Implantation of paclitaxel-eluting stent impairs the vascular compliance of arteries in porcine coronary stenting model

Atherosclerosis 202 (2009) 144–151

Implantation of paclitaxel-eluting stent impairs the vascular compliance of arteries in porcine coronary stenting model Serdar Farhan a , Rayyan Hemetsberger b , Johannes Matiasek b , Christoph Strehblow b , ¨ Petneh´azy c , Zsolt Petr´asi c , Alexandra Kaider d , Noemi Pavo b , Aliasghar Khorsand b , Ors Dietmar Glogar b , Kurt Huber a , Mariann Gy¨ongy¨osi b,∗ a

3rd Department of Medicine (Cardiology and Emergency Medicine) Wilhelminenhospital, Vienna, Austria b Department of Cardiology, Medical University of Vienna, Austria c Institute of Diagnostic Imaging and Radiation Oncology, University of Kaposvar, Hungary d Core Unit for Medical Statistics and Informatics, Medical University of Vienna, Austria Received 30 January 2008; received in revised form 7 April 2008; accepted 8 April 2008 Available online 29 April 2008

Abstract Background: The impaired compliance of large and medium-sized muscular arteries has been shown to correlate with the risk of adverse cardiovascular events. We assessed coronary artery distensibility using simultaneous intracoronary ultrasound and pressure wire measurements in porcine coronary arteries after implantation of paclitaxel-eluting (PES) and bare metal stents (BMS) and compared this with the histopathology of the arterial wall injury. Methods: PES and BMS were implanted into porcine left coronary arteries under general anesthesia. At 1-month follow-up (FUP) the endothelium-dependent and endothelium-independent vascular compliances were measured after intracoronary infusion of 10−6 M acetylcholine for 2.5 min, and intracoronary bolus of 100 ␮g nitroglycerine, respectively. The arterial stiffness index, distensibility and reflexion index were calculated in stented arteries (n = 25 PES and n = 25 BMS), and correlated with histopathologic and histomorphometric changes of the vessel wall. Results: In spite of smaller neointimal area, the fibrin deposition, medial thickening, vascular wall inflammation scores and arterial remodeling index were elevated and endothelialization was impaired in arteries with PES. Arteries with PES exhibited significantly worse endotheliumdependent vascular compliance: the stiffness (p < 0.001) and reflexion index (p < 0.001) were significantly higher and the distensibility index (p < 0.001) lower as compared with the arteries with BMS. The endothelium-independent vascular reaction was similarly impaired in arteries with PES, as the stiffness index (p < 0.001) and the distensibility index (p < 0.001) differed significantly between the PES and BMS groups. Incomplete endothelialization (r = 0.617, p < 0.001) was significantly associated with the endothelium-dependent increased vascular stiffness. The increased fibrin score (r = 0.646, p < 0.001), vessel wall inflammation (r = 0.657, p < 0.001) and medial thickening (r = 0.672, p < 0.001) correlated significantly with the endothelium-independent stiffness index. Conclusions: Implantation of PES impairs the coronary artery wall structure and the endothelium-dependent and independent vessel wall dynamics more than does the implantation of BMS. © 2008 Elsevier Ireland Ltd. All rights reserved. Keywords: Stents; Nitroglycerine; Acetylcholine; Vasodilation; Coronary pressure

1. Introduction ∗

Corresponding author at: Department of Cardiology, Medical University of Vienna, W¨ahringer G¨urtel 18-20, A-1090 Vienna, Austria. Tel.: +431 40400 4618; fax: +431 40400 4216. E-mail address: [email protected] (M. Gy¨ongy¨osi). 0021-9150/$ – see front matter © 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.atherosclerosis.2008.04.039

The impaired compliance of large and medium-sized muscular arteries has been shown to correlate with the risk of development of cardiovascular diseases [1–3]. The vasomotor functions (compliance and distensibility) of native human coronary arteries undergoing atherosclerotic remodeling

