Effect of platelet inhibition on platelet phenomena in cardiopulmonary bypass in pigs

Antithrombotic therapy in the coronary vein graft patient

Coronary artery bypass grafting is an important therapeutic modality in the treatment of the patient with coronary artery disease; however, long-term results are limited by the development of saphenous vein graft disease early and late after operation. The pathogenesis of early vein graft occlusion is primarily thrombotic, while that occurring later frequently involves thrombosis superimposed on intimal hyperplasia or vein graft atherosclerosis. We describe the role of various platelet inhibitors and anticoagulants in the prevention of saphenous vein graft occlusion following coronary artery bypass grafting.

The pathogenesis and prevention of aortocoronary vein bypass graft occlusion and restenosis after arterial angioplasty: role of vascular injury and platelet thrombus deposition

Vascular injury during aortocoronary vein bypass graft surgery and arterial angioplasty initiates platelet thrombus deposition and mandates antithrombotic therapy, starting before the procedures to maximize protection against occlusion. This has been shown in studies in animals and in patients undergoing aortocoronary vein bypass graft operation where dipyridamole therapy was started before the operation, heparin was given intraoperatively and combined dipyridamole and aspirin therapy was started 7 hours after operation and markedly reduced vein graft occlusion in patients with grafts at both high and low risk for occlusion without increasing bleeding. Other alternative regimens, particularly preoperative dipyridamole followed postoperatively with aspirin alone, offer a promising future. Therapy should be continued for at least 1 year and perhaps indefinitely. Control of coronary risk factors appears important for long-term therapy to try to retard the atherosclerotic and occlusive process that leads to approximately 50% vein graft attrition by 10 years after operation. The possible role of cod liver oil and internal mammary artery bypass is discussed. Arterial angioplasty appears to cause deep arterial injury that activates both platelets and the coagulation system. These potentiate each other to form macroscopic mural thrombus within 1 hour in more than 90% of arteries that manifested deep arterial injury in pigs. Acute platelet thrombus deposition was retarded but not eliminated by only certain platelet-inhibitor agents. Implications for ongoing trials, current empiric therapy and future therapy are discussed.

[Hematologic changes induced by extracorporeal circulation]

The plasma and cellular changes seen during the use of extracorporeal circulatory circuits define the system's degree of haemocompatibility. Heparin is still very much used to prevent activation of the blood clotting mechanisms and to reduce their effects. The fall in concentration of the clotting factors and their inhibitors is usually moderate; it is due to haemodilution, particularly important in cardiac surgery and during plasma exchanges. Fibrinolysis is often activated. In cardiac surgery, it is seen in nearly 20% of cases straight after the end of the ECC, and in nearly 80% of cases during the ECC. In all cases of resin haemoperfusion, there is an early transitory fibrinolytic burst, seen only rarely during haemodialysis and plasma exchanges. This phenomenon is usually well controlled by the natural inhibitors; it can be prevented by antifibrinolytic drugs. Cellular changes show the same trends during cardiac surgery, haemoperfusion and haemodialysis. Thrombopaenia is seen within a few minutes starting of ECC. It is caused by platelet activation, with aggregate formation; these are then trapped by the microcirculation. Leukopaenia occurs at the same time, later followed by rebound; complement activation could be the main cause by forming aggregates of polymorphonuclear cells and monocytes. Intravascular mechanical haemolysis reaches significant levels only in a few cardiac surgical procedures. The great speed of activation of the plasma and platelet enzyme systems by the ECC circuits explains these early changes. They are not only due to direct effects of the physiological circulatory characteristics and to contact with artificial surfaces, but also to plasma-cell interactions and to the patients' reaction to these first alterations.

