Aspirin dose in stroke prevention: beautiful hypotheses slain by ugly facts

Antiplatelet Agents in Cardiovascular and Cerebrovascular Diseases

Aspirin has become a widely accepted platelet function inhibitor and is used to prevent arterial occlusions in coronary cerebral and peripheral vascular disease. The results of clinical studies with aspirin in the area of peripheral arterial occlusive disease are critically reviewed. Two thienopyridine compounds, ticlopidine and clopidogrel, have been effectively used in the prevention of myocardial infarction and stroke in several clinical trials, especially in the recently published CAPRIE-trial. Potent new platelet function inhibitors recently were developed. Intravenous treatment with abciximab, a new platelet membrane glycoprotein IIb/IIIa-inhibitor, effectively prevented coronary reocclusions in patients with high-risk coronary events. A series of promising new oral IIb/IIIa- inhibitors have been developed and may become effective drugs in the prevention of reocclusions in patients with periph eral vascular disease and in coronary or cerebral vascular dis ease. Key Words: Antiplatelet agents—Aspirin—Peripheral arterial disease—Ticlopidine—Clopidogrel—GPIIb/IIIa inhibitor.

Antiplatelet drugs: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

The article describes the mechanisms of action, pharmacokinetics, and pharmacodynamics of aspirin, dipyridamole, cilostazol, the thienopyridines, and the glycoprotein IIb/IIIa antagonists. The relationships among dose, efficacy, and safety are discussed along with a mechanistic overview of results of randomized clinical trials. The article does not provide specific management recommendations but highlights important practical aspects of antiplatelet therapy, including optimal dosing, the variable balance between benefits and risks when antiplatelet therapies are used alone or in combination with other antiplatelet drugs in different clinical settings, and the implications of persistently high platelet reactivity despite such treatment.

Effects of various doses of aspirin on platelet activity and endothelial function

Although aspirin has become an established medicine for cardiac and cerebrovascular diseases, the optimal dose remains unknown. We evaluated the optimal dose of aspirin on platelet activity and endothelial function by administering 11 healthy male volunteers (32 ± 6 years of age) doses of aspirin that were increased in a stepwise manner (0, 81, 162, 330 and 660 mg/day) every 3 days. Platelet activity was assessed as surface P-selectin expression (%) measured by flow cytometry and the platelet aggregation ratio. Endothelial function in the brachial artery was assessed by measuring flow-mediated dilation (FMD) before and after reactive hyperemia. Platelet aggregation and P-selectin expression were significantly and dose-dependently suppressed (81-660 mg), and the FMD ratio tended to increase from 0 to 162 mg, but decreased significantly at 660 mg. In conclusion, although aspirin suppressed platelet activity and even surface P-selectin expression, higher doses worsened endothelial-mediated arterial dilation.

Platelet cyclooxygenase inhibition by low-dose aspirin is not reflected consistently by platelet function assays: implications for aspirin "resistance"

OBJECTIVE

This study was conducted to assess the thromboxane (TX) dependence of biochemical and functional indexes used to monitor the effect of low-dose aspirin.

BACKGROUND

Functional assays of the antiplatelet effects of low-dose aspirin variably reflect the TX-dependent component of platelet aggregation. Previous studies of aspirin resistance were typically based on a single determination of platelet aggregation.

METHODS

We assessed the TXB(2) dependence of biochemical and functional indexes, as well as their intersubject and intrasubject variability during administration of the drug and after its withdrawal in 48 healthy volunteers randomized to receive aspirin 100 mg daily for 1 to 8 weeks.

RESULTS

Serum TXB(2) was uniformly suppressed by 99% of baseline. Urinary 11-dehydro-TXB(2), arachidonic acid-induced aggregation, and VerifyNow Aspirin (Accumetrics Inc., San Diego, California) showed stable, incomplete inhibition (65%, 80%, and 35%, respectively). Adenosine diphosphate- and collagen-induced aggregation was highly variable and poorly affected by aspirin, with an apparent time-dependent reversal. Inhibition of platelet cyclooxygenase activity was nonlinearly related to inhibition of platelet aggregation. Platelet function largely recovered by day 3 post-aspirin, independently of treatment duration. With any functional assay, occasionally "resistant" subjects were found to be "responders" on previous or subsequent determinations.

