Heparin as an adjunctive treatment after thrombolytic therapy for acute myocardial infarction

Optimizing adjunctive antithrombotic therapy in the treatment of acute myocardial infarction: a role for low-molecular-weight heparin

Thrombotic complications account for a large proportion of in-hospital deaths from acute myocardial infarction (MI). Although thrombolytic therapy has greatly improved clinical outcomes following MI, thrombin released during clot lysis has a prothrombotic effect, and the thrombolytic agents themselves may directly activate platelets. Antithrombotic therapy as an adjunct to thrombolysis improves the speed and extent of artery recanalization and reduces the incidence of secondary ischemic complications. The current treatment standard is unfractionated heparin (UFH) administered intravenously for 24-48 h. However, UFH has not been unequivocally shown to improve outcomes in large-scale, randomized clinical trials, and shows no evidence of benefit when used as an adjunct to streptokinase-based thrombolysis. Unfractionated heparin also has several clinical and practical disadvantages, such as the need for coagulation monitoring, difficulties attaining a stable and reliable anticoagulant effect, and the risk of hemorrhagic side effects. Low-molecular-weight heparin (LMWH) represents a safe and effective alternative antithrombotic therapy, with a stable and predictable anticoagulant effect, potential for use in combination with either fibrin-specific or streptokinase-based thrombolysis, no need for anticoagulation monitoring, and a low risk of hemorrhagic and other heparin-related complications. Several randomized clinical trials have shown that adjunctive LMWH is at least as effective as UFH in the acute phase of MI, is associated with fewer in-hospital recurrent ischemic events, and has an acceptable safety profile.

Enoxaparin as adjunctive antithrombin therapy for ST-elevation myocardial infarction: results of the ENTIRE-Thrombolysis in Myocardial Infarction (TIMI) 23 Trial

BACKGROUND

ENTIRE-TIMI 23 evaluated enoxaparin with full-dose tenecteplase (TNK) and half-dose TNK plus abciximab.

METHODS AND RESULTS

Patients (n=483) with ST-elevation MI presenting <6 hours from symptom onset were randomized to full-dose TNK and either unfractionated heparin (UFH) (bolus 60 U/kg; infusion 12 U/kg per hour) or enoxaparin (1.0 mg/kg subcutaneously every 12 hours+/-initial 30 mg intravenous bolus), or half-dose TNK plus abciximab and either UFH (bolus 40 U/kg; infusion 7 U/kg per hour) or enoxaparin (0.3 to 0.75 mg/kg subcutaneously every 12 hours+/-initial intravenous bolus of 30 mg). With full-dose TNK and UFH, the rate of TIMI 3 flow at 60 minutes was 52% and was 48% to 51% with enoxaparin. Using combination therapy, the rate of TIMI 3 flow was 48% with UFH and 47% to 58% with enoxaparin. The rate of TIMI 3 flow among all UFH patients was 50% and was 51% among enoxaparin patients. Through 30 days, death/recurrent MI occurred in the full-dose TNK group in 15.9% of patients with UFH and 4.4% with enoxaparin (P=0.005). In the combination therapy group, the rates were 6.5% with UFH and 5.5% with enoxaparin. The rate of major hemorrhage with full-dose TNK was 2.4% with UFH and 1.9% with enoxaparin; with combination therapy, it was 5.2% using UFH and 8.5% with enoxaparin.

CONCLUSIONS

Enoxaparin is associated with similar TIMI 3 flow rates as UFH at an early time point while exhibiting advantages over UFH with respect to ischemic events through 30 days. These findings with enoxaparin are achieved with a similar risk of major hemorrhage.

Issues Regarding the Use of Heparin Following Streptokinase Therapy

The success of thrombolytic therapy is dependent upon the balance of fibrinolytic activity and procoagulant activity. Streptokinase produces fibrin degradation products that have anticoagulant effects and may potentially protect against reocclusion. However, streptokinase also activates platelets and thrombin, and the prothrombotic effects may be more marked than after administration of recombinant tissue plasminogen activator (rt-PA). Administration of high-dose, delayed subcutaneous heparin after streptokinase and aspirin has been shown to have some benefits and some risks. The benefits and risks of adding intravenous heparin to aspirin and streptokinase have not been clearly defined.

