Editor's note: Figures, tables, and images are available in the PDF download accompanying this article, or at www.invasivecardiology.com.
Abstract: Peripheral artery chronic total occlusions (PCTOs) are frequently encountered during endovascular treatment of peripheral artery disease. Failure to successfully cross PCTOs accounts for the majority of unsuccessful endovascular procedures and associated complications. This review outlines a contemporary ultrasound-based approach to crossing femoropopliteal (FP) PCTOs based on a review of prospectively collected case report surveys, published evidence, and expert opinion compiled by the writing group members. The authors describe optimal imaging of PCTO lesions as well as key angiographic and ultrasound imaging features for determining the choice of antegrade, retrograde, or hybrid techniques, initial guidewire selection, guidewire escalation, and dissection re-entry approaches. These concepts are illustrated using clearly defined hierarchical steps and case examples. The writing group members recognize that while the algorithm provided may not encompass all clinical situations, it will serve as a foundation for establishing a systematic procedural strategy for crossing PCTOs to maximize crossing efficiency, treatment success, and patient safety.
J INVASIVE CARDIOL 2019;31(4):111-119.
Key words: chronic total occlusion, peripheral artery disease
Peripheral artery chronic total occlusions (PCTOs) are encountered in 40%-50% of patients presenting with intermittent claudication (IC) or critical limb ischemia (CLI) undergoing endovascular treatment of femoropopliteal (FP) and tibial or below-the-knee (BTK) arteries.1,2 FP-PCTOs often involve long vascular segments with heavy calcification and can occur at multiple levels.3,4 Therefore, treatment of infrainguinal PCTOs is technically challenging and failure to cross such lesions can be as high as 30%.5-8 Inability to cross a PCTO is the primary reason for revascularization failure and is associated with a higher risk of complications.9 Creating an algorithm to maximize the likelihood of successful PCTO crossing may help increase overall procedural success rates for complex endovascular procedures and ultimately improve patient outcomes.10
A multidisciplinary writing group of 11 high-volume PCTO operators provided guidance to develop best-practice recommendations based on a review of prospectively collected case reports, published evidence, and expert opinion. Our objective was to develop a procedural algorithm for crossing PCTOs that maximizes treatment success, efficiency, and patient safety.
PCTO Working Group
The PCTO working group comprised 11 experienced peripheral artery interventionalists (8 interventional cardiologists, 1 vascular surgeon, and 2 interventional radiologists) from United States centers. The working group members had three face-to-face and two web-based meetings between August 2017 and January 2018 to review relevant literature and 50 prospectively collected consecutive cases according to an agreed charter. The charter established the objective, scope of work, and the survey form for collecting de-identified infrainguinal PCTO case information through an online survey form. Supplemental Appendix S1 (supplemental materials available at www.invasivecardiology.com) illustrates the PCTO consensus group charter and survey forms.
PCTO survey results. Eighty-six percent of PCTO cases were FP and 14% were tibial or BTK; 64% of PCTO lengths were >150 mm and 12% were multilevel (involving FP and BTK arteries). Moderate-to-severe calcification was identified in 32% of cases. Contralateral common femoral artery (CFA) access was employed for crossing in 80% cases, antegrade CFA in 10%, tibiopedal artery in 8%, and retrograde SFA or popliteal in 2%. A hybrid crossing approach was used in 20%. A planned guidewire with a support catheter (wire-catheter approach) was initially selected in 92% of cases and a crossing device in 8%, with a switch from a wire-catheter approach to a crossing device required in 1 case. True lumen crossing was achieved in 76%, while use of re-entry devices or advanced subintimal tracking techniques were employed in 24%. Intravascular ultrasound (IVUS) was used in 16% of procedures.
PCTO algorithm. A PCTO crossing algorithm was subsequently developed through collaborative review of the survey data, representative PCTO case examples, and published manuscripts. These efforts were synthesized into algorithm figures, tables, and descriptive text that were periodically reviewed and edited by members of the working group and other contributing authors, leading to a final consensus version.
