Platelet survival in patients with substitute heart valves

Diagnosis of Coronary Embolism: A Review1

Coronary embolism is an uncommon but distinct clinical entity. It can be diagnosed clinically, and should be suspected when acute myocardial infarction occurs in association with an underlying condition which predisposes to embolism. The most common are valvular heart disease, a prosthetic heart valve, infective endocarditis, cardiomyopathy with mural thrombus and arrhythmia. The diagnosis may be obscured by atypical symptoms and transient ECG changes. The diagnosis is supported by the demonstration of normal coronary arteries by selective coronary arteriography. Treatment with long-term anticoagulants may prevent further emboli. Additional antiplatelet drugs are also necessary in patients with prosthetic heart valves.

Biomaterial-associated thrombosis: roles of coagulation factors, complement, platelets and leukocytes

Our failure to produce truly non-thrombogenic materials may reflect a failure to fully understand the mechanisms of biomaterial-associated thrombosis. The community has focused on minimizing coagulation or minimizing platelet adhesion and activation. We have infrequently considered the interactions between the two although we are generally familiar with these interactions. However, we have rarely considered in the context of biomaterial-associated thrombosis the other major players in blood: complement and leukocytes. Biomaterials are known agonists of complement and leukocyte activation, but this is frequently studied only in the context of inflammation. For us, thrombosis is a special case of inflammation. Here we summarize current perspectives on all four of these components in thrombosis and with biomaterials and cardiovascular devices. We also briefly highlight a few features of biomaterial-associated thrombosis that are not often considered in the biomaterials literature: The importance of tissue factor and the extrinsic coagulation system. Complement activation as a prelude to platelet activation and its role in thrombosis. The role of leukocytes in thrombin formation. The differing time scales of these contributions.

What really is blood compatibility?

The criteria for nonthrombogenicity are classically defined as long clotting times and minimal platelet deposition. The inability to point to unequivocal progress in the development of truly nonthrombogenic materials, highlights the inadequacy if not actually invalidity of these criteria. Our approach is to define nonthrombogenicity in terms of: (1) a thrombin production rate constant, kp < 10(-4) cm s(-1); (2) low platelet consumption and low degree of platelet activation (e.g., microparticle formation); (3) perhaps some platelet spreading; and (4) low complement and leukocyte activation. Only when the target becomes clear, will it be possible to identify clear strategies for producing the materials we need.

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.

Lack of influence of low-dose acetylsalicylic acid (100 mg daily) on platelet survival time, beta-thromboglobulin and platelet factor 4 in patients with peripheral arterial occlusive disease

In this study we investigated the influence of low-dose (100 mg daily) acetylsalicylic acid (ASA) on 111-In-platelet survival time (PST) and on plasma levels of beta-thromboglobulin (beta-TG) and platelet factor 4 (PF 4) in 30 patients (median age: 60 years) with arteriographically proven peripheral arterial occlusive disease in a chronic stable phase. We observed no significant changes of PST during therapy with ASA (weighted mean: 169.8----166 [median] hours; multiple hit: 168.3----170.6 hours), and also the plasma levels of beta-TG (median: 31.8----32.3 ng/ml) and of PF 4 (3.6----3.9 ng/ml) remained unchanged.

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)

Influence of acetylsalicylic acid (1.0 g/day) on platelet survival time, beta-thromboglobulin and platelet factor 4 in patients with peripheral arterial occlusive disease

In this study we investigated the influence of acetylsalicylic acid (ASA) 1.0 g/day on 111-In-platelet survival time (PST) and on plasma levels of beta-thromboglobulin (beta-TG) and platelet factor 4 (PF 4) in 37 patients (median age: 63.4 years) with arteriographically proven peripheral arterial occlusive disease (PAOD) in a chronic stable phase. We found a slight but significant increase of PST during therapy with ASA (weighted mean (WM): 184.3----193.2 [median] hours, p less than 0.05; multiple hit (MH): 182.4----192.8 hours, p less than 0.005) for the total group of patients. Concerning the influence of risk factors of PAOD on PST during ASA-therapy, there was a significant increase of PST only in the nondiabetics (WM: 180.3----204.6 hours, p less than 0.01; MH: 176.8----195.3 hours, p less than 0.01). There was a negative correlation between the baseline values of PST and their increase following ASA therapy (WM: r = -0.63; p less than 0.0001; MH: r = -0.61, p less than 0.0001). The pretreatment levels of beta-TG--but not PF 4--were significantly (p less than 0.001) elevated compared to healthy controls. Therapy with ASA caused a significant decrease in the plasma levels of beta-TG (median: 30.4----26.6 ng/ml, p less than 0.001) and PF 4 (2.95----2.2 ng/ml, p less than 0.01).

