed in this patient population. For patients with POAD without additional indicators of vascular risk, the protective effect of aspirin is unclear and dependent on the balance of risks and benefits in the individual patient. For patients undergoing peripheral revascularization, ticlopidine and aspirin in combination with dipyridamole are effective in maintaining patency after bypass procedures and following angioplasty/femoral endarterectomy. The efficacy of thienopyridines in peripheral angioplasty is uncertain, and the optimum timing and duration of APT relative to intervention are not known.

Resistance to antiplatelet drugs: current status and future research

Platelet reactivity and activation are important factors during the development of atherothrombotic processes and subsequent ischaemic complications. Pharmacological agents that suppress platelet function are proved to be the most efficient in the prevention and treatment of thrombotic complications. As the activation of platelets during thrombus generation involves many complex and redundant pathways, simultaneous use of different antiplatelet drugs that are directed against different targets have been effective in reducing adverse clinical events. The main antiplatelet drugs are aspirin (which inhibits thromboxane synthesis), thienopyridines (which block P2Y12 receptors) and glycoprotein IIb/IIIa antagonists (which block glycoprotein IIb/IIIa receptors). In recent years, resistance or nonresponsiveness to antiplatelet therapy has been reported and, more importantly, are linked to the occurrence of adverse cardiovascular events. New treatment strategies to overcome nonresponsiveness are being sought. A focus on the development of simple, reproducible and user friendly point-of-care methods to determine aspirin/clopidogrel responsiveness should be undertaken to assist clinicians in tailoring antiplatelet therapy to the individual patient.

Aspirin resistance in stroke: 2004

Aspirin is a well-established medication in the treatment of atherothrombotic vascular disease. However, despite aspirin treatment a substantial number of patients experience recurring ischaemic episodes. Aspirin resistance denotes those situations when it is unable to protect individuals from thrombotic complications, or when it fails to produce an anticipated effect in laboratory tests of platelet function. There are various laboratory techniques with which to evaluate the effectiveness of aspirin and other antiplatelet drugs. It has been estimated that in 5-60% of patients, aspirin does not achieve adequate efficacy in various measures of platelet activity. Some studies have revealed that vascular patients shown by laboratory tests to be aspirin-resistant are at an increased risk of major vascular events. The suggested mechanisms of aspirin resistance, among others, include genetic polymorphisms, alternate pathways of platelet activation, aspirin-insensitive thromboxane biosynthesis, drug interactions, or low aspirin dose. An increase in the dosage of aspirin or conversion to clopidogrel or clopidogrel plus aspirin might be beneficial in the management of those patients who are aspirin resistant. Additional work is required to improve and validate laboratory tests of platelet function, so that they may become useful tools for selecting the most appropriate antiplatelet therapy for an individual patient. Improvements in antiplatelet treatment strategies in the future should lead to a reduction in premature vascular events.

Aspirin Resistance in Cardiovascular Disease: A Review

BACKGROUND

Aspirin is effective at reducing the cardiovascular event rate in defined patient groups. The introduction of antiplatelet therapies other than aspirin and the concept of aspirin resistance have led to critical reappraisal of current treatment. This review aims to clarify the evidence for aspirin resistance in patients with atherosclerosis.

METHODS

Medline search was performed to identify publications concerned with antiplatelet effects of aspirin and failure of aspirin therapy. Manual cross referencing was also performed.

RESULTS AND CONCLUSION

Wide variations in the rate of aspirin resistance (5.5-75%) have been reported. The lack of consensus on an appropriate definition and the number of different tests used to investigate aspirin resistance needs to be addressed. There are few studies where the primary aim was to document aspirin resistance or aspirin non-response. Further work should aim to investigate if aspirin resistance is clinically important and, if it is, what treatments may be beneficial to the at risk patient.