Reduced heart rate variability and new-onset hypertension: insights into pathogenesis of hypertension: the Framingham Heart Study

Heart rate variability (HRV) is a useful noninvasive tool to assess cardiac autonomic function. The purpose of this study was to (1) compare measures of HRV between hypertensive and normotensive subjects and (2) examine the role of HRV as a predictor of new-onset hypertension. The first 2 hours of ambulatory ECG recordings obtained from 931 men and 1111 women attending a routine examination at the Framingham Heart Study were processed for HRV. Three time-domain and 5 frequency-domain variables were studied: standard deviation of normal RR intervals (SDNN), percentage of differences between adjacent normal RR intervals exceeding 50 milliseconds, square root of the mean of squared differences between adjacent normal RR intervals, total power (0.01 to 0.40 Hz), high frequency power (HF, 0.15 to 0.40 Hz), low frequency power (LF, 0.04 to 0.15 Hz), very low frequency power (0.01 to 0.04 Hz), and LF/HF ratio. On cross-sectional analysis, HRV was significantly lower in hypertensive men and women. Among 633 men and 801 women who were normotensive at baseline (systolic blood pressure <140 mm Hg and diastolic blood pressure <90 mm Hg and not receiving antihypertensive treatment), 119 men and 125 women were newly hypertensive at follow-up 4 years later. After adjustment for factors associated with hypertension, multiple logistic regression analysis revealed that LF was associated with incident hypertension in men (odds ratio per SD decrement [OR], 1.38; 95% confidence interval [CI], 1.04 to 1.83) but not in women (OR, 1.12; 95% CI, 0.86 to 1.46). SDNN, HF, and LF/HF were not associated with hypertension in either sex. HRV is reduced in men and women with systemic hypertension. Among normotensive men, lower HRV was associated with greater risk for developing hypertension. These findings are consistent with the hypothesis that autonomic dysregulation is present in the early stage of hypertension.

Relation of coagulation parameters to patency and recurrent ischemia in the Thrombolysis in Myocardial Infarction (TIMI) Phase II Trial

Current protocols for use of tissue-type plasminogen activator in acute myocardial infarction include heparin estimated by the activated partial thromboplastin time (aPTT). Recent reports indicate a risk of recurrent ischemic events with long aPTT values. Longer aPTT values in the Thrombolysis in Myocardial Infarction-II (TIMI II) Trial, obtained within the first 48 hours, were associated with patency at 18 to 48 hours and better left ventricular function at discharge (average 9.6 days), but also with emergency catheterizations within the first 48 hours and, weakly, with recurrent ischemia during the first 18 hours. A moderate decrease in fibrinogen, compared with a "small" decrease, was also associated with patency, but a "large" decrease was associated with hemorrhagic events. Patency was associated with higher fibrinogen values and higher plasminogen values at baseline. The aPTT results support frequent monitoring during the first 24 to 48 hours to ensure optimal clinical outcome. The coagulation factor results suggest that there may be an optimum window for fibrinogenolysis in this setting.

Thrombolytic therapy and proteolysis of factor V

OBJECTIVES

We sought to determine the extent of Factor V proteolysis during thrombolytic therapy.

BACKGROUND

Thrombin- or Factor Xa-activated Factor V is an essential cofactor in the prothrombinase complex. In purified systems, plasmin, the major product of thrombolytic therapy, is known to first activate then inactivate Factor V.

METHODS

We used quantitative gel electrophoresis and Western blotting to analyze the cleavages in plasma Factor V after thrombolytic therapy.

RESULTS

The addition of streptokinase to plasma resulted in the activation then inactivation of Factor V, confirming previous results using purified reagents. We also identified the Factor V fragments resulting from the action of thrombin and plasmin. After thrombolytic therapy, there was considerable Factor V cleavage. The cleavage patterns were consistent with the action of plasmin, with little evidence for the action of thrombin. In the Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries trial (n = 17), we observed an average 58% loss of intact Factor V at 6 h (range 1% to 91%). Samples from the Thrombolysis in Myocardial Infarction trial, Phase II (n = 12), collected on a shorter time scale, showed a loss of up to 99% at 50 min, with the loss of intact Factor V associated with the plasma concentration of plasminogen activator. Samples from patients with bleeding (n = 12) showed extensive Factor V cleavage.

CONCLUSIONS

Factor V cleavage in thrombolytic therapy is primarily plasmin mediated, rapid and often extensive. It is likely that transient increases, as well as longer term losses, of Factor V cofactor activity play a role in both ischemic and hemorrhagic events subsequent to thrombolytic therapy. The extensive loss of Factor V in some patients may affect the estimation of heparinization.

