Prevention of coronary artery reocclusion and reduction in late coronary artery stenosis after thrombolytic therapy in patients with acute myocardial infarction. A randomized study of maintenance infusion of recombinant human tissue-type plasminogen activator

Relative efficacy of antithrombin compared with antiplatelet agents in accelerating coronary thrombolysis and preventing early reocclusion

BACKGROUND

Optimal coronary thrombolysis should be prompt and persistent. Although activation of platelets and increased thrombin activity have been associated with clinical thrombolysis, the role of each in delaying thrombolysis or inducing early coronary reocclusion has been difficult to define.

METHODS AND RESULTS

In conscious dogs with coronary thrombosis induced by electrical current, we assessed the impact on the rapidity of thrombolysis and the incidence of reocclusion of two types of adjunctive treatment given concomitantly with intravenous tissue-type plasminogen activator (t-PA): 1) inhibition of platelet function with a peptide mimetic antagonist of platelet glycoprotein IIb/IIIa receptors or with lysine acetylsalicylic acid (ASA) and 2) inhibition of thrombin activity with recombinant hirudin or with heparin. ASA but not the receptor antagonist shortened the time to thrombolysis with t-PA (20 +/- 13 [mean +/- SD] minutes with ASA, 36 +/- 15 minutes with receptor antagonist, and 43 +/- 16 minutes with the saline control). Reocclusion occurred promptly after completion of the infusion of t-PA in all seven dogs given saline. Reocclusion was delayed and prevented in some dogs within 90 minutes after the end of the infusion of t-PA by both antiplatelet agents but still occurred in 42% despite continued inhibition of platelet function (i.e., three of six dogs given ASA and two of six given receptor antagonist). In contrast, inhibition of thrombin activity with recombinant hirudin in a dose that prolonged the partial thromboplastin time modestly (1.5-2-fold) resulted in accelerated lysis (19 +/- 10 minutes) and prevention of reocclusion in each of six dogs. Heparin given in doses that elicited similar prolongation of the partial thromboplastin time did not accelerate lysis nor prevent reocclusion, which occurred in five of six dogs.

CONCLUSIONS

Inhibition of thrombin by recombinant hirudin facilitates thrombolysis and maintains patency of coronary arteries recanalized with t-PA particularly effectively. The benefit conferred may reflect direct anticoagulant effects plus diminished activation of platelets secondary to decreased thrombin activity.

Kistrin, a polypeptide platelet GPIIb/IIIa receptor antagonist, enhances and sustains coronary arterial thrombolysis with recombinant tissue-type plasminogen activator in a canine preparation

BACKGROUND

Kistrin is a 68-amino acid polypeptide from the venom of the Malayan pit viper Agkistrodon rhodostoma, which inhibits the platelet GPIIb/IIIa receptor. Its effect on thrombolysis, reocclusion, and bleeding associated with administration of recombinant tissue-type plasminogen activator (rt-PA) was studied in a canine model of coronary artery thrombosis.

METHODS AND RESULTS

Coronary patency was monitored for 2 hours by ultrasonic flow probe and repeated coronary angiography. The rt-PA was given as 0.45-mg/kg bolus injections at 15-minute intervals until recanalization or to a maximum of four boluses. Four groups of four or five dogs were studied: a control group that received intravenous heparin (4,000-unit bolus and 1,000 units each hour) and three groups that received heparin and 0.48, 0.24, or 0.12 mg/kg kistrin, administered as a 10% bolus injection and an infusion during a 60-minute period. In the control group, reflow occurred in four of five dogs within 37 +/- 47 minutes but was followed by cyclic reflow and reocclusion. Kistrin at a dose of 0.48 and 0.24 mg/kg reduced the time to reflow to 6 +/- 5 and 10 +/- 3 minutes, respectively, and abolished reocclusion. With 0.12 mg/kg kistrin, reflow occurred in all four animals, within 27 +/- 23 minutes, and reocclusion occurred in two animals. Kistrin induced a dose-related prolongation of the template bleeding time: with 0.48 mg/kg kistrin, the bleeding time was prolonged from 3.8 +/- 1.3 minutes before infusion to 29 +/- 2 minutes during infusion, but it was shortened to 8.3 +/- 2.6 minutes at 90 minutes after the end of infusion. Kistrin also caused a dose-related inhibition of platelet aggregation with ADP and collagen: with 0.48 mg/kg kistrin, platelet aggregation was abolished during the infusion but had partially recovered toward the end of the observation period. Pathological examination of recanalized coronary arterial segments of dogs given 0.48 or 0.24 mg/kg kistrin revealed widely patent arteries with some platelets layered on the damaged intimal surface.

