Canadian Blog: Platelet-Active Drugs
If one compares the absolute benefits of aspirin prophylaxis in these trials with those achieved in the “primary” prevention of MI in patients with chronic stable angina, it becomes apparent that the level of cardiovascular risk in the control population (ie, those receiving placebo) is a major determinant of the absolute benefit of antiplatelet therapy.
Table 5—Primary Prevention Trials of Aspirin vs Placebo
| Trial | Subjects | Aspirin, mg | Follow-up,yr | Placebo Event Rate,%/yr | AspirinRR |
| US Physicians | Healthy men (22,071) | 325 every other day | 5.0 | 0.7 | 0.82 |
| PPP | High-risk men and women (4,495) | 100 daily | 3.6 | 0.8 | 0.71 |
| HOT114 | Hypertensive patients (18,790) | 75 daily | 3.8 | 1.1 | 0.85 |
| UK Doctors | Healthy men (5,139) | 500 daily | 5.8 | 1.4 | 1.03 |
| TPT | High-risk men (5,085) | 75 daily | 6.3 | 1.6 | 0.83 |
| SAPAT | Stable angina patients (2,035) | 75 daily | 4.2 | 3.7 | 0.71 |
As a PPP = Primary Prevention Project; HOT = Hypertension Optimal Treatment; TPT = Thrombosis Prevention Trial; SAPAT = Swedish Angina Pectoris Aspirin Trial. fValues in parentheses are No vascular Events (•) Avoided per 1,000 Treated per Year 12-, 10- TPT SAPAT Major_ с Bleeds (Ш) Caused per 1,000 Treated per Year ppp US Phys Placebo major vascular events per 100/years. 
Figure 1. Absolute benefit and bleeding risk of aspirin therapy in primary prevention. The data are plotted from placebo-controlled aspirin trials in different settings that are characterized by variable cardiovascular risk, as noted on the abscissa. The benefit (•) is reported on the left ordinate axis as the number of subjects in whom an important vascular event (ie, nonfatal MI, nonfatal stroke, or vascular death) is prevented by treating 1,000 subjects with low-dose aspirin for 1 year.
The bleeding risk (□) is reported on the right ordinate axis as the number of subjects in whom a major bleeding complication is caused by treating 1,000 subjects with low-dose aspirin for 1 year. For each of the six trials, a couple of symbols denote the benefit (•) and the bleeding risk (□) associated with long-term aspirin prophylaxis. HOT = Hypertension Optimal Treatment; US Phys = United States Physicians’ Health Study; PPP = Primary Prevention Project; UK Doc = British Doctors Trial; TPT = Thrombosis Prevention Trial; SAPAT = Swedish Angina Pectoris Aspirin Trial.
Result, the first step in deciding whether to consider aspirin for primary prophylaxis is an assessment of the 5-year to 10-year risk for that individual of developing a cardiovascular event. From that information and from the known risk reduction expected from aspirin therapy, one can estimate the number of vascular events that would be avoided. Individuals then can assess whether they believe that this likely benefit outweighs the expected toxicity related to GI bleeding and hemorrhagic stroke.
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