Cardiac valve prostheses: pathological and bioengineering considerations

Cardiac valve replacement with mechanical prosthetic or bioprosthetic devices enhances patient survival and quality of life. Nevertheless, prosthesis-associated complications are frequent and contribute significantly to outcome. Thromboembolic complications are the most important problems in patients with mechanical valves, necessitating chronic anticoagulation in all patients receiving them. In contrast, patients with bioprosthetic valves, composed of chemically treated animal tissues, generally do not require anticoagulants. However, bioprostheses fail frequently by degeneration, especially that involving cuspal calcification. This paper reviews the pathological and bioengineering considerations in the selection of cardiac prosthetic valves and the management of patients who have received these devices. The significance, morphology, and pathogenesis of the observed major complications and other alterations during function are described in detail. Contemporary investigative trends are summarized, including studies of inhibition of mineralization and other degenerative changes in bioprostheses, improved design rigid mechanical valves with pyrolytic carbon occluders and the development of central-flow, flexible polymeric leaflet valves.

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.

Antithrombotic therapy in children and adolescents

This report documents our experience with long term antithrombotic therapy (acenocoumarol plus aspirin) in 31 children and adolescents, from 5 months to 16 years of age. The valves replaced were mitral in 20 patients, aortic in 4, mitral-aortic in 4 and tricuspid in 3; the overall follow-up time was of 1336 months. Anticoagulant requirement in each children was not in correlation with age, but a significant increase (p less than 0.01) was found in association with sexual development. Our total incidence of embolic episodes was 1.49/1000 patient-months. The embolic incidence on adequate anticoagulated patients was 0.74/1000 patient-months and 93.7% of all patients were free of thrombo-embolic accidents up to 96 months of follow-up. Minor haemorrhage in relation to an excess of anticoagulant was 1.49/1000 patient-months. There has been only one major bleeding episode associated with severe sepsis, with an incidence of 0.74/1000 patient-months. No major difficulties were found in the management of anticoagulant treatment and its association with antiplatelet drugs in children.

Radiolabeled blood cells: techniques and applications

During the past few years, the clinical and experimental applications of autologous blood cells labeled with gamma-emitting radionuclides, i.e., technetium-99m (Tc-99m) and indium-111 (In-111) have continued to grow rapidly. Investigations have centered around developing techniques that would label blood cells efficiently without alteration of cell viability and their pathophysiologic integrity. Experimental and clinical applications have been aimed at the studies of in vivo cell kinetics and detecting abnormal lesions by external imaging. These will be discussed in detail together with their advantages and disadvantages, and the future directions will be outlined.

Effects of platelet-modifying drugs on arterial thromboembolism in baboons. Aspirin potentiates the antithrombotic actions of dipyridamole and sulfinpyrazone by mechanism(s) independent of platelet cyclooxygenase inhibition

To resolve questions of drug actions, efficacy, and interactions for platelet-modifying agents used clinically, we have compared the relative capacities and mechanisms of aspirin, dipyridamole, sulfinpyrazone, and dazoxiben to prevent arterial thromboembolism in a baboon model. In 136 studies the agents were given twice daily by oral administration both singly and in combination. The antithrombotic efficacy of a given therapy was determined by its capacity to interrupt steady-state platelet utilization induced by thrombogenic arteriovenous cannulae. When given alone, dipyridamole and sulfinpyrazone reduced the rate at which platelets were utilized by thrombus formation in a dose-dependent manner with essentially complete interruption by dipyridamole at 10 mg/kg per d. In contrast, neither aspirin (2-100 mg/kg per d) nor dazoxiben (20-100 mg/kg per d) decreased cannula platelet consumption detectably despite the striking reduction in the capacity of platelets to produce thromboxane B2. However, aspirin, but not dazoxiben, potentiated the antithrombotic effects of dipyridamole and sulfinpyrazone in a dose-dependent fashion without changing the pharmacokinetics for any of the agents. Complete potentiation required aspirin at 20 mg/kg per d to be given with each dose of dipyridamole. Because dazoxiben's blockade of platelet thromboxane A2 production was not associated with antithrombotic potentiation, and because complete potentiation by aspirin required a dose that fully inhibited vascular production of prostaglandin I2 (PGI2), we conclude that aspirin's potentiating effect on dipyridamole is independent of PGI2 production or inhibition of thromboxane A2 formation. In addition, because frequent repeated and synchronous dosing of aspirin was necessary, aspirin's potentiating effects appear to be produced by mechanism(s) unrelated to its potent, irreversible inhibition of platelet cyclooxygenase.