CONCLUSIONS

Platelet cyclooxygenase activity, as reflected by serum TXB(2) levels, is uniformly and persistently suppressed by low-dose aspirin in healthy subjects. However, the effect of aspirin is variably detected by functional assays, potentially leading to misclassification of "responder" as "resistant" phenotypes owing to poor reproducibility of functional measurements. The nonlinearity of the relationship between inhibition of TX production and inhibition of platelet function has important clinical implications.

Antiplatelet drugs: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition)

This article about currently available antiplatelet drugs is part of the Antithrombotic and Thrombolytic Therapy: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). It describes the mechanism of action, pharmacokinetics, and pharmacodynamics of aspirin, reversible cyclooxygenase inhibitors, thienopyridines, and integrin alphaIIbbeta3 receptor antagonists. The relationships among dose, efficacy, and safety are thoroughly discussed, with a mechanistic overview of randomized clinical trials. The article does not provide specific management recommendations; however, it does highlight important practical aspects related to antiplatelet therapy, including the optimal dose of aspirin, the variable balance of benefits and hazards in different clinical settings, and the issue of interindividual variability in response to antiplatelet drugs.

Does Prior Aspirin Use Reduce Stroke Mortality?

BACKGROUND

Millions of people around the world regularly consume aspirin, but its value in determining stroke severity is still not clear. It has been stated that prior aspirin use might reduce the severity of ischemic stroke by reducing the size of fibrin-platelet emboli or by ameliorating platelet hyperaggregability that occurs in the microcirculation. However there are only few large studies focusing on the early outcome of stroke patients including both ischemic stroke and primary intracerebral hemorrhage patients with prior aspirin therapy.

REVIEW SUMMARY

We retrospectively analyzed the medical records of 2509 consecutive stroke patients who were hospitalized in the 2nd Neurology Clinic of Haydarpaşa Numune Education and Research Hospital, Istanbul, Turkey, during the period of 1993-2003. We compared the early mortality rates in the patients with prior aspirin use versus the patients without. Of the patients without prior aspirin use, 20.1% died during the first 3 weeks of the stroke, while only 11.5% of the patients with prior aspirin use died during the same period. The difference in early mortality rate between 2 groups was extremely significant (P = 0.0008). Logistic regression analysis indicated that aspirin use was a significant (P < 0.01) and independent predictor of early stroke mortality.

CONCLUSION

Daily low dose (100-300 mg) aspirin has a protective effect in reducing the risk of early death in stroke.

The dose of aspirin for the prevention of cardiovascular and cerebrovascular events

BACKGROUND

Low dose aspirin (ASA) (75-325 mg daily) is commonly used for the secondary prevention of cardiovascular and cerebrovascular events, as recommended by US national guidelines. Questions remain, however, as to whether there is a difference in the efficacy and safety across this low dose range.

SCOPE

Double-blind controlled studies, meta-analyses, and observational analyses were reviewed to assess the body of evidence regarding the safety and efficacy of aspirin dosing.

FINDINGS

Only one double-blind study directly compared two doses of aspirin within the recommended low dose range. No difference in efficacy or safety was observed, although there was a trend toward greater efficacy with ASA 325 mg vs. ASA 81 mg. Three meta-analyses did not find a difference in bleeding events within the low dose range, while one found that higher doses were associated with more events. One meta-analysis found ASA 75-150 mg was as effective as ASA 160-325 mg. Observational analyses of low dose (75-325 mg) ASA from two large controlled trials differed in their results. One study found no difference in the number of cardiovascular/cerebrovascular events, and a significant improvement in mortality with higher doses, while the other found that higher doses were associated with more events.

CONCLUSION

There does not appear to be a difference in safety across the low dose range of 75-325 mg based on randomized controlled trial data. Furthermore, ASA 325 mg daily appears to be at least as effective as 75 mg daily. Since the optimal dose of ASA for primary and secondary prevention of events in the broad population is uncertain, dosing considerations should include an evaluation of a patient's individual clinical status as well as an overall cardiovascular and cerebrovascular benefit vs. risk assessment.

