Published online before print September 8, 2009, doi: 10.1161/CIRCINTERVENTIONS.108.844985
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Original Articles |
Stent Thrombosis in Sweden
A Report From the Swedish Coronary Angiography and Angioplasty Registry
From the Uppsala Clinical Research Centre (B.L., J.L., S.K.J.), Uppsala University Hospital, Uppsala, Sweden; Department of Medicine (J.C.), Kalmar Hospital, Kalmar, Sweden; Department of Cardiology (O.F.), Örebro University Hospital, Örebro, Sweden; Department of Cardiology (F.S.), Lund University Hospital, Lund, Sweden; and Department of Cardiology (U.S.), University Hospital, Linkoping, Sweden.
Correspondence to Bo Lagerqvist, MD, PhD, Uppsala Clinical Research Center, Uppsala University Hospital, 751 85 Uppsala, Sweden. E-mail bo.lagerqvist{at}ucr.uu.se
Received December 16, 2008; accepted July 16, 2009.
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Abstract |
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Background— The objective was to evaluate the role of risk factors and stent type for stent thrombosis (ST) using a large real world registry.
Methods and Results— We evaluated all consecutive coronary stent implantations in Sweden from May 1, 2005, to June 30, 2007. All cases of ST, documented in the Swedish coronary angiography and angioplasty registry until September 21, 2008, were analyzed. ST was registered in 882 of 73 798 stents. Acute coronary syndromes, insulin-treated diabetes mellitus, smoking, previous coronary intervention, warfarin treatment, small stent diameter, and stenting in restenotic, complex, or bypass graft lesions had the strongest association with ST in the multivariable statistical model. There were considerable differences in the frequency of ST between different stent brands. The overall risk of ST was lower in drug-eluting stents compared with bare metal stents (adjusted risk ratio, 0.79; 99% CI, 0.63 to 0.99). However, from 6 months after stent implantation and onward, the risk for ST was higher in drug-eluting stents compared with bare metal stents (adjusted risk ratio, 2.02; 99% CI, 1.30 to 3.14).
Conclusions— ST is a multifactor disease, and the incidence varies considerably between patients based on clinical, vessel, and stent characteristics. For drug-eluting stents compared with bare metal stents, the risk pattern was biphasic; initially, bare metal stents demonstrated a higher risk of ST; whereas after the first months, ST risk was higher with drug-eluting stents. Our findings highlight the need for prospective randomized studies with head-to-head comparisons between different stents.
Key Words: stent • thrombosis • myocardial infarction • angioplasty • revascularization
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Introduction |
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Coronary stenting reduces the rate of clinical and angiographic restenosis.1,2 Stent thrombosis (ST) is an infrequent event after percutaneous coronary interventions (PCI) but remains a serious complication in terms of morbidity and mortality.3 Drug-eluting stents (DES) reduce the rate of restenosis and the need for target lesion revascularizations compared with bare metal stents (BMS). However, concerns have been raised about an increased rate of late and very late ST with DES.4
Clinical Perspective on p 401
A number of predictors for ST have been proposed and identified both after BMS and DES; factors such as diabetes mellitus and renal failure,3,5–7 bifurcation lesions, vessel diameter and stent length,3,5 residual coronary dissections,8–10 and underexpansion11 of the stent have been shown to be independently associated with increased rates of ST. ST is also more frequently reported after acute coronary syndromes4,7,12 and particularly in association with premature discontinuation of thienopyridine therapy.12–14
Relevant information about incidences of ST of different stent brands and predictors of ST requires analysis of a high number of patients and procedures. The aim of this study was therefore to assess incidences and predictors of ST after PCI in the Swedish Coronary Angiography and Angioplasty Registry (SCAAR) that provides data from an entire country with complete and continuous registration of all deaths and all repeat angiographies and PCIs.
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Methods |
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Study Population
This study included all stents used in Sweden from May 1, 2005, to June 30, 2007, and the follow-up regarding occurrence of ST was performed until September 21, 2008. Stents implanted until the end of 2007 at 1 small center (Skövde Hospital) were excluded, because this hospital was not recording their subsequent coronary angiographies in SCAAR during this period. Also, stents with missing data were excluded.
