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Relationship between aspirin/clopidogrel resistance and intra-stent thrombi assessed by follow-up optical coherence tomography after drug-eluting stent implantation

Junbeom Park, Dong-Ho Shin, Byeong-Keuk Kim, Ae-Young Her, Yong Hoon Kim, Hyun Hee Choi, Jung-Sun Kim, Young-Guk Ko, Donghoon Choi, Yangsoo Jang, Myeong-Ki Hong
DOI: http://dx.doi.org/10.1093/ehjci/jet088 1181-1186 First published online: 8 May 2013

Abstract

Aims No data exist regarding the relationship between aspirin/clopidogrel resistance and intra-stent thrombi on follow-up optical coherence tomography (OCT) after drug-eluting stent (DES) implantation. The purpose of this study was to evaluate the relationship between aspirin/clopidogrel resistance and intra-stent thrombi on the follow-up OCT in DES-treated patients.

Methods and results A total of 308 DES-treated patients who underwent follow-up OCT and simultaneous measurement of aspirin reaction unit (ARU) and P2Y12 reaction unit (PRU) using the VerifyNow assay system were selected for the study. Aspirin and clopidogrel resistance were defined as ARU ≥550 and PRU ≥275, respectively. Intra-stent thrombi were detected in 29 patients (9.4%). The mean time interval from DES implantation to OCT was 195 ± 133 days (202.9 ± 103.0 days for patients with intra-stent thrombi vs. 194.7 ± 136.0 days for patients without intra-stent thrombi; P = 0.750). There were no significant differences between patients with and without intra-stent thrombi with regard to the incidence of aspirin resistance (13.8 vs. 11.1%, respectively; P = 0.630) or clopidogrel resistance (72.4 vs. 50.5%, respectively; P = 0.056). The percentage of uncovered struts was 17.9 ± 15.8% in patients with intra-stent thrombi and 12.7 ± 17.3% in patients without intra-stent thrombi (P = 0.098). Stent length was significantly longer in patients with intra-stent thrombi (22.9 ± 6.0 vs. 19.4 ± 5.0 mm, P = 0.006). Multivariate logistic regression analysis showed that stent length (odds ratio = 1.152, 95% confidential interval 1.025–1.295; P = 0.017) was the only independent risk factor for the presence of intra-stent thrombi on OCT.

Conclusion This OCT study suggested that the presence of intra-stent thrombi may not be associated with aspirin/clopidogrel resistance in DES-treated patients.

  • Platelet resistance
  • Optical coherence tomography
  • Intra-stent thrombi

Introduction

Pathological studies reported that the main mechanism of late stent thrombosis following drug-eluting stent (DES) implantation was delayed and poor endothelial coverage over the stent struts.1,2 A recent optical coherence tomography (OCT) study showed that the presence of an uncovered stent strut was an associated risk factor for late stent thrombosis following DES implantation.3 Other factors known to have an effect on thrombus formation include the pharmacodynamic response of antiplatelet agents, type of stent implanted, and the anatomic characteristics of lesions.4,5 Previous studies reported that the occurrence of stent thrombosis in DES-treated patients was significantly associated with a poor response of the platelets to clopidogrel.6,7 However, there are no published data to evaluate the relationship between the platelet response to antiplatelet therapy and intra-stent thrombi detected by follow-up OCT after DES implantation. Therefore, this relationship between the platelet response to antiplatelet therapy and the formation of intra-stent thrombi following DES implantation was evaluated.

