Effect of platelet turnover on whole blood platelet aggregation in patients with coronary artery disease

BACKGROUND

Previous studies have demonstrated considerable variation in the antiplatelet effect of aspirin.

OBJECTIVES

To investigate the impact of platelet turnover on the antiplatelet effect of aspirin in patients with stable coronary artery disease (CAD) and to identify determinants of platelet turnover.

METHODS

Platelet turnover was evaluated by measurements of immature platelets and thrombopoietin in 177 stable CAD patients on aspirin monotherapy, including 85 type 2 diabetics and 92 non-diabetics. Whole blood platelet aggregation was determined using the VerifyNow(®) Aspirin test and multiple electrode aggregometry (MEA, Multiplate(®) ) induced by arachidonic acid (AA) (1.0 mm), adenosine diphosphate (ADP) (10 μm) and collagen (1.0 μg mL(-1) ).

RESULTS

Immature platelet levels significantly correlated with MEA (r = 0.31-0.36, P-values < 0.0001) and the platelet activation marker sP-selectin (r = 0.19, P = 0.014). Contrary to the VerifyNow(®) test, MEA significantly correlated with variations in platelet count (r = 0.45-0.68, P-values < 0.0001). Among patients with residual platelet reactivity according to AA, there were significantly more diabetics (61% vs. 41%, P = 0.027) and higher levels of sP-selectin (77.7 ± 29 vs. 70.2 ± 25 ng mL(-1) , P = 0.070) and serum thromboxane B(2) (0.81 [0.46; 1.70] vs. 0.56 [0.31; 1.12] ng mL(-1) , P = 0.034). In a multivariate regression analysis, immature platelet levels were determined by thrombopoietin levels (P < 0.001), smoking (P = 0.020) and type 2 diabetes (P = 0.042).

CONCLUSIONS

The antiplatelet effect of aspirin was reduced in CAD patients with an increased platelet turnover. Once-daily dosing of aspirin might not suffice to adequately inhibit platelet aggregation in patients with an increased platelet turnover.

Reduced platelet response to aspirin in patients with coronary artery disease and type 2 diabetes mellitus

INTRODUCTION

Diabetes mellitus is complicated by accelerated atherosclerosis, resulting in an increased risk of coronary artery disease (CAD) and thrombosis. Despite the proven benefits of aspirin, previous studies indicate a reduced cardiovascular protection from aspirin in diabetic patients. We aimed to investigate whether diabetes mellitus influenced the platelet response to aspirin in patients with CAD.

MATERIALS AND METHODS

Platelet aggregation and activation were evaluated during aspirin treatment in 85 diabetic and 92 non-diabetic patients with CAD. Adherence to aspirin was carefully controlled. All patients had CAD verified by coronary angiography and were taking 75 mg non-enteric coated aspirin daily.

RESULTS

Diabetic patients showed significantly higher levels of platelet aggregation compared to non-diabetic patients evaluated by VerifyNow® Aspirin (p=0.03) and Multiplate® aggregometry using arachidonic acid (AA) 0.5 mM (p=0.005) and 1.0 mM (p=0.009). In addition, platelet activation determined by soluble P-selectin was significantly higher in diabetics compared to non-diabetics (p=0.005). The higher AA-induced aggregation was associated with higher levels of HbA(1c). Compliance was confirmed by low levels of serum thromboxane B(2) (below 7.2 ng/mL). Diabetics had significantly higher levels of serum thromboxane B(2) (p<0.0001).

CONCLUSIONS

Diabetic patients with CAD had significantly higher levels of both platelet aggregation and activation compared to non-diabetic patients with CAD despite treatment with the same dosage of aspirin. These findings may partly explain the reduced cardiovascular protection from aspirin in diabetic patients.