Safety and feasibility of dobutamine-atropine stress testing in hypertensive patients

Dobutamine stress testing is increasingly used for the diagnosis and functional evaluation of coronary artery disease. The aim of this study was to assess the hemodynamic profile, safety, and feasibility of dobutamine stress testing in hypertensive patients. Dobutamine (up to 40 micrograms/kg per minute)-atropine (up to 1 mg) stress echocardiography was performed for the detection of myocardial ischemia in 1164 patients with limited exercise capacity (age, 60 +/- 12 years; 761 men); 446 patients were known to have hypertension. The test was considered feasible when 85% of the maximal heart rate and/or an ischemic end point (new or worsened wall motion abnormalities, ST segment depression, or angina) was achieved. No myocardial infarction or death occurred during the test. Dobutamine induced a significant increase of heart rate in patients with and without hypertension (59 +/- 25 and 63 +/- 23 beats per minute, respectively). Peak rate pressure product was similar in patients with and without hypertension (18,566 +/- 4584 and 18,230 +/- 4508). Hypotension (systolic pressure drop > 40 mm Hg) during the test was more frequent in hypertensive patients (7% versus 4% in normotensive, P < .05). Independent predictors of hypotension were baseline systolic pressure greater than 140 mm Hg (odds ratio, 6.9; 95% confidence interval, 3.4 to 14), older age (odds ratio, 1.04; 95% confidence interval, 1.01 to 1.07), and medication with calcium channel blockers (odds ratio, 1.8; 95% confidence interval, 1.1 to 3.5). The prevalence of ventricular tachycardia was similar (4.1%) in both groups. Episodes of 10 beats or more (0.06% of patients) were terminated promptly by intravenous metoprolol administration. Dobutamine stress testing was considered feasible in 91% of patients with and 92% of patients without hypertension. Dobutamine-atropine stress echocardiography is a safe and feasible method for the assessment of hypertensive patients referred for evaluation of myocardial ischemia. Despite the higher prevalence of dobutamine-induced hypotension in these patients, the feasibility of the test is comparable to that in individuals without hypertension.

Placebo-controlled biofeedback blood pressure effect in hypertensive humans

The role of biofeedback in blood pressure control remains ill-defined because of nonspecific (placebo) effects, small study numbers, and the technical limitations of continuous pressure feedback. Clarification of its potential is awaited by those seeking a nonpharmacological approach to blood pressure control. This study examines the capability for systolic pressure lowering of 5 mm Hg or more using continuous pressure feedback in a statistical sample of untreated, well-characterized, mildly hypertensive individuals. Subjects were randomized in a double-blind study to active or placebo biofeedback. Placebo consisted of a modified contingency approach, using a partial disguise based on a digital high pass filter with 15 elements. Blood pressure-lowering capability was assessed during two laboratory sessions. Continuous visual feedback resulted in 11 of 28 subjects on active treatment and 12 of 28 on placebo treatment lowering their systolic pressure by 5 mm Hg or more (11 +/- 5.6 and 12 +/- 8.4 mm Hg, respectively; P = NS). Prestudy pressure was well-matched (153 +/- 9/97 +/- 4 and 154 +/- 8/98 +/- 4 mm Hg, respectively). An initial small difference in diurnal profile did not change. These findings indicate that among mildly hypertensive individuals, almost half can lower systolic pressure at will for short periods. This capability is independent of the real or placebo nature of the feedback signal. We conclude that there is no specific short-term biofeedback pressure-lowering capability in hypertensive individuals. Further exploration is needed to determine whether specific components of the placebo effect can be delineated, whether personality characteristics influence the response, and whether further biofeedback training can alter the outcome.

Contemporary use of and future roles for heparin in antithrombotic therapy

Although heparin therapy is an established component of the prevention and treatment of thromboembolic disease, recent advances have resulted in improvements in the clinical use of this agent. Studies have shown that weight-based dosing influences significantly both the time to reach a therapeutic intensity of anticoagulation and the incidence of thromboembolic recurrence. It is now considered the standard of care. A growing understanding of the variability among activated partial thromboplastin time (aPTT) reagents and the influence of these differences on aPTT outcomes has led to the use of reagent-specific therapeutic ranges for heparin monitoring. Many practitioners now choose to adjust the therapeutic range to correspond to heparin serum concentrations of 0.2-0.4 U/mL rather than the more common practice of prolonging aPTT to 1.5-2.5 times the mean normal aPTT. Pharmaceutical companies have developed low molecular weight heparins to minimize adverse effects associated with unfractionated heparin. More specific thrombin inhibitors are also under investigation with the aim of improving clinical outcomes in coronary syndromes now treated with heparin. Low molecular weight heparins or specific thrombin inhibitors are unlikely to replace unfractionated heparin in the near future. Therefore, optimum dosing and appropriate monitoring of heparin are critically important in the management of thromboembolic disease.

Thrombolysis with recombinant unglycosylated single-chain urokinase-type plasminogen activator (saruplase) in acute myocardial infarction: influence of heparin on early patency rate (LIMITS study). Liquemin in Myocardial Infarction During Thrombolysis With Saruplase

OBJECTIVES

The Liquemin in Myocardial Infarction During Thrombolysis With Saruplase (LIMITS) study was instituted to evaluate and characterize the effect of a prethrombolytic heparin bolus (5,000 IU) on the efficacy and safety of saruplase in patients with acute myocardial infarction.

