Role of heparin after intravenous thrombolytic therapy for acute myocardial infarction

Cardiac metabolic effects of heparin differentiate between patients with normal and stenotic coronary arteries

We studied the effects of heparin, given as 12,500 units intravenously, on cardiac metabolism during catheterization of the coronary sinus at rest and during repeated rapid atrial pacing in 8 patients with stable angina pectoris, positive stress tests and coronary arterial disease and in 8 patients with normal coronary arteries without objective signs of ischemic heart disease. Heparin did not influence angina, ST-segment depression or myocardial lactate production induced by pacing in the group with diseased coronary arteries. In both groups, heparin increased the arterial levels (70%) and the myocardial uptake (40-50%) of free fatty acids, the latter only during non-ischemic conditions. Myocardial net uptakes of glucose, lactate and glutamate and the release of alanine were reduced by heparin in the subjects with normal coronary arteries but not in those with ischemic heart disease. Myocardial oxygen consumption was unchanged. In the patients with normal coronary arteries, the levels of free fatty acid in the arteries were positively related to myocardial uptake of fatty acids and the release of citrate but inversely related to cardiac uptake of lactate and glucose. These relations were lacking in the patients with diseased coronary arteries. The metabolic effects of heparin on the heart, therefore, were diminished in patients with ischemic heart disease when compared to controls. This is probably due to an altered regulation of substrate preference in ischemic hearts.

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.

Hypercoagulable state after thrombolytic therapy in patients with acute myocardial infarction (AMI) treated with streptokinase

Fibrinogen activity was studied in 70 patients with AMI who were treated with an intravenous infusion of SK (800,000 U/30 min or 1.5 mill U/60 min). Patients received a continuous infusion of heparin after thrombolytic therapy was completed. 800,000 U and 1.5 mill U SK recanalized infarct-related arteries at a rate of 78%. Early re-infarction occurred in 6% in each group. Upon admission to the hospital patients showed a hypercoagulable state that may be related to an elevated level of fibrinogen and HMW fibrinogen (70.5 +/- 2 vs 65 +/- 2% in patient and normal plasmas, respectively) that changed to a transitory hypercoagulable state indicated by decreased fibrinogen levels after SK treatment. Forty-eight hours after SK, a new fibrinogen hyperfunction, related to an increase in fibrinogen level and especially HMW synthesized fibrinogen (82 +/- 1 or 81 +/- 1%, 800,000 and 1.5 mill U SK, respectively) was observed, which was neutralized by heparin therapy (1,660 U/h with continuous infusion). The elevated levels of fibrinogen (363 +/- 21 vs 240 +/- 8 mg/dl in patient and normal plasmas, respectively) and HMW fibrinogen (70 +/- 3% with both SK hypercoagulable state that is not neutralized by the heparin dose were compared with those whose arteries recanalized. The former group had a higher concentration of fibrinogen (197 +/- 31 vs 147 +/- 18 mg/dl), HMW fibrinogen (78 +/- 0.5 vs 74 +/- 0.3%, respectively), and FPA (130 +/- 3 vs 6 +/- 4 pmol/ml) and more extensive fibrin gel formation kinetics (gelation rate 3.3 +/- 1.4 vs 1.1 +/- 0.2 OD/s x 10(-4), respectively) than the second group. The hypercoagulable state found in patients with acute myocardial infarction undergoing thrombolytic therapy may be related mainly to the progression of HMW fibrinogen and fibrinogen levels.

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)

A randomized pilot trial of brief versus prolonged heparin after successful reperfusion in acute myocardial infarction

Controversy exists as to whether and how long heparin treatment is necessary after infarct vessel recanalization. To determine the role of heparin, patients with suitable angiographic features after reperfusion therapy were randomly allocated to receive a brief infusion of intravenous heparin for less than or equal to 24 hours (group 1), adjusted to a partial thromboplastin time of 2 times control or a prolonged infusion for greater than or equal to 72 hours (group 2), using the same titration mechanism. Patients were excluded for complex intimal dissections, large residual filling defects, less than Thrombolysis in Myocardial Infarction grade 3 flow pattern or greater than 50% residual stenosis. Heparin was sustained except for discontinuation 2 to 4 hours before periaccess sheath removal, or if significant bleeding (greater than or equal to 2 units blood transfusion) occurred. The primary endpoints were 1-week patency determined by repeat catheterization or recurrent ischemia, or both, and the incidence of bleeding complications. Fifty patients were randomized, 25 in both groups. Baseline variables were similar; 14 group 1 and 15 group 2 patients received thrombolytic treatment; 20 patients in each group had coronary angioplasty. Two documented reocclusions occurred in both groups. Significant bleeding complications occurred in 0 of 25 (0%) group 1 versus 6 of 25 (24%) group 2 patients (p less than 0.05). Thus, in low-risk patients after successful reperfusion, prolonged heparin therapy does not protect against rethrombosis and is associated with a significantly higher rate of bleeding complications. Therefore, prolonged heparin therapy for greater than 24 hours does not appear to be justified in low-risk patients with successful reperfusion.

