11 The Use of Antithrombotic Drugs in Artery Disease

A comparison of postoperative thrombotic potential following abdominal aortic surgery, carotid endarterectomy, and femoro-popliteal bypass

Postoperative changes in procoagulant, anticoagulant, and antifibrinolytic factors were compared in patients undergoing abdominal aortic surgery, carotid endarterectomy, and femoro-popliteal bypass. There were increases in plasma fibrinogen (P < 0.01) and factor VIII coagulant (P < 0.01) levels following all three procedures. There were decreases in antithrombin III (P < 0.01) and protein C (P < 0.01), and increases in thrombin-antithrombin complex levels (P < 0.01) in the abdominal aortic group only. There were no significant changes in type 1 plasminogen activator inhibitor levels following any of the procedures. The results indicate that all three procedures are associated with an increased potential for thrombosis due to increases in procoagulant factors. However, patients undergoing abdominal aortic surgery are particularly at risk due to concurrent decreases in natural anticoagulant factors. Specific antithrombotic therapy should be considered for all patients undergoing vascular surgery, but particularly for those undergoing major procedures such as abdominal aortic surgery.

Abnormally high circulation levels of tissue plasminogen activator and plasminogen activator inhibitor-1 in patients with a history of ischemic stroke

We evaluated 106 subjects with and 109 subjects without a history of ischemic stroke. All were attending a metabolic ward. The two groups were compared for major risk factors for ischemic events. A positive family history for ischemic complications of atherosclerosis was more common in subjects with a history of stroke than in those without; moreover, plasma levels of plasminogen activator inhibitor-1 (PAI-1) and tissue-type plasminogen activator (TPA) were higher in patients with documented previous events. A strong positive significant correlation was found between TPA and PAI-1 levels, and an interaction between age and TPA was observed when the sample was stratified according to ages being above or below 70 years. When the patient population was analyzed according to the number of ischemic events, it was found that 62 of the 106 subjects with a history of stroke had experienced more than one ischemic event. Under these conditions, the levels of TPA and PAI-1 still correlated with the occurrence of previous ischemic episodes. As in the whole patient sample, TPA was the strongest discriminator. We conclude that in subjects attending a metabolic ward, TPA and PAI-1 levels consistently help identify subjects with a history of cerebral ischemic episodes and that TPA is the strongest discriminator.

Fibrinogen and mechanisms of thrombosis. A difficult link

Epidemiological observations indicate that high plasma fibrinogen levels are strongly correlated to the frequency of two major thrombotic complications of atherosclerosis: stroke and myocardial infarction. Thrombosis is increasingly recognized as a central mechanism in stroke and myocardial infarction, and fibrinogen is involved in events thought to play a major role in thrombosis. Therefore, elucidation of the relationship between fibrinogen and thrombosis may strengthen the predictive value of this protein and define new interventions against stroke and myocardial infarction. In addition, advances in the understanding of the atherogenic potential of several risk factors of coronary heart disease took advantage of information emerging from the measurement of the factor in population-based studies. Thus, it is conceivable that measuring plasma fibrinogen to predict stroke and myocardial infarction is a major direction to be followed to gain insight into the thrombogenic potential on this protein and inspire new strategies against thrombotic complications of atherosclerosis.

The effect of anaesthetic agents on platelet function

This paper reviews studies which have investigated the effect of anaesthetic agents on platelet function. The results of these studies suggest that halothane is the only agent in current use which inhibits platelet function in concentrations used clinically. Nitrous oxide appears to cause only a modest inhibition, while enflurane and isoflurane appear to have minimal or negligible effects. There is no current evidence that intravenous induction agents, opiates, or muscle relaxants affect platelet function. Reports indicate that local anaesthetic agents inhibit platelet aggregation, but only at concentrations far greater than peak plasma concentrations found during clinical use. Epidural anaesthesia may be associated with a reduction in platelet aggregation through a mechanism unrelated to direct local anaesthetic inhibition. The clinical significance of the effect of halothane on platelet function is not known. However, it is possible that halothane may affect bleeding or thrombotic complications in a similar manner to other 'anti-platelet' drugs.

Anticoagulants in the prevention of venous thromboembolism

For 50 years, the key to successfully preventing venous thrombosis (VT) or pulmonary embolism (PE) among high-risk patients has been the judicious use of anticoagulants: first through full doses of oral anticoagulants and more recently through low-dose heparin prophylaxis. Low-dose heparin has become the standard of comparison for other preventive methods, since it is relatively safe and simple, its ability to prevent approximately 65% of the subclinical VT found by leg scanning after elective general surgery is well known, and recent meta-analysis of the many pertinent published clinical trials (large and small) strongly suggests a much greater benefit: a 65% reduction in the risk of postoperative death from major PE. In addition, there are trials that have also found low-dose heparin to be effective in general medical patients, although its value in this clinical setting is much less well documented. Although several effective approaches other than low-dose heparin are available, many of these tend to be either more cumbersome (intermittent external leg compression) or probably less powerful (graded pressure elastic stockings). There are situations where low-dose heparin prophylaxis fails, most obviously after orthopaedic surgery where the use of more complex regimens, including adjusted-dose heparin treatment and various schedules of warfarin prophylaxis, becomes appropriate. Recent progress has come from the intensive clinical exploration of various low molecular weight heparin fractions or fragments which appear to be effective after once daily administration to general surgical patients and show great promise of effectiveness and safety after hip surgery. The level of warfarin effect needed for VT prophylaxis has also been reinvestigated, with trials suggesting a need for less warfarin and a lower prothrombin time effect than was previously thought to be appropriate. Given that any attempts to minimize mortality from PE in hospital patients must rely on the widespread and systematic use of simple, safe, and cost-effective preventive methods, it is hoped that these advances will help move anticoagulant prophylaxis further out of the realm of clinical research and into that of common clinical practice.

