Are agents affecting platelet functions clinically useful?

Infarct size reduction: a review of the clinical trials

The most important finding to emerge from this review of experimental and clinical studies is that the earlier therapy is begun after the onset of symptoms of acute MI, the greater the potential for reduction of infarct size and possibly mortality. It is difficult to define a precise time after which therapy would not have an effect, since the clinical trials for each drug group vary significantly in respect to time of therapy initiation. In experimental studies, major salvage of ischemic myocardium occurs when the drug is given within two hours of coronary artery occlusion. If drug therapy is begun four to six hours postocclusion, then only minor or no reductions in infarct size will occur. The ability of any drug or intervention to reduce infarct size in humans would be optimized if therapy were begun less than four hours of onset of symptoms. With the realization of the wavefront phenomenon and the potential salvage of myocardium at risk with reperfusion, the introduction of reperfusion in the clinical setting with thrombolytic agents or other procedures becomes highly desirable. Clot-selective thrombolytic agents, such as tissue plasminogen activator, diminish the adverse effects and high costs of intracoronary thrombolytic therapy or PTCA. Consequently, it is probable that the initial procedure of choice would be the use of clot-selective thrombolytic therapy. Thrombolytic therapy only lyses thrombi and does not affect the underlying causes of the coronary artery occlusion. Therefore, therapy to reduce the chances of reinfarction and death must also be initiated. Percutaneous transluminal coronary angioplasty, in selected patients, should reduce the reocclusion rate. Beta-adrenoceptor blocking agents appear to be an excellent therapy for reducing mortality when administered chronically; these agents reduce myocardial oxygen consumption and reverse the imbalance between oxygen supply and oxygen demand caused by activation of the sympathetic nervous system and actions of catecholamines. Since thrombus formation has occurred at least once in patients who survive an MI, it is probable that the conditions for thrombus formation still exist. Therefore, institution of antiplatelet aggregating drugs, such as aspirin, would seem to be an appropriate prophylactic regimen. Beta blockers and possibly nitroglycerin have desirable effects when thrombolysis is unavailable. The efficacy of calcium-channel blocking agents on reduction of infarct size appears to be limited, although in the setting of stable and unstable angina postinfarction, these agents can play an important role.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of Ticlopidine upon platelet function, haemorrhage and post-operative thrombosis in patients undergoing suprapubic prostatectomy

In order to study in its protective effect against post-operative thrombosis, Ticlopidine was administered to one out of two randomized groups of patients undergoing suprapubic prostatectomy. Bleeding time, platelet function, and haematological parameters were followed. Blood loss during operation and post-operatively were estimated, transfusions required and subjective tolerance were recorded. Among twenty-six evaluated patients receiving Ticlopidine, positive leg scans were found in four, whereas among twenty patients treated with acenocoumarol post-operatively, positive leg scans were found in seven patients. The difference is not statistically significant, thus Ticlopidine appears to offer a protection against deep venous thrombosis comparable to that of acenocoumarol. A score is introduced for comparison of the results of aggregation tests. Platelet aggregability diminished during the use of the drug, but the adhesiveness was not influenced by Ticlopidine. The bleeding time was prolonged in both groups studied. Ticlopidine did not raise per-operative blood loss and caused only slightly more haematuria post-operatively. The subjective tolerance was good.

Management of patients with established ("completed") cerebral infarction

Management of patients with sudden neurological deficit must be based on complete knowledge of the underlying cause. In about 80% of such patients, a careful history and examination will lead to a precise etiologic and anatomic diagnosis. If the deficit is vascular in etiology, therapy and prognosis depend on its stage of evolution.

Intracoronary thrombolysis in evolving myocardial infarction

After experimental studies in dogs confirmed the feasibility and safety of rapid intracoronary thrombolysis by local infusion of Thrombolysin (streptokinase and plasmin), intracoronary thrombolysis was attempted in 20 patients with evolving myocardial infarction who were hospitalized within 3 hours from the onset of symptoms during the day and within 2 hours at night. Thrombolysin was infused in the immediate vicinity of the site of coronary occlusion using a 0.85 mm outer diameter catheter advanced through the lumen of the Judkins catheter. Reperfusion was achieved in four patients after an average of 43 minutes of Thrombolysin infusion at a rate of 2000 IU/min and in 15 patients after an average of 21 minutes of Thrombolysin infusion at a rate of 4000 IU/min. The failure to open the artery in one patient may have been caused by our inability to advance the infusion catheter close to the site of occlusion. Rethrombosis occurred in one patient 8 days after reperfusion and 2 days after discontinuation of anticoagulants because of a history of chronic alcoholism. Wall motion and perfusion studies showed improvement following reperfusion. Patency of the artery was achieved an average of 4 hours after the onset of symptoms. The need for earlier reperfusion is emphasized.