S. Farhan et al. / Atherosclerosis 202 (2009) 144–151

processes have also been investigated through intravascular and Doppler ultrasonographic techniques and coronary angiography [4–6]. However, the assessment of coronary pressure (measured by means of intracoronary pressure wire) offers a more attractive platform to analyze the pulse wave amplitudes of the coronary arteries directly, and to measure the endothelium-dependent or independent vasodilatory responses of the artery, being in direct association with the endothelial function and the endothelium-independent arterial compliance, respectively [7–9]. The characteristic features of an injured coronary artery after stent implantation include different degrees of fibrin deposition, local inflammation, media necrosis and hemorrhagia around the stent struts [10]. These changes occur earlier and are more pronounced after implantation of drug-eluting stents (DES) as compared with bare metal stents (BMS), even if DES have been shown to prevent restenosis and reduce the need for repeated revascularization [11–13]. In contrast, sirolimus stent implantation resulted in a long-term endothelial dysfunction in humans [6], while paclitaxel-eluting stents (PES)-induced vasoconstriction during exercise measured by quantitative angiography in humans [14]; both phenomena have been suggested to be in association with delayed healing and late thrombosis. Because the atherosclerotic process in large and small coronary arteries plays a central role in the pathogenesis of myocardial ischemia in coronary artery disease, early detection of vessel atherosclerosis and impaired vessel distensibility is of great importance. Accordingly, the aim of the present study was to investigate the endothelium-dependent and independent vascular compliance of coronary arteries after implantation of paclitaxel-eluting stents and BMS, in direct relation to the morphological injury of the vascular wall in porcine coronary arteries. 2. Material and methods Intracoronary stents (15 mm length, diameter of 2.75–3.5 mm) were implanted in 25 domestic pigs (weight 18–30 kg, 50% male) under general anaesthesia, in accordance with the guidelines of the preclinical experiments involving coronary stents [10,15,16]. The PES (biostable polymer-based moderate-release 2.0 ␮g/mm2 stent surface paclitaxel-eluting stainless steel stent with tubular slotted tube stent design, experimental stents, not for human use) or BMS (same stent design as the PES) was placed in the left anterior descending (LAD) or circumflex (LCx) coronary artery. The animals were randomized to receive the PES in the LAD or LCx, while the BMS was implanted in the corresponding other left coronary artery. The experiments were conducted in the Institute of Diagnostics and Oncoradiology, University of Kaposvar, Hungary in accordance with the “Principle of laboratory animal care” (NIH publication No. 86-23, revised 1985).

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2.1. Animal preparation and stent implantation All animals were pretreated with a loading dose of 100 mg aspirin and 300 mg clopidogrel 1 day before stent implantation. The pigs fasted overnight and the anesthesia was initiated and maintained as described previously [17]. After arteriotomy of the right femoral artery, coronary angiography and stenting was performed using standard methods as described previously [17]. During the 1-month followup (FUP) [10,15,16] the pigs received 100 mg aspirin and 75 mg clopidogrel per os each day. One month after stent implantation, coronary angiography, intravascular ultrasound (IVUS) (CVIS/Boston Scientific Corporation, San Jose, CA, USA) and pressure wire measurements (PressureWire, Radi Medical Systems, Uppsala, Sweden) were performed 5–10 mm distal to the stent, in order to record the coronary systolic–diastolic vasomotion and the pressure at the same site, not influenced by side branch or bifurcation. Subsequently, the animals were euthanized with 10 ml saturated potassium chloride. The hearts were then explanted and the coronary arteries were pressure fixed, dissected and embedded in Technovit 9100 (stented part) and paraffin (proximal and distal reference segments). 2.2. Quantitative angiography For the quantitative angiographic analysis, the FUP minimal lumen and reference diameters and the percentage diameter stenosis (%DS) of the stented segment were automatically calculated using the edge detection algorithm (ACOMPC, Siemens, Germany). 2.3. Intravascular ultrasound The IVUS catheter (Atlantis, Boston Scientific Corporation, San Jose, CA, USA) was tracked over the pressure wire up to a position distal to the stented segment. The IVUS examination of the artery was performed at 0.5 mm/s using a motorized pullback device (Boston Scientific Corporation). All IVUS studies were recorded on a super VHS tapes for offline analysis (TapeMeasure, Indec Corporation, Mountain View, CA, USA). The reproducibility and the intra- and interobserver variability of the intravascular ultrasound analyses in our laboratory have been detailed elsewhere [18]. 2.4. Pressure wire measurements Before introducing it into the coronary artery, the 0.014-in. pressure wire was calibrated. Subsequently it was introduced and positioned distal to the stent [7–9]. Coronary artery pressure was recorded at baseline for at least 1 min, and during maximal hyperemia. An injection of bolus of nitroglycerin (100 ␮g) was administered prior to pressure measurement and after flush of the artery with repeated bolus of 10 ml 0.9% NaCl solution and waiting for 10 min, intracoronary acetylcholine was given (10−6 M for 2.5 min) [6].