The SCAAR Registry
SCAAR records consecutive patients from all centers (n=30) performing coronary angiography and PCI in Sweden. The treating physician reports each procedure online through a web interface directly from the catheterization laboratory. In early 2004, an interactive method for registration of restenosis was implemented in SCAAR. During the registration of a coronary angiography or PCI, a mandatory question regarding the existence of any type of restenosis has to be answered by the treating physician. A detailed interactive presentation of all previously (in Sweden) treated coronary segments is displayed. Information about date, hospital, and coronary segment together with name and dimension of stents used is shown. The treating physician is told to record any restenosis and, from April 2005, information about ST in the specified stents. A list of the most important recorded variables that are in accordance with international recommendations15 is presented in Table 1
. The risk of ST for the different stents used in Sweden are reported on the home page of SCAAR.16
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Definitions
An ST is defined in SCAAR as an angiographic occlusion of a previously implanted stent with an acute clinical presentation. Early STs are defined as those reported within 30 days after stenting. A bifurcation lesion is recorded if there is a stenosis in a bifurcation with a side branch of at least 2 mm in diameter, or if the treating physician regards the use of several guide wires as desirable. Diameter of the stent is defined as the nominal diameter of the stent balloon or the nominal diameter of any larger balloon if the stent was postdilated. Chronic total occlusions are defined as occlusions with a probable duration of at least 3 months.
Only stent types implanted at >500 occasions in Sweden during the study period are separately analyzed and presented in this study. The names of the different stents are presented as they are entered in the database. The Multi-Link Vision and the Multi-Link MiniVision (Abbott Laboratories, Abbott Park, Ill) are both referred to as "Vision"; Medtronic Driver coronary stent and Medtronic Micro-Driver coronary stent (Medtronic Inc) as "Driver"; Cordis Cypher and Cypher Select (Cordis Corporation, Miami, Fl) as "Cypher"; Flexmaster F1 (Abbott Laboratories) as "Flexmaster"; Medtronic Endeavor as "Endeavor"; Braun Coroflex Blue (B. Braun Melsungen AG, Germany) as "Coroflex Blue"; Sorin Chrono stent (Sorin SPA, Italy) as "Chrono"; Boston Scientific Taxus Express2, Taxus Liberté, and Liberté stents as "Taxus Express," "Taxus Liberté," and "Liberté," respectively (Boston Scientific Corporation).
Statistical Analyses
Baseline characteristics were summarized with means and standard deviations for continuous variables and percentages for discrete variables. Cumulative event rates were estimated by the Kaplan-Meier method. The primary end point was the registration of ST, which was defined as occlusions of a previously implanted stent with acute clinical presentation in SCAAR. All individual stents were followed until September 21, 2008, until death occurred or until restenosis/occlusion in the particular stent was reported. The relative risk (RR) of the primary end point was calculated using Cox proportional hazard method. For calculation of the adjusted RR, all factors in Table 1
were forced into the model together with the treating hospital and year of the procedure. The variables "Class of stenoses"17 and "Severity of disease" were dichotomized into B2/C and A/B1 lesions and 3-vessel or left main disease versus the rest. Only stents with no missing data are presented with exception for the variables "Hypertension" and "Hyperlipidemic treatment" where the missing data (1.6% and 1.8%, respectively) were recoded as "unknown" and included in the analyses. In most calculations, the name of the stent was a covariate in the model except when the effect of "DES/BMS" was studied; in that case, the variable "name of stent" was substituted by DES/BMS.
To handle possible different results at different hospitals, we also tested our model stratifying for different hospitals. However, this resulted in a materially unchanged result.
All calculations were performed with a focus on individual stents. Thus, the data are presented from the stent perspective with patient and procedure data linked to the individual stents. Because of the size of the database, 99% confidence intervals are presented. This study is, however, exclusively descriptive and should only be regarded as hypothesis generating. All analyses were performed with the use of SPSS statistical software, version 15.0 (SPSS, Inc, Chicago, Ill).
The authors had full access to the data and take responsibility for its integrity. All authors have read and agreed to the manuscript as written.
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Results |
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Patient Characteristics and Stents
During the study period, 78 878 stents were implanted in Sweden. In the analyses, 1112 stents (1.4%) were excluded from 1 hospital with lacking registration of subsequent angiographies, and 3968 stents (5.0%) were excluded due to incomplete background information. There was no difference in the rates of reported ST event rate between the analyzed and the excluded cohorts (1.2% event rate, RR, 0.89, 95% CI, 0.67 to 1.17). In the final analysis, 73 798 stents from 47 197 procedures and 42 150 patients were included. From angiographies performed after the index PCI, 882 cases of ST were registered, of which almost half, 433 (49%) occurred within 30 days after stent implantation. The follow-up time was 661±332 days (mean±SD), with a variation from 83 to 1239 days.
