Prevention of arterial thromboembolism with acetylsalicylic acid. A controlled clinical study in patients with aortic ball valves

Comparison of high-dose with low-dose aspirin in patients with mechanical heart valve replacement treated with oral anticoagulant

BACKGROUND

There are no reported studies on the safety and efficacy of low-dose aspirin with low-intensity oral anticoagulation in patients with heart valve replacement. In this study, we compared the use of 100 mg/d aspirin with 650 mg/d aspirin in the prevention of systemic embolism and vascular death in patients with heart valve replacement who were being treated with oral anticoagulants with a target international normalized ratio (INR) of 2.0 to 3.0.

METHODS AND RESULTS

Four hundred nine of 416 consecutive patients who had cardiac valve replacement were randomized in open allocation into one of two groups; both groups were treated with oral anticoagulant therapy with a target INR of 2.0 to 3.0. Two hundred seven patients who received 100 mg/d aspirin for an average of 24.1 months were compared with 202 patients who received 650 mg/d aspirin for an average of 21.7 months in a randomized-treatment, open-allocation study. There were no significant differences in systemic embolism, vascular death, or total death rates between the low- and high-dose aspirin treatment groups (0.5 and 1.1, 1.2 and 0.5, and 4.6 and 2.5 per 100 patients/y, respectively). The total number of hemorrhagic events was 13.4 per 100 patients/y in the high-dose aspirin group and 7.9 per 100 patients/y in the low-dose aspirin group (P = .035), but the rate of bleeding was influenced by dipyridamole in the 650-mg aspirin group.

CONCLUSIONS

In patients with mechanical heart valve replacements, low-dose aspirin (100 mg/d) in conjunction with oral anticoagulants at an INR of 2.0 to 3.0 is as effective as the use of high-dose aspirin (650 mg/d) in the prevention of systemic embolism.

Efficacy and safety of combined anticoagulant and antiplatelet therapy versus anticoagulant monotherapy after mechanical heart-valve replacement: a metaanalysis

We performed a metaanalysis of five randomized controlled trials to compare the efficacy and safety of combined oral anticoagulant and antiplatelet therapy versus oral anticoagulants alone after prosthetic heart-valve replacement. The combined regimen reduced embolism and overall mortality by approximately 67% (pooled odds ratio [OR] 0.33; 95% confidence interval [CI] 0.16 to 0.69; p = 0.0032) and 40% (OR 0.60; 95% CI 0.32 to 1.12; p = 0.11), respectively, but increased the risk of hemorrhage by approximately 65% (OR 1.65; 95% CI 1.15 to 2.39; p = 0.0069) and of major gastrointestinal hemorrhage by approximately 250% (OR 3.47; 95% CI 1.43 to 8.40; p = 0.0058). It is estimated that for every 1.6 patients who had their stroke prevented by combination therapy, there was an excess of one major gastrointestinal bleed. This metaanalysis suggests that the benefits derived from the enhanced antithrombotic potential of combined therapy outweigh the toxic effects resulting from the enhanced anticoagulant potential of this regimen.

Frequency of prosthetic valve-related complications with very low level warfarin anticoagulation combined with dipyridamole after valve replacement using St. Jude Medical prostheses

The safety of a very low level of anticoagulation combined with dipyridamole in a rheumatic population (mean age 31 +/- 13 years) with the St. Jude Medical (SJM) prosthesis has not yet been tested. Furthermore, no data are available on the safety of relatively infrequent monitoring of anticoagulation levels and of the necessity for different therapeutic targets according to valve position, number of risk factors, and other baseline risk factors for thromboembolism. In this study, the performance of the SJM prosthesis was tested using a target international normalized ratio (INR) of 2.0 to 2.5 combined with dipyridamole 300 mg/day applied uniformly to all patients. Clinical, biochemical, and echocardiographic data were acquired prospectively in 200 consecutive patients at 3-month intervals. Follow-up (mean 27 +/- 13 months) was complete in 95% of patients. Thirteen patients died (2.9%/patient year). Severe left ventricular dysfunction was the cause of death in 10 of 13 patients. Probability of survival (Kaplan-Meier) was 0.92 at 36 months and of event-free survival 0.84 at 36 months. The median INR was 2.0 +/- 0.9. Valve obstruction did not occur, and there were 3 thromboembolic events (0.6%/patient year). Incidence of bleeding was 1.6%/patient year (n = 7) and was major (hemorrhagic stroke) in 1 (0.2%/patient year). Thus, the SJM prosthesis performs very well despite the use of very low level warfarin anticoagulation combined with dipyridamole. A 3-month assessment of the anticoagulation level is safe. Left ventricular dysfunction rather than valve-related complications is the leading cause of mortality in this population.