Indications for PCTO intervention. Percutaneous intervention of FP-PCTO lesions is appropriate in patients with lifestyle-limiting claudication (Rutherford class [RC] 2-3) who have failed or are intolerant of pharmacological and exercise therapy as well as in patients with CLI (RC 4-6).11,12 Treatment of BTK-PCTO is appropriate in patients with two- or three-vessel infrapopliteal disease and ischemic rest pain (RC 4), minor (RC 5) or major tissue loss (RC 6).12,13 In some instances, it may also be appropriate to treat infrapopliteal PCTOs in claudicants (RC 2-3) to preserve the patency of proximal interventions and establish at least one-vessel runoff to the foot.
PCTO imaging. Preintervention imaging and review of FP- and BTK-PCTO morphology are crucial for planning a CTO intervention. Defining inflow vessels, PCTO location, lesion morphology, outflow vessels, and the arterial supply to ischemic tissue are vitally important to procedural approach and success. For procedure planning, digital subtraction angiography (DSA) performed via selective catheter placement in or just proximal to the vessel(s) of interest is preferred over “stepping-table” or “bolus-chase” imaging.14 Selective DSA allows better delineation of the PCTO proximal and distal caps, associated calcification, side branches, collateral vessels, and distal-vessel reconstitution. Prolonged filming is often necessary to record delayed enhancement of postocclusion reconstituted and pedal vessels from collateral or retrograde circulation, thereby minimizing the risk of failing to identify angiographically occult or hibernating arteries.15
For FP- and BTK-PCTOs, delineation of contralateral CFA, bilateral iliac vessels, and ipsilateral CFA bifurcation along with infrapopliteal vessels to the foot is required. Contralateral oblique projections are best to delineate iliac artery lesions, while ipsilateral oblique projections are often needed to view the CFA bifurcation. Popliteal, infrapopliteal, and pedal artery angiograms can be best obtained via catheter placement in the distal superficial femoral artery (SFA) when feasible. Standard anteroposterior and lateral oblique projections should be obtained in all cases to allow visualization of the complex vascular anatomy of the foot.15 Moreover, due to the frequent presence of variant and altered pedal anatomy in patients with foot wounds, selective biplane pedal angiography with liberal vasodilator use may help identify target vessels to ischemic tissue and wounds.15
The use of ultrasound (US) to visualize vascular structures and interventional devices has been an important addition during preprocedure planning and intraprocedural guidance, respectively. During PCTO intervention, extravascular US (EVUS) can be used to obtain vascular access, identify the cap of a PCTO, visualize cap penetration, traverse the body of the CTO, or confirm true-lumen access in the target vessel.16,17 These functions are served without any radiation exposure or use of iodinated contrast, but may require a dedicated technologist. Another option might be to consider carbon-dioxide angiography for FP-CTO revascularization; however, BTK imaging may be suboptimal with this imaging technique in patients with chronic kidney disease and heavy vascular calcification of BTK vessels.18
Computed tomography angiography (CTA) and magnetic resonance angiography (MRA) with and without contrast enhancement (flow-sensitive dephasing MRA) have high accuracy in detecting peripheral artery anatomy and lesion morphology and are being increasingly used for preprocedure planning.19,20
Proper selection of one or more of the aforementioned imaging techniques will provide the operator with an in-depth understanding of the anatomic features of the target PCTO and help establish a safe, effective, and efficient strategy for crossing the occluded vascular segment.