Methods

Study population

A total of 308 patients who underwent the follow-up OCT and the simultaneous aspirin and clopidogrel resistance test by the VerifyNow-P2Y12 assay after DES implantation between August 2009 and May 2012 were selected from the OCT registry database of our institute. The choice of DES was made by the operators at the time of implantation and included 107 sirolimus-eluting stents (Cypher™, Cordis, Miami, FL, USA), 17 paclitaxel-eluting stents (Taxus™, Boston Scientific, Natick, MA, USA), 66 zotarolimus-eluting stents (Endeavor Sprint™, Medtronic, Santa Rosa, CA, USA), 88 biolimus-eluting stents (Nobori®, Terumo Corporation, Tokyo, Japan), and 30 everolimus-eluting stents (Xience™, Abbott Vascular, Santa Clara, CA, USA). DES implantation was performed using conventional techniques. Unfractionated heparin was administered as an initial bolus of 100 IU/kg, with additional boluses administered during the procedure to achieve an activated clotting time of 250–300 s. Dual antiplatelet therapy (aspirin and clopidogrel) was given to all patients until the follow-up OCT was performed. Patients were eligible for the study if there were no specific exclusion criteria: (i) untreated significant left main coronary artery disease; (ii) congestive heart failure (left ventricular ejection fraction <30%); (iii) renal insufficiency (baseline creatinine ≥2.0 mg/dL); (iv) bifurcation lesions treated with stent implantation in both the main vessel and the side branch; (v) lesions treated with a bare-metal stent; (vi) lesions with an incomplete evaluation of the stent with OCT; (vii) poor OCT image quality; and (viii) aspirin reaction unit (ARU) and P2Y12 reaction unit (PRU) were not checked at the same time as the follow-up OCT. This study was approved by the Institutional Review Board of our institute, and written informed consent was obtained from each patient.

OCT procedure and analysis

OCT was performed with two OCT systems (Model M2 Imaging System and C7-XR™ Imaging System, LightLab Imaging, Inc., St Jude Medical, St Paul, MN, USA). In the Model M2, the occlusion catheter was positioned proximal to the stent, and a 0.014-inch wire-type imaging catheter was positioned distal to the stent. During image acquisition, the occlusion balloon (Helios, Avantec Vascular Corp., CA, USA) was inflated to 0.4–0.6 atm, and lactated Ringer's solution was infused at 0.5–1.0 mL/s. The imaging wire was pulled from the distal to the proximal with a motorized pull-back system at 1.0 mm/s.8 The frequency-domain OCT system (Model C7-XR™) was developed to generate frames at much higher rates and thus allow faster pull-back speeds. OCT images were generated at 100 frames/s while the catheter was pulled back at 20 mm/s. A non-occlusive contrast medium was continuously flushed through a guiding catheter at a rate of 4–5 mL/s for 3–4 s. Continuous images were acquired and stored digitally for subsequent analysis.

All OCT images were analysed at a core laboratory (Cardiovascular Research Center, Seoul, Korea) by analysts who were blinded to patient and procedural information. Cross-sectional OCT images were analysed at 1-mm intervals. Stent and luminal cross-sectional areas (CSA) were measured, and neointimal hyperplasia (NIH) CSA was calculated as the stent CSA minus the luminal CSA. The mean values were reported in this study. The percentage of NIH CSA was calculated as NIH CSA × 100/stent CSA. NIH thickness (i.e. the distance between the endoluminal surface of neointima and the strut), was measured inside the struts with a line as perpendicular as possible to the neointima and strut.9 An uncovered strut was defined as having the NIH thickness of 0 µm.9,10 A malapposed strut was defined as a strut that was detached from the vessel wall (Cypher™ ≥160 μm; Taxus™ ≥130 μm; Endeavor® ≥110 μm; Xience™ ≥100 μm; and Nobori® ≥130 μm).1012 The percentage of malapposed or uncovered struts in each stented lesion was calculated as (the number of malapposed or uncovered struts/total number of struts in all cross-sections of the lesion) × 100, respectively. Stent struts across major side branches (diameter ≥2 mm) were excluded from analysis. The inter- and intra-observer agreement for the measurement of NIH thickness and CSA in our laboratory were reported in a previous study.13 Intra-stent thrombi were defined as signal-rich, low-backscattering protrusions (= white thrombi), or high-backscattering protrusions inside the artery lumen with signal-free shadowing (= red thrombi) on OCT images (Figure 1).14,15

Figure 1

Representative optical coherence tomographic image of intra-stent thrombi (arrow).