CONCLUSIONS

Kistrin increases the rate and extent of thrombolysis with a reduced dose of rt-PA, and it prevents reocclusion. At an effective dose, it is associated with a transient prolongation of the bleeding time and inhibition of platelet aggregation. Kistrin may offer promise as adjunctive treatment to thrombolytic agents in patients with acute myocardial infarction.

Role of new anticoagulants as adjunctive therapy during thrombolysis

Procoagulant activity may persist during coronary thrombolysis and result in either delay in the time to recanalization or recurrent thrombosis. Although heparin and aspirin form the mainstay of current therapy, recurrent thrombosis occurs despite adjunctive heparin therapy during thrombolysis. Newer agents that inhibit thrombin by antithrombin III-independent mechanisms, or that inhibit earlier steps in the coagulation cascade, have been shown to be effective in the experimental preparation of coronary thrombolysis. Because heparin-antithrombin III is a relatively inefficient inhibitor of thrombin bound to fibrin, agents such as hirudin or small peptide inhibitors of the thrombin-active site appear to be more effective inhibitors of clot-associated thrombin activity. Inhibition of early steps in the coagulation cascade with the inhibitor of tissue factor-factor VIIa complex, or with activated protein C, also appears to be an effective anticoagulant strategy. In experimental preparations all of these agents have shown superiority in preventing recurrent thrombosis compared with heparin, and in some cases they appear to accelerate the rate of clot lysis.

In vivo thrombin inhibition enhances and sustains arterial recanalization with recombinant tissue-type plasminogen activator

The effects of heparin and the synthetic competitive thrombin inhibitor (2R,4R)-4-methyl-1-[N2-(3-methyl-1,2,3,4-tetrahydro-8-quinolinesulfon yl)-L-arginyl]-2-piperidinecarboxylic acid monohydrate (Argatroban) on thrombolysis with recombinant tissue-type plasminogen activator (rt-PA) was studied in groups of six or seven rabbits with arterial thrombosis. The model consisted of a whole-blood clot produced in a 1-cm isolated femoral arterial segment with superimposed endothelial damage and distal high-grade stenosis. rt-PA was injected as an intravenous bolus of 0.45 mg/kg body wt at 15-minute intervals until recanalization, or up to a maximum of four boluses. In seven rabbits given an intravenous injection of 17 mg/kg aspirin, rt-PA induced transient reflow in only one animal. In seven rabbits that received intravenous heparin (200 units/kg over 60 minutes), rt-PA administration produced reflow in five animals, which was persistent in two rabbits. Combined administration of aspirin and heparin in seven rabbits was associated with similar rt-PA-induced recanalization. rt-PA administration in six rabbits given intravenous Argatroban (100 micrograms/kg/min for 60 minutes) caused recanalization in five, with persistent patency in three. In six rabbits given aspirin and Argatroban, rt-PA caused recanalization in all, with persistent patency in five animals. Reflow occurred significantly more rapidly with Argatroban (14 +/- 7 minutes) than with heparin (35 +/- 11 minutes), reflow was obtained with fewer boluses of rt-PA in combination with Argatroban (median value of one bolus) than with heparin (median value, three boluses), and reocclusion after reflow was less frequent with Argatroban (0 of 11 versus 5 of 10 rabbits). Furthermore, the degree of thrombolysis determined by pathological analysis was significantly more extensive with Argatroban than with heparin, and patency persisted during a 3-hour observation period, despite elimination of Argatroban from the circulation. Thus, Argatroban, relative to heparin, enhances and sustains thrombolysis with rt-PA. It may offer promise as an adjunctive agent for thrombolytic therapy of arterial thrombosis.

Lysis of plasminogen activator-resistant platelet-rich coronary artery thrombus with combined bolus injection of recombinant tissue-type plasminogen activator and antiplatelet GPIIb/IIIa antibody