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Intracoronary pulse contour analysis was performed in offline mode, blinded to the type of the stent, using ImageJ software (ImageJ 1.30v, Wayne Rasband, National Institutes of Health, USA). The coronary pressure curves were calibrated and the systolic and diastolic pulse amplitudes, and the amplitudes of the reflected waves were measured after maximal vasodilation with intracoronary nitroglycerin and acetylcholine. 2.5. Coronary artery distensibility measurements The largest luminal area of the distal part of the coronary artery (distal to stent) was assessed by IVUS by tracing the lumen-intima border at peak diastole, and the smallest luminal area at peak systole within one cardiac cycle. The peak diastole and systole were determined by simultaneous recording of the electrocardiogram. Changes in intracoronary pressure during one cardiac cycle (P) were measured and documented by pressure wire. The distensibility index of the coronary artery was defined as [(A/A)/P] × 1000 mmHg−1 , where A represents the luminal area change between diastole and systole, and A represents the largest luminal area in diastole [19,20]. The dA/dP ration could be influenced by a change in blood pressure, therefore, another index, β, which is considered to be independent of changes in blood pressure, was obtained. This was done essentially by normalizing dimension change to the diastolic mean diameter according to the following equation: β = [ln(Psys /Pdias )]/(D/D), where Psys represents the systolic pressure, Pdias the diastolic pressure of the coronary artery, D the difference between systolic and diastolic lumen diameters, and D the diastolic luminal diameter [4,20]. For calculation of the reflexion index, the amplitudes of the reflected waves were measured [21]. All reported measurements represent the average of two independent observers from different cardiac cycles. 2.6. Histopathology and histomorphometry of the stented arteries The histopathology and histomorphometry of the explanted stented coronary arteries were performed blinded to the stent type in accordance with the relevant standards of preclinical studies [10,15,16]. Five sections of the stented arteries were stained with hematoxylin-eosin and analyzed histopathologically: proximal and distal reference segments approximately 5 mm proximal or distal to the stent, the proximal stent edge, the middle part of the stent (stent body) and the distal stent edge. Inflammatory response caused by the stents was analyzed by counting of chronic inflammatory cells 5 fields in randomly chosen around the stent and near the lumen. Fibrin deposition was analyzed by Ladewig staining. Endothelialization was evaluated by staining with von Willebrand factor. For each stent segment, scores (from 0 to 3) of vessel wall inflammation, fibrin deposition, hemorrhagia and, media necrosis were determined. 0 indicated “no changes”

and 3 reflected “the most severe changes”. Endothelialization was evaluated with a score system ranging between 3 (absent) and 0 (complete). Scoring of the histopathological alterations and mean injury score was determined according to Schwartz [15] and Schwartz et al. [16]. The quantitative analysis included the cross-sectional areas of the external elastic lamina (EEL), internal elastic lamina (IEL), and lumen of each stented section, and the lumen area of each non-stented sections, measured with digital morphometry. Neointimal thickness was measured as the distance from the inner surface of each stent strut to the luminal border. From the measured data, the following parameters were calculated: (1) percentage area stenosis (%AS) [(neointimal area/IEL area) × 100]; (2) remodeling index (EEL area of stent body/EEL area of proximal reference segment); (3) proximal edge effects (neointimal area at proximal reference/neointimal area of stent body); and (4) distal edge effects (neointimal area at distal reference/neointimal area of stent body) [15,16]. For each stent, the mean histopathological and histomorphometric values of the 3 stent sections were entered into the analysis. 2.7. Statistical analysis Continuous parameters in the PES and BMS stent groups were expressed as means ± standard deviation (S.D.). In case of skewed distribution and ordinal categorical variables, median values (quartiles) are given. Categorical variables were characterized as percentages. The continuous outcome variables were compared using analysis of variance (ANOVA). According to the randomized cross-over study design, the relevant factors (stent types and locations) were considered in the ANOVA. Skewed outcome variables were analyzed non-parametrically applying the Wilcoxon Rank Sum test. The p-values yielded by the multiple comparisons were corrected for multiplicity by using the method of Bonferroni–Holm. This procedure controls the overall type I error at the level of 5%. The correlation between two parameters was calculated by using the Pearson correlation coefficient. With focus on the outcome variables, the following separate analytical groups were created: histopathological scores, histomorphometric parameters, endothelium-dependent and endothelium-independent vascular compliance parameters and correlations between the coronary compliance and histological data. The statistical analyses were performed with SPSS for Windows version 11.5.

