The Angiographic Step-Up and Step-Down: A Surrogate for Optimal Stent Expansion by Intravascular Ultrasound

Methods

This study was reviewed and approved by our center’s institutional review board. We prospectively screened clinically stable patients undergoing diagnostic coronary arteriography for inclusion in the study. All patients provided written, informed consent. Patients found to have a de novo stenosis >50% who were candidates for coronary stenting were included in the trial unless exclusion criteria were present. Exclusion criteria included acute myocardial infarction, planned use of multiple stents in the same lesion, planned treatment of lesions >30 mm in length, tapering of the artery >1 mm over the planned length of the stent, excessive tortuosity of proximal segments, extreme angulation of the lesion, chronic total occlusion, or anatomy unfavorable for delivery of the IVUS probe. The patients were then randomized to standard sizing or step-up and step-down sizing as the angiographic goal. The procedures were performed by high-volume operators (>250 percutaneous coronary interventions [PCIs] per year; JCB, KDS).
Stent deployment techniques. Standard angiographic sizing was performed by visual estimation or online quantitative coronary analysis (QCA) using the least foreshortened angiogram after administration of 200 µg of intracoronary nitroglycerin. Operators chose pretreatment modality, stent and balloon size and inflation pressure according to their clinical practice. In general, the balloon and stent size were selected to achieve a goal diameter 100–110% of the nominal artery size. The operator determined all other adjunctive medications. The angiographic goal for the standard approach was 0% residual stenosis without dissection within the stented segment. Postdilatation was performed using Quantum Maverick™ balloons (Boston Scientific Corp., Natick, Massachusetts) as needed to achieve angiographic success.
Step-up, step-down sizing was performed as described above, except the angiographic goal was defined as an appearance of a step-up and step-down at the proximal and distal stent edges, respectively, with complete angiographic stent expansion along the length of the stent. Larger balloons were used as needed after stent deployment for postdilatation to achieve the angiographic goal. Extreme care was taken to not inflate the postdilatation balloon outside of the stent.
IVUS analysis. IVUS was performed using automatic pullback with the Atlantis™ SR 40 MHz catheter (Boston Scientific) and the ClearView Ultra™ IVUS imaging system (Boston Scientific) after 200 µg of intracoronary nitroglycerin was administered. Incomplete stent strut apposition, dissection within the stent or grossly suboptimal stent expansion were treated at the discretion of the operator. IVUS was performed after the intervention and was not used to assist in stent or balloon sizing. IVUS images were recorded onto high-resolution VHS tape for further analysis. Additional therapy based on the IVUS images was performed at the discretion of the operator. Operator interpretation of the IVUS images and additional therapy were blinded to the IVUS reviewer.
A blinded investigator (TH) independently reviewed the IVUS images after all 25 patients were entered into the study. Cross-sectional area of the proximal and distal reference vessels and minimum stent area were measured by online planimetry (Figure 1). Stent expansion index was calculated according to Multicenter Ultrasound Stenting In Coronaries (MUSIC) criteria which defined an optimal stent expansion index as a minimum stent area >80% of the average reference lumen areas for MSA >9.0 mm2 or >90% for MSA ≤9.0 mm2.⁵
Quantitative coronary analysis. QCA was performed after all 25 patients were entered into the study by a blinded investigator using electronic digital calipers. The proximal and distal reference segments and minimum lumen diameter were measured on both the preintervention and postintervention images.
Statistical analysis. Statistical analysis was performed using SAS (SAS, Inc., Cary, North Carolina) and Microsoft Excel software. The averaged data were compared using a two-tailed Student’s t-test. Categorical values were compared using Fisher’s exact test. The Wilcoxon two-sample test was used to compare nonparametric data. Statistical significance was based on a p-value <0.05.