Platelet number and volume during myocardial infarction in relation to infarct size

Platelet number and mean platelet volume (MPV) were measured in 100 patients with acute chest pain, 41 with acute myocardial infarction (AMI), 33 with angina pectoris (AP) and 26 with non-coronary event (NCE), and compared with 21 controls. We found no significant difference in platelet count on admission in the patient groups, but it was lower compared with controls. There were no significant differences in MPV between the patient groups nor between patients and controls. Thirty patients with AMI were followed for 10 days and showed an initial 12% fall in platelet count followed by a 36% increase. Initially there was an increase in MPV (2%) followed by a fall (8%). The fall in platelet count and increase in MPV correlated with infarct size (maximum activity of lactate dehydrogenase (LDH)) and might reflect consumption of platelets. The precise role of platelets in the process of infarction is still unknown.

Myocardial infarction centres: the way forward

In the era of primary PCI, a strategy of admitting patients to the nearest hospital should be obsolete. Instead, a prehospital diagnostic strategy should be implemented in order to: (1) refer patients directly to interventional centres, thereby eliminating delay at local hospitals; (2) alert the interventional centre, thereby reducing door to balloon times; (3) initiate adjunctive medication in the prehospital phase.

The platelet polymorphism PlA2 is a genetic risk factor for myocardial infarction

OBJECTIVES

Platelet glycoprotein (GP) receptor IIb/IIIa plays a key role in the development of myocardial infarction (MI), and Pl(A2) is a polymorphism in the gene encoding this receptor. The prevalence of Pl(A2) shows pronounced geographical variation and has to our knowledge not been presented for a Scandinavian population before. Platelets from Pl(A2)-positive individuals show increased aggregability compared with platelets from Pl(A2)-negative individuals, and Pl(A2) genotypes might be associated with MI. The purpose of this study was to investigate the relation between the Pl(A2) polymorphism and MI in a large Scandinavian population.

DESIGN

Case-control study. We included patients with angiographically verified CAD with and without previous MI and a group of healthy individuals matched for age, race, and sex.

RESULTS

We studied the frequency of Pl(A2) in 1191 healthy individuals and 1019 patients with coronary artery disease (CAD). Amongst these patients, 529 subjects had suffered an MI previously. Pl(A2) was present in 28% of healthy individuals, 28% of patients with CAD but no MI, and in 35% of patients with CAD and MI. The difference between healthy individuals and MI patients was significant (P = 0.002). Furthermore, a graded relationship between the number of Pl(A2) alleles and the risk of MI was seen (P = 0.011). Associations between Pl(A2) and traditional cardiovascular risk factors as well as mean platelet volume were investigated. We found a significant interaction between Pl(A2) and serum cholesterol.

CONCLUSION

In our Scandinavian study population the common platelet polymorphism Pl(A2) is significantly associated with an increased risk of MI, but not of CAD. Clinically, typing for Pl(A2) might have implications for antiplatelet therapy of patients with MI.

n-3 polyunsaturated fatty acids and coronary thrombosis

Studies of Greenland Eskimos showed that a very high intake of marine n-3 fatty acids markedly inhibited platelet reactivity and suggested that intake of these fatty acids might prevent coronary thrombosis. Later studies with lower, more practical doses of n-3 fatty acids also have shown a platelet inhibitory effect of n-3 fatty acids, albeit fairly marginal. Furthermore, n-3 fatty acids have little effect on measures of blood coagulability and may slightly decrease fibrinolysis. In animal models, n-3 fatty acids often have been shown to inhibit thrombosis, but again the doses have tended to be very high. Finally, there has been little effect of (low-dose) n-3 fatty acids in clinical trials in humans on the incidence of myocardial infarction. Overall, there is little evidence for a major antithrombotic effect of practical doses of n-3 fatty acids on coronary thrombosis. This does not exclude a beneficial effect of n-3 fatty acids on coronary heart disease as suggested from clinical trials, but the major effect may be antiarrhythmic rather than antithrombotic.

Incomplete thromboxane inhibition with 100 mg of intravenous acetylsalicylic acid in patients with acute ST elevation myocardial infarction: a placebo-controlled pilot trial

BACKGROUND

Acetylsalicylic acid (ASA) is now a standard treatment of acute myocardial infarction (AMI). ASA inhibits thromboxane A(2) (TXA(2)) production by blocking the constitutive cyclooxygenase (COX)-1 enzyme, but only to a small degree the inducible COX-2. COX-2 is induced by increased concentrations of cytokines, which is related to an enhanced inflammatory response. Previously, we have found a complete inhibition of TXA(2) synthesis in healthy volunteers after intravenous administration of 50 mg of ASA. We measured in a randomized, placebo-controlled pilot trial the effect of 100 mg of ASA injected intravenously on TXA(2) synthesis in AMI patients treated with streptokinase.