BACKGROUND

Heparin has been used after thrombolytic therapy for acute myocardial infarction to prevent reocclusion of the infarct-related artery.

METHODS

The study was designed as a randomized, parallel-group, double-blind, multicenter trial. Patients were treated within 6 h of onset of symptoms with either a bolus of 5,000 IU of heparin (Liquemin) (n = 56, HSH group) or placebo (n = 62, PSH group) before thrombolytic treatment with saruplase given as a 20-mg bolus followed by an infusion of 60 mg over 60 min. Thirty minutes after completion of thrombolysis, an intravenous heparin infusion was administered for 5 days. Before coronary angiography was performed at 6 to 12 h after start of lysis, an additional bolus of 5,000 IU heparin was given to all patients. End points studied were patency of the infarct-related artery, changes in the hemostatic system and bleeding complications.

RESULTS

In the HSH group (heparin-saruplase-heparin), 78.6% of patients had an open infarct-related vessel (Thrombolysis in Myocardial Infarction [TIMI] flow grade 2 or 3) compared with 56.5% in the PSH group (placebo-saruplase-heparin) (intention-to-treat analysis, p = 0.01). No significant difference was observed between the two groups with regard to changes in fibrinogen and fibrin/fibrinogen degradation products. A total of eight bleeding complications (14.3%) were observed in the HSH group and five (8.1%) in the PSH group; no cerebrovascular event occurred, and no allergic reaction was reported. A total of 12 patients died during the hospital stay, 3 in the HSH group (5.4%) and 9 in the PSH group (14.5%).

CONCLUSIONS

In acute myocardial infarction, the administration of a heparin bolus before thrombolytic therapy with saruplase is associated with a significantly higher patency at angiography 6 to 12 h after the start of thrombolysis without any appreciable increase in risk of bleeding.

Heparin as adjunctive therapy to coronary thrombolysis in acute myocardial infarction

For many years anticoagulation has played a role in the prevention and management of thromboembolic complications associated with acute myocardial infarction. However, the role of heparin therapy after pharmacologic thrombolysis in myocardial infarction remains controversial. Debate continues regarding the necessity of heparin treatment after thrombolytic therapy as well as the mode by which it is administered. The purpose of this review is to summarize the findings of clinical trials designed to evaluate the effectiveness and safety of heparin as an adjuvant agent to thrombolytic therapy in acute myocardial infarction. Data regarding the clinical effectiveness of heparin are presented. Information and recommendations regarding the optimal dose, route of administration, timing of initiation, and duration of heparin treatment are provided.

Persistent thrombin generation in humans during specific thrombin inhibition with hirudin

BACKGROUND

The degree to which antithrombotic drugs suppress thrombin generation is unknown. Because hirudin, unlike antithrombin III, binds intravascular thrombin rapidly and selectively to yield a circulating inactive complex of 3- to 4-hour half-life, we used intravenous hirudin in humans to investigate the course of thrombin generation during and early after anticoagulation with this potent, direct antithrombin.

METHODS AND RESULTS

Intravascular thrombin was measured with an ELISA for the thrombin-hirudin complex formed during and for 18 hours after stopping a 6-hour infusion of hirudin at 0.1, 0.2, and 0.3 mg.kg-1.h-1 in three groups of six patients each. With free hirudin in 20- to 10,000-fold molar excess of thrombin and peak activated partial thromboplastin times of 2.3 to 3.0 times baseline, mean plasma thrombin-hirudin complex increased from 794 +/- 85 pg/mL (mean +/- SEM) 15 minutes after the start of the infusion to 1617 +/- 151 pg/mL at 6 hours of infusion to 2667 +/- 654 pg/mL at 24 hours. During the 24-hour observation period, plasma concentration of fragment 1.2 (the peptide released during conversion of prothrombin to thrombin) never fell below baseline but rather increased transiently during the hirudin infusion. Plasma concentrations of thrombin-antithrombin III complex (in ng/mL) decreased from 4.34 +/- 0.40 at baseline to 1.64 +/- 0.13 at 6 hours (P < .001) and gradually increased after stopping the infusion to 5.7 +/- 0.87 at 24 hours (nonsignificant compared with baseline).

CONCLUSIONS

Measurement of thrombin-hirudin complex may be used as a marker of thrombin generation in humans. Persistent accumulation of thrombin-hirudin complex and generation of fragment 1.2 during and after completion of potent anticoagulation with hirudin suggest thrombin generation is not blocked by high-affinity thrombin inhibition. The persistent formation of thrombin during declining plasma levels of hirudin may contribute to the pathogenesis of rethrombosis early after antithrombin therapy or during inadequate anticoagulation.

