High-Definition IVUS System for Sizing BVS and Post-Dilation Balloon to Optimize BVS Expansion

A number of intravascular ultrasound (IVUS) series have shown that stent underexpansion, smaller post-procedure lumen dimensions, residual reference segment stenosis, and the presence of thrombus or dissection after stenting are the predictors of stent restenosis or thrombosis.^1-3 Recent meta-analyses^4,5 of IVUS trials demonstrated that IVUS-guided stenting with drug-eluting stents (DES) significantly reduced the incidence of major adverse cardiac events, including stent thrombosis. Likewise, a recent large-scale, prospective, multicenter, non-randomized study of 8583 patients showed that IVUS guidance compared with angiography significantly reduced the risk of stent thrombosis, myocardial infarction, and major adverse cardiac events within 1 year following DES implantation.⁶

It has been shown that minimal stent area (MSA) determined by IVUS is a strong predictor of in-stent restenosis after DES deployment. A recent IVUS study⁷ reported that among patients with in-stent restenosis, the rate of stent underexpansion defined as an MSA <5.0 mm² was 69% with the second-generation DES. Likewise, stent thrombosis or restenosis occurred in 67% of lesions with an MSA <5.0 mm² after sirolimus-eluting stent deployment.⁸ A recent registry study⁹ showed that the best cut-off value of MSA for predicting in-stent restenosis was 5.4 mm² in patients who underwent stenting with everolimus-eluting stents. In the AVID (Angiography Versus IVUS Directed coronary stent placement) trial¹⁰, optimal stent expansion was defined as the MSA ≥90% of the distal reference vessel lumen area after bare metal stent deployment.

Bioresorbable vascular scaffolds (BVS) have emerged as a potential major breakthrough for treating coronary lesions. In fact, the need for vessel scaffolding and drug delivery is temporary. Once the processes of recoil and hyperplasia have ended, vessel healing will continue to follow. The use of IVUS facilitates more precise determination of target vessel diameter and lesion length, assures optimal stent expansion, and provides immediate delineation of edge dissections. Given the unique mechanical properties of BVS and the procedural details for successful implantation, IVUS is particularly helpful in vessel sizing before stenting. Underexpansion and malapposition of struts during percutaneous coronary intervention (PCI) are associated with a higher rate of restenosis and stent thrombosis. IVUS has been previously demonstrated to improve stent expansion and reduce the rates of target vessel revascularization.^7-10 However, to date, there are no data on the technique of optimal BVS expansion by IVUS. Here, we present the use of high-definition (HD) IVUS for sizing BVS and post-dilation balloon to optimize BVS expansion. 

Case Presentation

A 78-year-old man with a history of hypertension and hyperlipidemia was admitted with unstable angina. Heart catheterization demonstrated a critical stenosis in the proximal left circumflex coronary artery (LCX) (Figure 1A). The lesion was predilated with a 3.0 x 15 mm non-compliant balloon. A Kodama HD IVUS catheter (the ACIST HDi HD IVUS system) was advanced 10 mm distal to the lesion in the LCX and pulled back automatically at a speed of 1.0 mm/s. The online IVUS measurements showed that the average distal reference lumen diameter and external elastic membrane diameter (EEM) were 3.4 mm and 4.0 mm, respectively (Figure 1B). At the lesion site, the lumen area was 4.6 mm² after pre-dilation (Figure 1C). The average proximal reference lumen diameter was 3.75 mm (Figure 1D). The lesion length was 15 mm. Since the average distal reference lumen diameter was 3.4 mm by IVUS, a 3.5 x 18 mm Absorb GT1 BVS (Abbott Vascular) was implanted at 16 atmospheres. Angiography showed no residual stenosis (Figure 2A). After BVS implantation, the Kodama HD IVUS catheter was re-advanced into the LCX and pulled back automatically, measuring minimal BVS area at 7.24 mm² (Figure 2B). Since the average distal reference EEM diameter was 4.0 mm, a 4.0 x 15 mm non-compliant balloon was advanced and post-dilation was performed from the distal edge of BVS to the proximal edge. After post-dilation, angiography showed no difference compared to angiography performed after BVS implantation (Figure 2C). The Kodama HD IVUS catheter was re-advanced into the LCX and pulled back automatically. The online IVUS analysis showed that minimal BVS area had now increased to 8.13 mm² (Figure 2D). Figure 2E shows the struts of the BVS well apposed to the vessel wall, clearly visible as a double-layer density representing the endoluminal and abluminal edges of the struts.

