Aspirin-resistant patients as a group have reduced response to clopidogrel. Furthermore, we have identified a unique group of dual drug-resistant patients who may be at increased risk for thrombotic complications after PCI.

The proportion of circulating RPs strongly correlates with response to antiplatelet therapy in patients with stable CAD. Large platelets exhibit increased reactivity despite dual antiplatelet therapy, compared with smaller platelets.

Circulating platelets are heterogeneous in size and structure. Whether this translates into differences in platelet function and efficacy of antiplatelet therapy is unclear. Hence, we decided to investigate the activation patterns among different platelet populations differentiated by size, and to compare the inhibitory effects of aspirin in these populations. Circulating platelets from 9 healthy volunteers were separated by size and stratified into the largest and smallest quintiles. Platelets were stimulated with 75 μM arachidonic acid (AA), 10 μM ADP or 25 μM TRAP. Alpha-granule protein secretion and expression (P-selectin, VWF, fibrinogen), surface-protein activation (activated integrin αIIbβ3) were assessed. Platelet thromboxane B(2) (TxB(2)) synthesis following AA stimulation was measured in vitro before and after incubation with 265 μM aspirin. Reticulated (juvenile) platelets were assessed using thiazole orange staining. A greater number of large platelets in the largest quintile were reticulated compared with the smallest quintile (6.1 ± 2.8% vs. 1.2 ± 1.5% respectively, p < 0.001). Larger platelets also synthesized more TxB(2) than small platelets both before (1348 ± 276 pg/mL vs. 1023 ± 214 pg/mL, respectively, p = 0.01) and after aspirin (1029 ± 190 pg/mL vs. 851 ± 159 pg/mL, respectively, p = 0.03). After stimulation with each agonist, a greater proportion of large platelets bound fibrinogen, VWF, P-selectin and activated integrin αIIbβ3 than small platelets both in the presence and in the absence of in vitro aspirin. In an in vitro setting, large platelets appear to be more active than small platelets and continue to be more active even after in vitro aspirin. Platelets exhibit heterogeneity in size and structure. Whether this translates into platelet function and efficacy of antiplatelet therapy is unclear. We evaluated platelet functional properties and the effects of aspirin on separated platelet subpopulations in an in vitro setting. Platelets were sorted into the largest and smallest size quintiles using flow cytometry forward scatter. Alpha-granule protein release, dense granule content, surface protein activation and thromboxane synthesis were significantly greater in large platelets compared with small platelets, before and after stimulation with arachidonic acid, ADP and TRAP. Even after incubation with aspirin, large platelets continued to be more active than small platelets. In conclusion, large platelets are more active than small platelets and aspirin fails to eliminate these differential activation properties.

Hyperhomocysteinemia promotes atherosclerosis and is most commonly caused by B-vitamin deficiencies, especially folic acid, B(6), and B(12); genetic disorders; certain drugs; and renal impairment. Elevated homocysteine promotes atherosclerosis through increased oxidant stress, impaired endothelial function, and induction of thrombosis. Prospective studies have shown that elevated plasma homocysteine concentrations increase risk of cardiovascular disease by twofold and risk of cerebrovascular disease to a lesser degree. Hyperhomocysteinemia should be identified in patients with progressive or unexplained atherosclerosis and treated appropriately. Treatment of hyperhomocysteinemia is primarily through vitamin supplementation; folic acid and vitamins B(6) and B(12) are the mainstay of therapy. Betaine and 5-methyl tetrahydro-folate are also effective in lowering homocysteine levels. Treatment of moderately elevated plasma homocysteine in patients without atherosclerosis should be deferred until the completion of randomized outcome trials.