Crossing Femoropopliteal and Tibial PCTOs
General principles. While crossing a PCTO, the operator is tasked with making a number of key decisions. This includes selecting vascular access and approach (antegrade and/or retrograde), a crossing strategy (guidewire or dedicated crossing device), and a definitive therapy (ie, stent vs non-stent). While the focus of this review is on PCTO crossing, it is important to understand that these factors also have a profound impact on the definitive treatment delivered to the occluded arterial segment.9
While ascribing to a hierarchical approach for successful crossing of PCTOs, flexibility with altering the approach or strategy based on intraprocedural findings is important.21 This flexible strategy, which is focused on using all possible approaches to successfully, efficiently, and safely traverse a PCTO, is termed the “hybrid approach.”22 More specifically, it refers to shifting the “base of operations” during the procedure. The base of operations constitutes guidewire or crossing device positioning within the PCTO segment relative to the proximal or distal cap during an antegrade or retrograde crossing, respectively.22 The base of operations could be shifted beyond the distal or proximal cap when more advanced techniques are employed to achieve subintimal to true-lumen access, along with the selection of definitive therapy. These advanced techniques include use of re-entry devices, subintimal tracking and re-entry (STAR), limited antegrade subintimal tracking (LAST), controlled antegrade retrograde tracking (CART), and reverse CART (RCART).23 The hybrid approach for crossing PCTOs should be differentiated from a hybrid revascularization technique, which refers to the combined use of open surgical and endovascular approaches to treat advanced PAD.24
Assessment of PCTO features. A PCTO cap refers to the point of occlusion of a vascular segment as defined by angiography and/or EVUS. Identification of cap morphology may guide decisions regarding the proper engagement of the lesion while considering bridging collaterals and side branches with a guidewire supported within a compatible catheter or with a crossing device. The PCTO cap can be blunt or tapered, and the CTOP classification provides a validated approach for using proximal or distal cap morphology to select an initial crossing strategy (Figure 1).25 From a cranio-caudal direction, CTOP cap morphology designates type I as a concave proximal cap and a concave distal cap; type II as a concave proximal cap and a convex distal cap; type III as a convex proximal cap and a concave distal cap; and type IV as a convex proximal cap and convex and distal cap.
In addition to PCTO cap morphology the length of the occluded segment, ambiguity of the proximal cap or vessel course, and the quality of the target vessel may also impact selection of an initial crossing strategy.6 Figure 2 depicts a simplified scheme for considering an initial antegrade, retrograde, or hybrid approach.
Initial approach and escalation. Based on the review of our case survey, CTOP type II caps were most common and type III were least common. Short (≤50 mm) or intermediate (51-150 mm) PCTOs with type I cap morphology and in the absence of severe calcification were most likely to be crossed antegrade. Long (>150 mm) PCTOs with type IV cap morphology were more often crossed from a retrograde approach. A simplified framework outlining a procedure plan for crossing of FP- and BTK-PCTOs is depicted in Figure 3.
Initial antegrade PCTO crossing via a contralateral CFA or an ipsilateral CFA access is preferred when the proximal PCTO cap is tapered and the occluded segment is either short (≤50 mm) or intermediate length (51-150 mm). An antegrade dissection re-entry strategy may be needed for a long occluded length (>150 mm), especially when it involves the popliteal or BTK arteries or the presence of type III or IV stent fracture. If antegrade dissection re-entry is not successful, operators should consider retrograde crossing, provided there are adequate distal access vessel or interventional collaterals.26
While placement of an antegrade sheath at the level of the CFA is appropriate for crossing FP-PCTOs, it is preferable to achieve a distal SFA or popliteal artery positioning for antegrade crossing of BTK-PCTOs. This can be achieved using a 90 cm sheath inserted from the contralateral CFA. This minimizes the use of contrast and provides incrementally better support while crossing BTK-PCTOs. A retrograde approach is commonly employed for CLI; however, it should be used more selectively and cautiously in claudicants and in the presence of more than 1 vessel supplying the foot. Table 1 illustrates factors that should be considered prior to retrograde FP and BTK intervention.