The stents with reported ST differed from the stents without ST regarding several of the presented background characteristics as presented in Table 1.
The timing of ST is illustrated in Figure 1. During the first 30 days after stent implantation, ST had been reported in 0.6% of the stents. After 1 month, there was an abrupt decrease in the rate of new occurring ST with a rate during the subsequent 12 months of around 0.5% ST per year. After the first 12 months, the rate of ST was stabilized at 0.3 to 0.4% per year.
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Patient and Clinical Factors
Insulin-treated diabetes mellitus was a strong predictor of ST, whereas diabetes without insulin treatment was not (Table 2). However, during the first 30 days after the index PCI, there was a trend toward more ST also in stents implanted in patients with diabetes mellitus without insulin treatment. Acute coronary syndromes and smoking were also strongly related to ST, early as well as during the entire follow-up period. Previous PCI and coronary artery bypass grafting were also strongly related to ST in the unadjusted analysis. The number of stents implanted per patient was also related to the risk of ST in each of the stents, particularly during the early period after treatment. Furthermore, severity of coronary disease (3-vessel or left main disease) was strongly related to the unadjusted incidence of ST, but after adjustment, this association disappeared. Age was an inverse independent predictor of ST, with a reduced adjusted risk with increasing age as reported in Table 2. Finally, complete revascularization was associated with lower risk of ST.
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Medication
Medication with aspirin, clopidogrel, and warfarin before the procedure were all related to the unadjusted risk. However, after adjustment, the association disappeared for clopidogrel. Aspirin given before the patient arrived in the catheterization laboratory was associated with a favorable outcome. The effect of aspirin pretreatment was pronounced during the first month with an adjusted risk for early ST reduced by half. Interestingly, treatment with glycoprotein IIB/IIIA blocker was not associated with the incidence of ST. Similarly, anticoagulation with only low-molecular weight heparin or with bivalirudin compared with standard heparin did not affect the adjusted risk of ST in this study.
Lesion and Stent Factors
Complex stenoses (B2/C) and restenotic lesions were independently associated with higher risk of ST, whereas bifurcations were not. Stenting of chronic total occlusions was strongly associated with early ST only. ST was more often detected in the left anterior descendent coronary artery than in the left circumflex coronary artery or in the right coronary artery. Left main stem stenting was uncommon (1.4%) and not associated with increased risk of ST compared with stenting in the left anterior descendent artery. Stenting in bypass graft (96% vein grafts) was associated with an overall increased adjusted risk, but this was not apparent during the first 30 days. Also, the diameter, but not the length, of the stent was highly inversely and independently associated with ST. For every 1-mm increase in stent diameter, the adjusted RR of ST was reduced by half.
Different Stents
There were considerable differences between the different stent brands both regarding the total risk of ST and regarding the timing of the events. For stents such as the Coroflex Blue and Driver, most of the ST occurred during the first weeks, whereas for most of the other stents the majority of ST occurred later. After adjustment, the differences were smaller but still considerable. Some of the stents had 2 to 2.5 times higher risk of ST compared with other stents. The differences between the different types of DES were small and statistically nonsignificant.
Drug-Eluting Versus BMS
Figure 2 illustrates the occurrence of ST when the different stents were classified as BMS or DES. There seemed to be different patterns of the event curves of these 2 types of stents. With BMS, the risk of ST was higher initially after stent implantation. When DES/BMS were used as a covariate in the statistical model instead of the different stents, the risk for ST was lower in the DES group compared with BMS both during the first month and when the entire period was analyzed (Table 2). After the first month, the event curves for DES had a higher slope than the BMS group resulting in crossing crude event curves at 1 year and a break-even at 2.5 years in the adjusted curves. After the recommended time of clopidogrel treatment (6 to 12 months), the incidence of ST was higher with DES compared with BMS. When only analyzing the time period after 6 months, the overall risk of reported ST was higher with DES with 198 ST reported in 25 302 DESs and 71 ST in 37 507 BMSs. The adjusted RR of ST in DES compared with BMS during this time period was 2.02 (99% CI, 1.30 to 3.14).
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Discussion |
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To our knowledge, this report from the national SCAAR registry is the largest single study to date on ST, reporting more than 880 events from more than 73 000 stent implantations.