Resistance of coronary occlusive thrombus to thrombolytic therapy, found in some patients with acute myocardial infarction, may be due to the presence of platelet-rich coronary clot. Reperfusion therapy in such patients may require the development and evaluation of alternative strategies in animal models. Therefore, platelet-rich coronary artery thrombus was developed by excision, eversion (inside out) and reanastomosis of a 1 cm segment of the left circumflex coronary artery in anesthetized dogs maintained on heparin antiocoagulation. Blood flow was restored in 25 of 27 dogs. Thrombotic occlusion of the everted segment graft with primarily platelet-rich thrombus or thrombus containing platelet-rich and erythrocyte-rich zones, persisting for at least 30 min, occurred within 4.5 +/- 3.5 min (mean +/- SD) in 20 of these 25 dogs. In 5 of these 20 dogs (group I, control), stable occlusion, as monitored with an ultrasound flow probe and coronary angiography, was maintained during a 2 h observation period. In group II (n = 5), intravenous bolus injections of recombinant tissue-type plasminogen activator (rt-PA) at a dose of 0.45 mg/kg body weight at four 15 min intervals did not cause reperfusion in four dogs and produced cyclic reperfusion and reocclusion in one dog. In group III (n = 5), a single intravenous bolus injection of 0.8 mg/kg of the F(ab')2 fragment of a murine monoclonal antibody (7E3) against the human platelet GPIIb/IIIa receptor [7E3-F(ab')2] produced stable reperfusion in two of the five dogs, whereas occlusion persisted in the other three. In group IV (n = 5), injection of 7E3-F(ab')2 (0.8 mg/kg) followed by rt-PA (0.45 mg/kg) caused stable reperfusion without reocclusion in all dogs (p less than 0.05 versus rt-PA alone and p less than 0.01 versus control). This study confirms that platelet-rich occlusive coronary thrombus is very resistant to lysis with intravenous rt-PA. However, this resistance may be overcome by the combined use of a reduced dose of rt-PA and the antiplatelet GPIIb/IIIa receptor antibody 7E3. The results indicate that platelet-rich thrombus resistant to thrombolytic agents may be dispersed pharmacologically without resort to mechanical recanalization. The present dog model may be useful in investigating specific strategies for the dispersion of resistant platelet-rich coronary thrombus.

Consequences of reocclusion after successful reperfusion therapy in acute myocardial infarction. TAMI Study Group

To determine the clinical consequences of reocclusion of an infarct-related artery after reperfusion therapy, we evaluated 810 patients with acute myocardial infarction. Patients were admitted into four sequential studies with similar entry criteria in which patency of the infarct-related artery was assessed by coronary arteriography 90 minutes after onset of thrombolytic therapy. Successful reperfusion was established acutely in 733 patients. Thrombolytic therapy included tissue-type plasminogen activator (t-PA) in 517, urokinase in 87, and a combination of t-PA and urokinase in 129 patients. All patients received aspirin, intravenous heparin and nitroglycerin, and diltiazem during the recovery phase. A repeat coronary arteriogram was performed in 88% of patients at a median of 7 days after the onset of symptoms. Reocclusion of the infarct-related artery occurred in 91 patients (12.4%), and 58% of these were symptomatic. Angiographic characteristics at 90 minutes after thrombolytic therapy that were associated with reocclusion compared with sustained coronary artery patency were right coronary infarct-related artery (65% versus 44%, respectively) and Thrombolysis in Myocardial Infarction (TIMI) flow 0 or 1 (21% versus 10%, respectively) before further intervention. Median (interquartile value) degree of stenosis in the infarct-related artery at 90 minutes was similar between groups: 99% for reoccluded (value, 90/100%) compared with 95% for patent (value, 80/99%). Patients with reocclusion had similar left ventricular ejection fractions compared with patients with sustained patency at follow-up. However, patients with reocclusion at follow-up had worse infarct-zone function at -2.7 (value, -3.2/-1.8) versus -2.4 (SD/chord) (value, -3.1/-1.3) (p = 0.016). The recovery of both global and infarct-zone function was impaired by reocclusion of the infarct-related artery compared with maintained patency; median delta ejection fraction was -2 compared with 1 (p = 0.006) and median delta infarct-zone wall motion was -0.10 compared with 0.34 SD/chord (p = 0.011), respectively. In addition, patients with reocclusion had more complicated hospital courses and higher in-hospital mortality rates (11.0% versus 4.5%, respectively; p = 0.01). We conclude that reocclusion of the infarct-related artery after successful reperfusion is associated with substantial morbidity and mortality rates. Reocclusion is also detrimental to the functional recovery of both global and infarct-zone regional left ventricular function. Thus, new strategies in the postinfarction period need to be developed to prevent reocclusion of the infarct-related artery.

Management of the patient following coronary thrombolysis

Myocardial salvage can be maximized by the early institution of thrombolytic therapy and aspirin. Certain patients may benefit from the administration of intravenous heparin, beta blockers, or nitroglycerin. The routine use of percutaneous transluminal coronary angioplasty (PTCA) or calcium-channel blockers does not appear to be warranted. Recurrent myocardial ischemia should be vigorously treated with medical therapy and there may be value in cardiac catheterization, followed by PTCA or bypass surgery, depending upon the extent of myocardium at risk and the underlying coronary anatomy. Long-term morbidity and mortality may be reduced by instituting aspirin and beta blockers as well as by modifying risk factors. There is no evidence for the long-term benefit from any calcium-channel blocker. Oral anticoagulation may be warranted in those patients with a mural thrombus, congestive heart failure, or atrial fibrillation. ACE inhibitors may be of value in the presence of left ventricular dysfunction and certainly in the presence of symptomatic congestive heart failure. Antiarrhythmic therapy is generally indicated only for symptomatic or life-threatening arrhythmias. Residual myocardial ischemia should be sought by exercise testing, and those patients with poor exercise tolerance generally warrant cardiac catheterization in consideration for revascularization.

