Postdilatation following Coronary Stent Deployment: Lesion and Procedural Characteristics Associated with an Increase in Stent D

First-generation balloon-expandable stents such as the Palmaz-Schatz stent were often deployed at low pressure using compliant delivery balloons. Overhang of the deployment balloon outside the stent increased the risk of edge dissection when high-pressure inflation was used. Intravascular ultrasound (IVUS) studies showed that stents deployed in this fashion were often underexpanded and postdilatation with higher pressures or larger balloons was required to optimize stent expansion.¹ Postdilatation following stent implantation was shown to increase the mean stent minimal lumen diameter (MLD) by 16%, and the mean stent minimal cross-sectional area (CSA) by 35%.^2,3
Improvements in stent design and delivery systems have enabled modern stents to be deployed at high pressure. This has resulted in a reduced use of postdilatation after stent deployment.⁴ The purpose of this study was to examine the use of postdilatation and its impact on stent expansion and lumen dimensions in patients undergoing coronary stenting using modern stent delivery systems.

Methods

Patient selection. We performed a prospective observational study documenting a series of native vessel percutaneous coronary interventional (PCI) cases reflecting routine clinical activity at our center. The data collection period ran from September 26, 2003 to October 12, 2004. Three consultant cardiologists agreed to take part in this study (RHS, RAP, JLM). All data relating to the PCI procedure were collected in real time by SA, who was present in the catheterization laboratory during the procedures. Trial-specific information was transcribed to a computer database system running as an adjunct to our routine clinical audit database tool. All procedures were performed according to agreed protocols, with the acquisition of specific angiographic images for subsequent offline quantitative coronary angiographic (QCA) analysis.
A total of 499 individual lesions in 391 patients treated with coronary stent implantation were recorded during the study period. Out of 499 cases, no reference vessel diameter (RVD) measurements were available in 8 cases and no MLD following postdilatation in 1 case. The remaining 490 lesions (representing 385 patients) were included in the analysis. Postdilatation was performed in 41.2% (202/490) of cases. Angiographic evaluation. Digital angiographic records were analyzed offline by SA. QCA was performed using an automated edge detection system, CAAS II (Cardiovascular Angiography Analysis System, Pie Medical Imaging, Maastricht, The Netherlands). This system has been extensively validated in previous studies.⁵ Investigators were required to record paired orthogonal images of target lesion segments as part of routine PCI practice.
QCA was used to measure the RVD, diameter stenosis and the MLD within the stent before and after postdilatation. Optimal stent deployment was defined as a stent MLD ≥ 90% of the RVD. This was calculated as an average of the proximal and distal RVDs. Procedural technique. All procedures were performed via the femoral or radial routes. Intravenous heparin was given at the start of the procedure to maintain an activated clotting time of 220–300 seconds. The choice of guidewires, balloons and stents was left to the discretion of the operators. The decision to carry out postdilatation was made by the operator based on the angiographic images available at the time of the procedure. The offline QCA analysis data were not available during the procedure. All patients received aspirin 300 mg and clopidogrel 300–600 mg preprocedure. Aspirin 75 mg was continued indefinitely and clopidogrel 75 mg was continued for a minimum of 4 weeks post procedure.
Statistical analysis. We examined the ability of postdilatation to increase the stent MLD. This analysis consisted of comparing the MLD immediately after stent deployment with the final MLD using a paired t-test. The results were expressed as mean absolute and relative differences with 95% confidence intervals. We explored factors associated with an increase in the MLD following postdilatation. These included the use of predilatation, baseline RVD, stent deployment pressure, balloon vessel (BV) ratio of the postdilatation balloon and residual stenosis after stent implantation. The diameter of the postdilatation balloon was calculated from the manufacturer’s reference chart depending on the inflation pressure used. Residual stenosis after stent implantation was calculated as RVD - stent MLD/RVD x 100.
Categorical variables are presented as absolute values and percentages. Continuous variables are presented as a mean with standard deviation. All analyses were performed with SPSS version 11.0. A p-value