Antiplatelet drugs: clinical pharmacology and therapeutic use

Because platelets are so important in thrombus formation, drugs which inhibit platelet function (the 'antiplatelet drugs') have considerable potential as antithrombotic agents. Among the antiplatelet drugs, only aspirin, sulphinpyrazone, dipyridamole, hydroxychloroquine, and clofibrate have had wide clinical trial. Their effects on platelet metabolism differ. Aspirin prevents platelet prostaglandin synthesis by acetylating and irreversibly inactivating platelet prostaglandin synthetase, while sulphinpyrazone is a reversible inhibitor of the same enzyme. Both aspirin and sulphinpyrazone impair the platelet release reaction and reduce platelet aggregation, but neither prevents platelet adhesion to the subendothelium or to foreign surfaces. On the other hand, dipyridamole reduces platelet adhesion as well as aggregation, probably by inhibiting phosphodiesterase and so raising platelet cyclic AMP levels. The effects of hydroxychloroquine and clofibrate have been less well defined. As the antiplatelet drugs form a diverse group of substances with differing effects on platelet function, it is hardly surprising that every potential clinical application of each antiplatelet drug or drug combination has had to be tested separately, and that these drugs have not proved to be equally effective. One or more antiplatelet drugs have now been evaluated in each of the following situations.

Fresh autologous blood in open-heart surgery. Influence on blood requirements, bleeding and platelet counts

The role of fresh autologous blood on haemostasis was studied in 30 patients undergoing aortic valve replacement. All the patients were extremely haemodiluted during the perfusion by using a non-haemic priming solution and withdrawal of 15% of the blood volume at the start of operation. In half of the patients, the autologous blood was retransfused immediately after the termination of perfusion. In the other half, donor blood was given in this period, while the retransfusion of autologous blood was delayed until three hours postoperatively. An increase of circulating platelets was found after the withdrawal of blood and replacement with double the amount of Ringer's acetate. Significantly less donor blood and plasma was transfused in the patients receiving early transfusion of autologous blood. An average reduction of 36% donor blood and 45% plasma was obtained. The blood losses were also less in these patients, but the differences were not significant.

Cardiopulmonary bypass. Microembolization and platelet aggregation

Particulate microemboli and in vitro platelet aggregation were studied in blood of patients during cardiac operations with an electronic particle size analyzer. A small gradient of microemboli developed on passage of blood through a bubble oxygenator but not through a membrane oxygenator. However, with both types of oxygenators, there was a sustained increase in the volume of microemboli in cardiotomy return blood which was much greater than in aterial blood. After cardiopulmonary bypass with both oxygenators, there was a comparable reduction in the volume of circulating platelets which exceeded that of the hemoglobin concentration, indicating platelet loss exceeded that that expected from hemodilution alone. However, the total volume and mean size of platelet aggregates induced in blood of patients after membrane oxygenation was significantly greater than similar measurements after bubble oxygenation. This study shows that membrane oxygenation reduces particulate microembolization and preserves platelet function in patients undergoing cardiac operations when compared to bubble oxygenation.

Platelet aggregation: effects of cardiopulmonary bypass

This study was designed to determine whether reduction in platelet aggregate microembolization during the first 30 minutes of cardiopulmonary bypass is due to thrombocytopenia or to decreased ability of platelets to aggregate. The total volume of platelet aggregates induced in blood by adenosine diphosphate (ADP) was measured with a Coulter counter. The volume of platelets in blood was calculated by multiplying hemocytometry platelet counts by the mean platelet volume. Immediately before cardiopulmonary bypass, the total volume of aggregates induced in blood by ADP (2muM) was reduced when compared to normal donors because of (1) a slight fall in the volume of platelets, and (2) reduction in the percentage by volume of platelets which aggregated. After 30 minutes on bypass, the volume of both platelets and aggregates fell, but a greater percentage of platelets aggregated. This indicates that reduction of platelet aggregate formation during cardiopulmonary bypass is due to thrombocytopenia. It also suggests that anesthesia, surgical trauma and heparinization alter platelet reactivity more than cardiopulmonary bypass.