Thrombolytic therapy in acute myocardial infarction--selected recent developments

Thrombolytic therapy is the established treatment of choice for most eligible patients with acute myocardial infarction. Early initiation of treatment and early, complete and maintained patency of the infarct-related coronary artery are desirable, because these variables correlate with a reduction in mortality. As a consequence, considerable efforts have been undertaken to develop new pharmacological agents that serve these purposes. Among these, new plasminogen activators such as reteplase (r-PA), saruplase (scuPA), and staphylokinase are in clinical development, and DSPA (bat t-PA) and antibody-targeted plasminogen activators (ScuPA-59D8) have undergone extensive animal testing. Anticoagulants such as recombinant hirudin, hirulog, argatrobane, and Factor Xa inhibitors, as well as antiplatelet agents on the basis of monoclonal antibody 7E3 offer promise as adjunctive therapy to thrombolysis or to invasive intracoronary procedures.

Safety observations from the pilot phase of the randomized r-Hirudin for Improvement of Thrombolysis (HIT-III) study. A study of the Arbeitsgemeinschaft Leitender Kardiologischer Krankenhausärzte (ALKK)

BACKGROUND

Adjunctive therapy for thrombolysis in acute myocardial infarction consists of platelet inhibition with aspirin and thrombin inhibition with heparin. Thrombin inhibition may be improved by the use of hirudin as indicated by experimental and phase II clinical studies. The randomized, double-blind phase III r-Hirudin for Improvement of Thrombolysis study (HIT III) compared a recombinant hirudin (HBW 023) with heparin. The primary end point was the incidence of death or reinfarction.

METHODS AND RESULTS

Seven thousand patients with acute myocardial infarction and a duration of symptoms of less than 6 hours were to be randomized to receive intravenous heparin (70 IU/kg body wt bolus and 15 IU.kg-1.h-1) or hirudin (0.4 mg/kg body wt bolus and 0.15 mg.kg-1.h-1) infused over 48 to 72 hours and adjusted to an activated partial thromboplastin time of 2 to 3.5 times baseline values. In a pilot phase, 1000 patients receiving front-loaded alteplase for thrombolysis were to be recruited by 93 German centers. After enrollment of 302 patients, the trial was stopped after an increased rate of intracranial bleeding was observed in the hirudin group (5 of 148, 3.4%) compared with the heparin group (0 of 154). The overall stroke rate was 3.4% in the hirudin group and 1.3% in the heparin group. Other major bleeding occurred in five versus three patients and ventricular rupture occurred in three versus one patient in the hirudin and heparin groups, respectively. There were 19 in-hospital deaths, with 13 of them from the hirudin group.

CONCLUSIONS

Although the number of patients was too small for a definite benefit-risk assessment, at the dosage tested, hirudin in combination with front-loaded alteplase and aspirin may be associated with an increased rate of intracranial hemorrhage. Our findings are consistent with the observations of the GUSTO-II and TIMI-9 trials, where higher doses of another recombinant hirudin were used. Therefore, the therapeutic range of hirudin as an adjunct to thrombolysis may be smaller than previously thought, and reappraisal of dose finding should be considered.

Prevention of thrombus formation and growth by antithrombin III and heparin cofactor II-dependent thrombin inhibitors: importance of heparin cofactor II

Heparin (HEP) prevents thrombus formation (TF) and thrombus growth (TG), by accelerating thrombin (THR) inhibition by antithrombin III (ATIII). Recent studies suggest that dermatan sulphate which catalyzes thrombin inhibition by heparin cofactor II (HCII), can inhibit TF and TG as effectively as HEP. This study compared the antithrombotic effects of HEP and another agent, Sulodexide (SLX) which catalyzes thrombin inhibition by ATIII and HCII simultaneously. TF was induced in rabbit jugular veins, using the stasis/hypercoagulation model. TG was measured as the accretion of 125I-fibrin onto existing thrombi in rabbit jugular veins. HEP and SLX inhibited TF when given in doses of 10 and 5 anti-thrombin U/kg, respectively. SLX (16 anti-thrombin U/kg or 260 micrograms/kg) was more effective than HEP (120 anti-thrombin U/kg or 800 micrograms/kg) in preventing TG when administered either as a bolus or by continuous infusion. These data suggest that agents which accelerate THR inhibition by both ATIII and HCII simultaneously, can inhibit TF and TG with less systemic anticoagulation than comparable antithrombotic doses of HEP.

Transesophageal echocardiography before and during direct current cardioversion of atrial fibrillation: evidence for "atrial stunning" as a mechanism of thromboembolic complications

OBJECTIVES

The purpose of this study was to evaluate the usefulness of transesophageal echocardiography before electrical cardioversion in patients with atrial fibrillation and to determine the mechanism of thromboembolism after cardioversion.