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.

The interaction of varying doses of dipyridamole and acetyl salicylic acid on the inhibition of platelet functions and their effect on bleeding time

1 In normal volunteers maximum reductions in platelet functions, collagen aggregation, adhesion and PF4 availability, were achieved using combined doses of 50 mg three times daily dipyridamole + 180 mg ASA or 75 mg three times daily dipyridamole + 120 mg ASA daily. 2 These doses did not prolong the bleeding time. 3 A synergistic effect has been demonstrated with 25 mg dipyridamole three times daily and 60 mg ASA. 4 At higher doses the effects on platelet functions were additive up to the maximal response. 5 The effect of low doses of ASA on platelet function was cumulative. 6 As lower doses of ASA in the combination studied inhibit platelet functions maximally without altering the bleeding time and probably without inhibiting prostacyclin, we suggest that these combinations of dipyridamole and ASA merit consideration in future clinical trials.

Role of antiplatelet agents in cerebrovascular disease

It is now generally accepted by neurologists that most transient ischaemic attacks, particularly in the carotid artery territory, have a thromboembolic basis. These emboli are, for the most part, fibrin-platelet aggregates. Others which contain atheromatous debris are more likely to produce longer lasting neurological deficits. If one assumes this hypothesis then it is reasonable to employ drugs which interfere with platelet aggregation in order to prevent cerebrovascular symptoms and signs. Acetylsalicylic acid (aspirin) prevents aggregation by inhibiting the 'release reaction' initiated by thromboxane A2. This inhibition lasts for the life of the affected platelets. Recent trials in the United States and Canada have demonstrated a positive clinical benefit from the employment of aspirin in patients suffering from transient cerebral ischaemic attacks and amaurosis fugax. There was a reduction or cessation of the attacks in both males and females and a 50% reduction of stroke morbidity and mortality in males.

Prostaglandins, their intermediates and precursors: cardiovascular actions and regulatory roles in normal and abnormal circulatory systems

The characterization of newly found unstable metabolites of arachidonic acid has provided new perspectives for cardiovascular regulatory mechanisms and new insights into disorders of the circulatory system. Since these intermediates are often more potent on and more specific for cardiovascular structures than the classical prostaglandins, they are more likely candidates as physiologic mediators of circulatory events. Their instability in vitro need not preclude these roles; on the contrary, the limited pharmacology described to date suggests that they function purely as local hormones. As such, changes in the rate of generation of these unstable but potent compounds would provide an excellent control system. The stable prostaglandins may represent only overflow of degradation products of the active mediators associated with pathologic events. For example, the dicovery of prostacyclin and the realization that this prostaglandin and not PGE2 is the primary metabolite of arachidonic acid in blood vessels emphasizes the need to reinterpret many of the previously held hypotheses that proposed that prostaglandins of the E series contributed to the regulation of vessel tone and blood pressure, Moreover, the contribution made by abnormal prostaglandin mechanisms to hypertensive disease should now take into account that a deficiency of prostacyclin and not PGE2 could be a major factor causing the elevated tension developed in vascular smooth muscle and the augmented vessel responsiveness to stimuli associated with hypertension.

The treatment of the hemolytic-uremic syndrome with inhibitors of platelet function

In four patients with clinical and laboratory manifestations of the hemolytic-uremic syndrome, the administration of aspirin and dipyridamole was associated with a dramatic and rapid increase in the platelet count. In three of the four patients there was also improvement in neurologic or renal function. No subject experienced bleeding or other untoward effects. We conclude that a trial of aspirin and dipyridamole therapy is warranted early in the course of the hemolytic-uremic syndrome.

Influence of antiplatelet drugs on platelet-surface interactions

Platelet aggregates are important in the thromboembolic complications of prosthetic devices, and drugs that alter platelet function have shown promise in clinical trials. Results with these drugs have increased insight into the interaction of platelets with artificial materials, particularly when the clinical experience has been correlated with the results of in vitro models. In a bead column/surface contact model, the characteristic interactions of platelets with artificial surfaces resemble effects of adding thrombin ADP to platelet-rich plasma. Anti-inflammatory agents and other antiplatelet drugs inhibit these reactions. Results in an in vivo model, survival of 51 Cr-labeled platelets in sheep bearing arteriovenous shunts of test materials, correlate systems indicate that the initial events upon contact of a surface with plasma proteins persistently influence the long-term behavior of that surface toward blood. A review of clinical studies discusses the effect of antiplatelet drugs on platelet survival and also on thrombotic complications of heart valve replacement and other conditions employing prosthetic devices.