METHODS AND RESULTS

Nineteen patients with AMI treated with streptokinase were randomized to 100 mg of ASA or placebo injected intravenously. Se-TXB(2) and bleeding time were measured before and after drug administration. One hundred and eighty minutes after intravenous ASA administration, treatment with oral ASA was initiated. We found a significant decrease in serum concentrations of TXB(2) after 30, 60 and 180 min following ASA injection compared to placebo, but in none of the patients was complete inhibition of TXA(2) production achieved. No significant change in bleeding time could be demonstrated.

CONCLUSION

Intravenous ASA in a dosage of 100 mg did not completely prevent TXA(2) production in AMI patients treated with streptokinase. This may be due to synthesis of TXA(2) by the inducible COX-2 enzyme and/or to a transcellular metabolism in platelets of prostanoids generated by endothelial cells.

Six months' clinical and angiographic follow-up of a flexible, coiled stainless steel stent in long, native coronary artery lesions

This study was conducted to evaluate the short- and long-term clinical and angiographic results of implantation of a flexible, coiled stainless steel stent, the Freedom Coronary Stent. During the study period this stent was used as an alternative to the Palmaz-Schatz PS153 coronary stent in long or tortuous lesions. The study was designed as a prospectively planned outcome analysis. Implantation of Freedom stents was attempted in 62 consecutive patients (56% males, mean age 63+/-10 years) with a total of 65 coronary lesions. Indications for stent implantation were: restenosis, 8%; recoil, 26%; visible dissection, 32%; threatening occlusion, 15%; chronic total occlusion, 18%. The average stent length was 30+/-16 mm and 67% of the lesions were type C. Rate of successful stent implantation, acute complications, angiographic restenosis after 6 months and major cardiac events (death, myocardial infarction, target vessel revascularization) during follow-up were assessed. The success rate of stent implantation was 94%. One patient died after an emergency bypass operation and one patient suffered a subacute stent thrombosis, which was successfully treated with re-percutaneous transluminal coronary angioplasty (PTCA). There were no Q- or non-Q myocardial infarctions. Clinical follow-up was carried out in 56 patients (97%) and 57 vessels were assessed by angiography (93%). Mean length of the follow-up period was 6.8+/-2.3 months. During the 6 months' follow-up period, one patient died, two patients suffered an acute non-Q myocardial infarction and eight patients had revascularization of the target vessel. Major cardiac event rate for all patients where stent implantation was intended was 23%. Angina CCS class declined from 3.0+/-0.9 to 1.1+/-0.8 (p < 0.01) before PTCA to follow-up. Overall restenosis rate was 28%. In 14 lesions with a stented segment length of <20 mm, the restenosis rate was 21%; in 31 lesions with a stented segment length > or =20 and <30 mm, the restenosis rate was 26%; and in 13 lesions with a stented segment length of > or =30 mm, the restenosis rate was 42%. Although there was a high procedural success rate after implantation of the Freedom stent in long or tortuous lesions, problems with high restenosis rates in long lesions remain unresolved.

[Abciximab (ReoPro) in the treatment of acute coronary syndromes]

Platelet activation plays a major role in the pathophysiology of acute coronary syndromes (ACS). Inhibition of platelet function is the basic pharmacological treatment of ACS. Platelet membrane glycoprotein IIb/IIIa inhibitors, a new class of potent antiplatelet agents, have been used in the treatment of ACS and in the prevention of complications after percutaneous coronary interventions (PCI). Several large clinical trials have demonstrated the effectiveness of this class of agents. The first of these agents to show beneficial effects after coronary interventions was the mouse/human chimeric Fab fragment antibody c7E3 abciximab (ReoPro). The purpose of this article is to describe the pharmacology of abciximab and to review the results of the clinical trials carried out with the drug in patients with ACS, treated either with or without acute/elective PCI.