Results

Baseline procedural and angiographic characteristics. The study population was divided into two groups depending on whether postdilatation was performed. The baseline characteristics of these groups were similar as shown in Table 1. The angiographic and procedural details are described in Table 2. Postdilatation was performed more frequently in larger vessels and in cases involving the right coronary artery than the circumflex artery. The mean (SD) RVD of the right coronary artery was significantly greater than the circumflex artery, 3.22 (0.50) mm versus 2.87 (0.51) mm, respectively (p Balloons used for postdilatation. Noncompliant balloons were used for postdilatation in 91% (184/202) of cases. The most frequently used balloon used for postdilatation was the Quantum (Boston Scientific Corp., Natick, Massachusetts). This was used in 88% (178/203) of cases. In 2.5% (5/202) of cases, the stent delivery balloon was used for postdilatation. In these cases, a second stent was often deployed distal to the first stent. The delivery balloon of the second stent was used to perform additional dilatation of the first stent.
Impact of postdilatation on stent MLD and optimal stent expansion. In the 202 cases where postdilatation was performed, the mean (SD) stent MLD before postdilatation was 2.50 (0.40) mm [95% CI 2.44–2.55 mm].
Following postdilatation, the stent MLD increased to a mean (SD) of 2.70 (0.38) mm [95% CI 2.65–2.75] p In those cases where postdilatation was not performed, optimal stent expansion was achieved in 64.9% (187/288) of them. Optimal stent deployment increased from 35.6% (72/202) to 56.9% (115/202) with postdilatation (Figure 1).
Despite the use of appropriately-sized stents in relation to the RVD, the final mean stent MLD was 92.2% of the mean RVD for the postdilatation group and 92.8% for the group with no postdilatation. In both groups, the final stent MLD was far short of the stent nominal diameter (Table 2).
The influence of stent deployment pressure on the response to postdilatation. The percentage increase in stent MLD following postdilatation was greater in those cases where the stent deployment pressure was ≤ 14 atm compared to those cases with a deployment pressure of > 14 atm, p The influence of baseline RVD and predilatation on response to postdilatation. The study population was divided into quintiles depending upon the baseline RVD. The percentage increase in stent MLD with postdilatation was similar across the different RVD groups, p = 0.90.
Predilatation was performed in 119/202 (58.9%) of the patients who subsequently went on to have postdilatation after stent deployment. The mean (SD) percentage increase in stent MLD with postdilatation was similar in patients with predilatation compared to direct stent implantation (9.89% [12.21] vs. 8.71% [9.87], respectively, p = 0.697).
The influence of balloon vessel (BV) ratio on postdilatation. Postdilatation was mainly performed with balloons that were larger than the stent nominal size and inflated to high pressures. The mean (SD) BV ratio used was 1.31 (0.17), with a mean (SD) inflation pressure of 15.00 (1.79) atm.
In only 3 cases was the BV ratio 1.23 was not associated with any significant improvement in the mean percentage increase in stent MLD with postdilatation (Table 3).
Residual stenosis after stent deployment in patients who underwent postdilatation. The mean (SD) residual stenosis after stent deployment was 13.98% (11.27), 95% CI 12.42–15.54%. The study population was divided into tertiles depending on the residual stenosis after coronary stenting. The mean (SD) percent increase in stent MLD with postdilatation was significantly greater in those cases where the residual stenosis was > 20% after stent deployment compared to those cases with a residual stenosis of p = 0.001 using analysis of variance, p Types of coronary stents used. A total of 19 different coronary stents were used. The most commonly used stent was the Cypher™ Select (Cordis Corp., Miami Lakes, Florida) in 22.2% (109/490) of all cases. The Taxus^® stent (Boston Scientific) was used in 11.0% (54/490) of cases, resulting in a drug-eluting stent (DES) use of 33.2%. The bare-metal stents (BMS) deployed included the Zeta (Guidant Corp., Santa Clara, California) in 15.1% (74/490); the Driver (Medtronic, Inc., Minneapolis, Minnesota) in 13.5% (66/490); the Amazonia (Minvasys, Gennevilliers, France) in 13.1% (64/490); the Vision (Guidant Corp.) in 8.6% (42/490); the Snapper (Minvasys) in 6.5% (32/490); and the Liberté (Boston Scientific) in 4.9% (24/490). The use of postdilatation was similar between DES and BMS cases: 41.7% (68/163) versus 41% (134/327), respectively, p = 0.876.