3. Experimental results The mean stent diameter for the PES and BMS were 3.0 ± 0.1 mm and 3.0 ± 0.2 mm. Coronary angiography revealed an in-stent stenosis with neointimal hyperplasia >50% DS in 4 PES and 6 BMS.

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3.1. Histopathological and histomorphometric results

4. Discussion

One month after implantation of the PES and BMS, histopathology revealed significantly more fibrin deposition, and higher vascular wall inflammation scores in the arteries with PES as compared with those with BMS (Table 1). Infiltration with significantly more chronic inflammatory cells was observed around the stent struts in PES (14 ± 2 cells/field vs. 3 ± 2 cells/field, p < 0.001), with similar distribution near the lumen. Von Willebrand staining indicated more complete endothelialization in the BMSs. The injury score was similar in the stent groups. PES implantation resulted in smaller neointimal area, as compared with BMS. In contrast, the media area and remodeling index were significantly larger in the PES group as compared with BMSs. Strong trends towards significantly worse edge effects but better lumen encroachment with smaller neointimal thickness und %AS were observed in PES.

Our study demonstrates that the stent-related structural impairment of the coronary vascular wall is associated with higher arterial wall stiffness, and lower vascular distensibility. The presented results highlight that coronary compliance may be severely impaired in the coronary sites with minimal neointimal hyperplasia and showing excellent long-term angiographic results after stenting [22].

3.2. Coronary artery compliance Intracoronary infusion of both nitroglycerine and acetylcholine caused significantly more pronounced coronary vessel vasodilatation and better arterial compliance parameters in arteries with BMS (Tables 2 and 3). Fig. 1 highlights the vasomotor responses of the coronary segments to nitrates and acetylcholine in both stent groups (Fig. 1). Typically, implantation of the PES was associated with more fibrin deposition and inflammation around the stent struts, but paradoxically less neointima, as compared with the BMS. The alteration in the arterial wall structural components resulting in vessel rigidity led to malfunctioning of the endothelium-dependent and independent vasodilation response with decreased pulse pressure amplitudes (Fig. 1). 3.3. Association between the histological findings and the vessel compliance parameters The endothelium-dependent vascular compliance parameters exhibited a significant positive correlation with the score for incomplete endothelialization, as the correlation with the stiffness index was r = 0.617, p < 0.001, and also with the reflexion index that was r = 0.577, p < 0.01. A weak negative correlation was observed between incomplete endothelialization and the distensibility index: r = −0.358, p < 0.05. The endothelium-dependent arterial stiffness index correlated with the vessel remodeling (r = 0.649, p < 0.001). The endothelium-independent stiffness index exhibited a significant association with the degree of fibrin deposition (r = 0.646, p < 0.001), vessel wall inflammation (r = 0.657, p < 0.001) and medial thickening (r = 0.672, p < 0.001).

4.1. Vascular wall structural changes after implantation of PES Histopathology of arteries stented with Taxus stent have shown fibrin deposition surrounding the stent struts, which persisted up to 8 months after stenting, as a sign of delayed healing [14,23]. Similarly, our study demonstrated that the extents of fibrin deposition, vascular wall inflammation, medial thickening and positive remodeling were more pronounced in the arteries with PES implantation. The relatively high dose of paclitaxel in the stents presented in our study might also contribute to the unfavorable histopathology of the traumatized vessel wall in spite of the favorable histomorphometric results.

4.2. Pressure wire as a tool of arterial compliance measurements Pulse contour analysis provides measurements that capture both capacitive (storage) and cushioning (oscillatory) arterial functions. The augmentation and arterial stiffness index and the pulse pressure parameters are used to determine large arterial vascular elasticity, stiffness and strain, while reflexion index is established for characterization of the small vessel smooth muscle tone. Increased arterial stiffness is associated with sustained elevations in blood pressure and accelerated atherosclerosis, arterial smooth muscle hyperplasia and hypertrophy [24]. As the coronary artery pressure curve differs from that of large-, mediumsized- and small peripheral arteries (due to increased filling of the coronary arteries during diastole, normally lower compliance than for the other arteries, and interaction of myocardial muscle contraction and intramural blood vessel elasticity) [2,25], the usual pulse wave analysis and their normal values may not be fully applied to the coronary vessels. Thus, we limited the usual pulse contour analysis of the coronary pressure curves displayed by pressure wire to the measurements of the pulse amplitudes and amplitudes of the reflexion wave. Nevertheless, the combination of pressure wire with IVUS allows the simultaneous observation of the coronary dimensions and pressure at the same site.