[Clinical studies on glycoprotein IIb/IIIa receptor antagonist]

Platelet activation plays a major role in the pathophysiology of acute coronary syndromes (ACS), and inhibition of platelet function is the basic pharmacological treatment of ACS. Platelet membrane glycoprotein IIb/IIIa inhibitors, a new class of potent antiplatelet agents, have been used in the treatment of ACS, as well as in the prevention of complications after percutaneous coronary interventions. The aim of this article is to describe the potential possibilities of platelet inhibition and to review the pharmacology of glycoprotein IIb/IIIa inhibitors, the results of the clinical trials with these agents, and their current use in the pharmacological treatment of ACS and in relation to percutaneous coronary intervention.

Pathophysiology of coronary thrombosis

Detailed knowledge of the pathophysiology as well as the dynamic nature of coronary thrombus formation provides a valuable tool for correct management and proper adjunctive therapy in patients with acute coronary syndromes. Coronary thrombosis is in the majority of cases caused by disruption or fissuring of an atherosclerotic plaque. At the lesion thrombogenic material will be exposed to the flowing blood leading to activation of platelets and the formation of a platelet clot. Simultaneously, the coagulation system is activated resulting in increased thrombin formation. Thrombin is a key mediator in arterial thrombosis, due to its effect on both platelets and fibrin generation. Thrombin contributes to the stabilization of an initially loose platelet clot by generating cross-bound fibrin within the thrombus. During the course of an acute coronary syndrome, the patient presents changing chest pain and dynamic ischaemic ECG findings. This is likely to be related to the dynamic nature of the pathophysiology. The presence of a non-occlusive coronary thrombus may deprive the myocardium its normal blood flow and oxygen supply, leading to ischaemic pain. During lysis or embolization, blood supply may be restored, but the presence of thrombus fragments in the microcirculation holds the potential to sustained interference with myocardial metabolism. The emboli contain activated platelets which release vasoconstrictors that may compromise the microcirculation. Recurrent thrombus formation at the lesion site may result in occlusion of the artery adding to the dynamic nature of the clinical presentation. In conclusion, platelets, the coagulation system, and the endothelium cause a dynamic process of intermittent occlusion, vasospasm and embolization of thrombus material.

Dietary fish oil reduces microvascular thrombosis in a porcine experimental model

Microvascular thrombosis plays a significant role in the pathophysiology of ischaemic reperfusion injury. A fish oil-supplemented diet containing n-3 polyunsaturated fatty acids (PUFA) reduces thromboxane A(2) (TxA(2)) synthesis and, thus, vasoconstriction and platelet aggregation. The aim of this study was to elucidate whether n-3 PUFA in a porcine model of ischaemia and reperfusion injury 1) inhibit accumulation of platelets and fibrinogen in ischaemia-reperfusion injured tissue, 2) prolong the bleeding time, and 3) inhibit TxA(2) synthesis. Nine pigs were fed a standard diet supplemented with 7 g n-3 PUFA/day for 3 weeks. Nine pigs on the standard diet served as controls. Unilateral myocutaneous flaps were exposed to ischaemia for a period of 6 hours. Contralateral flaps were nonischaemic. Tissue contents of radioactive-labelled platelets and fibrinogen were measured after 4 hours of reperfusion. Platelet count, serum TxB(2), and the cutaneous bleeding time were measured before and after 3 weeks of diet. In the fish oil group, the accumulation of platelets was significantly reduced in all the myocutaneous flaps, except in the ischaemic skin part, when compared to control animals. Fibrinogen was significantly reduced in nonischaemic flaps, but not in ischaemic flaps. After the feeding period, the level of TxB(2) was significantly lowered in the fish oil group (p<0.01). No difference in the bleeding time was observed. Thus, dietary supplementation with n-3 PUFA inhibits the formation of microvasculatory thrombosis in this model.