Quantitative coronary angiography analysis

Quantitative coronary angiography analysis was performed using an offline computerized quantitative coronary angiographic system (CASS system, Pie Medical Imaging, Maastricht, The Netherlands) pre- and post-DES implantation, and the follow-up was performed in an independent core laboratory (Cardiovascular Research Center, Seoul, Korea). The minimal lumen diameter of treated coronary lesions and reference diameter were measured in the view that was the narrowest and not foreshortened.

Platelet function test

Blood samples for platelet function tests were obtained from patients in the catheterization room through the indwelling femoral or radial artery sheath before the administration of heparin and follow-up OCT. Blood samples were collected in Greiner Bio-One 3.2% citrate Vacuette tubes (Greiner Bio-One, Kremsmünster, Austria).16 Blood samples were kept at room temperature for, at least, 30 min before platelet function testing and used for testing within 3 h of blood collection. The platelet function tests were performed by experienced laboratory personnel blinded to patients' clinical data. The VerifyNow system (Accumetrics, San Diego, CA, USA) was used for measuring platelet function.16,17 The arachidonic acid-induced platelet aggregation was quantified with ARU, and platelet reactivity to adenosine diphosphate was quantified as PRU.16 Aspirin resistance was defined as ARU >550,18 and clopidogrel resistance was defined as PRU >275.16

Statistical analysis

Statistical analysis was performed using the SAS software (SAS 17.0., SAS Institute, Cary, NC, USA). Categorical data were presented as numbers and percentages, and compared using the χ2 statistic or Fisher's exact test. Continuous data were presented as mean ± standard deviation, and compared with Student's t-test. If the distributions were skewed, a non-parametric test was used. To determine the independent risk factors for intra-stent thrombi, multivariate logistic regression analysis was performed. Univariate variables with a P-value <0.1 and factors associated with the presence of intra-stent thrombi in a prior study (stent length, stent diameter, and uncovered struts)13 were entered into the multivariate logistic regression analysis. A P-value <0.05 was considered statistically significant.

Results

Intra-stent thrombi were detected in 29 patients (9.4%): white thrombi were observed in all patients without red thrombi. Baseline clinical characteristics between patients with and without intra-stent thrombi are listed in Table 1. Diabetes mellitus was significantly associated with a higher incidence of intra-stent thrombi compared with no intra-stent thrombi (51.7 vs. 27.9%, respectively; P = 0.008). Quantitative coronary angiographic analysis and follow-up OCT findings are shown in Table 2. The mean-time interval from DES implantation to OCT was 195 ± 133 days (202.9 ± 103.0 days for patients with intra-stent thrombi vs. 194.7 ± 136.0 days for patients without intra-stent thrombi; P = 0.750). Patients treated with the first generation DESs (Cypher™ and Taxus™) seemed to have more intra-stent thrombi than those treated with second generation DESs, but not statistically significant (12.6 vs. 7.0%, P = 0.096). Stent length was significantly longer in patients with intra-stent thrombi (22.9 ± 6.0 vs. 19.4 ± 5.0 mm, respectively; P = 0.006). The percentage of uncovered struts was greater in patients with intra-stent thrombi (17.9 ± 15.8 vs. 12.7 ± 17.3%, respectively; P = 0.098). There were no significant differences in the incidence of aspirin resistance between patients with and without intra-stent thrombi (13.8 vs. 11.1%, respectively; P = 0.630) or clopidogrel resistance between patients with and without intra-stent thrombi (72.4 vs. 50.5%, respectively; P = 0.056; Table 3). Independent risk factors for the presence of intra-stent thrombi on multivariate logistic regression analysis are shown in Table 4. Stent length (odds ratio = 1.152, 95% confidential interval 1.025–1.295; P = 0.017) was the only independent factor for the presence of intra-stent thrombi on OCT.