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Table 1 Histopathological and histomorphometric results of stented arteries 1 month after bare metal stent (BMS) and paclitaxel-eluting stent (PES) implantation PES (n = 25) Histopathology Fibrin depositiona Endothelializationc Vessel wall inflammationa Injury scorea Media necrosisa Hemorrhagiaa Histomorphometry Lumen area (mm2 ) Neointima area (mm2 ) IEL area (mm2 ) Media area (mm2 ) EEL area (mm2 ) Adventitial area (mm2 ) %Area stenosis (%) Maximal neointimal thickness (mm) Remodeling index Proximal edge effect Distal edge effect

1.63 ± 0.41 0.33 (0;0.5) 1.23 ± 0.4 1.39 ± 0.28 0.23 ± 0.38 0.51 ± 0.6 2.4 ± 0.89 1.1 ± 0.61 3.46 ± 0.72 1.73 ± 0.62 5.22 ± 1.1 6.11 ± 1.06 29.6 ± 11.6 0.28 ± 0.16 1.46 ± 0.37 0.68 (0.03;0.98) 0.26 (0.04;0.30)

BMS (n = 25)

p-Value

0.83 ± 0.45 0 (0;0.33) 0.56 ± 0.48 1.33 ± 029 0.12 ± 0.3 0.58 ± 0.76

<0.001b <0.001b <0.001b 0.249 0.141 0.345

2.02 ± 0.53 1.53 ± 0.69 3.55 ± 0.52 0.98 ± 0.43 4.63 ± 0.62 5.82 ± 0.46 35.2 ± 10.7 0.36 ± 0.12 1.0 ± 0.25 0.27 (0.04;0.31) 0.13 (0.02;0.015)

0.039 0.001b 0.305 <0.001b 0.012 0.11 0.019 0.03 <0.001b 0.015 0.017

IEL = internal elastic lamina, EEL = external elastic lamina, n.s. = non-significant. Values are expressed as means ± S.D. or medians (first quartiles). Uncorrected p-values are reported. a Score between 0 and 3 (no to most severe). b Represent significant differences between the stents after Bonferroni–Holm multiplicity test corrections. c Score between 0 and 3 (complete to no endothelialization). Table 2 Endothelium-dependent changes (after 10−6 M intracoronary infusion of acetylcholine for 2.5 min) in vascular dimensions, pressures and coronary vascular compliance of stented arteries with bare metal stent (BMS) and paclitaxel-eluting stent (PES) Endothelium-dependent vascular compliance

PES (n = 25)

Pressure-systole (mmHg) Pressure-diastole (mmHg) Lumen diameter diastole (mm) Systolic-diastolic change in lumen diameter (mm) β stiffness index Lumen area diastole (mm2 ) Systolic-diastolic change in lumen area (mm2 ) Distensibility index (mmHg−1 ) Pulse amplitude (mmHg) Reflexion index (%)

113 72.5 2.66 0.41 1.42 6.87 0.42 1.05 33.4 72.4

± ± ± ± ± ± ± ± ± ±

8 7.8 0.12 0.13 0.28 0.44 0.07 0.22 5.5 6.8

BMS (n = 25)

p-Value

± ± ± ± ± ± ± ± ± ±

0.867 0.558 <0.001a <0.001a <0.001a <0.001a <0.001a <0.001a 0.022 <0.001a

115 71.8 2.83 0.69 0.91 7.61 0.79 1.83 38.8 61.5

7 6.5 0.11 0.12 0.23 0.48 0.31 0.24 4.7 5.2

Values are expressed as means ± S.D. Uncorrected p-values are reported. a Represent significant differences between the stents after Bonferroni–Holm multiplicity test corrections. Table 3 Endothelium-independent (after 100 ␮g intracoronary nitroglycerine) changes in vascular dimensions, pressures and coronary vascular compliance of stented arteries with bare metal stent (BMS) and paclitaxel-eluting stent (PES) Endothelium-independent vascular compliance

PES (n = 25)

Pressure-systole (mmHg) Pressure-diastole (mmHg) Lumen diameter diastole (mm) Systolic-diastolic change in lumen diameter (mm) ␤ stiffness index Lumen area diastole (mm2 ) Systolic-diastolic change in lumen area (mm2 ) Distensibility index (mmHg−1 ) Pulse amplitude (mmHg) Reflexion index (%)

117 76.2 2.70 0.38 1.32 6.76 0.45 1.10 33.2 73.1

± ± ± ± ± ± ± ± ± ±

9 5.6 0.11 0.16 0.23 0.52 0.16 0.13 10.5 6.3

Values are expressed as means ± S.D. or medians (first quartiles). Uncorrected p-values are reported. a Represent significant differences between the stents after Bonferroni–Holm multiplicity test corrections.