What an interventional cardiologist should know about the pathophysiology of acute myocardial infarction

Basic knowledge of the sequence of cellular events that change the relative benign disease coronary atherosclerosis into a life-threatening acute coronary syndrome is of great importance for the interventional cardiologist in order to understand and choose the correct pharmacological and interventional management in patients with acute myocardial infarction. Plaque disruption, or fissuring, with superimposed thrombosis frequently complicates the course of coronary atherosclerosis. Small ruptures often remain clinically silent, whereas more extensive plaque rupture may lead to the development of unstable angina, acute myocardial infarction, and sudden cardiac death. The risk of plaque disruption depends more on plaque composition than on plaque size and stenosis severity. Major determinants of a plaque's vulnerability to rupture are: the size and consistency of the lipid-rich atheromatous core, the thickness of the fibrous cap covering the core, and inflammation and repair within the cap. The elevation of fibrinogen and C-reactive protein in patients with unstable angina may be markers of ongoing plaque inflammation. Both plaque vulnerability and rupture triggers are important for plaque disruption. The resultant thrombotic response, which is important for the clinical presentation and outcome, is in part determined by the reactivity of the circulating platelets and the balance between the fibrinolytic and coagulation systems. New ways of identification and treatment of the dangerous vulnerable plaques responsible for infarction and death and optimization of anti-thrombotic treatment are highly warranted in order to prevent and treat life-threatening coronary thrombosis.

Early administration of intravenous magnesium inhibits arterial thrombus formation

An antiplatelet effect of magnesium has been demonstrated in vitro and ex vivo, and this effect may be advantageous in patients with acute myocardial infarction to inhibit reocclusion after coronary angioplasty or thrombolysis. We investigated the antithrombotic in vivo effect of intravenous magnesium in a placebo-controlled, blinded study in 46 male Wistar rats. Thrombus formation was induced by standardized arteriotomy of the femoral artery and partial inversion of the vessel wall to produce a thrombogenic area. The vessel was transilluminated and visualized dynamically by in vivo microscopy. Thrombus area was measured every minute for 30 minutes after removal of the vessel clamp by image analysis techniques, and the number of visible emboli was registered. Animals were randomized into three groups: groups 1 and 2 received saline (control group, n = 15) or MgSO4 (Mg-early group, n = 15), respectively, during the entire infusion period. In group 3 intravenous saline was given during preparation of the arteriotomy followed by infusion of MgSO4 (Mg-late group, n = 16) from 10 minutes after removal of the vessel clamp. Thrombus area was significantly reduced by 75% in the Mg-early group (P < .005) but not in the Mg-late group compared with the control group. Mean number of emboli was reduced during magnesium infusion. The serum magnesium level increased to 2.2 (2.1 to 2.5) and 3.5 (3.0 to 4.2) mmol/L after infusion in the Mg-late and the Mg-early group, respectively. In the present study, intravenous infusion of MgSO4 significantly reduced thrombus formation in vivo but only when it was given before reperfusion. The antithrombotic effect of magnesium may be utilized in patients with acute myocardial infarction to reduce the rate of reocclusion. Magnesium infusion may also be of value in elective arterial angioplasty, but this option has not been investigated in clinical trials. However, correct timing of magnesium administration is critical to obtain an efficient antithrombotic effect.

Insights into the pathophysiology of unstable coronary artery disease

Coronary atherosclerosis without thrombosis is, in general, a benign disease. However, plaque disruption, or fissuring, with superimposed thrombosis, frequently complicates the course of coronary atherosclerosis. Small ruptures often remain clinically silent, whereas more extensive plaque rupture may cause the development of unstable angina, myocardial infarction, and sudden death. The risk of plaque disruption depends more on plaque type (composition) than on plaque size and stenosis severity. Major determinants of a plaque's vulnerability to rupture are: the size and consistency of the lipid-rich atheromatous core; the thickness of the fibrous cap covering the core; and ongoing inflammation and repair within the cap. Both plaque vulnerability (intrinsic disease) and rupture triggers (extrinsic forces) are important for plaque disruption. The former predisposes the plaque to rupture whereas the latter may precipitate it. The resultant thrombotic response, which is important for the clinical presentation and outcome, is portly determined by the reactivity of the circulating platelets and the balance between the fibrinolytic and coagulation systems. New ways of identification and treatment of the dangerous vulnerable plaques responsible for infarction and death, and optimization of antithrombotic treatment, are highly warranted in order to prevent and treat life-threatening coronary thrombosis.