Collaborative overview of randomised trials of antiplatelet therapy--I: Prevention of death, myocardial infarction, and stroke by prolonged antiplatelet therapy in various categories of patients. Antiplatelet Trialists' Collaboration

OBJECTIVE

To determine the effects of "prolonged" antiplatelet therapy (that is, given for one month or more) on "vascular events" (non-fatal myocardial infarctions, non-fatal strokes, or vascular deaths) in various categories of patients.

DESIGN

Overviews of 145 randomised trials of "prolonged" antiplatelet therapy versus control and 29 randomised comparisons between such antiplatelet regimens.

SETTING

Randomised trials that could have been available by March 1990.

SUBJECTS

Trials of antiplatelet therapy versus control included about 70,000 "high risk" patients (that is, with some vascular disease or other condition implying an increased risk of occlusive vascular disease) and 30,000 "low risk" subjects from the general population. Direct comparisons of different antiplatelet regimens involved about 10,000 high risk patients.

RESULTS

In each of four main high risk categories of patients antiplatelet therapy was definitely protective. The percentages of patients suffering a vascular event among those allocated antiplatelet therapy versus appropriately adjusted control percentages (and mean scheduled treatment durations and net absolute benefits) were: (a) among about 20,000 patients with acute myocardial infarction, 10% antiplatelet therapy v 14% control (one month benefit about 40 vascular events avoided per 1000 patients treated (2P < 0.00001)); (b) among about 20,000 patients with a past history of myocardial infarction, 13% antiplatelet therapy v 17% control (two year benefit about 40/1000 (2P < 0.00001)); (c) among about 10,000 patients with a past history of stroke or transient ischaemic attack, 18% antiplatelet therapy v 22% control (three year benefit about 40/1000 (2P < 0.00001)); (d) among about 20,000 patients with some other relevant medical history (unstable angina, stable angina, vascular surgery, angioplasty, atrial fibrillation, valvular disease, peripheral vascular disease, etc), 9% v 14% in 4000 patients with unstable angina (six month benefit about 50/1000 (2P < 0.00001)) and 6% v 8% in 16,000 other high risk patients (one year benefit about 20/1000 (2P < 0.00001)). Reductions in vascular events were about one quarter in each of these four main categories and were separately statistically significant in middle age and old age, in men and women, in hypertensive and normotensive patients, and in diabetic and nondiabetic patients. Taking all high risk patients together showed reductions of about one third in non-fatal myocardial infarction, about one third in non-fatal stroke, and about one third in vascular death (each 2P < 0.00001). There was no evidence that non-vascular deaths were increased, so in each of the four main high risk categories overall mortality was significantly reduced. The most widely tested antiplatelet regimen was "medium dose" (75-325 mg/day) aspirin. Doses throughout this range seemed similarly effective (although in an acute emergency it might be prudent to use an initial dose of 160-325 mg rather than about 75 mg). There was no appreciable evidence that either a higher aspirin dose or any other antiplatelet regimen was more effective than medium dose aspirin in preventing vascular events. The optimal duration of treatment for patients with a past history of myocardial infarction, stroke, or transient ischaemic attack could not be determined directly because most trials lasted only one, two, or three years (average about two years). Nevertheless, there was significant (2P < 0.0001) further benefit between the end of year 1 and the end of year 3, suggesting that longer treatment might well be more effective. Among low risk recipients of "primary prevention" a significant reduction of one third in non-fatal myocardial infarction was, however, accompanied by a non-significant increase in stroke. Furthermore, the absolute reduction in vascular events was much smaller than for high risk patients despite a much longer treatment period (4.4% antiplatelet therapy v 4.8% control; five year

Antiplatelet therapy following cardiac valve replacement-a comparative study of aspirin and ticlopidine