View this table:
Table 1

Baseline clinical characteristics

No intra-stent thrombiIntra-stent thrombiP-value
No. of patients27929
Age (years)62.3 ± 8.961.9 ± 8.30.818
Male (%)190 (68.1)18 (62.1)0.509
Diabetes mellitus (%)78 (27.9)15 (51.7)0.008
Hypertension (%)172 (61.5)16 (55.2)0.507
Dyslipidaemia (%)150 (53.6)13 (44.8)0.376
Current smoker (%)102 (36.7)7 (24.1)0.177
Acute coronary syndrome (%)94 (33.8)9 (31.0)0.767
Prior myocardial infarction (%)17 (6.2)3 (10.3)0.401
Prior PCI (%)29 (10.5)4 (13.8)0.591
Medications at follow-up OCT (%)
 Aspirin279 (100)29 (100)
 Clopidogrel279 (100)29 (100)
 ACEI57 (20.3)10 (35.7)0.187
 Statin254 (91.1)29 (100)0.243
 Beta-blocker209 (74.8)23 (78.6)0.757
  • ACEI, angiotensin-converting enzyme inhibitor; PCI, percutaneous coronary intervention; OCT, optical coherence tomography.

View this table:
Table 2

Quantitative coronary angiographic analysis and follow-up OCT findings

No intra-stent thrombi (n = 279)Intra-stent thrombi (n = 29)P-value
Target vessel (%)0.407
 Left anterior descending artery161 (57.7)19 (65.5)
 Left circumflex artery49 (17.6)6 (20.7)
 Right coronary artery69 (24.7)4 (13.8)
Quantitative coronary angiographic analysis
 Stent diameter (mm)3.2 ± 0.33.0 ± 0.40.028
 Stent length (mm)19.4 ± 5.022.9 ± 6.00.006
 Lesion length (mm)18.3 ± 6.221.2 ± 7.50.117
 Proximal reference vessel diameter (mm)3.3 ± 0.43.1 ± 0.50.029
 Distal reference vessel diameter (mm)2.9 ± 0.52.6 ± 0.60.029
 Pre-intervention minimal lumen diameter (mm)1.0 ± 0.50.9 ± 0.40.400
 Post-intervention minimal lumen diameter (mm)2.8 ± 0.42.6 ± 0.50.182
 Follow-up minimal lumen diameter (mm)2.5 ± 0.42.4 ± 0.50.351
Follow-up OCT
 Time interval from stent implantation to OCT (days)194.7 ± 136.0202.9 ± 103.00.750
 Mean stent cross-sectional area (mm2)4.5 ± 3.43.7 ± 3.70.244
 Mean lumen cross-  sectional area (mm2)4.1 ± 3.33.4 ± 3.50.297
 Mean NIH cross-sectional area (mm2)0.4 ± 3.30.3 ± 0.40.862
 Percentage of NIH cross-  sectional area (%)11.4 ± 10.79.0 ± 8.10.256
 Mean NIH thickness (μm)106 ± 9479 ± 570.040
 Percentage of uncovered struts (%)12.7 ± 17.317.9 ± 15.80.098
 Percentage of malapposed struts (%)1.6 ± 5.11.8 ± 3.30.843
  • NIH, neointimal hyperplasia; OCT, optical coherence tomography.

View this table:
Table 3

Aspirin or clopidogrel resistance in patients with and without intra-stent thrombi

No intra-stent thrombi (n = 279)Intra-stent thrombi (n = 29)P-value
Aspirin reaction unit (ARU)451.0 ± 65.0455.0 ± 67.70.757
P2Y12 reaction unit (PRU)276.3 ± 80.3287.4 ± 86.90.522
Aspirin resistance (%)31 (11.1)4 (13.8)0.630
Clopidogrel resistance (%)141 (50.5)21 (72.4)0.056
View this table:
Table 4

Independent risk factors for the presence of intra-stent thrombi on multivariate logistic regression analysis

Univariate analysisMultivariate analysis
OR95% CIP-valueOR95% CIP-value
Stent length1.1331.033–1.2430.0081.1521.025–1.2950.017
Diabetes mellitus2.7671.277–5.9950.010
Stent diameter0.2650.080–0.8780.030
Clopidogrel resistance2.3880.956–5.9680.059
Percentage of uncovered struts1.0160.996–1.0370.098
Use of sirolimus- or paclitaxel-eluting stentsa1.9070.883–4.1180.100
Mean neointimal hyperplasia thickness0.9580.904–1.0160.153
Post-intervention minimal lumen diameter0.4580.146–1.4410.182
Hypertension0.7710.357–1.6650.508
Male0.7670.347–1.6910.510
Aspirin resistance1.3170.429–4.0470.630
Acute coronary syndrome0.8810.382–2.0340.767
Age0.9950.953–1.0390.818
  • aCypher™ and Taxus™, respectively.

