Incidence, Predictors, and Outcome of Thrombosis After Successful Implantation of Drug-Eluting Stents FREE

Despite major improvements in antiplatelet therapy, thrombotic events remain the primary cause of death after percutaneous coronary interventions.^1- 2 Sirolimus-eluting stents and polymer-based paclitaxel-eluting stents have been shown to reduce neointimal hyperplasia and risk of restenosis without increasing the risk of stent thrombosis.^3- 7 Operators are now using drug-eluting stents for a wide variety of clinical and anatomic situations, many of which have not been evaluated in randomized studies.^8- 10 We analyzed the incidence, predictors, and clinical outcome of stent thrombosis at 9-month follow-up in an observational cohort study.

We identified 2229 consecutive patients who underwent successful implantation with sirolimus-eluting stents (1062 patients, 1996 lesions, 2272 stents) or paclitaxel-eluting stents (1167 patients, 1801 lesions, 2223 stents) between April 2002 and January 2004. Patients were treated at 2 community hospitals or 1 academic hospital in Germany and Italy. Patients with ST-elevation acute myocardial infarction (MI) less than 48 hours before the procedure, with intraprocedural stent thrombosis, and those treated with both types of stents were excluded. All patients were pretreated with ticlopidine or clopidogrel and aspirin; a loading dose of 300 mg of clopidogrel was given to patients not previously taking the agent. Aspirin was continued indefinitely and clopidogrel or ticlopidine for at least 3 months after sirolimus-eluting stent implantation and for at least 6 months after paclitaxel-eluting stent implantation. Stent implantation methods have been described previously.¹¹ Glycoprotein IIb/IIIa inhibitors were administered at the physician’s discretion. Standard qualitative and quantitative analyses and definitions were used for the angiographic analysis.¹²

All patients signed an informed consent document and local institutional review boards approved the study as planned.

Stent thrombosis was determined as the occurrence of any of the following events: angiographic documentation of partial or total stent occlusion detected within 30 days of the procedure (an acute clinical ischemic event in addition to angiographic documentation had to be present when the event occurred after 30 days), or sudden cardiac death or postprocedural MI after successful stent implantation not clearly attributable to another coronary lesion. Stent thrombosis cases were categorized according to the timing of occurrence into subacute (from procedure end through 30 days) and late (>30 days).

Major adverse cardiac events were defined as death (all-cause), Q-wave MI, target lesion revascularization, and target vessel revascularization.

Differences in proportions were tested with the χ² or Fisher exact test. SAS version 8.2 (SAS Institute Inc, Cary, NC) was used for data analysis.

A total sample of 2300 observations was computed to achieve 80% power at a 2-sided .05 significance level to detect a hazard ratio (HR) equal to or greater than 3.0 with a Cox regression of the log HR on a binary risk factor with a 25% or greater prevalence. The sample size was adjusted for an anticipated event rate of 1.5%.

Relationships of event incidence to covariates were investigated with univariate Cox regression models. The proportional hazard assumption was checked for all screened covariates and no relevant violations were found. The predictive robustness of univariate findings was subsequently tested by means of a bootstrap subset selection method in which multivariable Cox regression analysis using a stepwise elimination process was repeated for each of 1000 bootstrap samples.¹³ The relative frequency of selection of “important” variables was used as a criterion for inclusion of predictors in the final multivariable model. Variables were retained if the selection frequency of the bootstrap samples was at least 50% or if they were clinically relevant. Because of the small absolute number of events, we performed an internal validation process to test model adequacy and quantify “overfitting.”¹⁴

The proportion of total variability was estimated by means of the Nagelkerke Index (pseudo R²), which was calculated when the models were fitted to 1000 bootstrap replications (training sample) and to the original data (test sample). The Nagelkerke Index obtained from each bootstrap sample was then subtracted from the initial index value of the original population. The average of the differences was considered as a measure of optimism in the model fit, where optimism is the proportion of variability ascribed to overfitting. Finally, a corrected index was calculated by subtracting the average of the optimism estimates from the original Nagelkerke Index. The estimates of slope shrinkage, which were used to identify overparameterized models, were obtained for the multivariable models using the same bootstrapping validation process. Thus we were reassured that, although based on few events, overfitting did not substantially bias our final model.

Baseline, angiographic, and procedural characteristics are shown in Table 1. All stents were deployed successfully. There were no significant differences between the 2 stent groups regarding procedural complications and in-hospital outcome. The rates for Q-wave and non–Q-wave MI were 0.3% and 9%, respectively. There were 4 in-hospital deaths, of which 2 were determined to be caused by stent thrombosis.

At 9-month follow-up (available in all patients), 29 patients (1.3%) had stent thrombosis (9 [0.8%] in the sirolimus group and 20 [1.7%] in the paclitaxel group; P = .09). Fourteen patients had subacute thrombosis (0.6%), 4 in the sirolimus group and 10 in the paclitaxel group (0.4% vs 0.8%; P = .19) and 15 patients had late thrombosis (0.7%), 5 in the sirolimus group and 10 in the paclitaxel group (0.5% vs 0.8%; P = .30).