Discussion

To the best of our knowledge, this case is the first to show the impact of high-definition IVUS-guided BVS implantation and post-BVS balloon sizing for optimization of BVS expansion. We demonstrated that after post-dilation with a non-compliant balloon sized to the distal reference EEM diameter, the minimal BVS area was further expanded as compared to the same area after BVS implantation. The struts were well apposed and there was no evidence of edge dissection. This technique may reduce event rates. However, randomized trials are needed to investigate the impact of IVUS-guided strategy as compared with angiography, on the outcome of patients undergoing BVS implantation.

There is no established IVUS-guided protocol to optimize stent expansion. In a number of large IVUS trials^9-11, IVUS was not performed prior to stenting to size the stent and post-dilation balloon. Furthermore, post-dilation was not mandatory and no specific method was used for post-dilation. In the AVIO (Angiography Vs. IVUS Optimization) trial¹², Chiefo et al randomized 284 patients to IVUS-guided stenting vs angiography. In the IVUS-guided stenting group, after stenting, they sized the post-dilation balloon to the average of EEM diameter at the proximal, mid, and distal segments of the stent to optimize stent expansion. They defined optimal stent expansion as MSA >70% of balloon cross-sectional area (CSA), but it was achieved in only 48% of the lesions. Furthermore, they reported that minimal lumen diameter by angiography was significantly larger in those patients who received optimal stent expansion, but outcomes were not significantly different.

BVS struts are 150 µm thick and approximately two-thirds thicker than those of the second-generation DES. The ring structure of a BVS stent is also up to 30% tighter. These differences make deliverability and side branch management more challenging. In addition, BVS, in contrast to DES, cannot be over-expanded. Therefore, more precise device sizing by IVUS or optical coherence tomography (OCT) is essential. In the presence of circumferential calcification by IVUS or OCT, BVS cannot be adequately expanded and DES is preferred after performing rotational atherectomy. In the GHOST-EU registry (Gauging Coronary Healing With Bioresorbable Scaffolding Platforms in Europe)¹³, the cumulative incidence of target lesion failure with BVS was 2.2% at 30 days and 4.4% at 6 months, with an annualized rate of 10.1%, comparable to rates reported with DES. However, the rates of definite/probable scaffold thrombosis — 1.5% at 30 days and 2.1% at 6 months — are concerning and higher than those previously reported for the second-generation DES. Furthermore, the results of the ABSORB III study¹⁴ showed that stent thrombosis trended higher at 1 year (1.5% for BVS and 0.6% for the Xience stent [Abbott Vascular]), highlighting the importance of lesion selection and optimal BVS expansion guided by IVUS or OCT.

The ACIST HDi IVUS system offers improved image quality, speed of procedure, and image interpretation. The new system is considered to be the first high-definition IVUS system. It includes a touch panel interface, high-definition catheter, and fast pullback speed. The improved image quality of the ACIST HDi IVUS system is due to a combination of its 60 MHz transducer and powerful signal processing. The system produces imaging at high axial resolution (<40 µm vs the ~100 µm of other IVUS systems) and minimizes image noise without the need for vessel clearing, which can improve optimal stent sizing and expansion. In particular, as shown in Figure 2E, the apposition of BVS struts to the vessel wall can be clearly assessed by the ACIST HDi IVUS system.

Conclusions

A growing body of evidence supports the potential clinical benefits of BVS. Given its novel mechanical properties, improving the expansion of BVS by IVUS or OCT to avoid potential complications is of the utmost importance. The unique resolution of the ACIST HDi IVUS system allows for reliable evaluation of BVS expansion within our described technique of using the lumen and EEM diameters for sizing the BVS and the post-dilation balloon. Future randomized trials can further delineate the role of the ACIST HDi IVUS system for optimal BVS implantation and determine the system’s overall role in improving outcomes of patients receiving BVS.

References

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Disclosures: Dr. William Fonbah reports no conflicts of interest regarding the content herein. Dr. Massoud Leesar reports he is a consultant to ACIST Medical.

The authors can be contacted via Massoud A. Leesar, MD, FACC, Professor of Medicine, University of Alabama-Birmingham (UAB), UAB Heart and Vascular Center, at mleesar@uab.edu.