It is important to note that the definition of ST in our material is not identical to the definition of ST in any of the different Academic Research Consortium definitions. The SCAAR registry describes angiographically documented stent occlusions with acute clinical presentation. This definition corresponds well with the Academic Research Consortium definition for "definite ST" but excludes nonocclusive ST, which may explain that the rate of ST is lower than in other observational studies such as the Rotterdam-Bern report4 and the EVASTENT registry.6 The timing and rates of ST in our study correspond well with the report from the Spanish ESTROFA registry.7 "Probable ST" according to the Academic Research Consortium definition cannot be calculated from our registry material without using additional information from hospital records. However, in this study, use of the probable ST definition would most likely have had minor impact on the analyses of predicting factors of ST. The SCAAR definition of ST may underestimate the risk of ST in patients treated in the left main stem because of the fact that these patients are more likely to die outside the hospital before arriving at the catheterization laboratory.
In accordance with previous observations,7 one of the strongest predictors of a subsequent ST in our study was an acute indication for stenting. Thus, stents implanted in patients with an acute ST-elevation myocardial infarction had a 2.5-fold increase in risk and stents implanted in patients with unstable coronary artery disease had a near-doubled risk of developing ST compared with stents implanted in patients with stable coronary artery disease. This relation was even more pronounced for ST occurring early after stenting. Among the patient-related covariables, insulin-treated diabetes had a strong relationship with ST as found in other studies.5,6 Noninsulin-treated diabetes had a weaker association with ST except for the first month, where there was a statistically nonsignificant trend.
We found a relationship between age and ST with a lower rate of reported ST with higher age. However, this was only seen in the elderly patients (80 years and older) which may not necessary relate to a lower rate of ST but may rather be explained by a lower reangiography rate in this subpopulation.
Similar to the study by Mishkel et al,18 we found a higher risk of ST in smokers than in nonsmokers and exsmokers. This relationship seems logical and is supported by the relationship between smoking and elevated levels of coagulation markers.19,20 In our study, the severity of coronary artery disease was not independently related to the incidence of ST. After multivariable adjustment, stents in patients with 3-vessel or left main disease did not occlude more often than stents in patients with 1- or 2-vessel disease. On the other hand, the number of implanted stents at the index PCI was positively related to the risk of ST in each of the stents. Other patient-related factors such as gender, history of hypertension, medically treated hyperlipidemia, and previous myocardial infarction had very little impact on the risk of ST. Stents in patients with a history of previous PCI showed a strong trend toward increased risk of subsequent ST in correspondence with the high risk in stented restenosis lesions in our study. Others have also described an increased risk for ST after stenting restenotic lesions.5 A history of previous coronary artery bypass grafting was also related to an increased risk of subsequent ST corresponding to the increased risk of stents in bypass grafts compared with stents implanted in the left anterior descending artery. We have not found documentation for such a relationship in previous studies. In our study, we analyzed >7000 stents implanted in patients with a history of previous coronary artery bypass grafting. Most of these stents were implanted in the native coronary arteries, but almost 2200 of them were placed in bypass grafts. This increased risk of stenting in previously coronary artery bypass grafting-treated patients, and especially of stents implanted in grafts was not seen during the first month after implantation but only after including the entire follow-up period in the analysis. This may suggest a different pathoanatomic explanation for ST in bypass grafts with fibrin deposition and graft deterioration rather than increased platelet activation. Complete revascularization was associated with a lower risk. The reason for this is unclear and needs further studies.
Antithrombotic Medication
Antithrombotic medication is of great importance reducing the risk of ST. In the SCAAR, it is unfortunately not possible to monitor coherence to medication after a patient has left the catheterization laboratory. In our registry, only medical treatment with aspirin, clopidogrel, and warfarin given any time 24 hours before PCI or during the intervention is registered and can be analyzed. In many patients, pretreatment includes medications given in the emergency ward or in the ambulance just before the patient arrives at the catheterization laboratory. Thus, the variable "clopidogrel before PCI" is broadly defined, which might explain why it was not found to be a predictor of the occurrence of subsequent ST in our study. On the other hand, aspirin treatment before PCI was strongly associated with a lower risk of ST. The risk for early ST was reduced by half in the group of patients treated with aspirin before PCI. Warfarin treatment before PCI was associated with a higher risk for ST possible reflecting less use of antiplatelet agents for an extended period of time in these patients. However, also the number of stents implanted in warfarin-treated patients was lower. Treatment with GPIIb/IIIa receptor blockers, low-molecular weight heparin, or bivalirudin in the catheterization laboratory was not associated with any change in risk.