The mechanisms for the variability in antiplatelet effects of clopidogrel are not elucidated entirely. Immature (reticulated) platelets may modulate the antiplatelet effects of clopidogrel but must be measured using flow cytometry. Whether new automated detection techniques yield similar results is not known. The objectives of the study to evaluate the role of immature platelets assessed by an automated method in response to the antiplatelet effects of clopidogrel. Twenty-nine healthy volunteers had platelet studies performed before and 1 week after 75 mg daily dosing of clopidogrel. Immature platelet fraction (IPF) was determined using an automated particle counter. Subjects were stratified into tertiles based on the IPF. Platelet studies included light transmission aggregometry (LTA), and vasodilator stimulated phosphoprotein phosphorylation (VASP-P) determined by platelet reactivity index (PRI). Baseline platelet aggregation responses to 2, 5 and 20 μM ADP, were similar in all three tertiles, however they were greater in the upper than in the lower tertile of immature platelets after clopidogrel in response to 5 μM ADP (54% vs. 23%, P = 0.02), with concordant trends for the other two concentrations. PRI was also greater in the upper tertile after clopidogrel (71.2% vs. 57.8%, P = 0.04). The frequency of clopidogrel hyporesponsiveness (aggregation >50% in response to 5 μM of ADP) was also higher in the upper tertile when compared to lower tertile, (60%) versus (10%) respectively (P = 0.02). [corrected]. Immature platelets measured using an automated method, are associated with impaired response to antiplatelet effects of clopidogrel.

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Background-Use of upper-arm arterial occlusion to induce reactive hyperemia, and endothelium-dependent flowmediated dilation (FMD) of the brachial artery, induces greater conduit vessel dilatation than lower-arm occlusion. However, brachial artery ischemia after upper arm arterial occlusion may make this approach unreliable. We studied whether upper or lower arm occlusions differ in their ability to detect endothelial dysfunction in cigarette smokers, and its improvement with an antioxidant strategy. Methods and Results-Ten cigarette smokers with a Ͼ20 pack year history and 10 age-and gender-matched healthy controls participated in a 2-phase randomized controlled study of xanthine oxidase inhibition, using a 600-mg oral dose of allopurinol administered beforehand. Endothelium-dependent dilatation was assessed using ultrasound-Doppler after lower and upper arm occlusion. After lower arm occlusion, FMD was significantly impaired in smokers compared with controls (3.8Ϯ1.1% versus 8.7Ϯ2.2%; Pϭ0.001). However, after upper arm occlusion, brachial artery dilatation in smokers was higher (11.8Ϯ2.7%; PϽ0.0001 versus lower arm) and did not differ from controls (9.4Ϯ2.9%; Pϭ0.3). There was no difference in endothelium-independent dilatation to sublingual nitroglycerin between smokers and controls. Inhibition of xanthine oxidase with allopurinol improved lower arm FMD (3.8Ϯ1.1 to 10.1Ϯ1.9%; PϽ0.0001), but did not improve upper arm FMD (11.8Ϯ2.7 to 14.1Ϯ3.7%; Pϭ0.4). Conclusions-Although upper arm occlusion induces robust brachial vasodilatation, it cannot detect endothelial dysfunction induced by smoking or its improvement by inhibition of xanthine oxidase. The increase in brachial artery diameter with upper arm occlusion may be confounded by ischemia of the artery. Conduit artery FMD after release of lower arm occlusion appears to be a more valid method for assessment of endothelial function in humans. (Arterioscler

Aspirin 'resistance' (AR) is a phenomenon of uncertain etiology describing decreased platelet inhibition by aspirin. We studied whether (i) platelets in AR demonstrate increased basal sensitivity to a lower degree of stimulation and (ii) platelet aggregation with submaximal stimulation could predict responses to aspirin. Serum thromboxane B(2) (TxB(2)) levels and platelet aggregation with light transmission aggregometry (LTA) were measured at baseline and 24 hours after 325 mg aspirin administration in 58 healthy subjects. AR was defined as the upper sixth of LTA (> or = 12%) to 1.5 mM AA. Baseline platelet aggregation with submaximal concentrations of agonists [ADP 2 microM, arachidonic acid (AA) 0.75 mM, collagen 0.375 and 0.5 microg/ml] was greater in AR subjects compared with non-AR subjects, but not with higher concentrations (ADP 5 microM and 20 microM, AA 1.5 mM and collagen 1 microg/ml). Post-aspirin platelet aggregation was elevated in AR subjects with both submaximal and maximal stimulation. Baseline and post-aspirin serum TxB(2) were higher in AR subjects and decreased further with ex-vivo COX-1 inhibition, suggesting incompletely suppressed COX-1 activity. Pre-aspirin platelet aggregation to 0.75 AA demonstrated a dichotomous response with 29/58 subjects having aggregation < or = 15% and 29/58 subjects having aggregation > or = 75%. In the high aggregation group 28% had AR compared to 6% in the non-AR group (p = 0.04). In conclusion, platelets in AR subjects demonstrate increased basal sensitivity to submaximal stimulation, which could predict responses to antiplatelet therapy.