Effect of suloctidil on platelet function in patients with shortened platelet survival time

The effect of suloctidil (600 mg/day) on platelet survival time (PST) and plasma and urine betathromboglobulin (BTG) was studied in a double-blind, placebo-controlled six-week crossover trial in 13 patients with shortened PST (less than 110 hrs, exponential model). Mean PST after suloctidil (110.6 hrs) was significantly higher than in the placebo phase (94.5 hrs) (p = 0.04). Mean plasma BTG was significantly lower during the suloctidil phase (42.8 ng/ml) compared with the placebo phase (65.8 ng/ml) (p = 0.02), but there was no significant difference in urine BTG. These results suggest that suloctidil provides a platelet protective effect and therefore may be of benefit in reducing the frequency of platelet mediated thromboembolic events.

Labeled cells in the investigation of hematologic disorders

Radiolabeling techniques for white cells, platelets, and erythrocytes are reviewed. The early studies using diisopropylfluoro-32P contributed to an understanding of the production and circulation of the blood elements, and 51Cr proved useful in localizing sites of cell migration or destruction. 111In-oxine has further improved the understanding of blood cell organ sequestration, and permitted combined kinetic and organ imaging studies. Radionuclide labels have been essential for the elucidation of various hematologic disorders, such as the neutropenias, thrombocytopenias, anemias, and polycythemia. Many new treatments, including monoclonal antibodies, have been evaluated with radionuclides.

Indexes of intravascular hemolysis, quantification of coagulation factors, and platelet survival in patients with porcine heterograft valves

Ten patients with porcine heterograft valves who were not receiving anticoagulant agents were evaluated to determine the effect of the valve on red blood cell survival and on platelet activation and consumption as measured by (1) quantification of the coagulation mechanism, (2) platelet function studies, and (3) 51-chromium platelet survival time. There was no evidence of significant intravascular hemolysis as determined by the reticulocyte count, serum iron and iron binding capacity, serum bilirubin level, or lactic dehydrogenase activity. The coagulation profile and the platelet function studies were normal. No statistically significant difference was found in the platelet survival time in the 10 patients with porcine heterograft valves (half-life 3.2 +/- 0.8 days) and the 11 normal control subjects (half-life 3.6 +/- 0.6 days) (p greater than 0.2). The finding of a normal platelet survival time in patients with porcine heterograft valves is consistent with clinical experience indicating that this device is associated with a low incidence of systemic embolization, approximating 3% per year.

Hematologic manifestations and etiology of atrial myxoma

We have investigated the case of a woman, 44 years of age, who presented with erythrocytosis, thrombocytopenia and a right axial myxoma. Surgical removal of the tumor allowed correction of all hematologic abnormalities. A serum erythropoietin level obtained from the right atrium of the patient during pre-operative cardiac catheterization was twice those of 4 control patients (250 MIU vs. 131 MIU). Renal vein erythropoietin levels were normal. This implicates atrial myxomas as another tumor capable of erythropoietin production, and explains the heretofore mysterious polycythemia frequently reported to be associated with this tumor.

Platelet function studies in coronary artery disease. X. Effect of dipyridamole

To evaluate the effects of dipyridamole on blood platelet function in patients with coronary artery disease, platelet counts and aggregation were examined in aortic and coronary venous blood. Before administration of dipyridamole, platelet counts and aggregation in response to adenosine diphosphate were less (p less than 0.02) in coronary venous than in aortic blood. Dipyridamole administration (100 mg) resulted in an increase in platelet counts and platelet aggregation in coronary venous blood so that the differences in aortic and coronary venous blood values were eliminated. These phenomena were probably related to inhibitory actions of dipyridamole on platelet adhesion to atherosclerotic vessels. To further study the mechanism of action, the direct effects of dipyridamole on in vitro platelet aggregation were evaluated. Although dipyridamole, in the concentrations used, had no effect on in vitro platelet aggregation, it greatly potentiated the aggregation inhibitory actions of exogenous prostacyclin. In vivo potentiation of endogenous prostacyclin and inhibitory actions on platelet adhesion are the most likely mechanisms of the potentially beneficial actions of dipyridamole.