Place of drug therapy in the treatment of carotid stenosis

Carotid stenosis is an important cause of transient ischaemic attacks and stroke. The cause of carotid stenosis is most often atherosclerosis; contributing to the pathogenesis of the lesion are endothelial injury, inflammation, lipid deposition, plaque formation, fibrin, platelets and thrombin. Carotid stenosis accounts for 10-20% of cases of brain infarction, depending on the population studied. Despite successful treatment of selected patients who have had an acute ischaemic stroke with tissue plasminogen activator and the promise of other experimental therapies, prevention remains the best approach to reducing the impact of ischaemic stroke. High-risk or stroke-prone patients can be identified and targeted for specific interventions. At this juncture, treatment of carotid stenosis is a well established therapeutic target and a pillar of stroke prevention. There are two main strategies for the treatment of carotid stenosis. The first approach is to stabilise or halt the progression of the carotid plaque through risk factor modification and medication. Hypertension, diabetes mellitus, smoking, obesity and high cholesterol levels are closely associated with carotid stenosis and stroke; control of these factors may decrease the risk of plaque formation and progression. The second approach is to eliminate or reduce carotid stenosis through carotid endarterectomy or carotid angioplasty and stenting. Carotid endarterectomy, which is the mainstay of therapy for severe carotid stenosis, is beyond the scope of this review. Anticoagulants seem to play little role (if any) in the medical (i.e. non-surgical) treatment of carotid stenosis. Adoption of a healthy lifestyle combined with the reduction of risk factors has been shown to lead to a reduction in the extent of carotid stenosis. The medical treatment of carotid stenosis should be based on the triad of the reduction of risk factors, patient education, and use of antiplatelet agents.

Platelet-Active Drugs: The Relationships Among Dose, Effectiveness, and Side Effects

This article discusses platelet active drugs as part of the Seventh American College of Chest Physicians Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. New data on antiplatelet agents include the following: (1) the role of aspirin in primary prevention has been the subject of recommendations based on the assessment of cardiovascular risk; (2) an increasing number of reports suggest a substantial interindividual variability in the response to antiplatelet agents, and various phenomena of "resistance" to the antiplatelet effects of aspirin and clopidogrel; (3) the benefit/risk profile of currently available glycoprotein IIb/IIIa antagonists is substantially uncertain for patients with acute coronary syndromes who are not routinely scheduled for early revascularization; (4) there is an expanding role for the combination of aspirin and clopidogrel in the long-term management of high-risk patients; and (5) the cardiovascular effects of selective and nonselective cyclooxygenase-2 inhibitors have been the subject of increasing attention.

Antithrombotic and thrombolytic therapy for ischemic stroke

Antithrombotic therapy is the mainstay of treatment for stroke prevention. Multiple antiplatelet agents are now proven options for patients at risk for stroke, whereas warfarin anticoagulation remains the preferred therapy for most patients with atrial fibrillation. Recent clinical trials have clarified the role of anticoagulation in acute stroke and in secondary prevention of noncardioembolic stroke. Intravenous tissue plasminogen activator is the only approved therapy for patients with acute ischemic stroke. Intra-arterial thrombolysis is emerging as a promising therapy in selected patients.