Role of thrombin and thromboxane A2 in reocclusion following coronary thrombolysis with tissue-type plasminogen activator

Reocclusion of the coronary artery occurs after thrombolytic therapy of acute myocardial infarction despite routine use of the anticoagulant heparin. However, heparin is inhibited by platelet activation, which is greatly enhanced in this setting. Consequently, it is unclear whether thrombin induces acute reocclusion. To address this possibility, we examined the effect of argatroban [MCI9038, (2R,4R)-4-methyl-1-[N alpha-(3-methyl-1,2,3,4-tetrahydro-8- quinolinesulfonyl)-L-arginyl]-2-piperidinecarboxylic acid], a specific thrombin inhibitor, on the response to tissue-type plasminogen activator in a closed-chest canine model of coronary thrombosis. MCI9038 prolonged the thrombin time and shortened the time to reperfusion (28 +/- 2 min vs. 59 +/- 7 min in controls; mean +/- SEM, n = 5, P less than 0.01). At the highest dose, 2.5 mg/kg per hr, complete reocclusion was prevented in four of the five experimental animals, whereas reocclusion occurred in all five controls. However, reperfusion was complicated by cycles of decrease flow, which were abolished by the thromboxane A2 antagonist, GR32191. GR32191 at 1 mg/kg combined with MCI9038 at 0.5 mg/kg per hr prevented reocclusion, whereas, at these doses, either drug alone was without effect. In addition, thromboxane A2 biosynthesis, determined as excretion of its metabolite 2,3-dinor-thromboxane B2, was increased after reperfusion at all doses of MCI9038. These data demonstrate that thrombin impairs thrombolysis induced by tissue-type plasminogen activator in vivo and induces acute reocclusion. Furthermore, the response to thrombin inhibition may be impaired by continued formation of thromboxane A2.

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.

Fiberoptic study on the effects of transluminal angioplasty in experimental occlusive arterial thrombosis

Percutaneous transluminal coronary angioplasty has been proposed as definitive therapy for coronary recanalization of occluded coronary arteries in patients with acute myocardial infarction (AMI). The effects of transluminal angioplasty (TA) on experimental occlusive canine arterial thrombi that closely simulated the clinical condition was examined by a fiberoptic method. Experimental arterial thrombosis was produced by endothelial denudation and induction of luminal stenosis. Eighteen dogs that showed total occlusion of the iliac artery with thrombi were evaluated. Seven dogs (group A) with 6-hour-old thrombi received 20,000 IU/kg intravenous urokinase (UK) but did not show recanalization. TA was performed with a Gruentzig or Simpson-Robert balloon catheter and its effect was evaluated by a vascular fibroscope. Eight dogs (group B) with 6-hour-old thrombi underwent primary TA. After TA, less than 50% luminal obstruction with residual thrombi was visualized in five dogs (71%) of group A and four dogs (50%) of group B. Residual thrombi showed a doughnut-like or globular type shape and consisted of dense fibrin networks and compact platelet aggregates. All dogs in group B received 20,000 IU/kg intravenous UK after TA, but most of them showed progression of thrombus size despite UK infusion. In conclusion, the results suggest (1) that TA is effective in recanalization of an occluded artery with aged thrombus that is resistant to thrombolytic therapy and (2) that vascular fiberscope is a useful method for evaluation of the effects of TA on occlusive arterial thrombus.

Postinfarction angina

In conclusion, the PIA patient is at high risk, with higher early as well as late mortality. The pathophysiology of PIA is complex and may vary from patient to patient. The concepts of ischemia at a distance and ischemia in the infarct zone have led to a better understanding of early PIA. Coronary spasm may play an important role in most PIA patients as in the general population of patients with angina pectoris. Medical therapy is efficacious in many, although it may on rare occasion aggravate myocardial ischemia. Urgent coronary arteriography is generally safe and should be performed as soon as possible for medically refractory PIA. CABG appears to be safe in experienced hands, but its timing must be individualized. The IABP should be reserved for more unstable patients for fear of vascular complications. Randomized controlled trials such as the BARI Trial will further compare PTCA with CABG.

Adverse reactions to thrombolytic agents. Implications for coronary reperfusion following myocardial infarction

The use of thrombolytic agents to dissolve coronary artery thrombi causing acute transmural myocardial infarctions has been shown to decrease short term mortality, and improve left ventricular function, in patients with acute transmural myocardial infarction. Several thrombolytic agents are currently available which differ mainly in cost, antigenicity, and mechanism of action. Current investigations are being directed at finding safer, more effective thrombolytic agents and at developing optimal therapy following thrombolysis. The complications of thrombolytic therapy are for the most part minor and reversible. Immediate and delayed hypersensitivity to streptokinase is rare. Hypotension and arrhythmias commonly accompany myocardial reperfusion and are usually benign and self-limited. Haemorrhagic complications are the most frequent and serious problems following the use of thrombolytic agents. They can be lessened by the proper selection of patients to avoid those at high risk of bleeding. The avoidance of unnecessary arterial and venous punctures will decrease the incidence of minor but annoying local bleeding. Those agents which are activated at the site of thrombi will hopefully cause fewer bleeding episodes, but early experience with these agents has not been able to demonstrate a lower rate. With careful attention to patient selection and follow-up, thrombolytic agents can be safely and effectively used in the management of patients with acute myocardial infarction.