Comparative effects of aspirin, a synthetic thrombin inhibitor and a monoclonal antiplatelet glycoprotein IIb/IIIa antibody on coronary artery reperfusion, reocclusion and bleeding with recombinant tissue-type plasminogen activator in a canine preparation

The comparative effects of intravenous aspirin, the synthetic thrombin inhibitor (2R,4R)-4-methyl-1-[N2-(3-methyl-1,2,3,4-tetrahydro-8- quinolinesulfonyl)-L-arginyl]-2-piperidinecarboxylic acid monohydrate (Argatroban) and F(ab')2 fragments of monoclonal antibody 7E3 against platelet glycoprotein IIb/IIIa (7E3-F[ab']2) on thrombolysis, reocclusion and bleeding associated with 0.45 mg/kg body weight bolus injections of recombinant tissue-type plasminogen activator (rt-PA) were studied in a canine coronary artery thrombosis model. Coronary patency was monitored for 2 h both by flow probe and by coronary angiography. Four groups were studied: Group I = pretreated with 17 mg/kg intravenous aspirin (n = 6), Group II = pretreated with 200 micrograms/kg per min intravenous Argatroban for 60 min (n = 5), Group III = pretreated with aspirin and Argatroban (n = 5) and Group IV = pretreated with 0.8 mg/kg intravenous 7E3-F(ab')2 (n = 5). In Group I, reflow occurred in four of six dogs, but did not persist; reflow was induced in Group II in four of five dogs, persisting in one; in Group III, reflow occurred in all five dogs, persisting in four; in Group IV reflow was achieved in four of five dogs, persisting in two. The frequency of persistent reflow in Group III was significantly higher than in the combined Groups I and II (p = 0.012), whereas the time to reflow was significantly shorter in the groups receiving Argatroban than in the aspirin group (median 25 versus 55 min, p = 0.04). There were no significant differences between Groups III and IV.(ABSTRACT TRUNCATED AT 250 WORDS)

Progressive failure of coronary flow during reperfusion of myocardial infarction: documentation of the no reflow phenomenon with positron emission tomography

During reperfusion of a myocardial infarct, development of microvascular occlusion may result in regional hypoperfusion ("no reflow") despite a patent infarct-related artery. This study examined the extent and time course of no reflow with use of rubidium-82 positron emission tomography. In 12 anesthetized dogs, the left anterior descending coronary artery was occluded for 90 min and then freely reperfused. Regional myocardial perfusion was imaged by serial rubidium-82 positron emission tomography during coronary occlusion and every 30 min during reperfusion. After 4 h of reperfusion, infarct size and no reflow zone were measured postmortem by triphenyltetrazolium and thioflavin staining, respectively. Perfusion defects evident on rubidium-82 images during coronary occlusion rapidly resolved during the early reflow period. However, a recurrent perfusion defect appeared after 1 to 2 h of reflow in all dogs. The severity of recurrent perfusion defects progressed with time; after 5 min of reflow, relative perfusion in the left anterior descending artery territory was 97 +/- 6% of that in the normal circumflex artery region, but perfusion decreased progressively to 68 +/- 5% after 2 h (p less than 0.05) and to 55 +/- 4% after 4 h of reperfusion (p less than 0.05 versus 2 h). As measured by radioactive tracer microspheres, endocardial blood flow decreased similarly in the postischemic left anterior descending artery region from 1.2 +/- 0.2 ml/min per g after 5 min of reflow to 0.4 +/- 0.1 ml/min per g after 3 h of reflow (p less than 0.01). Residual infarct perfusion, measured by rubidium-82 after 4 h of reflow, was related to both infarct size (r = -0.88) and the extent of the no reflow zone (r = -0.84) in the postmortem left ventricular sections. Thus, serial positron emission tomography with rubidium-82 demonstrates a progressive loss of infarct perfusion, beginning 1 to 2 h after initial restoration of blood flow despite patency of the infarct-related artery. This phenomenon is probably a manifestation of progressive microvascular occlusion within the reperfused myocardium.