Results

Among 25 patients undergoing coronary stent implantation, 12 were randomized to standard sizing and 13 to step-up and step-down sizing as the angiographic goal (Figures 2 and 3). Patient and lesion baseline characteristics were not significantly different between the two groups (Tables 1 and 2).One patient in the step-up, step-down group did not meet the angiographic goal due to absence of a step-down at the end of the stent. One patient in the standard group had an angiographic step-up and step-down. The step-up, step-down group was treated with higher pressures (17.1 ± 1.3 vs. 15.5 ± 1.5 atm) and more inflations than the standard treatment group (Table 3). Residual angiographic stenosis, measured by QCA, was higher in the standard group compared to the step-up, step-down group: 5.5% versus -6.1% (p = 0.001) (Table 4). Minimum lumen diameter was larger in the step-up, step-down group: 3.01 ± 0.36 vs. 2.66 ± 0.32; p = 0.02.
The primary endpoint of optimal stent expansion was met by 4 patients (33%) treated with standard stent implantation and 12 patients (92%) treated with the step-up and step-down approach (p = 0.001). No patients in either group had unapposed stent struts or dissection on IVUS examination (Tables 4 and 5). There was no significant difference in creatine kinase-MB fraction between the two groups and there were no major adverse cardiac events on 6-month clinical follow up.

Discussion

The findings of this study suggest that coronary stenting using the angiographic step-up and step-down goal results in optimal stent expansion by IVUS criteria more often than the standard angiographic goal. This is the first study to demonstrate this finding.
Achieving the step-up, step-down goal was not due to oversizing, as the balloon-to-artery ratios based on nominal inflation pressure size were similar. Rather, the step-up, step-down group was treated with more frequent postdilatation with noncompliant balloons and higher pressures. This suggests that the semicompliant balloon of the stent delivery system is not ideal to yield a step-up and step-down or fully expand the stent despite using high pressures. This confirms the findings of other investigators.^4,8
Drug-eluting stents (DES) are now being used in more than 70% of interventions involving stents.⁹ Stent thrombosis is a relatively uncommon complication of coronary stenting. Older retrospective analyses noted an association between stent underexpansion and thrombosis in bare-metal stents (BMS).¹⁰ Recently, Fujii et al¹¹ associated suboptimal stent expansion by IVUS with thrombosis of DES as well. Thus, an angiographic correlate to IVUS-determined optimal stent expansion might be especially useful in preventing thrombosis of BMS and DES.
A potential concern with obtaining the step-up, step-down is oversizing of the stent leading to dissection or perforation. The risk of perforation correlates with a balloon-to-artery ratio >1:1.2.¹² The step-up, step-down sizing in our study averaged 6.1% greater than the reference vessel diameter by QCA, and in no case was the size greater than 15% of the reference diameter. In our clinical practice this angiographic endpoint is obtained by appropriately sizing the stent with the reference vessel, and if necessary, postdilating the stent with a larger noncompliant balloon. There were no edge dissections or perforations in the study or control group.
Study limitations. In one case, we were unable to obtain an angiographic step-up and step-down due to limited vessel compliance and calcification. Operator discretion dictated the use of more aggressive postdilatation in less compliant vessels. The small size of this study limits broad application of the step-up, step-down angiographic goal, and further investigation in larger randomized trials is warranted to validate our findings. The study was not designed to test our angiographic endpoint on specific stents. The operators chose the stents, and as a result, a wide variety were used. However, all stents were stainless steel, with 2 DES used in the standard group and 4 in the step-up, step-down group (Table 5).
Although there were no dissections or perforations related to either stent deployment strategy, this study was not powered to analyze for these adverse angiographic outcomes. However, this deployment strategy is routinely used in our clinical practice. On review of the National Cardiovascular Data Registry, there have been no stent-related perforations by the operators (JCB, KDS) in over 2,000 interventions.
In summary, the angiographic step-up and step-down led to better stent expansion and larger stent area in this study. Larger stent areas are associated with less restenosis and lower target lesion revascularization rates.^3,5–7 Suboptimal stent expansion is a risk factor for thrombosis BMS and DES.^10,11 We propose the use of the angiographic step-up and step-down as a practical and economical alternative to IVUS for optimal stent expansion.

Acknowledgement. We appreciate the support of Boston Scientific Corporation for providing the intravascular ultrasound catheters for this study.

References

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