A benefit-risk assessment of agents used in the secondary prevention of stroke

Stroke is a major cause of morbidity and mortality. Full assessment of stroke or transient ischaemic attack (TIA) patients is required to identify all risk factors and apply appropriate secondary preventative strategies. Antiplatelet therapies are effective in the secondary prevention of ischaemic stroke and can be justified despite adverse effects such as gastrointestinal haemorrhage. Aspirin (acetylsalicylic acid), aspirin plus dipyridamole, ticlopidine and clopidogrel are all of value but their adverse effect profiles vary significantly. Combinations of antiplatelet agents may offer additional benefit but not all combinations have been studied in stroke patients. Anticoagulation with agents such as warfarin is effective with coexisting atrial fibrillation and other conditions predisposing to cardioembolic stroke. Antihypertensive agents have been extensively studied in the primary prevention of stroke; however, relatively few trials of antihypertensive agents in the secondary prevention of stroke are available. The incidence of adverse effects of antihypertensive agents is relatively low and the benefit-risk profile would tend to favour their use in the secondary prevention of stroke. Recent studies of ACE inhibitors have identified an important role for these agents in the secondary prevention of stroke even in those who are normotensive and in those who have had a haemorrhagic stroke. The incidence of serious adverse effects with ACE inhibitors appears relatively low. Lipid-lowering agents may have a role to play in certain groups of patients with stroke. The incidence of adverse effects is relatively low with HMG-CoA reductase inhibitors. Cigarette smoking is an important risk factor for stroke and evidence is available that smoking cessation does reduce the individual's risk of stroke. Pharmacological agents are available to help smoking cessation. In patients with diabetes mellitus, intensive regimens with insulin and oral hypoglycaemic agents have so far not definitively been shown to reduce the incidence of macrovascular complications such as stroke. Tight glycaemic control has been shown to improve microvascular complications such as retinopathy, nephropathy and neuropathy and hence this is reason enough to advocate the use of these agents. Future developments in the treatment of diabetes may help. Secondary prevention of stroke has improved greatly over the past decade and hopefully will continue to improve. The use of pharmacological agents available currently and in the future will be clarified and refined as further clinical trials report.

Increased platelet aggregation and activation in peripheral arterial disease

OBJECTIVES

patients with peripheral arterial disease (PAD) have a threefold increase in cardiovascular mortality. Standard antiplatelet treatment may not confer uniform benefit in different patient groups. This study aimed to compare platelet function in patients with lower limb PAD, carotid disease and abdominal aortic aneurysm (AAA) with age- and sex-matched healthy controls.

METHODS

patients with lower limb PAD (n = 20), carotid disease (n = 40), AAA (n = 13) and age/sex matched healthy controls (n= 20) were studied. Whole blood methods to detect spontaneous platelet aggregation (SPA), and adenosine diphosphate (ADP) and collagen-induced aggregation were used. The detection of platelet P-selectin and the PAC-1 antigen by flow cytometry were also used as markers of platelet activation and aggregation.

RESULTS

patients with lower limb PAD or AAA had higher baseline SPA compared to normal controls (p < 0.01). There was significantly higher collagen-induced aggregation in IC patients compared to normal controls (p < 0.01). However, there was no difference in ADP-induced aggregation between lower limb PAD and control patients. There was no difference in PAC-1 binding between control patients and the patients with lower limb PAD, carotid disease or AAA. Patients with carotid disease had a higher expression of P-selectin compared to normal controls (p < 0.05).

CONCLUSIONS

this study provides further evidence that platelet hyperactivity is present in patients with PAD despite the use of antiplatelet therapy. Further antiplatelet strategies may be indicated to protect these patients.

Collaborative meta-analysis of randomised trials of antiplatelet therapy for prevention of death, myocardial infarction, and stroke in high risk patients

OBJECTIVE

To determine the effects of antiplatelet therapy among patients at high risk of occlusive vascular events.

DESIGN

Collaborative meta-analyses (systematic overviews).

INCLUSION CRITERIA

Randomised trials of an antiplatelet regimen versus control or of one antiplatelet regimen versus another in high risk patients (with acute or previous vascular disease or some other predisposing condition) from which results were available before September 1997. Trials had to use a method of randomisation that precluded prior knowledge of the next treatment to be allocated and comparisons had to be unconfounded-that is, have study groups that differed only in terms of antiplatelet regimen.

STUDIES REVIEWED

287 studies involving 135 000 patients in comparisons of antiplatelet therapy versus control and 77 000 in comparisons of different antiplatelet regimens.

MAIN OUTCOME MEASURE

"Serious vascular event": non-fatal myocardial infarction, non-fatal stroke, or vascular death.