Thrombolytic activity of a novel plasminogen activator, LY210825, compared with recombinant tissue-type plasminogen activator in a canine model of coronary artery thrombosis

LY210825, a recombinant tissue-type plasminogen activator (rt-PA), which contains the kringle-2 and serine protease functional domains of native tissue-type plasminogen activator, was previously produced by site-directed mutagenesis in a Syrian hamster cell line. We studied the thrombolytic potential of this molecule in a canine thrombosis model. Male hounds (16-22 kg) were anesthetized; a 2.0-cm segment of the left circumflex coronary artery (LCX) was isolated proximal to the first main branch, and the dogs were instrumented with an electromagnetic flow probe to measure coronary blood flow. An occlusive thrombus was formed after injury of the intimal surface of the LCX with an electrical current applied by a needle-tipped anode placed distal to the electromagnetic flow probe. After 1 hour of occlusion, either LY210825 or rt-PA was administered intravenously according to the following protocols: 1) a 1-hour infusion of either 0.25 mg/kg LY210825 or 0.4 mg/kg rt-PA, 2) single injections of 0.15-0.6 mg/kg LY210825, and 3) a single injection of 0.45 mg/kg LY210825 and a 3-hour infusion of 1.0 or 1.7 mg/kg rt-PA. Plasma half-lives of LY210825 and rt-PA were 58 +/- 7 and 3.3 +/- 0.3 minutes, respectively. LY210825 produced more rapid reperfusion of the LCX than did rt-PA. In the third study, 90% of the rt-PA-treated vessels reoccluded within 1 hour after cessation of drug, whereas only 25% of the LY210825-treated vessels reoccluded during a 4-hour washout period. There were significant, but relatively small, reductions produced by both plasminogen activators on plasma fibrinogen and plasminogen (25-35% decreases). Because of its longer plasma half-life, LY210825 could be administered intravenously as a single injection. In a canine model of coronary artery thrombosis, LY210825 was a more effective thrombolytic agent than was rt-PA.

Acceleration of recombinant tissue-type plasminogen activator-induced thrombolysis and prevention of reocclusion by the combination of heparin and the Arg-Gly-Asp-containing peptide bitistatin in a canine model of coronary thrombosis

The effect of tissue-type plasminogen activator (t-PA) alone or in combination with heparin, the Arg-Gly-Asp-containing peptide bitistatin, or both heparin and bitistatin was evaluated on thrombolysis time and acute reocclusion in a canine model of coronary thrombosis. Thrombus formation was elicited by electrolytic injury with a needle electrode to the endothelial surface of the circumflex coronary artery in the open-chest, anesthetized dog in the presence of a flow-limiting critical stenosis. Thirty minutes after spontaneous coronary artery occlusion, t-PA (1 mg/kg i.v. over 90 minutes) was administered. Group 1 was given t-PA alone; reperfusion occurred at 78.2 +/- 5.6 minutes with a reperfusion incidence of 60% (6/10). Group 2 received t-PA plus heparin (100 units/kg plus 50 units/kg/hr); reperfusion occurred at 61.9 +/- 9.1 minutes with a reperfusion incidence of 90% (9/10). Group 3 received t-PA plus heparin plus bitistatin (30 micrograms/kg plus 3 micrograms/kg/min); reperfusion occurred at 47.3 +/- 7.6 minutes (p less than 0.05 versus group 1) with a reperfusion incidence of 90% (9/10). Group 4 received t-PA plus bitistatin, and reperfusion occurred at 51.8 +/- 8.5 minutes; however, the reperfusion incidence was only 60% (6/10). In groups 1, 2, and 4, acute reocclusion occurred in more than 80% of the reperfused dogs, whereas in group 3 reocclusion occurred in 22% (2/9) of the reperfused dogs (p less than 0.05 versus group 1). The dose of heparin used in this study increased activated partial thromboplastin times 1.5-2.0-fold over control.(ABSTRACT TRUNCATED AT 250 WORDS)

Heparin and infarct coronary artery patency after streptokinase in acute myocardial infarction

Anticoagulant therapy is frequently used after thrombolytic agents in the treatment of acute myocardial infarction (AMI) although it is unclear that such therapy will prevent subsequent infarct vessel reocclusion. The role of duration of heparin therapy in maintaining infarct artery patency was studied retrospectively in 53 consecutive AMI patients who received streptokinase therapy and underwent coronary angiography acutely and at 14 +/- 1 days. Of the 39 patients with initial infarct vessel patency, patency at follow-up angiography was observed in 100% (22 of 22) of those who received greater than or equal to 4 days of intravenous heparin but in only 59% (10 of 17) of those patients who received less than 4 days of heparin (p less than 0.05). Of the 14 patients not initially recanalized after streptokinase, patent infarct-related arteries at follow-up angiography were found in 3 of 8 (38%) treated with greater than or equal to 4 days of heparin therapy but in none of the 6 patients treated for less than 4 days (difference not significant). No significant difference in hemorrhagic complications was noted between the short- and long-term heparin treatment groups. Thus, greater than or equal to 4 days of intravenous heparin therapy after successful streptokinase therapy in AMI is more effective in maintaining short-term infarct vessel patency than a shorter duration of therapy and it may maintain the short-term patency of the infarct vessel in those patients who later spontaneously recanalize.