BACKGROUND

Thromboembolic complications after electrical cardioversion of atrial fibrillation have been attributed to the dislodgment of preexistent left atrial thrombus during the resumption of atrial contraction. Transesophageal echocardiography has been proposed as a method of screening patients for left atrial thrombus before cardioversion.

METHODS

Seventy transesophageal echocardiographic studies were performed in 66 patients, predominantly with nonvalvular atrial fibrillation, before direct current cardioversion. In addition, transesophageal echocardiography was performed during the cardioversion procedure in 15 patients and immediately after in 1 patient.

RESULTS

Left atrial thrombus was detected in one patient (1.4%), and cardioversion was cancelled. Thromboembolic complications occurred in 4 patients, none of whom had evidence of left atrial thrombus before cardioversion. Within 10 s of successful cardioversion, left atrial spontaneous echo contrast appeared in five patients, increased in one patient and was unchanged in nine patients. Patients with new or increased spontaneous echo contrast had more impaired atrial contraction and slower initial heart rates after cardioversion than those without. Left ventricular contraction was also impaired transiently by cardioversion.

CONCLUSIONS

Transesophageal echocardiographic detection of left atrial thrombus before direct current cardioversion is important but infrequent in patients with predominantly nonvalvular atrial fibrillation. The occurrence of thromboembolic complications in the absence of demonstrable left atrial thrombus and the new development of spontaneous echo contrast in association with the transient atrial dysfunction ("stunning") caused by cardioversion suggest that cardioversion may promote new thrombus formation, in which case all patients should receive full anticoagulant therapy at the time of cardioversion.

Effect of intravenous nitroglycerin on heparin dosage requirements in coronary artery disease

Patients admitted to the coronary care unit who received both intravenous nitroglycerin and heparin were studied to evaluate heparin dosage requirements. Physicians ordered all nitroglycerin and heparin doses as well as coagulation studies without knowledge of this study. Activated partial thromboplastin time (APTT) values obtained during steady-state heparin administration were considered therapeutic if the ratio of APTT/APTT-baseline was > or = 1.5. Sixty patients with myocardial infarction or unstable angina were included in the study. The initial therapeutic heparin dose of 1,014 +/- 151 units/hour produced an APTT ratio of 2.0 +/- 0.5. At the time of the initial therapeutic dose, the nitroglycerin dose was 110 +/- 108 micrograms/min. There was a significant correlation between the initial therapeutic dose and both total (r = 0.56; p = 0.0001) and lean (r = 0.26; p < 0.05) body weight. Comparison of patients with nitroglycerin doses < and > or = 100 micrograms/min revealed a significant difference in the initial therapeutic dose (971 +/- 147 vs 1,077 +/- 136 U/hour, p < 0.01), but not the initial therapeutic dose standardized to total body weight (14.0 +/- 2.5 vs 13.5 +/- 2.7 U/kg/hour). Similarly, analysis of variance revealed a significant difference in the initial therapeutic dose (p < 0.05), but not the initial therapeutic dose standardized to weight among 5 different nitroglycerin dosage ranges (10 to 533 micrograms/min). Neither aspirin use, thrombolytic therapy nor decreasing or discontinuing the nitroglycerin dose significantly affected heparin requirements. Thus, contrary to prior reports, clinically significant heparin resistance induced by nitroglycerin was not found.

Management of acute myocardial infarction in the very elderly

Coronary artery disease is highly prevalent among the elderly, and the incidence of myocardial infarction (MI) is high. Still, the notion of optimal treatment for the elderly patient with MI remains unclear. This review will first discuss some of the characteristics of the aging myocardium that impact on the care of elderly cardiac patients. Next, the therapeutic options and their appropriateness for the aged patient are presented. Thrombolytic and beta-blocker therapies are reviewed extensively since they remain among the controversial issues in geriatric cardiology. Other well-known as well as experimental therapies are also discussed.

The Role of Heparin in the Thrombolytic Treatment of Acute Myocardial Infarction

Objective:

To review the literature to determine the most appropriate role of heparin when using thrombolytic therapy.

Data Sources:

English-language journal articles published between 1985 and 1991.

Study Selection:

Ten trials comparing the effects of thrombolytic therapy with and without heparin. Desired outcomes that the authors were interested in were coronary vessel patency, reocclusion, recurrent ischemia, reinfarction, mortality, and bleeding complications.

Data Extraction:

Studies were assessed based on methodology.

Data Synthesis:

The concurrent use of heparin with tissue plasminogen activator (tPA) enhances coronary vessel patency without significantly increasing the risk of bleeding complications. The concurrent use of heparin with streptokinase enhances survival; however, the risk of bleeding associated with heparin use is unclear at present. Data are not currently available for anistreplase. Studies are underway to further address the role of heparin with thrombolytic therapy.