RESULTS

Overall, among these high risk patients, allocation to antiplatelet therapy reduced the combined outcome of any serious vascular event by about one quarter; non-fatal myocardial infarction was reduced by one third, non-fatal stroke by one quarter, and vascular mortality by one sixth (with no apparent adverse effect on other deaths). Absolute reductions in the risk of having a serious vascular event were 36 (SE 5) per 1000 treated for two years among patients with previous myocardial infarction; 38 (5) per 1000 patients treated for one month among patients with acute myocardial infarction; 36 (6) per 1000 treated for two years among those with previous stroke or transient ischaemic attack; 9 (3) per 1000 treated for three weeks among those with acute stroke; and 22 (3) per 1000 treated for two years among other high risk patients (with separately significant results for those with stable angina (P=0.0005), peripheral arterial disease (P=0.004), and atrial fibrillation (P=0.01)). In each of these high risk categories, the absolute benefits substantially outweighed the absolute risks of major extracranial bleeding. Aspirin was the most widely studied antiplatelet drug, with doses of 75-150 mg daily at least as effective as higher daily doses. The effects of doses lower than 75 mg daily were less certain. Clopidogrel reduced serious vascular events by 10% (4%) compared with aspirin, which was similar to the 12% (7%) reduction observed with its analogue ticlopidine. Addition of dipyridamole to aspirin produced no significant further reduction in vascular events compared with aspirin alone. Among patients at high risk of immediate coronary occlusion, short term addition of an intravenous glycoprotein IIb/IIIa antagonist to aspirin prevented a further 20 (4) vascular events per 1000 (P<0.0001) but caused 23 major (but rarely fatal) extracranial bleeds per 1000.

CONCLUSIONS

Aspirin (or another oral antiplatelet drug) is protective in most types of patient at increased risk of occlusive vascular events, including those with an acute myocardial infarction or ischaemic stroke, unstable or stable angina, previous myocardial infarction, stroke or cerebral ischaemia, peripheral arterial disease, or atrial fibrillation. Low dose aspirin (75-150 mg daily) is an effective antiplatelet regimen for long term use, but in acute settings an initial loading dose of at least 150 mg aspirin may be required. Adding a second antiplatelet drug to aspirin may produce additional benefits in some clinical circumstances, but more research into this strategy is needed.

Collaborative meta-analysis of randomised trials of antiplatelet therapy for prevention of death, myocardial infarction, and stroke in high risk patients

OBJECTIVE

To determine the effects of antiplatelet therapy among patients at high risk of occlusive vascular events.

DESIGN

Collaborative meta-analyses (systematic overviews).

INCLUSION CRITERIA

Randomised trials of an antiplatelet regimen versus control or of one antiplatelet regimen versus another in high risk patients (with acute or previous vascular disease or some other predisposing condition) from which results were available before September 1997. Trials had to use a method of randomisation that precluded prior knowledge of the next treatment to be allocated and comparisons had to be unconfounded-that is, have study groups that differed only in terms of antiplatelet regimen.

STUDIES REVIEWED

287 studies involving 135 000 patients in comparisons of antiplatelet therapy versus control and 77 000 in comparisons of different antiplatelet regimens.

MAIN OUTCOME MEASURE

"Serious vascular event": non-fatal myocardial infarction, non-fatal stroke, or vascular death.

RESULTS

Overall, among these high risk patients, allocation to antiplatelet therapy reduced the combined outcome of any serious vascular event by about one quarter; non-fatal myocardial infarction was reduced by one third, non-fatal stroke by one quarter, and vascular mortality by one sixth (with no apparent adverse effect on other deaths). Absolute reductions in the risk of having a serious vascular event were 36 (SE 5) per 1000 treated for two years among patients with previous myocardial infarction; 38 (5) per 1000 patients treated for one month among patients with acute myocardial infarction; 36 (6) per 1000 treated for two years among those with previous stroke or transient ischaemic attack; 9 (3) per 1000 treated for three weeks among those with acute stroke; and 22 (3) per 1000 treated for two years among other high risk patients (with separately significant results for those with stable angina (P=0.0005), peripheral arterial disease (P=0.004), and atrial fibrillation (P=0.01)). In each of these high risk categories, the absolute benefits substantially outweighed the absolute risks of major extracranial bleeding. Aspirin was the most widely studied antiplatelet drug, with doses of 75-150 mg daily at least as effective as higher daily doses. The effects of doses lower than 75 mg daily were less certain. Clopidogrel reduced serious vascular events by 10% (4%) compared with aspirin, which was similar to the 12% (7%) reduction observed with its analogue ticlopidine. Addition of dipyridamole to aspirin produced no significant further reduction in vascular events compared with aspirin alone. Among patients at high risk of immediate coronary occlusion, short term addition of an intravenous glycoprotein IIb/IIIa antagonist to aspirin prevented a further 20 (4) vascular events per 1000 (P<0.0001) but caused 23 major (but rarely fatal) extracranial bleeds per 1000.