Antegrade wire crossing of FP- or BTK-PCTO should be performed with a guidewire advanced through a compatible support catheter. Primary crossing device use is low and high-quality evidence regarding comparative effectiveness of a wire-catheter vs a primary crossing device is currently lacking. Upon successful crossing, the operator should confirm the target vessel true lumen positioning with angiography or IVUS. Primary technical success refers to successful crossing of the PCTO with the initially selected strategy (wire-catheter or crossing device). Secondary technical success involves switching from wire-catheter to a crossing device or vice versa. Use of a re-entry device is termed provisional technical success (Figure 4).6
A simplified PCTO guidewire selection scheme is shown in Figures 5A and 5B. If an initial guidewire approach is unsuccessful in either penetrating the proximal PCTO cap or traversing the occluded arterial segment, guidewire escalation or use of a crossing device is suggested.27 An escalation strategy should be ideally attempted for approximately 10 minutes, and if unsuccessful should lead to consideration of a retrograde approach.26 Supplemental Table S1 provides a description of key attributes of suggested guidewires used in PCTO interventions. A similar description of dedicated crossing devices and re-entry catheters is included in Supplemental Tables S2 and S3, respectively. It is relevant to state here that haptics of guidewire performance are often individualized and related to personal experience and performance with a toolset of wires.
Dissection re-entry. The crossing strategies described here are based on the direction from which the crossing tools (wires, catheters, or CTO crossing devices) are utilized in order to achieve procedural success. Hence, a crossing strategy is classified as antegrade, retrograde, or antegrade-retrograde (also called rendezvous techniques). If a crossing tool from either direction enters the subintimal space and requires simultaneous use of antegrade and retrograde devices, so-called “advanced” techniques may need to be employed. These techniques include use of re-entry devices, CART, RCART, LAST, STAR, and others.23 Continuing to advance a knuckled guidewire until it spontaneously re-enters the true lumen (usually at a distal bifurcation) is called STAR.28 LAST re-entry is achieved by using a guidewire with an acute (45°-60°, 1-2 mm) distal bend. During retrograde crossing, dissection is usually performed using a knuckle wire, whereas re-entry can be achieved by CART or RCART techniques. Inflating a balloon over the retrograde guidewire, followed by advancement of the antegrade guidewire into the distal true lumen is termed as CART.28 RCART involves inflating a balloon over the antegrade guidewire, followed by advancement of the retrograde guidewire into the proximal true lumen. The subintimal arterial “flossing” with antegrade-retrograde intervention (SAFARI) technique is also used.29 Tunneling, a variation of the rendezvous technique, involves passing of a retrograde or antegrade guidewire from an antegrade or retrograde catheter into a receiving catheter approaching from the contralateral direction.30 The re-back technique refers to use of a needle-based re-entry device into a retrograde balloon to facilitate guidewire externalization.31,32 Controlled dissection re-entry is performed using specialized re-entry devices. The double-balloon technique involves inflation of two abutted balloon catheters advanced over the retrograde and antegrade guidewires. This may help merge the subintimal planes and facilitate true-lumen advancement of the antegrade or retrograde guidewires.33 Some operators have also described the use of an antegrade .035˝ hydrophilic guidewire to disrupt the subintimal septa and allow true-lumen passage of the retrograde guidewire.
It is important to indicate that following an antegrade or retrograde guidewire or crossing-device attempt to penetrate the PCTO cap and cross the lesion, entry into the subintimal space can occur. Conceptual knowledge and technical expertise of dissection re-entry techniques are necessary to traverse the lesion by advancing through the subintimal space and ultimately gain access to or re-enter the true lumen of the target vessel. These techniques form the core skill set needed for predictable crossing of PCTO lesions.