Incidence of aspirin nonresponsiveness using the Ultegra Rapid Platelet Function Assay-ASA

We report the incidence of aspirin nonresponsiveness in a prospective, multicenter registry (n=422 patients) to be 23% using the Ultegra Rapid Platelet Function Assay-ASA, and determined a history of coronary artery disease to be associated with twice the odds of being an aspirin nonresponder (odds ratio 2.01, 95% confidence interval 1.189 to 3.411, p=0.009). Further prospective studies are needed to correlate aspirin nonresponsiveness to adverse clinical events.

Influence of aspirin on inflammatory markers in patients after acute myocardial infarction

The aim of this study was to assess the influence of aspirin on selected inflammatory markers in patients recovering from acute myocardial infarction (AMI). Patients participating in the Warfarin Aspirin Re-Infarction Study-II were randomized to either aspirin 160 mg/day or aspirin 75 mg/day + warfarin, or warfarin alone after AMI. After AMI, aspirin 160 mg/day was associated with significantly lower levels of high-sensitivity C-reactive protein and tumor necrosis factor-alpha than warfarin alone over 4 years. However, the same levels were not predictors for clinical end points.

Aspirin non-responsiveness as measured by PFA-100 in patients with coronary artery disease

INTRODUCTION

The purpose of the present study was to study the concept of aspirin resistance or non-responsiveness by investigating the response to long-term aspirin therapy in patients with a former acute myocardial infarction (AMI).

MATERIALS AND METHODS

Patients with an AMI (n=202) randomly assigned to aspirin 160 mg/day (n=71), aspirin 75 mg/day and warfarin (INR 2.0-2.5) (n=58) or warfarin (INR 2.8-4.2) (n=73) were evaluated by the PFA-100(R), biochemical variables and clinical events after a mean treatment period of 4 years.

RESULTS

The limit for being an aspirin non-responder was defined as the 95th percentile value in the warfarin alone group (196 s) with the epinephrine cartridge. In patients on aspirin alone 25/71 (35%) were non-responders and on the combination 23/58 (40%). With the adenosine diphosphate (ADP) cartridge only minor differences were found. The levels of thromboxane B(2) in both aspirin groups, in responders as well as in non-responders, were extremely low compared to the warfarin alone group. Evaluating both aspirin groups together (n=129), the levels of soluble P-selectin were significantly higher in non-responders as compared to responders (p=0.012). During the observation period of 4 years with limited number of events, there was a tendency for higher event rates in non-responders as compared to responders (36% vs. 24%, p=0.28).

CONCLUSIONS

In our evaluation of the PFA-100(R) a considerable number of post-AMI patients seemed to be non-responders to long-term aspirin therapy in doses of 75 and 160 mg/day. Circulating levels of P-selectin were higher in the non-responders. A tendency to higher incidence of clinical events among non-responders was observed.

Increased platelet aggregability during exercise in patients with previous myocardial infarction. Lack of inhibition by aspirin

The aim of the present study was to investigate the effects of acute exercise on platelet aggregability, blood coagulation, and fibrinolysis in patients with recent myocardial infarction, and to examine these effects in relation to two different antithrombotic regimens. Forty patients (mean age 60 years) were investigated 3 months after a myocardial infarction. They were randomized to antithrombotic treatment with either warfarin (INR 2.8-4.2) or aspirin 160 mg daily. They performed a standardized ergometer bicycle exercise test. Blood was drawn before and after the exercise. The platelet function tests included a platelet aggregate ratio (PAR), which, in the presence of aggregates, is<1. The coagulation products remained largely unchanged during the exercise, whereas the fibrinolytic activity and the catecholamine levels increased significantly. At baseline, PAR was lower in the warfarin group than in the aspirin group. During exercise, PAR was significantly reduced in both study groups (0.75 vs. 0.80), indicating increased platelet aggregability. Beta-thromboglobulin decreased in both groups. The increased platelet aggregability after exercise despite aspirin is probably due to activation by catecholamines. This implies that aspirin may have a limited antithrombotic effect during physical exercise and probably also in other situations with increased catecholamine levels.