The antiplatelet agents, aspirin and ticlopidine, are widely used to prevent thromboembolism following cardiac valve replacement. To compare the clinical effects of each platelet inhibitor, a daily dose of ticlopidine 300 mg was given to 50 patients who underwent aortic valve or mitral valve replacement with an average age of 56.9 years over a mean 52.6 months after surgery. 50 more patients with an average age of 50.2 years were given a daily dose of aspirin 81 mg over a mean 51.3 months after surgery. Warfarin was given to maintain thrombotest values at 10 to 25% (PT-INR at 1.6-3.0). The incidence of thromboembolism was low in both groups; 1.0/100 patient years in the ticlopidine group and 1.9 in the aspirin group. Hemorrhagic complications, hematuria and ecchymosis, showed an incidence of 2.9 in the ticlopidine group and 2.3 in the aspirin group. Slight increases in GOT and GTP were observed in 4 and 18% of cases and elevated total cholesterol and neutral fat in 2 and 18% of cases. No adverse reactions were reported. With the exception of a significant decrease in ADP-induced platelet aggregation in patients who took ticlopidine, there were no significant differences observed between the two groups.

Effect of laboratory variation in the prothrombin-time ratio on the results of oral anticoagulant therapy

BACKGROUND

Patients receiving long-term anticoagulant therapy may be subject to unnecessary risks of bleeding or thromboembolism because of variability in the commercial thromboplastins used to determine prothrombin time and consequent uncertainty about the actual intensity of anticoagulation.

METHODS

We explored the effect of this uncertainty on the benefits and risks of anticoagulation in patients with prosthetic heart valves, using models of thromboembolic and hemorrhagic complications as a function of the intensity of anticoagulation, with quality-adjusted life expectancy and average variable costs used to describe outcomes.

RESULTS

Anticoagulation provides a striking benefit for patients whose treatment is conducted within the recommended range of the international normalized ratio (INR)--i.e., 2.5 to 3.5--but if uncertainty about the laboratory results causes the intensity of anticoagulation to fall outside this range, the gain becomes smaller. Uncertainty about the true intensity of anticoagulation may reduce the potential gain in life expectancy, adjusted for quality of life, by more than half and may increase the ratio of costs to effectiveness to almost five times the optimal value. Variability in the intensity of anticoagulation is even greater if older recommendations advocating a higher level of anticoagulation are followed.

CONCLUSIONS

Uncertainty about the sensitivities of the commercially available thromboplastins used in the United States can have important clinical and economic effects. This problem could be eliminated if clinical laboratories uniformly reported the intensity of anticoagulation as the INR, by adjusting prothrombin-time ratios for variability in thromboplastins.

A comparison of aspirin with placebo in patients treated with warfarin after heart-valve replacement

BACKGROUND

Despite the use of warfarin, major systemic embolism remains an important complication in patients with heart-valve replacement. Although the addition of antiplatelet agents has the potential to reduce this complication, their efficacy and safety when given in combination with warfarin are uncertain.

METHODS

In a randomized, double-blind, placebo-controlled trial, we assessed the efficacy and safety of adding aspirin (100 mg per day) to warfarin treatment (target international normalized ratio, 3.0 to 4.5) in 370 patients with mechanical heart valves or with tissue valves plus atrial fibrillation or a history of thromboembolism.

RESULTS

A total of 186 patients were randomly assigned to aspirin and 184 to placebo, and they were followed for up to 4 years (average, 2.5). Major systemic embolism or death from vascular causes occurred in 6 aspirin-treated patients (1.9 percent per year) and 24 placebo-treated patients (8.5 percent per year) (risk reduction with aspirin, 77 percent; 95 percent confidence interval, 44 to 91 percent; P < 0.001). Major systemic embolism, nonfatal intracranial hemorrhage, or death from hemorrhage or vascular causes occurred in 12 patients assigned to aspirin (3.9 percent per year) and 28 patients assigned to placebo (9.9 percent per year) (risk reduction, 61 per cent; 95 percent confidence interval, 24 to 80 percent; P = 0.005); major systemic embolism or death from any cause occurred in 13 patients (4.2 percent) and 33 patients (11.7 percent), respectively (risk reduction, 65 percent; 95 percent confidence interval, 33 to 82 percent; P < 0.001); and death from all causes occurred in 9 patients (2.8 percent) and 22 patients (7.4 percent), respectively (risk reduction, 63 percent; 95 percent confidence interval, 19 to 83 percent; P = 0.01). Bleeding occurred in 71 patients in the aspirin group (35.0 percent), as compared with 49 patients in the placebo group (22.0 percent) (increase in risk, 55 percent; 95 percent confidence interval, 8 to 124 percent; P = 0.02); major bleeding occurred in 24 and 19 patients, respectively (increase in risk, 27 percent; 95 percent confidence interval, -30 to 132 percent; P = 0.43).