Antegrade dissection re-entry occurs when a guidewire or crossing catheter advances subintimally in a cranio-caudal direction. A looped polymer-jacketed guidewire is generally advanced toward the distal CTO cap without excessive torqueing or rotation. This technique of guidewire handling is called the knuckle-wire technique. It is important to keep the knuckled loop diameter relatively small to safely and reproducibly traverse an occluded peripheral arterial segment. This is accomplished by repeatedly retracting and advancing the guidewire to avoid the formation of an excessively wide loop. The distensible adventitial layer of the vessel has a lower risk of perforation or side-branch penetration with a tightly knuckled guidewire tip. Similar subintimal advancement can be performed with low-profile crossing devices such as the Viance (Medtronic). Creation of a limited subintimal space during antegrade dissection facilitates re-entry into the true lumen using guidewire (STAR or LAST) or re-entry device based methods. For example, the maximum length of the microcatheter lancet beyond the positioning balloon is 20 mm for the Offroad re-entry device (Boston Scientific) and 7 mm for the Outback device (Cordis) and Pioneer device (Medtronic). These devices are best suited for controlled re-entry in the FP segment, while the Enteer re-entry system (Medtronic), which is a self-orienting flat balloon, may be used in tibial artery locations.34 A description of re-entry devices is provided in Supplemental Table S3.
Retrograde dissection re-entry is often required to access the true lumen from a retrograde approach. The RCART method is most commonly used for this purpose. An angioplasty balloon is placed over the antegrade wire adjacent to the retrograde guidewire located in a different subintimal plane within the vessel architecture. Balloon angioplasty creates a connection between the two spaces and allows passage of the retrograde wire into the proximal vessel, followed by externalization with a 300 cm (or longer) guidewire. The ViperWire (CSI) is 335 cm long and provides excellent support. The ViperWire is preferred over the .010˝, 300 cm long RG3 guidewire (Asahi Intecc) and the .013˝, 350 cm R350 guidewire (Vascular Solutions) because it provides greater support for peripheral artery interventions and is the preferred wire for externalization.
The main reason for RCART failure is balloon undersizing. For this reason, RCART under IVUS guidance for appropriate balloon sizing can be performed. As described earlier, the CART technique can also be performed with retrograde balloon dilation of the subintimal track, followed by antegrade guidewire advancement into the distal true lumen. It may be possible to avoid using a 300 cm or longer guidewire during retrograde wire externalization by placing hemostats on the pedal access wire. It is then simple to pull enough guidewire through and advance a support catheter antegrade over essentially any wire, even with contralateral femoral access.
Although balloon-uncrossable lesions after guidewire externalization is rare, the balloon deployment in a forcible manner (BAD FORM) technique can be used in such situations. It involves tightly attaching a torque device on the guidewire exiting the balloon hub and pulling it across the lesion as a unit by applying tension on the guidewire at the opposite end.35
Application of PCTO crossing techniques and strategies. To illustrate application of the outlined PCTO crossing strategies and techniques in clinical use we present three case examples that were selected from the submitted case reports. Figure 6A-Panel 1 represents a mid SFA PCTO lesion in a patient with intermittent claudication despite optimal medical management. The lesion is classified as type IB based on a tapered proximal cap and PCTO length between 50-150 mm in the absence of suboptimal target vessel or involvement of the second or third popliteal artery segments. Based on these clinical and angiographic features, antegrade crossing with a .018˝ V-18 guidewire (Boston Scientific) supported within a compatible straight-tip catheter was selected (Figure 6A-Panel 2). The entry of the guidewire into the subintimal space was followed by limited antegrade dissection using a knuckle-wire technique, leading to a guidewire based re-entry into the true lumen of the target vessel (LAST; Figure 6A-Panel 3). The lesion was predilated and treated with drug-coated balloon angioplasty (Figure 6A-Panels 4 and 5).
Figure 7A-Panel 1 represents a distal popliteal artery BTK-PCTO lesion in a patient with CLI. The lesion is classified as type IIIB based on a tapered proximal cap and PCTO length between 50-150 mm and an ambiguous distal cap (Figure 7A-Panel 2). Based on these features, an initial antegrade crossing with a .014˝ Hi Torque Command guidewire (Abbott Vascular) supported within a compatible straight-tip catheter was selected. The entry of the guidewire into the subintimal space (Figure 7A-Panel 3) was followed by retrograde anterior tibial artery access and Confianza Pro 12 guidewire (Asahi Intecc) entry into the antegrade true lumen following RCART (the hybrid approach, shown in Figure 7A-Panels 4 and 5).