Different Stents
After adjustment for differences in background and stent factors, there were still large differences between the different stents regarding the risk of subsequent ST. Most of the differences in the risk of ST between individual stents appeared during the first few months after PCI. The Coroflex blue stent and to some extent the Titan2 stent were outliers with higher incidences of ST than the other stents.
The time pattern for ST differed remarkably between different stents. With stents such as Coroflex Blue and Driver, most ST occurred during the early follow-up period, but in other stents such as Cypher and Taxus Liberté, most of the ST were seen later. Whether the differences in risk level and risk pattern could be explained by other factors than the different properties of the stents is unclear. Residual coronary dissections and underexpansion of stents are known predictors of early ST but that are unfortunately not recorded in the SCAAR and therefore not possible to adjust for in the multivariable models. Stents with thin struts, such as Coroflex Blue, have a reputation of good deliverability and may be used as a "bail-out stents" or mainly used in situations where other stents have failed. To test the potential difference of the use of Coroflex Blue as a routine stent or a selected stent for difficult situations, we analyzed this stent type in 2 groups of equal size. One group (48.6%) in which Coroflex Blue was routinely used (at least 18% of all stents) in a hospital during a 3-month period was compared with another group with a lower average usage. The unadjusted risk of ST was equally high in the frequent as in the infrequent usage groups (RR, 0.79; 95% CI, 0.40 to 1.55). The Kaplan-Meyer event curves for these 2 groups were almost identical (not shown). This finding suggests that the high risk of ST with the Coroflex Blue stent was not mainly explained by a selective use of the stent in patients at high risk. One may speculate that the thin struts and poor radial force may increase risk of early ST. However, the selection of stent types is based on many factors impossible to control in a database as the SCAAR. It is possible that most of the difference in reported ST with different stent brands is explained by these concealed confounders rather than properties of the different stents. Therefore, we emphasize that these findings must be considered hypothesis generating and that the cause-effect relationships has to be assessed in prospective randomized studies.
The differences between the different DESs were smaller. The event curves for Cypher, Endeavor, and Taxus Liberté stents were almost superimposed implying only small differences between these stents.
DES Compared With BMS
The difference in time pattern between DES and BMS is remarkable. During the early period, up to 6 to 12 months, both the absolute and adjusted rates of ST were higher in the BMS group compared with the DES group, but after 6 months, the rate was higher in the DES group. This corresponds well with the findings in both the Western Denmark PCI registry21 and the Bern-Rotterdam material.4 In meta-analyses of randomized studies; initially, a trend toward higher rates of ST with DES after clopidogrel treatment cessation was found. However, after changing the definition of this event type and not regarding reintervention as a censuring event, there was no longer any increased risk.22
Limitations
As with similar retrospective evaluations of registry data, there are inherent limitations in this type of study mainly regarding unknown confounders. Despite appropriate statistical adjustments, there might remain important differences in baseline characteristics and/or selection criteria that are unrecorded. Important patient-related factors such as discontinuation of clopidogrel treatment are not registered in SCAAR and can therefore not be included in the different statistical models. This factor may be of importance when DESs are compared with BMSs but not when different BMSs are compared with other BMSs or DES to DES. Unregistered factors related to deliverability in complex vessels or lesions are more important when different stents are compared with each other. These limitations are of great importance for the interpretation of our findings.
It is important to note that the definition of ST in our material is not identical to the Academic Research Consortium definitions of ST. The SCAAR definition corresponds to definite ST but excludes nonocclusive ST. However, this fact does not affect the comparisons between types or brands of stents.
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Conclusions |
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In this nation-wide registry study of >73 000 stents, 882 clinically angiographically documented ST were found. ST is multifactorial and the incidence varies considerably between patients based on clinical, vessel, and stent characteristics. Our findings highlight the need for further prospective randomized studies with head-to-head comparisons between different stents. For DES compared with BMS, the risk pattern was biphasic; initially, BMS demonstrated a higher risk of ST, whereas after the first months, ST risk was higher with DES.
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Acknowledgments |
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The Swedish Coronary Angiography and Angioplasty Registry and the Uppsala Clinical Research Centre are funded by the Swedish Association of Local Authorities and Regions and the Swedish Heart Foundation.
Disclosures
None.
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References |
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