Reperfusion in acute myocardial infarction

During the past decade, the general acceptance of the primary role of thrombosis in acute myocardial infarction (AMI) has led to intense interest in the potential efficacy of reperfusion therapy, particularly thrombolytic therapy, in AMI. Accumulating evidence indicates that systemic thrombolytic therapy administered early after the onset of symptoms of AMI can restore infarct-related artery patency, salvage myocardium, and reduce mortality. Recommendations about the proper use of thrombolytic therapy, contraindications, and concomitant therapies (such as aspirin, heparin, nitrates, beta-adrenergic blocking agents, and calcium channel blockers) are reviewed. Although percutaneous transluminal coronary angioplasty (PTCA) is useful for subsets of patients with AMI (for example, patients with anterior infarctions with persistent occlusion of the infarct-related artery after thrombolytic therapy and those with cardiogenic shock), a conservative strategy, including angiography and PTCA only for postinfarction ischemia, is indicated for most patients with AMI in whom initial thrombolytic therapy is apparently successful. The use of PTCA after failed thrombolysis or as direct therapy for AMI seems promising, although further comparisons of PTCA and intravenous thrombolytic therapy are needed. Ongoing studies should help further define the risk-to-benefit ratio of various reperfusion strategies in different subsets of patients, define time limitations for reperfusion therapy, and provide data on therapeutic modalities that will limit reperfusion injury and therefore enhance salvage of myocardium.

Pharmacology of anistreplase

Eminase (anistreplase), or anisoylated plasminogen streptokinase activator complex (APSAC), is a reversibly inactivated lys-plasminogen-streptokinase activator complex that is given rapidly over a period of 2-5 minutes, and has a half-life in the circulation of 90-105 min. Eminase represents a significant advance in drug design for therapeutic thrombolysis based on convenience in administration, high gradient (especially initially) for diffusion into thrombus, improved fibrin binding, ability to lyse fibrin rapidly, improved stability in the circulation, long duration of action, and production of an antithrombotic state.

New approaches to treatment of myocardial infarction

Survival of patients with acute transmural infarction is largely related to the size of the myocardial infarction. The goal of thrombolytic therapy in acute myocardial infarction is maximal salvage of myocardium by reestablishment of flow in the occluded infarct-related artery and the establishment and maintenance of a patent infarct-related artery. Results of randomized trials show a significant reduction in mortality in patients who have undergone thrombolysis. A patent infarct-related artery, even in the absence of a change in left ventricular function, is associated with reduced mortality. The Thrombolysis in Myocardial Infarction Trial and the European Cooperative Trial showed that recombinant tissue-type plasminogen activator is superior to streptokinase in reestablishing flow in a totally occluded artery. Experimental and clinical evidence suggests that thrombolysis and thrombosis occur simultaneously, and that lysis appears to increase both thrombin and platelet activity. Effective reduction of thrombosis accelerates thrombolysis. Rethrombosis after thrombolysis is due to anchored residual thrombus, which alters the hemorrheology of blood flow and produces a highly thrombogenic substrate that is largely due to residual fibrin-bound thrombin. Platelet deposition is directly related to severity of residual stenosis and shear rate. Thrombin appears to be the most potent of the 5 potential stimulators of platelet activation during arterial thrombosis. Proper anticoagulation can play an important role in reducing thrombosis. Experimental evidence strongly supports the use of heparin during and after thrombolysis. A recently reported study shows continued reduction of residual stenosis after 1 month of vigorous anticoagulation with intravenous heparin and subsequent oral anticoagulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of platelet-rich arterial thrombosis by selective thrombin inhibition

The effect of heparin and of the synthetic competitive thrombin inhibitor (2R,4R)-4-methyl-1-[N2-(3-methyl-1,2,3,4-tetrahydro-8-quinolinesulfon yl)-L-arginyl]-2-piperidinecarboxylic acid monohydrate (argatroban) on platelet-rich arterial thrombosis was studied in a rabbit model, consisting of a 4-6-mm everted ("inside-out") femoral arterial segment. Intravenous injection of heparin (200 units/kg) failed to prevent occlusion within 60 minutes in all 10 rabbits, whereas intravenous argatroban infusion at a rate of 100 or 200 micrograms/kg/min for 60 minutes, which prolonged the thrombin time more than fourfold, prevented thrombosis in nine of 13 rabbits (p = 0.002 vs. i.v. heparin). Intra-arterial infusion of 200 units/kg heparin over 60 minutes prevented occlusion in six of nine rabbits (p = 0.003 vs. i.v. heparin), whereas intra-arterial argatroban at a rate of 100 micrograms/kg/min for 60 minutes prevented thrombosis in all 10 rabbits (p = 0.00001 vs. i.v. heparin). Patency of femoral arterial segments was maintained after the end of the intra-arterial heparin and intravenous or intra-arterial argatroban infusion for up to 3 hours despite normalization of the thrombin time and partial thromboplastin time. Pathologic examination of the graft revealed that the inverted adventitial surface was covered by layers of platelets without platelet aggregation or fibrin deposition. These findings indicate that thrombin plays an important role in platelet-rich arterial thrombosis, and that the thrombogenic stimulus is rapidly attenuated by short-term infusion of the synthetic thrombin inhibitor. Selective thrombin inhibition can constitute an alternative approach to the prevention of arterial occlusion after angioplasty or thrombolytic therapy in patients with unstable coronary syndromes.

