IVUS or OCT for Bioresorbable Stents?

Bioresorbable vascular scaffolds (BRS or BVS) are the next revolution for percutaneous coronary revascularization, as previously discussed.¹ Preliminary data for BRS showed the vessel is healed with near-normal endoluminal tissue growth approximately 3 years after BRS implantation. Beyond improving coronary blood flow as do all stents, BRS potentially have several distinct advantages over metal stents. Most noteworthy is that after implantation, BRS reabsorbs over time, restoring the vessel histology and function to its near-normal state. The neoendothelium (often referred to as a neo-media given the development of functioning smooth muscle cells) in the absence of a metal scaffold promotes vasomotion and normalization of luminal shear stress, and both functions somewhat protectively and reduces the potential of neoatherosclerosis. Bare-metal stents (BMS), drug-eluting stents (DES), and even BRS continue to have reports of late subacute thrombosis, bleeding, and restenosis. Bioresorption of the stent theoretically permits earlier termination of dual antiplatelet therapy and restenosis. The absence of a permanent metal stent also permits an easier approach to repeat interventions that might be complicated by a pre-existing stent, such as treatment of side branches. As we will soon be seeing BVS in our labs and after reading comments by Dr. Alfonso², I thought it worthwhile to review the role of intravascular imaging for the routine use of BVS in order to achieve best results. 

BRS arrives in the U.S. 

On July 5, 2016, the U.S. Food and Drug Administration approved the first fully absorbable stent to treat coronary artery disease. The Absorb GT1 Bioresorbable Vascular Scaffold System (BVS) (Figures 1-2), which releases the drug everolimus to limit the growth of scar tissue, is gradually absorbed by the body in approximately 3 years. 

According to the press release, the Absorb GT1 BVS is manufactured from a biodegradable polymer called poly (L-lactide) that is similar to materials used in other types of absorbable medical devices, such as sutures. Table 1 compares types of bioabsorbable stents. After stent placement, studies have demonstrated that the need for radial scaffolding to produce durable vessel patency is only a transient one, with a minimum duration of approximately 3 months.3 The device’s absorption by the body gradually eliminates the presence of foreign material in the artery once the stent is no longer needed. After absorption, there are only 4 very small platinum markers embedded in the walls of the artery, which help cardiologists identify where the Absorb GT1 BVS was originally placed. In approving the Absorb GT1 BVS, the FDA evaluated data from a randomized trial of 2008 patients, which compared the rate of major adverse cardiac events between the Absorb GT1 BVS and a drug-eluting metallic stent. After 1 year, the Absorb GT1 BVS group showed a major cardiac adverse event rate of 7.8%, which was clinically comparable to the rate of 6.1 percent observed in the control group. In addition, after 1 year, the rate of blood clots forming within the devices was 1.54% for the Absorb GT1 BVS and 0.74% rate for the control.

Possible adverse events that may be associated with the procedure to insert the Absorb GT1 BVS or with the Absorb GT1 BVS itself include allergic reactions to materials in the device or medications used during the procedure, allergic reaction to the drug everolimus, infection or irritation at the catheter insertion site, internal bleeding, the development of abnormal connections between arteries and veins, embolism, or other coronary artery complications that may require medical intervention and that could lead to death.

The Absorb GT1 BVS is contraindicated for patients who have a known hypersensitivity or allergy to everolimus or the materials used in the device, such as poly(L-lactide), poly(D,L-lactide), or platinum. It is also contraindicated for those who are not candidates for angioplasty, have sensitivity to contrast, or who cannot take long-term aspirin therapy along with other blood-thinning medications (antiplatelet agents).

Differences in structure prompt differences in implantation technique with BRS

In contrast to metallic DES, which are easily visualized angiographically given their radio-opaque metal struts, the BRS is made of a radiolucent polymer with larger struts (compared to metallic BMS or DES). Due to its limited expandability, BRS requires a change in percutaneous coronary intervention (PCI) practice before, during, and after implantation. To localize the BRS under x-ray during and after deployment (and after resorption), 4 radio-opaque markers are incorporated at both edges of the scaffold (and on the accompanying Abbott balloon). These small markers are still difficult to see, but do help visualization of the ends of the scaffold.

Compared to Abbott Vascular’s Xience DES, the strut geometry of the Absorb BVS is similar, but the strut thickness is nearly twice as big at 150 microns (Xience strut thickness is 83 microns). The reason for the thicker struts is that the polymer is weaker than metal, thus the BRS designs use larger struts to ensure adequate radial force and expansion without exceeding scaffold integrity.

The more fragile nature of the BRS limits expansion beyond its rated expansion diameter. This feature impacts the importance of correct stent size selection. If the BVS is not sized correctly (undersized in particular), the option to over-expand the stent, like metal stents, is unavailable. To reach full expansion and apposition against the vessel wall, the relatively inelastic polymer structure limits the BVS’s ability to expand (<0.5 mm) compared to BMS or DES (about 1.5 mm).

Lesion assessment and preparation for BVS implantation

The larger struts and polymer of the BVS make it a less maneuverable stent, which will present some difficulty in crossing complex, calcific lesions, bifurcations, or previously implanted metal stents.

