Antiplatelets Overview

Antiplatelet Trialist Collaboration. Collaborative overview of randomised trials of antiplatelet treatment. Part I prevention of death, myocardial infarction and stroke by prolonged antiplatelet therapy in various categories of patients. BMJ 1994 308 81-106. 

Gent M. A systematic overview of randomised trials of antiplatelet agents for the prevention of stroke, myocardial infarction and vascular death. In Hass WK, Easton ID, editors. Ticlo-pidine, platelets and vascular disease. New York Springer-Verlag 1993 p99-116. 

Antiplatelet Trialists Collaboration. Collaborative overview of randomised trials of antiplatelet therapy— III reduction in venous thrombosis and pulmonary embolism by antiplatelet prophylaxis among surgical and medical patients. BMJ. 1994 308 235-246. 

Collaborative overview of randomised trials of antiplatelet therapy. II. Maintenance of vascular graft or arterial patency by antiplatelet therapy. BMJ 1994 308 159-168. 

While the data above does not resolve the question of the importance of prostacyclin sparing by low dose aspirin, the results of the Second Antiplatelet Trialist overview support (nonsignificant trend, P=0.06) the use of lower doses of aspirin (44). In this respect, trials using less than 160 mg per day of aspirin showed a 28% odds reduction in vascular events, compared with 26% and 20% odds reduction in trials employing 160 to 325 mg per day and 500 to 1,500 mg per day respectively (4,44). 

A fuller description of die methods used in the APT overview can be found elsewhere (8), but Figures 27.1 and 27.2 respectively summarise the derivation of a proportional od leduction and corresponding absolute risk reduction. In Figure 27.1 the overall typical odds ratio for patients with a prior MI is the ratio of the odds of a serious vascular event among patients allocated antiplatelet treatment to the corresponding odds among control patients. This typical odds ratio of 0.75 corresponds to a typical odds reduction of 25%. 

The APT 1994 overview thus established conclusively the value of antiplatelet therapy for a wide range of patients at high risk of occlusive arterial disease. The proportional reductions in vascular events were shown to be similar in men and women, in middle age and old age, in hypertensive and normotensive patients, and in diabetic and non-diabetic patients (8). 

The APT 1994 overview showed that any excess risk of intracranial bleeding with antiplatelet therapy was small, at most one or two per 1000 patients per year in trials of long-term (more t one month) treatment (8). Among patients at high ri of arterial occlusive disease, therefore, the large absolute reductions in serious vascular events produced by antiplatelet therapy (Figure 24.2) fer outweigh any absolute hazards. 

In the 1994 APT overview fetal extracranial bleeding was rare, and there was no clear excess with prolonged antiplatelet therapy. Most major extracranial bleeds are gastrointestinal, and a meta-analysis of the placebo-controlled trials demonstrated that prolonged aspirin treatment increased the risk of gastrointestinal bleeding by around 1.5 to 2-fold (70). However, unlike serious vascular events, such bleeds rarely result in permanent disability or death. 

Antiplatelet Trialists Collaboration. Collaborative overview of randomised trials of antiplatdet therapy... 

The Antiplatelet TriaUsts Collaboration (1994) Collaborative Overview of Randomised Trials of Antiplatelet Therapy. I Prevention of Death, Myocardial Irrfarction, and Stroke by Prolonged Antiplatelet Therapy in Various Categories of Patients, Br. Med. J. 308, 81 106. 

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