Platelet and vascular function during coronary thrombolysis with tissue-type plasminogen activator

Platelet activation may limit the response to tissue-type plasminogen activator (t-PA) during coronary thrombolysis in humans. As an index of platelet activation, we assessed thromboxane A2 biosynthesis during coronary thrombolysis with intravenous t-PA in patients with acute myocardial infarction. Urinary 2,3-dinor-thromboxane B2, a metabolite of thromboxane A2, was increased to a peak of 3,327 +/- 511 pg/mg creatinine (n = 12) following administration of intravenous t-PA and remained elevated for 48 hours. This increase was abolished by pretreatment with aspirin 325 mg orally (n = 6), indicating de novo biosynthesis of thromboxane A2 rather than washout of preformed metabolites during reperfusion. Prostacyclin (PGI2) biosynthesis, determined by excretion of 2,3-dinor-6-keto-PGF1 alpha, also increased after t-PA administration. However, this increase was less pronounced in patients who reperfused (28 +/- 3.3 ng.hr/mg creatinine) than in patients who failed to reperfuse (118 +/- 30 ng.hr/mg creatinine, p less than 0.05). These data provide evidence of platelet activation during coronary thrombolysis with t-PA. In patients who reperfuse, the reduction in PGI2 biosynthesis may be a marker of reperfusion injury to the vasculature and may further amplify platelet activation.

Selective decrease in lysis of old thrombi after rapid administration of tissue-type plasminogen activator

The safety of thrombolytic therapy of acute myocardial infarction could be improved if a method were developed to dissolve fresh occlusive coronary thrombus without simultaneously dissolving hemostatic thrombi outside the coronary arteries. This study is based on the assumption that, in a patient with evolving acute myocardial infarction, hemostatic thrombi are likely to be older than the thrombus responsible for occlusion of the coronary artery. It explored whether the relative rates of lysis of fresh and old thrombi could be influenced by the rapidity of recombinant tissue-type plasminogen activator (rt-PA) administration. In each of 17 dogs, two 1 h and two 24 h old thrombi were produced by inserting copper coils into both jugular and both femoral veins. After 24 h and 1 h, respectively, the coils with the thrombi were removed, weighed and inserted into the adjacent carotid and femoral arteries. A 1 mg/kg body weight dose of rt-PA was given either over 180 or over 30 min. The coils were removed and weights of the residual thrombi determined at the end of the 180 min infusion (Group I), at the end of the 30 min infusion (Group IIA) and 45 min after the 30 min infusion (Group IIB). The 24 h old thrombi were lysed significantly less than the 1 h old thrombi in all three experimental groups: 53.9 +/- 4.8% (mean +/- SE) versus 86.1 +/- 2.5% in Group I (p less than 0.001), 16.6 +/- 3.5% versus 65.2 +/- 6.0% in Group IIA (p less than 0.001) and 21.6 +/- 5.4% versus 91.7 +/- 1.7% in Group IIB (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

After coronary thrombolysis and reperfusion, what next?

Thrombolytic therapy for the removal of intravascular thrombi was introduced when streptokinase was first given to humans 40 years ago, the same year the American College of Cardiology was founded. Streptokinase was first administered to patients with acute myocardial infarction in 1959. Today, thrombolytic therapy has been established to offer significant benefits to patients with acute myocardial infarction provided they are brought to medical attention early enough after the onset of symptoms. The two major agents, streptokinase and recombinant tissue-type plasminogen activator (rt-PA), have been shown to result in reperfusion of infarct-related arteries, to salvage ischemic myocardium, to improve myocardial performance and to reduce mortality. In spite of these impressive gains, this novel therapy has shortcomings. The interval from the start of thrombolytic treatment to coronary reperfusion varies significantly from patient to patient and may, at times, be too long to produce a real benefit in terms of salvage of ischemic myocardium. The rate of reocclusion lies somewhere between 10% and 20% and appears not to be influenced by concomitant heparin anticoagulation. The rate of bleeding complications even with the "fibrin-specific" rt-PA is higher than anticipated and may range from 10% to 30%. As a consequence, intensive efforts are being directed at the development of improved thrombolytic agents and for adjunctive therapy evaluating better anticoagulants than heparin and better antiplatelet agents than aspirin. This review is a status report summarizing where we are in thrombolytic therapy in acute myocardial infarction, where we need to improve treatment results and what is being done mainly at the preclinical level to bring about such improvements.