Conclusions:

Based on data currently available, it appears that beginning intravenous heparin during the first hour of the tPA infusion and continuing heparin for 24 hours appears most appropriate and has a minimal risk of bleeding. Delaying heparin for 9–12 hours after the streptokinase infusion appears to produce clinical benefits; however, it is not clear whether these benefits outweigh the potential risks of bleeding.

Role of heparin in coronary thrombolysis

Although the benefits of coronary thrombolysis are well established, the optimal therapeutic strategy for ensuring rapid and sustained coronary artery patency remains controversial. The available data suggest that the success of coronary thrombolysis depends not only on the induction of clot lysis, but also on the extent to which procoagulant activity that promotes recurrent thrombosis is inhibited. Procoagulant activity increases almost immediately in patients treated with fibrinolytic agents, and persistent increases in thrombin activity have been associated with recurrent coronary thrombosis. Heparin administered intravenously appears to markedly attenuate the thrombin activity associated with thrombolysis and, in patients treated with tissue plasminogen activator (t-PA), prevents early recurrent coronary thrombosis. The results of clinical trials of coronary thrombolysis indicate that conjunctive treatment of patients with heparin improves survival compared with treatment with fibrinolytic agents alone. Although recent clinical trials in which patients were treated with streptokinase suggested that 12,500 units of heparin administered subcutaneously twice daily decreases mortality, this dosage regimen does not induce therapeutic levels of anticoagulation within the first 24 h in most patients. The failure to achieve early therapeutic anticoagulation may account for the lack of mortality benefit in trials in which patients given t-PA were treated with conjunctive subcutaneous heparin therapy. Thus, the available experimental and clinical data suggest that intravenous heparin should be given to patients treated with fibrinolytic agents for acute myocardial infarction.

Advances in thrombolytic therapy

Alteplase and saruplase are more fibrin-specific thrombolytic drugs than anistreplase. These and the thrombolytic drugs of the first generation (streptokinase and urokinase) have shortcomings and limitations. The prolonged intravenous maintenance infusions have been replaced by a bolus injection, accelerated infusions, or the combined intravenous administration of thrombolytic agents. Numerous truncated alteplase or saruplase molecules have been constructed by deletion and domain substitution or hybrids made of the two molecules without gaining in thrombolytic potency. Recombinant staphylokinase and plasminogen activator from bat saliva have some interesting properties and are being investigated. Thrombus-targeted thrombolytic drugs were constructed using monoclonal antibodies against fibrin fragments or against epitopes of activated platelets. Fibrin-specific thrombolytic drugs require the concomitant use of a potent antithrombotic drug to prevent reocclusion. Whether hirudin or synthetic thrombin inhibitors are superior to heparin and whether novel antiplatelet agents, including monoclonal antibodies to platelet receptors and disintegrins, are more effective than aspirin is under clinical investigation. The place of stable analogues of prostacyclin during thrombolytic treatment is still unsettled.

Conjunctive enhancement of enzymatic thrombolysis and prevention of thrombotic reocclusion with the selective factor Xa inhibitor, tick anticoagulant peptide. Comparison to hirudin and heparin in a canine model of acute coronary artery thrombosis

BACKGROUND

Effective thrombolytic recanalization of an occluded coronary vessel is often limited by acute thrombotic reocclusion, which has galvanized the search for effective adjunctive or conjunctive antithrombotic agents.

METHODS AND RESULTS

Recombinant versions of tick anticoagulant peptide (rTAP) and hirudin (rHIR) are highly selective and potent polypeptide inhibitors of factor Xa and thrombin, respectively. The comparative antithrombotic efficacies of rTAP, rHIR, and heparin, administered conjunctively with recombinant tissue-type plasminogen activator (rt-PA), on thrombolytic reperfusion and reocclusion, were determined in a canine model of occlusive coronary artery thrombosis with a superimposed critical stenosis. In this model, a platelet-rich occlusive thrombus was formed after damage to the intimal surface of the left circumflex coronary artery induced by electrolytic injury. Fifteen minutes after occlusion, the dogs received a systemic intravenous administration of either saline (control), heparin (200 units/kg bolus + 2 units/kg/min, heparin (HEP) 200 or 100 units/kg bolus + 1 unit/kg/min, HEP 100), rHIR (50 or 100 micrograms/kg/min, rHIR 50 or 100, respectively), or rTAP (100 micrograms/kg/min, rTAP 100) followed 15 minutes later by rt-PA (100 micrograms/kg bolus + 10 micrograms/kg/min over 90 minutes). Infusions of the conjunctive agents were discontinued 60 minutes after termination of rt-PA. The incidence and time (mean +/- SEM) to thrombolytic reperfusion were determined for control (five of 12; 68.0 +/- 7.8 minutes), HEP 100 (six of eight; 40.1 +/- 8.3 minutes), HEP 200 (six of eight; 39.8 +/- 9.5 minutes), rHIR 50 (six of eight; 51.7 +/- 14.6 minutes), rHIR 100 (eight of eight; 19.5 +/- 4.2 minutes), and rTAP 100 (eight of eight; 22.8 +/- 10.0 minutes). The incidence and time to reocclusion after rt-PA were determined for control (four of five; 45.7 +/- 12.5 minutes), HEP 100 (four of six; 18.2 +/- 10.7 minutes), HEP 200 (five of six; 26.2 +/- 20.7 minutes), rHIR 50 (four of six; 47.3 +/- 21.6 minutes), rHIR 100 (six of eight; 89.8 +/- 5.9 minutes), and rTAP 100 (three of eight; 54.0 +/- 16.3 minutes). All of the dogs that reoccluded in the rHIR 100 group did so after termination of the inhibitor infusion, whereas two of the three dogs in the rTAP 100 group that reoccluded did so during the inhibitor infusion. Coronary artery blood flow was characterized by intermittent periods of reocclusion and recanalization in all groups except rTAP 100.