CONCLUSIONS

In patients with mechanical heart valves and high-risk patients with prosthetic tissue valves, the addition of aspirin to warfarin therapy reduced mortality, particularly mortality from vascular causes, together with major systemic embolism. Although there was some increase in bleeding, the risk of the combined treatment was more than offset by the considerable benefit.

Single-dose effect of enteric-coated aspirin on platelet function and thromboxane generation in middle-aged men

OBJECTIVE

To evaluate the effect of a single dose of enteric-coated aspirin (ECA) in three different dosages on platelet function and thromboxane generation in middle-aged men.

DESIGN AND METHODS

In a nonblind, nonplacebo-controlled, crossover study, a single dose of ECA (50, 250, or 1000 mg) was given in a tablet form to a group of healthy, middle-aged men. Ten subjects, aged 50-67 years, volunteered to participate in this study. Platelet functions including bleeding time, platelet aggregation, adenine nucleotides, beta-thromboglobulin, platelet factor 4, thromboxane generation, and aspirin measurement were determined.

RESULTS

Before ECA ingestion, the intracellular adenine nucleotides (adenosine triphosphate, adenosine diphosphate) were decreased, and both beta-thromboglobulin and platelet factor 4 were increased. These observations suggested that platelets were activated in vivo in middle-aged men. These findings returned to normal within 8 hours after the ingestion of ECA, and maintained normal for at least two days. Bleeding time was significantly prolonged at 8 and 24 hours compared with that before ingestion of ECA 1000 mg (p < 0.05). The generation of platelet thromboxane was maximally inhibited by approximately 40 percent in the samples 8 hours after ECA ingestion. Abnormal values of adenine nucleotides, beta-thromboglobulin, and platelet factor 4 returned to normal within 8 hours. Arachidonic acid-induced platelet aggregation was inhibited compared with that before treatment (p < 0.01) and the inhibitory effect was maintained for at least three days. Adenosine diphosphate- and epinephrine-induced aggregations were less inhibited than those induced by arachidonic acid. Inhibitory effects of ECA on platelet aggregation were dose dependent.

CONCLUSIONS

Our study indicates that platelets are activated in middle-aged men and that a single dose of ECA 50 mg is safe and can inhibit thromboxane synthesis and platelet aggregation. These results suggest that a daily dose of ECA 50 mg may be useful for blocking platelet activation and preventing thrombosis.

Interruption of thrombosis and hemostasis by anti-platelet agents

Blood platelets play a fundamental role in hemostasis and thrombosis. The physiological role of platelets is to preserve the integrity of the cardiovascular system such that upon injury platelets are activated and serve to interrupt a potentially deleterious situation (i.e., hemorrhage). However, platelets are also involved in the pathophysiology of thrombosis in which the normal repair process is grossly exacerbated, resulting in interruption of blood flow to vital tissues. Thus, a critical balance exists between the normal function of blood platelets to preserve tissue function and a pathophysiological function in which tissue is damaged due to thrombosis and subsequent ischemia. Numerous anti-platelet agents have been developed in an attempt to interrupt the thrombotic process without severely compromising the hemostatic state of patients. This article will be devoted to a discussion of a commonly utilized anti-thrombotic agent, aspirin, as well as a class of newer, possibly more effective anti-thrombotic agents referred to collectively as fibrinogen receptor antagonists.

Antithrombotic treatment of cerebrovascular disease

The most common type of cerebrovascular disease is ischaemia or infarction from atherothrombosis or cardiac embolism. Antithrombotic treatment with an antiplatelet agent or anticoagulant assumes a prior clinical classification into categories of transient ischaemic attack (TIA) or minor stroke, acute partial stable stroke, stroke-in-progression, and completed stroke. Aspirin reduces the risk of stroke, myocardial infarction, and death after TIA or minor stroke secondary to atherothrombosis. Aspirin is effective in both sexes at a dose of 300 or 1200 mg/day. Ticlopidine (500 mg/day), a new antiplatelet agent, is more effective than aspirin in preventing stroke and death in patients with TIA or minor stroke. Ticlopidine (500 mg/day) is effective in preventing recurrent stroke, myocardial infarction, or vascular death in patients with completed stroke. Aspirin has not been directly shown to be effective after completed stroke. No clear evidence exists for the use of anticoagulants in atherothrombotic cerebral vascular disease in patients presenting with TIA or minor stroke, acute partial stable stroke, stroke-in-progression, or completed stroke. Anticoagulation for rheumatic valvular heart disease is effective in preventing recurrent embolism. Long-term anticoagulation of patients with mechanical prosthetic valves protects against initial embolism and prevents recurrent embolism. The addition of aspirin (500-1000 mg/day) to warfarin reduces the rate of cerebral embolism from mechanical prosthetic heart valves but is associated with increased bleeding. The addition of dipyridamole (400 mg/day) to warfarin may be more effective than aspirin in reducing the rate of cerebral embolism from mechanical prosthetic heart valves and has fewer bleeding side-effects. Anticoagulation during the hospital phase of myocardial infarction reduces the incidence of systemic embolism/stroke. Long-term anticoagulation of patients after the hospital phase of myocardial infarction reduces the incidence of systemic embolism/stroke, recurrent myocardial infarction and death. Prophylactic anticoagulant treatment of patients with non-valvular atrial fibrillation reduces the incidence of embolism, but the optimal duration of treatment is not known. Immediate anticoagulation of patients with completed cardioembolic stroke is safe and effective in preventing recurrent embolism.