BMS (n = 25)

p-Value

± ± ± ± ± ± ± ± ± ±

0.783 0.257 <0.001a <0.001a <0.001a 0.001a <0.001a <0.001a 0.026 0.002a

119 70.0 2.82 0.68 0.96 7.32 0.79 1.61 49.1 67.2

9 5.4 0.12 0.14 0.22 0.44 0.22 0.21 10.3 5.1

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Fig. 1. Local responses of the coronary arteries to bare metal (BMS) and paclitaxel-eluting stent (PES) implantation with corresponding histopathological changes. (A) Moderate vs. small neointimal hyperplasia after implantation of BMS vs. PES. Fibrin accumulation around struts with severe inflammation and granulomatous reaction at a single stent strut in the coronary artery with PES (arrows). (B) Coronary pulse pressure after nitroglycerine administration presenting increased pulse pressure amplitude and higher vascular distensibility to vasodilatators in arteries with BMS as compared with PES. X axis: time in s, Y axis: mmHg. (C) Vasoreactive response of the stented arteries after intracoronry infusion of actylcholine; in contrast with BMS, PES implantation resulted in lower distensibility and pulse pressure amplitudes. X axis: time in s, Y axis: mmHg.

4.3. Endothelium-dependent and independent vasodilatory responses of the stented coronary arteries Dysregulation of the balance between the production and degradation of collagen and elastin of the vascular wall, mainly by stimulation of an inflammatory milieu contributes to vascular stiffness, increase in the intima-medial thickness and vascular smooth muscle hypertrophy [1]. The pathomorphological changes documented for large elastic arteries were also found in the stented coronary arteries in our experiments, in the form of increased fibrin deposition, infiltration of the vessel wall with inflammatory cells, vessel remodeling and impaired endothelialization. The delayed endothelialization was associated with a higher

endothelium-dependent vascular stiffness and increased reflexion indices. Similarly to the findings of others [4–6], we also observed that the structural and functional abnormalities of the coronary arteries with arterial remodeling determine the local endothelium-dependent vascular response. Further pathological structural elements, such as fibrin deposition, vessel wall inflammation and medial thickening, correlated with the endothelium-independent vascular stiffness component. Thus, quantitative assessment of vessel compliance would be an alternative method of predicting the extent of atherosclerotic damage of coronary arteries, including not only the magnitude of lumen encroachment but also the functional injury of the vessel beyond the stents. Nakatani et al.

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reported that the elastic property of the LAD in humans was impaired even in the absence of angiographically significant stenosis [19]. It has also been shown, that impaired coronary endothelium-dependent vasodilatation aggravated the functional and structural changes of coronary arteries which further accelerates atherogenesis, promotes plaque rupture, increases in vasoconstriction, decreases the dilator reserve and thus promotes thrombogenesis [26]. Accordingly, the vascular wall changes might be related to development of late thrombosis. 4.4. Limitations It has been shown that the contiguous conduit and resistance arteries of the coronary arterial bed display different responses to the vasodilatory stimuli of acetylcholine: resistance vessel constriction was observed with increasing severity of hypertension [27]. However, in our study the animals were all normotensive. In addition, with the combined use of pressure wire with intravascular ultrasound, we could directly associate the vascular compliance parameters with the histopathological findings. Naturally, the vascular injury in the coronary arteries of healthy pigs differs from that in human atherosclerotic arteries. However, experimental animal studies with coronary arterial injury caused by stent overdilatation suggest an important relationship between inflammation, vascular injury and neointimal growth, which is similar to that of human coronary arteries [28].

5. Conclusion Our study has demonstrated the ability to detect and monitor impaired pulsatile function of the coronary arteries after stenting. The presented structural and functional alterations of the vascular wall are accompanied by reduced vasodilatory capacity, which may be of clinical importance in long-term outcome after stenting, and particularly after DES implantation.

Acknowledgments We thank Ewald Schober for the kind assistance with the pressure wire measurements. The study was supported by Verein zur F¨orderung der Forschung-ATVB, B and P Research GmbR and Verein zur F¨orderung der Forschung im Bereich der experimentellen und klinischen Kardiologie, Vienna, Austria.

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