CONCLUSIONS

Aspirin (or another oral antiplatelet drug) is protective in most types of patient at increased risk of occlusive vascular events, including those with an acute myocardial infarction or ischaemic stroke, unstable or stable angina, previous myocardial infarction, stroke or cerebral ischaemia, peripheral arterial disease, or atrial fibrillation. Low dose aspirin (75-150 mg daily) is an effective antiplatelet regimen for long term use, but in acute settings an initial loading dose of at least 150 mg aspirin may be required. Adding a second antiplatelet drug to aspirin may produce additional benefits in some clinical circumstances, but more research into this strategy is needed.

Cyclo-oxygenase-1 and cyclo-oxygenase-2 and cardiovascular system

Prostaglandins play important roles in the pathophysiological mechanism of action of platelets and endothelial cells in the cardiovascular system. The two isoforms of cyclo-oxygenase, respectively cyclo-oxygenase-1 and cyclo-oxygenase-2, are differently expressed in these cells. Activated platelets show a relatively large amount of cyclo-oxygenase-1, whereas endothelial cells have the gene for cyclo-oxygenase-2, the expression of which follows cell activation. In the atherosclerosis lesion, prostaglandin synthesis is mainly mediated by the inducible cyclo-oxygenase-2 expressed in macrophages/foam cells, smooth muscle cells and endothelial cells. Aspirin, a selective platelet cyclo-oxygenase-1 inhibitor still remains the most extensively studied antiplatelet agent, even though there is growing evidence that many other compounds could be valuable either in association, or alternatives in antithrombotic therapy.

Antiplatelet therapy in the elderly. Aspirin, ticlopidine-clopidogrel, and GPIIb/GPIIIa antagonists

Antiplatelet agents including aspirin, dipyridamole, the thienopyridines, and the GPIIb/IIIa antagonists have collectively demonstrated their ability to have a significant impact on the incidence of recurrent MIs, strokes, and other vascular ischemic events in the geriatric population. Low-dose aspirin also seems to be effective and safe for the primary prevention of ischemic heart disease in men considered at high risk. There is no evidence that the recommendations from these studies had increased relevance to younger adults, and the studies considering age as a variable found antiplatelet agents had either similar or increased benefit in older patients. In view of the relatively reduced adverse effects of these agents when compared with their potential therapeutic benefit, it is important that they be considered in all older patients for secondary prevention and in certain high-risk groups for primary prevention of cardiovascular morbidity and mortality.

[Aspirin and cerebral ischemic accidents]

At the acute phase of cerebral infarction, two recent large studies found that the use of aspirin reduces both mortality and the risk of the recurrence of stroke. In primary prevention, aspirin nearly halves the risk of myocardial infarction but does not reduce that of stroke. Concerning the secondary prevention of atherothrombotic brain infarcts, aspirin has been the most extensively studied drug, and is efficient between 50 mg and 1.3 g. In spite of the efficacy of other antiplatelets in this indication--ticlopidine (500 mg), clopidogrel (75 mg) and dipyridamole (400 mg)--aspirin remains the most cost-effective, doses between 100 and 300 mg being the most widely used. Cardiac diseases with a high embolic risk require the use of oral anticoagulation. In nonvalvular atrial fibrillation, the choice of antithrombotic drugs depends on risk stratification: oral anticoagulants are indicated in high-risk subjects, whereas aspirin is recommended in low-risk subjects and when oral anticoagulants are contraindicated. Studies with associations of aspirin and other antiplatelets are required to increase the yield of this medication in high-risk subjects, in parallel with efforts to detect and to treat the vascular risk factors.