Discussion

The angiographic final stent MLD has been shown to be a strong inverse predictor of restenosis.⁶ IVUS studies have shown a similar relationship with minimal stent cross-sectional area and target vessel revascularization (TVR).⁷ This has led to the “bigger is better” concept, with the aim being to maximize stent MLD within the natural restraints of reference vessel size. The use of postdilatation to increase stent MLD and minimal lumen area (MLA) has been shown to reduce TVR rates.^8,9 The present study demonstrates that even with modern stent delivery systems, adjunctive postdilatation can increase the stent MLD and improve the number of cases with optimal stent deployment. When postdilatation was performed using high-pressure oversized noncompliant balloons the average increase in stent MLD was 0.2 mm. Postdilatation was performed in approximately 40% of cases. The decision to perform postdilatation is based on operator preference, attempts to improve suboptimal angiographic results and IVUS-guided stenting. In patients in whom postdilatation was not performed, optimal stent expansion was only achieved in 65% of cases. A significant number of cases might therefore have benefited from postdilatation had it been performed more frequently.
The increase in stent MLD achieved with postdilatation in this study is similar to that reported in the POSTIT trial.¹⁰ In this IVUS-based study of 146 patients, the mean stent MLD increased from 2.6 to 2.8 mm with postdilatation. Adjunctive postdilatation was performed, as in our study, with noncompliant balloons, achieving a BV ratio of at least 1.1. The authors reported that optimal stent deployment was achieved in 42% of patients who underwent postdilatation.
A number of important observations from this study are relevant to routine PCI practice. First, the increase in stent MLD with postdilatation was independent of baseline RVD and the use of balloon predilatation. Second, postdilatation was useful and improved the result significantly when the stent deployment pressure was ≤ 14 atm. In cases where there was a mismatch between the proximal and distal RVD of the target vessel, the stent was often deployed at low pressure to avoid distal edge dissection. Postdilatation of the larger proximal segment was part of the strategy to achieve optimal stent expansion and not related to lesion stiffness or residual stenosis. Third, the greatest impact of postdilatation was seen in those cases with a residual stenosis of > 20% after stent deployment. Lesions with heavy calcification or large plaque burden are likely to have increased resistance to stent expansion. The immediate angiographic result after stent deployment should be carefully reviewed for stent underexpansion. Further studies are required to examine whether quantitative assessment of the post stent deployment images may improve patient selection for postdilatation. Fourth, a BV ratio of > 1.23 for postdilatation was not associated with any further benefit in increasing the stent MLD. Our practice is to use short (8–12 mm) noncompliant balloons for postdilatation with a nominal diameter 0.5 mm larger than the stent delivery balloon, deployed to ≥ 16 atm. Care should be taken to avoid vessel injury outside the stent margins with postdilatation.
Modern stent delivery systems use semicompliant balloons that are manufactured to achieve a certain nominal diameter at a specific pressure. Inflation of the balloon at pressures greater than nominal can result in a slight increase in the final diameter. Manufacturers provide a compliance chart for each device, which describes the expected stent diameter for a range of deployment pressures. These values are derived from calliper or other measurements of balloons inflated in air or in a water bath. Some manufacturers report only the compliance data of the delivery balloon without the stent. Studies have shown that the actual stent diameters achieved after deployment are much smaller than those predicted by the manufacturer’s compliance chart.¹¹ Stent delivery balloon underexpansion is the principal reason for this shortfall. The semicompliant nature of modern stent delivery balloons may not be able to overcome the resistance of coronary lesions, particularly those with extensive fibrocalcified plaque. Noncompliant balloons are manufactured to achieve a nominal size, with little further increase in diameter despite increasing the inflation pressures.¹² Noncompliant balloons that are oversized compared to the nominal stent diameter may achieve more uniform and greater expansion, resulting in better stent deployment. However, even noncompliant balloons are unable to overcome the resistance of very hard lesions, as shown by the 57% optimal expansion rate in those cases where postdilatation was performed. Sometimes a suboptimal stent may be acceptable when the lesion is unyielding to initial postdilatation, and further balloon inflations with oversized balloons may result in complications such as vessel dissection and perforation.
Randomized clinical trials have shown that drug-eluting stents (DES) reduce the in-stent restenosis and TVR rates compared to bare-metal stents.¹³ The local delivery of antiproliferative drugs prevents neointimal hyperplasia and late loss that occurs following coronary stenting. This may result in operators using adjunctive postdilatation less frequently after DES implantation compared with BMS. Our study showed that similar rates of postdilatation were used after DES and BMS deployment. Adequate DES expansion may be important to ensure optimal drug delivery, with close contact between the stent struts and the vessel wall. High-pressure postdilatation has been reported to be important in optimizing stent expansion in cases where DES are used for the treatment of in-stent restenosis.¹⁴ It has been reported that stent thrombosis after sirolimus-eluting stent implantation is associated with stent underexpansion.¹⁵ The authors showed that 80% of the cases with stent thrombosis had a stent MLA of 2. A smaller stent minimal diameter after DES deployment has also been shown to be associated with an increased risk of TVR.^16,17 Further studies are required to investigate whether postdilatation after DES deployment will reduce the risk of TVR and stent thrombosis.
Potential disadvantages of postdilatation included increased costs, longer procedure duration due to additional catheter exchanges, potential for vessel trauma outside the stented segment and risk of vessel perforation or dissection.
Study limitations. The major limitation in this study is the lack of clinical follow up to assess the impact of postdilatation on the rates of TVR and restenosis. Previous studies have used IVUS for the assessment of stent expansion and MLD. In this study, QCA was used, which may underestimate the impact of postdilatation. However, the results of this study were very similar to the IVUS-based POSTIT study. Most interventional cardiologists base their decision to perform postdilatation upon the angiographic appearances following stent deployment. Therefore, an angiography-based study is more representative of real-world practice.

Conclusion

A significant proportion of patients did not achieve optimal stent deployment with modern stent delivery systems. One-third of patients with suboptimal stent deployment did not undergo postdilatation. When postdilatation was performed with oversized noncompliant balloons, an increase in stent MLD and the proportion of patients with optimal stent deployment were achieved. Cases with a lower stent deployment pressure or a more significant residual stenosis after coronary stenting had the most to gain from postdilatation.

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