Discussion

This study showed that OCT-detected intra-stent thrombi were observed in 9.4% of DES-treated patients. The only independent risk factor for the presence of intra-stent thrombi was stent length. This is the first study to show no significant relationship between the aspirin/clopidogrel resistances and the development of intra-stent thrombi.

The presence of intra-stent thrombi may be associated with the extent of neointimal coverage over the stent struts. An angioscopic study reported that the incidence of intra-stent thrombi and the extent of incomplete neointimal coverage were greater in sirolimus-eluting stent-treated patients than in bare-metal stent-treated patients.19 Previous OCT studies also showed that the percentage of uncovered struts was greater in the lesions with intra-stent thrombi than in the lesions without intra-stent thrombi.13,20 Intra-stent thrombi were observed in 9.4% of the enrolled patients in this study. The reported incidence of intra-stent thrombi varied from 14 to 40%, depending on the imaging modality (OCT or angioscopy), type of DES, and the time interval from stent implantation to follow-up imaging studies.13,1922 Stent length was the only independent risk factor for the presence of intra-stent thrombi in this study, and this finding was consistent with the findings from previous OCT studies.13,20

Interestingly, intra-stent thrombi still occurred despite continuous, dual antiplatelet treatment up until the follow-up OCT examination was performed. Therefore, dual antiplatelet therapy did not completely suppress intra-stent thrombi formation, and factors such as uncovered struts, antiplatelet resistance, and flow dynamics may influence intra-stent thrombi formation.13,20 The relationship between intra-stent thrombi and antiplatelet resistance was evaluated in this study. Although there was a greater tendency towards clopidogrel resistance in patients with intra-stent thrombi in the univariate analysis, statistical significance for clopidogrel resistance was not achieved in the multivariate logistic regression analysis.

One study reported that high on-treatment platelet reactivity (>235 PRU) measured with the VerifyNow assay is associated with post-discharge events after DES implantation, including stent thrombosis.6 However, other studies showed that an ideal cut-off value to predict a risk of cardiovascular events was a PRU >208.23,24 A PRU value ≥275 by the VerifyNow assay was identified as the optimal cut-off value to predict 30-day major adverse cardiovascular events in DES-treated Korean patients.16 Even though a higher PRU value for clopidogrel resistance was used in this study, the clopidogrel resistance was still observed in more than half of the enrolled patients. The high incidence of clopidogrel resistance in this study might be partly explained by the prevalent cytochrome P450 2C19 loss-of-function polymorphism among Korean patients: this polymorphism which is associated with poor clopidogrel metabolism and its resistance was reported as high as 50–60% in Korean patients.25,26 The optimal method to quantify platelet reactivity and the threshold definition for high on-treatment platelet reactivity to adenosine diphosphate have been controversial.27,28 Regardless of measuring methods, there is a large intra-individual variability in platelet reactivity.29 In addition, platelet reactivity is influenced by various factors such as patient compliance, presence of other medications, and tobacco use.30 Despite Bonello et al.28 proposed criteria for high on-treatment platelet reactivity, they have very low positive predictive values. The variability of platelet reactivity within each patient over time likely explains the results in this study. Therefore, routine use of platelet function testing to guide clinical practice in patients undergoing elective DES implantation is not recommended.31,32

Study limitations

This was a non-randomized and retrospective study, which is inherently associated with the possibility of some selection bias. Additionally, post-intervention OCT and platelet function test data were not available.

Conclusion

During maintenance of dual antiplatelet treatment, aspirin/clopidogrel resistance may not influence the formation of intra-stent thrombi detected on the follow-up OCT in DES-treated patients.

Funding

This study was supported by grants from the Korea Healthcare Technology R&D Project, Ministry for Health, Welfare & Family Affairs, Republic of Korea (no. A085012).

Conflict of interest: none declared.

Footnotes

  • The first two authors contributed equally to this paper.

References

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