A new thrombolytic regimen for acute myocardial infarction using combination half dose tissue-type plasminogen activator with full dose streptokinase: a pilot study. KAMIT Study Group

Because a previous study utilizing a combination of recombinant tissue-type plasminogen activator (rt-PA) and urokinase demonstrated reduced reocclusion rates compared with rates obtained with rt-PA alone, this study was conducted to determine whether the combination of rt-PA and streptokinase might achieve similar results at reduced cost. Forty patients with acute myocardial infarction were treated with a 1 h infusion of rt-PA (50 mg) and streptokinase (1.5 million U) administered within 6 h (mean 3.6 +/- 1.2) of symptom onset. Emergency coronary arteriography revealed patency of the infarct-related artery in 30 (75%) of 40 patients. With the addition of coronary angioplasty in those who had unsuccessful thrombolytic reperfusion, the early patency rate was increased to 98%. In-hospital mortality rate (2.5%) and the incidence of significant bleeding requiring transfusion (15%) were low. Angiographically documented reocclusion of the infarct vessel occurred in 3 (8%) of 37 patients by day 7. Regional wall motion of the infarct zone improved by 0.9 +/- 0.9 SD/chord (p less than 0.0005), and ejection fraction increased 3.6 +/- 8% units (p less than 0.05) between immediate and day 7 studies. In contrast to the price of full dose rt-PA ($2,300) or rt-PA with urokinase ($3,500), the cost of this regimen was $1,230. This pilot study demonstrates that at half the cost, a combination of half dose rt-PA with full dose streptokinase offers high infarct vessel patency, recovery of ventricular function, a low rate of reocclusion and few bleeding complications.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of thrombolytic agents: selected hematologic, vascular and clinical events

Plasminogen activators of distinct structure and biochemical action seem to be more equivalent than unique regarding induced blood changes and clinical complications. All of the activators ultimately degrade substrate through plasmin, resulting in a striking hypocoagulable state characterized primarily by a decrease in fibrinogen concentration. Infusion regimens are inversely proportional to the half-life of the activator, which is relatively long with anisoylated plasminogen streptokinase activator complex (APSAC), intermediate for streptokinase (SK) and urokinase (UK), and very short for recombinant tissue plasminogen activator (rt-PA) and recombinant single-chain urokinase plasminogen activator (scu-PA). After therapy is discontinued, hypofibrinogenemia persists until activator is cleared from the blood, then is slowly corrected over 48 hours, regardless of which thrombolytic agent has been used. Coagulation and platelet activity may be transiently accentuated soon after administration of the agent. Hypercoagulability contributes to vascular reocclusion, especially when acting in concert with the thrombogenic influences of residual thrombus and the original ruptured atherosclerotic plaque. In the first 3 to 4 hours after symptom onset, coronary artery reperfusion can be achieved with all of the thrombolytic agents in 50 to 60% of patients, with a greater thrombolytic potential of rt-PA over SK in thrombi of greater than 4 hours' duration. After coronary artery reperfusion, reocclusion occurs in 10 to 20% of patients, more often after rt-PA than SK treatment. Antiplatelet agents such as aspirin decrease the incidence of reocclusion and when added to either SK or rt-PA, decrease mortality after acute myocardial infarction by half. APSAC appears to have a maximal beneficial effect in reducing mortality even without aspirin.(ABSTRACT TRUNCATED AT 250 WORDS)

Angioplasty after thrombolysis in the treatment of evolving myocardial infarction

Potent pharmacological agents that rapidly induce coronary thrombolysis reduce morbidity and mortality from evolving myocardial infarction especially when administered early after the onset of ischemia. However, recanalization frequently unmasks residual, high-grade stenoses that can impair reflow, predispose to reocclusion, and limit salvage of myocardium. Coronary angioplasty performed immediately after recanalization induced by thrombolysis reduces the severity of stenosis and can enhance salvage. Unfortunately, complication rates are higher with emergency compared with delayed angioplasty. Thus, mechanical recanalization early after thrombolysis should be reserved for patients with signs or symptoms of recurrent ischemia or for those in whom pharmacological recanalization has failed but a large amount of myocardium remains at risk.

Antithrombotic therapy after myocardial reperfusion in acute myocardial infarction

The problem of post-thrombolytic reocclusion can be approached in several ways. 1) Better thrombolytic agents with longer duration of effects and more powerful properties aimed at enhanced clot lysis and anticoagulation are under study. 2) The combination of high dose heparin and low dose aspirin is proposed for all patients with an acute myocardial infarction treated with thrombolytic agents. 3) Peptide inhibitors of thrombin and monoclonal antibodies against platelet glycoprotein receptors and adhesive macromolecules are potentially effective inhibitors of platelet aggregation and thrombus formation during or after thrombolytic therapy.