Antithrombotic therapy in cardiac disease: an approach based on pathogenesis and risk stratification

An approach to the management of thrombosis and embolism in various cardiovascular disorders is discussed. This approach is based on current knowledge of pathogenesis and risk of thromboembolism. Rational therapeutic guidelines are formulated along the lines of anatomic location (arterial circulation, cardiac chambers or prosthetic valves), pathophysiology (activation of platelets or the coagulation system, or both), and degree of thromboembolic risk. With clear understanding of these factors, it may be possible to determine the most suitable platelet inhibitor or anticoagulant regimen for the individual patient, and whether these agents should be given singly or in combination.

Platelet inhibitor agents in cardiovascular disease: an update

Platelets interact with the coagulation and fibrinolytic systems in the maintenance of hemostasis. However, these physiologic mechanisms may become pathologic, requiring prevention and treatment. In this review, the following clinical developments are analyzed: 1) the role of platelets in thrombogenesis; 2) the pharmacology of platelet inhibitory agents; and, most important, 3) the results of recent randomized trials of platelet inhibitor agents in different cardiovascular disorders. Aspirin reduces mortality and infarction rates in unstable angina and significantly decreases vascular mortality in acute myocardial infarction. Platelet inhibitors decrease mortality and recurrent cardiovascular events in the chronic phase after myocardial infarction. They also decrease vein graft occlusion rates after coronary bypass surgery. Although platelet inhibitors are beneficial in preventing acute vessel occlusion during coronary angioplasty, they are ineffective in preventing chronic restenosis. Antiplatelet agents, combined with warfarin, reduce thromboembolic events in patients with a mechanical prosthesis. Platelet inhibitors are also effective in secondary prevention of vascular events in patients with cerebrovascular disease. Finally, the use of aspirin for primary prevention of cardiovascular disease is still evolving, particularly in individuals at high risk. In conclusion, platelet inhibitors are effective in patients with a variety of cardiovascular disorders. The best studied, most inexpensive and least toxic agent is aspirin at a daily dose of 160 to 325 mg. Studies using new platelet inhibitor agents with different mechanisms of action are currently underway.

Major bleeding in outpatients treated with warfarin: incidence and prediction by factors known at the start of outpatient therapy

PURPOSE

To determine the incidence of major bleeding in outpatients treated with warfarin and to identify predictive factors known at the start of therapy.

PATIENTS AND METHODS

The records of 565 patients starting outpatient therapy with warfarin upon discharge from a university hospital were reviewed. Follow-up information was obtained for 562 patients (99.5%). Bleeding was classified as major or minor using explicit criteria. The cumulative incidence of bleeding was estimated by means of survival analysis. Independent risk factors for major bleeding were identified using Cox regression analysis in 375 randomly chosen patients; they were tested in the remaining 187 patients.

RESULTS

Major bleeding occurred in 65 patients (12%) and was fatal in 10 patients (2%). The cumulative incidences of major bleeding at one, 12, and 48 months were 3%, 11%, and 22%, respectively. The monthly risk of major bleeding decreased over time, from 3% during the first month of outpatient therapy to 0.3% per month after the first year of therapy. Five independent risk factors for major bleeding--age 65 years or greater, history of stroke, history of gastrointestinal bleeding, a serious comorbid condition (recent myocardial infarction, renal insufficiency, or severe anemia), atrial fibrillation--predicted major bleeding in the testing group; the cumulative incidence of major bleeding at 48 months was 2% in 57 low-risk patients, 17% in 110 middle-risk patients, and 63% in 20 high-risk patients.

