Background-Aspirin inhibits platelet activation and reduces atherothrombotic complications in patients at risk of myocardial infarction and stroke. However, a sufficient inhibition of platelet function by aspirin is not always achieved. The causes of this aspirin resistance are unknown. Methods and Results-Patients undergoing coronary artery bypass grafting (CABG) have a high incidence of aspirin resistance. To evaluate functional and biochemical responses to aspirin, platelet-rich plasma was obtained before and at days 1, 5, and 10 after CABG. Thromboxane formation, aggregation, and ␣-granule secretion were effectively inhibited by 30 or 100 mol/L aspirin in vitro before CABG, but this inhibition was prevented or attenuated after CABG. Whereas the inhibition of thromboxane formation and aggregation by aspirin in vitro partly recovered at day 10 after CABG, oral aspirin (100 mg/d) remained ineffective. The inducible isoform of cyclooxygenase in platelets, COX-2, has been suggested to confer aspirin resistance. In fact, immunoreactive COX-2 was increased 16-fold in platelets at day 5 after CABG, but the COX-2 selective inhibitor celecoxib did not alter aspirin-resistant thromboxane formation. By contrast, the combined inhibitor of thromboxane synthase and thromboxane receptor antagonist terbogrel equally prevented thromboxane formation of platelets obtained before (control) and after CABG. Conclusions-Platelet aspirin resistance involves an impairment of both in vivo and in vitro inhibition of platelet functions and is probably due to a disturbed inhibition of platelet COX-1 by aspirin. (Circulation. 2003;108:542-547.)

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The success of coronary artery bypass graft surgery (CABG) depends mainly on the patency of the graft vessels. Aortocoronary vein graft disease is comprised of three distinct but interrelated pathological processes: thrombosis, intimal hyperplasia and atherosclerosis. Early thrombosis is a major cause of vein graft attrition during the first month after CABG, while during the remainder of the first year, intimal hyperplasia forms a template for subsequent atherogenesis, which thereafter predominates. Platelets play a crucial role in the pathophysiology of graft thrombosis and aspirin is the primary antiplatelet drug that has been shown to improve vein graft patency within the first year after CABG. Nevertheless, a significant number of grafts still occlude in the early postoperative period despite 'appropriate' aspirin treatment. Moreover, laboratory investigations showed that the expected inhibition of platelet function is not always achieved. This has been called 'aspirin nonresponse' or 'aspirin resistance', although a uniform definition is lacking. The finding that a considerable number of patients show an impaired antiplatelet effect of aspirin after CABG brought new insight into the discussion concerning poor patency rates of bypass grafts: the early period after CABG shows a coincidence of an increased risk for bypass thrombosis (amongst others, due to platelet activation and endothelial cell disruption of the graft) and an increased prevalence of aspirin resistance. Hitherto, the underlying mechanisms of aspirin resistance are uncertain and largely hypothetical; amongst others, increased platelet turnover, enhanced platelet reactivity, systemic inflammation, and drug-drug interaction are discussed. Up to now available data concerning the clinical outcome of aspirin resistant CABG patients are limited, and there is evidence that platelets of patients with graft thrombosis are more likely to be resistant to aspirin compared with patients without thrombotic events. Many publications concerning aspirin resistance are available today, but reports addressing this topic in CABG patients are sparse. This review summarises recent insights into the antiplatelet treatment after CABG and describes the clinical benefit, but also the therapeutic failure of the well-established drug aspirin. Moreover, possible pharmacological approaches to improve antithrombotic therapy in aspirin nonresponders among CABG patients are discussed.

Aspirin reduces major atherothrombotic events across a wide spectrum of patients with atherosclerotic disease. The occurrence of ischemic events despite of aspirin treatment is a failure of therapy, often denoted 'clinical aspirin resistance'. This is distinguished from laboratory assays showing an insufficient inhibition of platelet function, which indicate 'laboratory aspirin resistance'. Laboratory aspirin resistance has been reported in up to 60% of patients after stroke or peripheral arterial disease, up to 70% in stable coronary heart disease and even up to 80% in acute myocardial infarction. However, this data must be interpreted carefully because of small sample sizes and potential confounding factors such as compliance, co-morbidities and large differences between the laboratory methods used for detection. During the past years, evidence has accumulated that laboratory aspirin resistance is associated with an increased incidence of major atherothrombotic events, with an up to 13-fold increased risk of events in patients with cardiovascular disease. Thus, an individualized antiplatelet therapy will have to consider the possibility of aspirin resistance, and the identification of aspirin non-responders may improve antiplatelet therapy in future. Whether an increased dose of aspirin or another antiplatelet drug (e.g. clopidogrel) instead or in addition to aspirin should be given is unclear. Prospective trials are underway which address this issue. This review gives an overview on the various clinical studies that have investigated the prevalence and clinical importance of laboratory aspirin resistance. Moreover, therapeutic options, as well as future perspectives are discussed.