Ex vivo response to aspirin differs in stroke patients with single or recurrent events: a pilot study

The dose of aspirin for secondary stroke prevention and the clinical meaning of ex vivo platelet abnormalities are debated. We assessed prospectively 39 noncardioembolic stroke patients in which 300 mg/day aspirin had proved effective (n=24) or ineffective (n=15) to prevent recurrent ischemic events. We evaluated platelet aggregation induced by arachidonic acid, adenosine diphosphate and epinephrine, and the sensitivity of platelets to increasing concentrations of the synthetic thromboxane mimetic U46619. Aggregation studies were repeated while subjects received 300 (study phase 1), and 600 (study phase 2) mg/day aspirin, respectively. Overall, arachidonic acid-induced platelet aggregation was less effectively inhibited during study phase 1 compared to phase 2. Arachidonic acid and epinephrine promoted a stronger platelet aggregation in aspirin nonresponders than in aspirin responders while taking 300 mg/day aspirin. On the other hand, 600 mg/day effectively inhibited platelet function in both clinical groups. A lower sensitivity to thromboxane receptors was also found during phase 1 of the study, although the response was similar between aspirin responders and nonresponders. This pilot study suggests that 300 mg/day aspirin is less effective than 600 mg/day to block the cyclooxygenase pathway in noncardioembolic stroke and, incomplete cyclooxygenase inhibition is associated with recurrent thromboembolic events despite adequate aspirin compliance. It is likely that patients could receive a more efficacious stroke prevention if the dose of aspirin is tailored to individual needs as reflected by laboratory findings.

Antiplatelet therapy. Aspirin, ticlopidine/clopidogrel, and anti-integrin agents

Aspirin is the most widely employed antithrombotic agent in use today and has a proven role in the prevention and acute management of atherosclerosis-associated arterial thrombotic events. More recently developed antiplatelet agents have been found to have specific prophylactic roles associated with percutaneous coronary intervention and other clinical settings. This article outlines pharmacologic considerations and current clinical knowledge relevant to the use of aspirin, ticlopidine, clopidogrel, and the GPIIbIIIa antagonists in the management of thrombotic disorders.

Prevention of myocardial infarction and stroke by aspirin: different mechanisms? Different dosage?

More than 50 randomized trials have documented the efficacy and safety of aspirin as an antiplatelet agent and a cardiovascular drug. However, the optimal dose for preventing coronary and cerebral thrombosis has long been a cause of debate. For patients with ischaemic heart disease the range recommended for the prevention of a secondary event, based on strong clinical evidence, is 75-160 mg aspirin/day. For patients with cerebrovascular disease, recommendations range from 30-1300 mg/day. If these patients require a higher dose of aspirin it suggests that a different mechanism of action is involved. This paper considers hypotheses and reports the findings of recent clinical trials. The SALT study compared aspirin with placebo in 1360 patients with TIA or minor ischaemic stroke. It showed an 18% reduction in the risk of stroke or death in patients receiving 75 mg aspirin/day. Five other trials of 55,000 patients with ischaemic cerebrovascular disease compared the protective effect of aspirin (range 30-300 mg/day) with placebo, clopidogrel, or oral anticoagulants. Aspirin was better than placebo, safer than oral anticoagulants, and no different from clopidogrel. The implications of these findings are discussed. Mechanistic studies and randomized clinical trials strongly suggest that the mechanism of action and dose requirement of the antithrombotic effect of aspirin in patients with cerebrovascular disease is the same as that for ischaemic heart disease.

Advances in Management of Carotid Atherosclerosis

Carotid artery stenosis is a common and potentially treatable cause of stroke. Stroke risk is increased as the degree of carotid stenosis increases, as well as in patients with neurological symptoms referable to the stenosed carotid artery. Carotid stenosis can be quantified by ultrasound imaging, magnetic resonance angiography, or conventional angiography. Medical treatment with platelet antiaggregants reduces stroke risk in some patients; other patients are best treated with carotid endarterectomy. Experimental treatments for carotid stenosis, including carotid angioplasty with or without stenting, are under investigation. We summarize the current literature and provide treatment recommendations for patients with atherosclerotic carotid artery disease.