CONCLUSION

These findings provide a quantitative basis for evaluating the risk of major bleeding in individual patients at the start of outpatient therapy with warfarin. Whether the risk of bleeding can be reduced in high-risk patients without reducing the benefit of therapy remains to be determined.

Prothrombin time ratio and other factors associated with bleeding in patients treated with warfarin

We previously showed that the risk of major hemorrhage in patients with venous thromboembolism treated with warfarin was strongly related to duration of anticoagulant therapy. We here report the results of a more detailed analysis of factors other than duration of warfarin therapy associated with the risk of hemorrhage in these patients. Almost 7% of patients had a major hemorrhage on warfarin and an additional 23.7% had at least one minor bleeding episode. Age, female sex, and congestive heart failure were associated with small increases in the risk of major hemorrhage but not with the risk of minor bleeding. A prothrombin time ratio greater than 2.5 was associated with a fourteen-fold increase in the risk of a major hemorrhage (95% CI 5.1, 42.7), but major hemorrhages occurred in patients on warfarin at all measured values of the prothrombin time ratio. Taken together with the findings from our previous analysis, the study suggests that prevention of bleeding in patients on warfarin would best be accomplished by minimizing the duration of warfarin therapy, by scrupulous monitoring of the prothrombin time ratio, and by considering the "therapeutic range" for the prothrombin time ratio to be somewhat less than 2.0-2.5.

Aspirin in cardiovascular disease

Although other mechanisms may be contributory, the antithrombotic properties of aspirin derive predominantly from its platelet-inhibitory effects. These are mediated via irreversible acetylation of platelet cyclo-oxygenase with subsequent blockade of platelet thromboxane synthesis. Long term administration of doses of aspirin as low as 20mg daily depresses platelet thromboxane formation by more than 90%; however, higher doses appear to be necessary to prevent thromboxane-dependent platelet activation in vivo. While there is evidence of biochemical selectivity with low doses of aspirin, significant reduction of the platelet-inhibitory eicosenoid, prostacyclin, occurs even at dosages ranging from 20 to 40mg daily. The ability of aspirin to prevent the occurrence or recurrence of vaso-occlusion has been extensively investigated. In the secondary prevention of myocardial infarction 7 placebo-controlled trials involving more than 15,000 patients have been completed. The dose of aspirin varied from 300 to 1500mg daily. Although none of the individual trials produced statistically significant reductions in total or coronary mortality, taken together the results are highly suggestive of a beneficial effect of aspirin. Similarly, 2 recent studies in patients with unstable angina demonstrated a protective effect of aspirin against acute myocardial infarction and death. While each study employed widely different doses of aspirin (324mg and 1250mg daily) similar reductions in mortality were reported. The effects of aspirin on the prevention of coronary artery bypass graft occlusion have been evaluated in 9 trials. Aspirin in doses of 100 to 975mg daily was shown to be of benefit in preventing early (less than 6 months) graft occlusion, particularly when therapy was started within 24 hours of operation. In patients with prosthetic vascular grafts of the lower limbs, aspirin has been shown to reduce platelet deposition, however further controlled trials will be required to establish the patient population most likely to benefit and, as in all these studies, the optimum dose of aspirin to employ. In patients with prosthetic heart valves it is clear that aspirin alone is insufficient to prevent thromboembolic complications and when administered as an adjunct to anticoagulant therapy it is associated with a high incidence of bleeding. In contrast, there is convincing evidence from several studies for the efficacy of aspirin in doses of 990 to 1300mg daily in the prevention of stroke and death in patients with transient ischaemic attacks.(ABSTRACT TRUNCATED AT 400 WORDS)