CONCLUSIONS

The potent antithrombotic effects of rTAP in this model directly implicate de novo thrombin formation as a major source of thrombin activity within the highly thrombogenic residual thrombus. These findings suggest that direct inhibition of prothrombinase activity may be an effective strategy in the development of a new class of conjunctive agents.

Fibrinolytic parameters and hemostatic monitoring: identifying and predicting patients at risk for major hemorrhagic events

The use of thrombolytic therapy has dramatically altered the treatment of acute myocardial infarction and is rapidly spreading from large medical centers to community hospitals throughout the country. The widespread use of thrombolytic therapy will benefit a wide range of people, but the potential risks of this form of therapy must be understood. Hemorrhage is one of the major risks of thrombolytic therapy. This review will focus on the data available from a number of recent, large trials of thrombolytic therapy for acute myocardial infarction with respect to laboratory parameters that may predict hemorrhagic complications and/or help with their management. We will discuss both conclusions drawn from currently available data and address future research directions.

Eicosanoids, other fatty acid metabolites and the cardiovascular system: are the present antithrombotic approaches rational?

Antiplatelet +/- anticoagulant drugs are currently used as the standard treatment to prevent and treat thrombosis. While this approach is beneficial, it is not optimal. Recent evidence suggests that constituents of the vascular endothelium play an important role in regulating vessel wall thrombogenecity, thereby inhibiting thrombogenesis. These include constituents such as PGI2, tissue plasminogen activator, thrombomodulin and the lipoxygenase fatty acid metabolite derived from linoleic acid, 13-hydroxyoctadecadienoic acid (13-HODE). Consequently, new strategies have been developed to maximize the use of these agents for antithrombotic therapy. We will review these different approaches, discuss their rationale, and based upon recent experimental data, introduce an alternative approach for antithrombotic therapy which may circumvent a number of limitations and side-effect of the currently used drugs.

Thrombolysis, anticoagulation, and reocclusion

Based on apparent higher recanalization rates of the infarct-related artery, preferential use of thrombolytic agents with high clot specificity has been proposed for treating patients with acute myocardial infarction. In the Thrombolysis in Myocardial infarction (TIMI-I) and European Cooperative Group studies, higher reperfusion rates were observed with alteplase compared with streptokinase, causing many to assume that the former would achieve a greater reduction in early hospital mortality. However, the Gruppo Italiano per lo Studio della Streptochinasi nell'Infarto Miocardico (GISSI-2) and its associated International Study Group failed to show any differences in 15-day mortality between these agents in more than 20,000 patients. This apparent lack of correlation between reperfusion rates and early mortality may be explained in part when one considers that recanalization or patency rates measured at a given point in time, such as 90 minutes after onset of therapy, fail to define the subsequent vessel status. Early reocclusion is the major reason for this and is a major limitation to the clinical efficacy of thrombolytic drugs. Following recanalization, residual fibrin-bound thrombin adherent to the site of arterial injury from plaque rupture strongly promotes rethrombosis. Although antiplatelet and antithrombin agents such as aspirin and heparin help to decrease rethrombosis, these agents are far from ideal. Thrombolytic agents that produce a significantly prolonged systemic thrombolytic state, such as streptokinase and anistreplase, are likely to result in less rethrombosis. Therefore, a systemic fibrinolytic state would appear to be an advantage rather than a disadvantage, particularly because the incidence of intracerebral hemorrhage does not appear to be greater with their use compared with agents producing less systemic fibrinolysis.(ABSTRACT TRUNCATED AT 250 WORDS)