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New data from the HORIZONS AMI trial suggest that inadequate very early antithrombin and antiplatelet therapy is one of the strongest drivers of early stent thrombosis in ST segment elevation myocardial infection (STEMI) patients undergoing primary percutaneous coronary intervention (PCI) with stenting, with the take-home message to give all patients a heparin bolus and 600 mg of clopidogrel as soon as possible — either in the ambulance or as soon as they arrive in the emergency department. Cigarette smoking was one of strongest drivers of late stent thrombosis.

"We can do a few simple things to bring stent thrombosis rates down. Get heparin and a potent antiplatelet agent into the patients as fast as possible, and reinforce the importance of stopping smoking," commented HORIZONS investigator Dr George Dangas (Columbia University Medical Center, New York).

The HORIZONS AMI trial, the largest trial of primary PCI ever conducted, involved 3602 STEMI patients within 12 hours of symptom onset undergoing primary PCI who were randomized to treatment with heparin plus a GP IIb/IIIa inhibitor or to bivalirudin alone. The primary results showed that bivalirudin monotherapy resulted in less major bleeding, comparable rates of ischemia, and improved survival compared with heparin plus a GP IIb/IIIa inhibitor at 30 days and one year.

The current analysis of stent thrombosis in the trial, presented here by Dangas during the i2 Summit at the American College of Cardiology 58th Scientific Session, was based on the 3203 patients who received a stent. Dangas reported that there were 107 cases of definite or probable stent thrombosis (3.3%) by one year. This could be broken down into an incidence of 0.9% in the first 24 hours (acute), 1.6% between one and 30 days (subacute), and 1% between one month and one year (late). "The 3.3% figure at one year sounds high, but these are patients with acute ST-elevation MI who are getting a stent, and these rates are not different from those seen in other studies of similar patients," he said.

He added that acute, subacute, and late stent thrombosis appear to be related to different factors, with lack of potent adjunctive pharmacological therapy one of the driving factors for acute and subacute events, while cigarette smoking made a large contribution to late stent thrombosis.

Early Heparin Bolus

The most important factor influencing acute stent thrombosis appeared to be use of early nonrandomized heparin before patients had been randomized to the study drug. Patients who received such treatment had significantly lower rates of stent thrombosis within the first 24 hours regardless of which treatment group they were assigned to.

HORIZONS AMI: Effect of Prerandomization Heparin on Incidence of Acute Stent Thrombosis

Dangas noted that about half the patients in the study received a prerandomization bolus of heparin. "This is something that just makes sense to do. Often bivalirudin or IIb/IIIa blockers are not stored in the emergency department, and many doctors like to get something in quick, so they give a heparin bolus. Every center has their own way of doing things—in Europe, heparin is often given in the ambulance, while in the US it is given in the ER [emergency room]. And we saw that this practice seems to have a big benefit on acute stent thrombosis. This cheap and easy strategy seems to have an important effect," he said.

A 600-mg loading dose of clopidogrel also appeared to be important in reducing stent thrombosis, particularly subacute stent thrombosis in the bivalirudin group. Dangas commented: "Again, this simple intervention of 600-mg clopidogrel seemed to have an effect. Early upfront intense antiplatelet therapy is of prime importance in these patients." He added: "So we have a clear message from this data: Give potent antiplatelet therapy and a bolus of heparin as soon as possible to ST-elevation-MI patients. This is a practical and viable solution."

Bivalirudin: Different Effect Early and Late

While stent thrombosis within one year occurred with similar frequency in both randomized treatment arms, acute stent thrombosis was more common with bivalirudin, especially within the first five hours, whereas stent thrombosis between 24 hours and one year was more common in the heparin-GP-IIb/IIIa-blocker arm.

HORIZONS AMI: Stent Thrombosis in Bivalirudin vs Heparin/GP-IIb/IIIa-Blocker Arms, Differential Effect With Time

On this observation, Dangas noted that optimizing adjunctive pharmacotherapy appears to be particularly important to reduce acute and subacute stent thrombosis with bivalirudin and that more use of early heparin and high doses of clopidogrel or the newer, more potent, antiplatelet agents approaching the market could further improve outcomes with bivalirudin. He also suggested that a prolonged bivalirudin infusion of about four to six hours may reduce acute stent thrombosis, but this would need further investigation to make sure it did not increase bleeding risk.

"It is really the first five hours that are important," Dangas commented. "The delay in the onset of action of clopidogrel could be key here, especially in ST-elevation-MI patients who may have decreased absorption. This is why we need superactive, more potent antiplatelet agents."

Other factors that affected acute and subacute stent thrombosis in HORIZONS AMI were vessel flow, lesion characteristics, and number and length of stents, while patient-related factors including cigarette smoking were predictors of late stent thrombosis. The type of stent implanted (drug eluting or bare metal) did not affect the risk of stent thrombosis during any time interval up to one year.

HORIZONS AMI: Independent Predictors of Acute Stent Thrombosis

HORIZONS AMI: Independent Predictors of Subacute Stent Thrombosis

HORIZONS AMI: Independent Predictors of Late Stent Thrombosis

i2 Summit at American College of Cardiology 58th Scientific Session: Late-breaking session 2406-9. Presented March 29, 2009.

The complete contents of Heartwire , a professional news service of WebMD, can be found at www.theheart.org, a Web site for cardiovascular healthcare professionals.

Pearls for Practice

• Inadequate very early antithrombin and antiplatelet therapy is one of the strongest drivers of early stent thrombosis, both acute and subacute, in STEMI patients undergoing primary PCI with stenting. Therefore, all patients should be given a heparin bolus and 600 mg of clopidogrel as soon as possible, either in the ambulance or immediately on arrival in the emergency department.
• Other factors that predicted acute and subacute stent thrombosis were vessel flow, characteristics of the lesion, and number and length of stents. Cigarette smoking and other patient-related factors increased the likelihood of late stent thrombosis. Risk for stent thrombosis for up to 1 year was not affected by choice of stent type (drug eluting or bare metal).