Thrombotic and bleeding complications of prosthetic heart valves

A review of articles published since 1979 indicates that thrombotic and bleeding complications account for about 50% of valve-related complications in patients with bioprosthetic aortic and mitral valves and for approximately 75% of the complications in patients with mechanical valves. Although compromised by lack of standard definitions and by variability in reporting and follow-up, the data suggest that the linearized rate of both thrombotic and bleeding complications in patients with aortic bioprostheses is approximately half that for aortic mechanical prostheses (2% versus 4%), but is approximately equal for both bioprostheses and mechanical valves in the mitral position (approximately 4%), and for mechanical and bioprosthetic aortic and mitral valves in combination. However, linearized rates for fatal thrombotic and bleeding events are two to four times higher in patients with mechanical prostheses. The adequacy of warfarin anticoagulation is the most important factor affecting thrombotic and bleeding complications in patients with mechanical valves and over shadows the dubious importance of other phenomena such as atrial fibrillation and left atrial thrombus. Short-term warfarin anticoagulation or the use of long-term platelet inhibitors, or both, do not appear to reduce the incidence of thrombotic complications in patients with aortic bioprostheses but increase bleeding. For mitral bioprostheses, the postoperative use of warfarin for three months or aspirin indefinitely is as effective in preventing thromboembolism as long-term warfarin. Acute prosthetic valve endocarditis is associated with a 13 to 40% incidence of thrombotic complications. Likewise, the recurrence rate of cerebral emboli is high (20-30%) in patients with prosthetic valves who are not anticoagulated. Bioprostheses are strongly preferred for women who wish to bear children; fetal wastage occurs in 25 to 30% of pregnant women with mechanical heart valves who receive either warfarin or heparin, or a combination of the two. Heparin, however, greatly increases the risk of maternal bleeding. In children, the efficacy of platelet inhibitors without warfarin anticoagulation is unproven; nearly all serious strokes occur when warfarin is omitted; and permanent disability from warfarin-related bleeding is rare. All prosthetic cardiac valves initiate coagulation and affect the dynamic equilibrium between activated procoagulants and endogenous anticoagulants. Warfarin is the only available oral exogenous anticoagulant.(ABSTRACT TRUNCATED AT 400 WORDS)

Aspirin and other antiplatelet drugs in the prophylaxis of thrombosis

Aspirin is of proven value as an antithrombotic drug. In unstable angina it reduces the risk of death and myocardial infarction by half. After a myocardial infarction it reduces the risk of death by about 10% and of coronary incidence (coronary death or definite myocardial infarction) by about 25%. These effects appear to be additive with those of beta-blocking drugs. Aspirin also reduces the risk of occlusion of aortocoronary saphenous vein grafts by about half. In transient cerebral ischaemia, aspirin may reduce the risk of stroke and death by 50%. In most clinical trials to date the daily dose of aspirin ranges from 325 mg to 1400 mg. Interest in very low doses of aspirin (less than 60 mg daily) is considerable but has yet to be translated into proven clinical benefit. Dipyridamole has not been shown to be effective as an antithrombotic when used alone. Its antiplatelet action ex vivo may be enhanced by combination with aspirin but clinical trials have shown relatively little advantage of the combination over aspirin alone. Sulphinpyrazone has not become established as a first line antithrombotic drug. Epoprostenol is useful in extracorporeal circulations to prevent platelet consumption and possibly in severe inoperable peripheral vascular disease.

Antithrombotic therapy in patients with valvular heart disease and prosthetic heart valves

Indications and the type of antithrombotic therapy for the prevention of thromboembolism in patients with valvular heart disease, mechanical prosthetic heart valves and bioprosthetic heart valves are discussed. The evidence for these clinical recommendations is described and graded into five levels. The indications for anticoagulation in patients with valvular heart disease are chronic or paroxysmal atrial fibrillation, sinus rhythm with a very large left atrium, severe left ventricular dysfunction or presence of heart failure or a history of previous thromboembolism. Anticoagulant therapy is administered to prolong the prothrombin time to 1.5 to 2.0 times control, using rabbit brain thromboplastin (standardized international normalized ratio = 3.0 to 4.5). Risk factors for thromboembolism in patients with prosthetic heart valves are discussed. Because intracardiac thrombus formation may start during and continues early after operation, restarting heparin therapy 6 hours after operation and continuing it for the duration of the hospitalization is advised. For mechanical prosthetic heart valves, oral anticoagulation as outlined plus dipyridamole is advised indefinitely. Platelet inhibitor therapy alone is insufficient. For bioprosthetic heart valves, heparin is followed by oral anticoagulation as outlined for 3 months after mitral or aortic valve replacement and indefinitely after mitral valve replacement if there is atrial fibrillation or a very large left atrium; aspirin may be recommended indefinitely after aortic valve replacement. Antithrombotic therapy is also considered for four special situations: noncardiac surgery, prosthetic valve endocarditis, anticoagulation after a thromboembolic event, and antithrombotic therapy during pregnancy.