A total of 71% (10/14) of the subacute cases occurred within 1 week of the procedure (median, 4 days) and 53% (8/15) of the late thrombosis cases occurred within 3 months of the procedure (median, 57 days). Seven cases (24%) presented as death, 20 (69%) as nonfatal MI, and 2 (7%) as unstable angina.

At follow-up the case-fatality rate—including death at presentation—was 45% (13/29). Angiographic documentation of thrombosis was available in 12 of the 22 patients (55%) with stent thrombosis (including both patients that presented with unstable angina) that did not present with death. In-hospital data were available for all patients who presented with acute MI and had no angiographic evidence of stent thrombosis.

At 9-month follow-up, target lesion revascularization was performed in 141 patients (6.3%) (58 [5.5%] in the sirolimus group vs 83 [7.1%] in the paclitaxel group; P = .13). Major adverse cardiac events occurred in 242 patients (10.8%) (109 [10.3%] in the sirolimus group vs 133 [11.4%] in the paclitaxel group; P = .42).

The incidence of stent thrombosis according to selected patient characteristics and the univariate predictors of cumulative stent thrombosis are shown in Table 2. Five of 17 patients with premature antiplatelet therapy discontinuation had stent thrombosis; in 1 of them only clopidogrel was discontinued. Independent predictors of subacute, late, and cumulative stent thrombosis are shown in Table 3. The key predictors of stent thrombosis were premature antiplatelet therapy discontinuation, renal failure, bifurcation lesions, diabetes, and low ejection fraction. For subacute thrombosis, stent length was also a predictor: for each 1-mm increase in length, there was 1.03 times greater risk of thrombosis.

In a large cohort of consecutive patients undergoing drug-eluting stent implantation, we noted a 9-month cumulative stent thrombosis incidence of 1.3%, substantially higher than rates reported in major clinical trials (0.4% at 1 year for sirolimus and 0.6% at 9 months for paclitaxel).^3,5 With widespread availability of drug-eluting stents, the scope of percutaneous coronary intervention has been expanded to more complex lesions and patients. In our study, 27% of the population had diabetes and 79% of the lesions were complex. The clinical consequences of stent thrombosis were severe, with a case-fatality rate of 45%.

Similar to previous reports of both bare-metal stents^2,16- 17 and drug-eluting stents,^18- 19 our study found that premature discontinuation of antiplatelet therapy was the most important predictor of stent thrombosis after implantation. Thrombosis occurred in 29% of patients who prematurely discontinued dual antiplatelet therapy, making treatment adherence of paramount importance. Nonetheless, the absolute numbers of events in our cohort were small, leading to wide confidence intervals for estimation of effect magnitude.

In addition to premature discontinuation of antiplatelet therapy, other key predictors of stent thrombosis were renal failure, bifurcation lesions, diabetes, low ejection fraction, and, for subacute thrombosis, stent length.

Renal failure has been linked to cardiac disease with microvascular and metabolic abnormalities that may predispose to thrombus formation.^20- 21 Renal failure also has been associated by numerous studies with an increased mortality rate despite successful coronary intervention.^22- 24

Regarding bifurcational lesion location, pathology studies have suggested that arterial branch points are foci of low shear and low flow velocity and are sites predisposed to the development of atherosclerotic plaque, thrombus, and inflammation.^25- 27 The observed associations of diabetes,^28- 32 low ejection fraction,^2,33 and stent length² with stent thrombosis are also consistent with previous reports.

In our study, stent type did not emerge as an independent predictor of thrombosis. Of concern, however, we found an almost double incidence of stent thrombosis after paclitaxel compared with sirolimus stent implantation. These findings are consistent with those of the ISAR-DESIRE (Intracoronary Stenting and Antithrombotic Regimen–Drug-Eluting Stents for In-Stent Restenosis) trial of drug-eluting stents for in-stent restenosis, in which sirolimus-eluting stents tended to be associated with a better outcome than paclitaxel-eluting stents.⁶ However, without larger numbers it is difficult to make firmconclusions; subsequent randomized trials will be critically important.

A limitation of the current study was the lack of angiographic documentation in all cases adjudicated as stent thrombosis. Nevertheless, the definition of stent thrombosis that we proposed is similar to the one used in important recent studies extended beyond 30 days.^3,19

In conclusion, the incidence of stent thrombosis at 9 months after successful drug-eluting stent implantation in consecutive real-world patients was 1.3%. Premature antiplatelet therapy discontinuation, bifurcation lesions, and low ejection fraction were identified as independent predictors of subacute, late, and cumulative stent thrombosis. In addition, stent length was also recognized as a predictor of subacute thrombosis, and renal failure and diabetes as predictors of both subacute and cumulative stent thrombosis. The clinical consequences were death in 45% of patients and nonfatal MI in the majority of the others.