Unlike BMS or DES procedures, to facilitate BRS passage, expansion, and apposition, the extent of plaque and accurate vessel sizing is critical. Intravascular ultrasound imaging (IVUS) or optical coherence tomography (OCT) are used to identify lesion characteristics before implant, select the correct stent size, and provide confidence in adequate stent expansion and strut apposition after implant. 

IVUS vs OCT for BVS?

Accurate identification of vessel size and difficulties with lesion preparation and device delivery are critical to successful BVS implantation and outcomes.  Intravascular imaging thus becomes more important. Are the differences (Table 2) between IVUS and OCT imaging enough to select one over the other for best BVS results? IVUS displays the vessel with deeper penetration (8-9 mm) compared to OCT, which visualizes the plaque approximately 1-2 mm deep. IVUS may more easily identify the calcified or fibrous plaques, one of several complicating factors for BVS implantation. Compared to IVUS, OCT has better resolution (approximately 10 vs 100 microns) and is better able to see the struts in detail, a feature that is useful in the post-deployment assessment. However, OCT requires additional contrast injections; IVUS does not. While visualization of the lumen and struts is intuitive with OCT, it should be noted that OCT is not intended for use in large vessels (>3.5mm), nor for ostial left main or right coronary ostial lesions. 

Deployment technique for BRS: Comments from Dr. David Rizik, Scottsdale, Arizona⁴

Like many labs in the United States, we have not yet had experience with the BVS. I asked our colleague Dr. David Rizik to describe the important technique changes he implemented when he implanted the first BVS in the United States. Dr. Rizik, said, “Important deployment techniques for the routine implantation of Absorb BVS must include adequate vessel preparation and predilation with NC [non-compliant] balloons, with a balloon to artery ratio of 1:1. Accurate device-artery sizing with the routine use of intravascular imaging is especially important and even more so during the early ‘learning curve’ experience. While OCT use has been advocated, given that the scaffold is well visualized using this modality, lack of availability of OCT should not be viewed as a barrier to BRS use. The utilization of IVUS pre-implantation for BVS to ascertain the true lumen dimension is also an effective imaging modality in achieving durable results. High-pressure post-dilatation (i.e., 16 atm) should be a common practice. For optimal results, operators are strongly encouraged to use appropriately sized, NC balloons for post-dilatation to the scaffold nominal dimension, but not to exceed 0.5 mm greater than the scaffold implanted. In sum, meticulous implant technique and routine application of high-resolution imaging modalities, already routinely integrated in our cath lab diagnostic armamentarium, are encouraged for optimizing BVS results.”

Considerations for complex lesions with a calcium arc of 180 degrees of vessel circumference include using extra PCI support, such as a stronger guide wire, buddy wire, or early use of guide extension device. Lesion preparation with Rotoblator (Boston Scientific) may be needed. Predilation with a NC balloon and reassessment for BRS suitability should be considered. Especially for highly tortuous vessels, DES may be a better option. For such complex lesions, defaulting to a more rugged DES may result in the best outcome. Small vessels (<2.5 mm) or vessels larger than 3.75 mm in diameter may not be suitable for BRS. Only three sizes of BRS scaffolds (2.5, 3, and 3.5 mm diameters) were tested in the ABSORB trials; thus, stent selection should be made based on the best approximation to the distal reference lumen diameter. The BRS length should cover the diseased segment completely to reduce chances of stent edge dissection.   

Dual antiplatelet therapy for bioresorbable stents

Numerous studies support the use of standard approaches to dual antiplatelet therapy (DAPT) for BMS/DES and BRS for at least 1 year. Although some studies suggested use of a reduced duration of DAPT therapy in newer-generation DES, at least 12 months of DAPT therapy for BRS should be used until longer-term data is available. 

The bottom line

Correct lesion preparation and vessel sizing is critical to BRS. Table 3 summarizes the indications for IVUS and OCT. Imaging with either IVUS or OCT, pre and post deployment, is important if not critical in achieving successful BVS (as well as other PCI) outcomes.

References

1. Kern, M.  Bioabsorbable stents – where are we now? Cath Lab Digest. June 2012;20(6). Available online at https://www.cathlabdigest.com/articles/Bioabsorbable-Stents-%E2%80%93-Where-Are-We-Now. Accessed August 23, 2016.
2. Ambrosia A. Role of imaging in bioresorbable stent procedures. Diagnostic and Interventional Cardiology. July 21, 2016. Available online at:  https://www.dicardiology.com/article/role-imaging-bioresorbable-stent-procedures. Accessed August 23, 2016.
3. Serruys PW, Luijten HE, Beatt KJ, Geuskens R, de Feyter PJ, van den Brand M, et al. Incidence of restenosis after successful coronary angioplasty: a time-related phenomenon. A quantitative angiographic study in 342 consecutive patients at 1, 2, 3, and 4 months. Circulation. 1988 Feb; 77(2): 361-371.
4. David Rizik, MD, Scottsdale Heart, Scottsdale, Arizona. Personal Communication, July 30, 2016.