Pharmacology of platelet inhibitors

Although many drugs have inhibitory effects on platelet function, none of them inhibits all of the mechanisms that may be involved in the various forms of thrombosis. Choice of suitable drugs is hampered by lack of full knowledge concerning the reactions that make the major contributions to the formation of arterial thrombi at sites of repeated vessel wall injury or on atherosclerotic lesions. Drugs such as aspirin that inhibit the arachidonate pathway in platelets can only be expected to be effective against thromboembolic events in which the generation of thromboxane A2 plays a major part. If thrombin and fibrin formation are dominant, oral anticoagulant agents or heparin should be beneficial; thus, experimental evidence indicates that with repeated vessel wall injury, the formation of platelet fibrin thrombi on the vessel wall is probably influenced more by inhibitors of thrombin generation than by the subendothelial constituents such as collagen. Agents like prostacyclin that raise platelet cyclic adenosine monophosphate (AMP) levels in platelets by stimulating adenylate cyclase are potent inhibitors of the reaction of platelets to all aggregating and release-inducing stimuli, but these agents are not suitable for long-term administration. The effect of dipyridamole on platelet cyclic AMP levels is weak, and this drug may act through other effects on platelets or on other cells. Indeed, several of the drugs that have been tested in clinical trials may exert their effects through unrecognized mechanisms. Many combinations of drugs have been used to affect platelets or platelets and coagulation. This practice has been based on the theory that because several mechanisms may be involved in thrombus formation, combinations of drugs that inhibit different mechanisms may be beneficial.

Clinical trials evaluating platelet-modifying drugs in patients with atherosclerotic cardiovascular disease and thrombosis

Aspirin has been convincingly shown to reduce stroke and death in men with transient ischemic attacks (it may possibly be beneficial to women also), myocardial infarction and death in patients with unstable angina, thromboembolic complications associated with artificial heart valves in patients receiving oral anticoagulants (although gastrointestinal bleeding is prohibitive with this combination), and thrombotic occlusion of silicone rubber arteriovenous cannulae in uremic patients undergoing hemodialysis. In addition, aspirin may possibly decrease occlusion of saphenous vein aortocoronary grafts and venous thrombosis in men after hip replacement, although these reports require confirmation. Aspirin is ineffective in the secondary prevention of stroke and has unproven benefit in the secondary prevention of myocardial infarction. Dipyridamole in combination with oral anticoagulation decreases the thromboembolic complications associated with mechanical heart valves. The combination of aspirin and dipyridamole prevents both early and late occlusion of saphenous vein aortocoronary bypass grafts and protects renal function in patients with membranoproliferative glomerulonephritis. The relative importance of combining aspirin and dipyridamole compared with either agent used singly remains to be established. Sulfinpyrazone reduces the thrombotic occlusion of arteriovenous cannulae and early occlusion of saphenous vein aortocoronary grafts. The reported benefit in the secondary prevention of myocardial infarction is controversial.

Pharmacology of platelet inhibition in humans: implications of the salicylate-aspirin interaction

The current dispute over the effects of "low" vs "high" doses of aspirin should take into consideration the pharmacokinetics of this drug. In fact, different pharmaceutical formulations of aspirin may deliver little or no aspirin to the systemic blood. This was the case, for instance, in healthy volunteers taking 320 mg of compressed aspirin or 800 mg of enteric-coated aspirin. In all instances thromboxane B2 generation in serum was fully inhibited. Platelet cyclooxygenase might therefore be effectively acetylated by exposure to aspirin in the portal circulation, whereas vascular cyclooxygenase could be spared. Thus aspirin formulations ensuring complete first-pass deacetylation should be sought rather than "low" or "high" doses of unspecified aspirin formulations. Regardless of the type and dose of aspirin administered, salicylate is formed and accumulates in the circulation. It may antagonize the effects of aspirin on cyclooxygenase, at least in acute conditions. As an example, after administration of 1 g of salicylate to healthy volunteers, when plasma levels of the drug were about 75 micrograms/ml, the effect of 40 mg iv aspirin (given 40 min later) on platelet cyclooxygenase and aggregation was significantly diminished. In contrast, in patients undergoing saphenectomy, the same dose of salicylate (1 g) gave plasma drug levels of about 25 micrograms/ml; salicylate was unable to prevent the inhibitory effect on platelets of 40 mg iv aspirin (given 1 hr later) but did act on vascular prostacyclin. Thus the combination of salicylate with aspirin at an appropriate dose and blood level ratio may result in almost complete dissociation of the drug's effect on platelets and vessels in man.(ABSTRACT TRUNCATED AT 250 WORDS)