Unstable angina

Unstable angina can manifest as an array of symptom complexes. In some patients, medical therapy will stabilize the episodes of angina, and only predismissal exercise testing or angiography (or both) will be necessary. At the other end of the spectrum are patients with rest angina or multiple episodes of silent ischemia who are refractory to medical therapy and experience undetected microinfarction. Most of these patients require immediate catheterization and subsequent intervention with intra-aortic balloon pulsation, percutaneous transluminal coronary angioplasty, or coronary artery bypass grafting. An entire spectrum of manifestations exists between these two extremes. One challenge during the 1990s will be better stratification of patients with unstable angina so that safe, efficient, cost-effective treatment strategies can be appropriately applied to all patients.

Aspirin, prostaglandin E1 and Quin-2 AM-induced platelet dysfunction: restoration of function by noradrenalin

Studies from our laboratory have demonstrated that adrenalin can restore the function of drug-induced refractory platelets to the action of physiological agonists via a novel mechanism (membrane modulation). In various disease states and clinical conditions the circulating levels of noradrenalin (NA) increase several fold more than adrenalin. Therefore, in this study the influence of NA on three well characterized platelet refractory models has been evaluated. Aspirin-exposed platelets were obtained for these studies from blood of donors who had taken one baby aspirin (80 mgs) per day for four consecutive days. Prostaglandin-exposed platelets were obtained by disaggregating ADP-induced aggregates through addition of prostaglandin E1 (1 microM). Finally, low calcium platelets were obtained by buffering cytosolic free calcium with a calcium specific cell permeant fluorophore, Quin-2 AM (60 microM). Drug-exposed platelets did not aggregate irreversibly when stirred with arachidonate (0.45 mM) or NA 5 microM). However, when treated with NA first, drug-treated platelets regained their sensitivity to the action of arachidonate and aggregated irreversibly. The ability of NA to restore the sensitivity of drug-induced refractory platelets was effectively blocked by yohimbine (10 microM), an alpha 2 adrenoceptor antagonist. Results of these studies suggest that NA, similar to the action of adrenalin, can activate membrane modulation and restore the sensitivity of platelets to the action of physiological agonists under a variety of experimental conditions.

Measuring plasma fibrinogen to predict stroke and myocardial infarction

Epidemiological observations indicate that high plasma fibrinogen levels are strongly correlated with the frequency of two major thrombotic complications of atherosclerosis, stroke and myocardial infarction. Thrombosis is increasingly recognized as a central mechanism in stroke and myocardial infarction, and fibrinogen is involved in events thought to play a major role in thrombosis. Therefore, elucidation of the relationship between fibrinogen and thrombosis may strengthen the predictive value of this protein and suggest new treatment to prevent stroke and myocardial infarction. The current data relating fibrinogen to thrombosis are not easy to reconcile with the available epidemiological observations. In addition, advances in understanding the atherogenic potential of several risk factors for coronary heart disease have used information on the measurement of the risk factors in population-based studies. Thus, measuring plasma fibrinogen to predict stroke and myocardial infarction may be important in gaining insight into the thrombogenic potential of this protein and in inspiring new strategies against the thrombotic complications of atherosclerosis.

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.

Coronary artery bypass graft failure--an autoimmune phenomenon?

Plasma anticardiolipin antibody (ACA) was measured in 83 patients having coronary artery bypass graft surgery and results were correlated with the incidence of early (1-2 weeks) and late (12 months) graft occlusion, as judged by angiography. There was an association between preoperative ACA level and the incidence of late graft occlusion in relation to both number of patients with an occlusion and number of distal anastomoses occluded. 8 of 15 patients (53.3%) whose maximum ACA level exceeded 4 SD of the mean of controls had a late graft occlusion. When the ACA titre was 2-4, 0-2, or less than 0 SD above the mean of the controls, the occlusion rates were 3 of 13 (23.1%), 3 of 33 (9.1%), and 1 of 15 (6.7%), respectively (p less than 0.03). The incidence of a postoperative rise in ACA level was higher in patients who had had a myocardial infarction in the past than in those with a history of angina only (chi 2 = 4.08, p less than 0.05). This observation supports the notion that one mechanism of ACA production is an immune response to myocardial injury. These results raise the further possibility that the ACA, or related antiphospholipid antibodies, may play a part in progressive coronary vessel disease.