Volume 141, Issue 2, Pages e89S-e119S (February 2012) Antiplatelet Drugs  John W. Eikelboom, MBBS, Jack Hirsh, MD, FCCP, Frederick A. Spencer, MD, Trevor P. Baglin, MBChB, PhD, Jeffrey I. Weitz, MD, FCCP  CHEST  Volume 141, Issue 2, Pages e89S-e119S (February 2012) DOI: 10.1378/chest.11-2293 Copyright © 2012 The American College of Chest Physicians Terms and Conditions

Figure 1 Arachidonic acid metabolism and mechanism of action of aspirin. Arachidonic acid, a 20-carbon fatty acid containing four double bonds, is liberated from the sn2 position in membrane phospholipids by several forms of phospholipase, which are activated by diverse stimuli. Arachidonic acid is converted by cytosolic prostaglandin H synthases, which have both COX and HOX activity, to the unstable intermediate prostaglandin H2. The synthases are colloquially termed “cyclooxygenases” and exist in two forms, COX-1 and COX-2. Low-dose aspirin selectively inhibits COX-1, and high-dose aspirin inhibits both COX-1 and COX-2. Prostaglandin H2 is converted by tissue-specific isomerases to multiple prostanoids. These bioactive lipids activate specific cell membrane receptors of the superfamily of G-protein-coupled receptors. COX = cyclooxygenase; DP = prostaglandin D2 receptor; EP = prostaglandin E2 receptor; FP = prostaglandin F2α receptor; HOX = hydroperoxidase; IP = prostacyclin receptor; TP = thromboxane receptor. CHEST 2012 141, e89S-e119SDOI: (10.1378/chest.11-2293) Copyright © 2012 The American College of Chest Physicians Terms and Conditions

Figure 2 Maximal capacity of human platelets to synthesize TXB2, rate of TXB2 production in healthy subjects, and relationship between the inhibition of platelet cyclooxygenase activity and TXB2 biosynthesis in vivo. Left, The level of TXB2 production stimulated by endogenous thrombin during whole-blood clotting at 37°C.34, 35 Center, The metabolic fate of TXA2 in vivo and the calculated rate of its production in healthy subjects on the basis of TXB2 infusions and measurement of its major urinary metabolite. Right, The nonlinear relationship between inhibition of serum TXB2 measured ex vivo and the reduction in the excretion of thromboxane metabolite measured in vivo.31 TXA2 = thromboxane A2; TXB2 = thromboxane B2. CHEST 2012 141, e89S-e119SDOI: (10.1378/chest.11-2293) Copyright © 2012 The American College of Chest Physicians Terms and Conditions

Figure 3 The absolute risk of vascular complications is the major determinant of the absolute benefi t of antiplatelet prophylaxis. Data are plotted from placebo-controlled aspirin trials in different clinical settings. For each category of patients, the abscissa denotes the absolute risk of experiencing a major vascular event as recorded in the placebo arm of the trials. The absolute benefi t of antiplatelet treatment is reported on the ordinate as the number of subjects in whom an important vascular event (nonfatal MI, nonfatal stroke, or vascular death) is actually prevented by treating 1,000 subjects with aspirin for 1 year. MI 5 myocardial infarction. CHEST 2012 141, e89S-e119SDOI: (10.1378/chest.11-2293) Copyright © 2012 The American College of Chest Physicians Terms and Conditions