Evaluation of platelet kinetics following porcine and mechanical valve replacement

Because thromboembolic pathogenesis in mechanical prosthetic valves (MP) might involve selective MP platelet consumption with consequent reduced platelet survival, it has been suggested that lowered platelet half-life (T 1/2) detection provides identification of postoperative thrombotic risk. Porcine heterografts (PH) possess less thromboembolic hazards than MP; thus, comparative evaluation of PH versus MP platelet T 1/2 is germane to the test's thrombotic predictive value, yet platelet kinetics in PH require elucidation. Accordingly, 51chromium platelet T 1/2 was determined in 16 patients with substituted valves (10 PH and 6 MP). Average platelet T 1/2 in PH (2.9 +/- 0.6 d) was similar (p greater than 0.05) to MP (3.2 +/- 0.6 d), and both were considerably lower (p less than 0.05) than platelet T 1/2 in normal subjects (NL, 5.1 +/- 0.5 d). Furthermore, platelet T 1/2 was below the lowest NL platelet T 1/2 (4.0 d) in seven PH and three MP patients. Thus, frequency and extent of decreased platelet survival in PH are common and equal to MP. These results indicate that platelet half-life does not afford thromboembolic predictability in patients with mechanical valves and, instead, favors prosthetic-induced physical trauma causing abnormal platelet survival.

Trial of dipyridamole-aspirin in recurring venous thrombosis

38 patients (26 men) with recurring venous thromboembolism (RVTE) were enrolled in a prospective double-blind, placebo-controlled trial of dipyridamole (DPY), 100 mg a day, and aspirin (ASA), 1200 mg a day. Platelet survival (51Cr labelling of autologous platelets) was measured every 6 months for 18 months. 19 patients were randomised to treatment with DPY and ASA, and 1 had new venous thrombosis (after 15 months of treatment); 19 received placebo and 7 had new venous thrombosis (4--16 months later (chi 2 = 5.70; p< 0.05). DPY-ASA increased platelet survival whereas placebo treatment did not. The results suggest that in patients with RVTE and abnormal platelet survival time DPY in combination with ASA decreases the frequency of new venous thrombosis. Peptic ulcers developed in 2 patients treated with DPY-ASA.

The effect of Coumadin upon thrombus forming on foreign surfaces

Experience with the Stagnation Point Flow Experiment has revealed the earliest microscopic events surrounding thrombus inception and growth on foreign surfaces in a perfectly defined flow field. Using 40 and 80 X magnification, the effects of Coumadin upon these events was studied. These events include leukocyte adherence, fibrin deposition and platelet aggregation, deviation of flow streamlines around these aggregates, and subsequent downstream thrombus growth. Surface density of leukocytes was measured as a function of time and shear, and served as an index of the quantity of thrombus on the surface, which in turn, allowed direct comparisons of thrombus growth rate between experiments. Coumadin decreases the adhesive force between leukocytes or thrombi and the foreign surface. Emboli are more frequent with Coumadin, but are smaller. Thrombus inception is not altered. These results suggest that clinical benefit obtained from use of Coumadin derives from reduction in size of emboli.

Microcalorimetric measurements of heat production in platelets of patients with prosthetic heart valves

The heat production in resting platelets of 12 patients with prosthetic heart valves was found to be significantly decreased (p less than 0.005), P = 49 +/- 9 fW/cell. In a matched control group of eight patients with heart valve disease, the corresponding heat production value in resting platelets was P = 65 +/- 11 fW/cell. The present results indicate that the overall metabolic activity in platelets of patients with prosthetic heart valves is abnormally low.

Platelet suppressant therapy in patients with prosthetic cardiac valves. Relationship of clinical effectiveness to alteration of platelet survival time

Platelet survival time (SURV) has correlated with thromboembolism in patients with prosthetic cardiac valves. Sulfinpyrazone increases SURV. SURV (autologous labeling with 51Chromium) was measured in 126 patients who had aortic or mitral valve replacement. These patients were followed prospectively. Ninety-four with shortened SURV received sulfinpyrazone; 32 with normal SURV were not treated with platelet suppressants. Eighty-seven patients were anticoagulated with warfarin--67 with shortened SURV and 20 with normal SURV. Eleven patients have had thromboembolism, and all had shortened SURV (2.4 +/- 0.08 days; average half-time +/- SEM; normal 3.7 +/- 0.03 days; n = 26) none had an increase of SURV with sulfinpyrazone (2.3 +/- 0.09 days). Of 83 patients with shortened SURV who did not have embolism, sulfinpyrazone increased SURV in 59 (71%) 2.6 +/- 0.05 to 2.9 +/- 0.06 days). Of 35 patients with shortened SURV who failed to increase SURV with sulfinpyrazone, 11 (31%) had embolism; none of 59 (0%) with an increase of SURV with sulfinpyrazone had thromboembolism. These results suggest that patients with thromboembolism after prosthetic cardiac valve replacement have shortened SURV and that patients treated with slufinpyrazone who have thromboembolism do not have an increased SURV.