Figure 8 illustrates a case of a long (330 mm) FP-CTO in a claudicant with a previous failed crossing attempt. CTO cap assessment revealed CTOP type II (Figure 8A-Panels 1 and 2). Figure 8A-Panel 3 depicts two-vessel pedal run-off. The lesion is classified as type IIIC and successfully crossed using a hybrid approach. An unsuccessful antegrade attempt using guidewire escalation with Regalia (Asahi Intecc) followed by Astato 20 (Asahi Intecc) was initially employed. Following passage of the antegrade guidewire in a distal SFA subintimal dissection plane similar to the prior crossing attempt, retrograde crossing via a posterior tibial approach was performed (Regalia guidewire in a .035˝support catheter; Figure 8A-Panel 4). The base of operations was established in the mid SFA by retrieving the antegrade .035˝ support catheter to accept the retrograde guidewire (the rendezvous technique; Figure 8A-Panel 5) followed by wire externalization and drug-coated balloon treatment. The selection of the mid SFA base of operation allows placement of a short mid SFA stent at the re-entry zone without stenting the distal SFA and popliteal artery.
Study limitations. This case-survey based consensus document on PCTO crossing reflects contemporary strategies and techniques employed by a select group of experienced operators. It presents a relatively limited perspective on the use of specialized crossing devices consistent with the use of such devices in clinical practice today. Moreover, as most surveyed PCTOs were FP, this consensus document focuses more intently on FP-PCTO crossing. The working group is currently planning a dedicated tibial or BTK-PCTO crossing algorithm document employing a similar case-survey based methodology.
The United States PCTO crossing algorithm presented in this manuscript is designed to provide a simplified, hierarchical, and reproducible approach for crossing FP- and BTK-PCTOs. Given the high prevalence of total occlusions observed during endovascular treatment of symptomatic PAD, the authors believe that familiarity with these techniques can provide operators with a standardized framework for tackling this complex lesion type. The algorithmic approach creates a platform for standardized practice and training directed toward performing successful, efficient, and safe PCTO interventions.
Acknowledgment. The authors wish to acknowledge J. Aaron Grantham, MD; Stéphane Rinfret, MD, SM; R. Michael Wyman, MD; M. Nicholas Burke, MD; Dimitri Karmpaliotis, MD; Nicholas Lembo, MD; Ashish Pershad, MD; David E. Kandzari, MD; Christopher E. Buller, MD; Tony DeMartini, MD; William L. Lombardi, MD; and Craig A. Thompson, MD, MMSC, for providing inspiration and guidance in creating this manuscript.
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From 1the University of Texas Southwestern Medical Center, Dallas, Texas; 2Veteran Affairs North Texas Health Care System, Dallas, Texas; 3University Hospitals Harrington Heart and Vascular Institute, Cleveland, Ohio; 4Advanced Cardiac and Vascular Amputation Prevention Centers, Grand Rapids, Michigan; 5Rocky Mountain Regional VA Medical Center, Denver, Colorado; 6University of Colorado School of Medicine, Aurora, Colorado; 7Texas Tech University Health Sciences Center, Lubbock, Texas; 8Holy Name Medical Center, Teaneck, New Jersey; 9The Surgical Clinic, PLLC, Nashville, Tennessee; 10Baptist Hospital of Miami, Miami, Florida; 11Minneapolis Heart Institute, Abbott Northwestern Hospital, Minneapolis, Minnesota; 12The Cardiac and Vascular Institute, Gainesville, Florida; and 13Columbia University Medical Center and Columbia University College of Physicians and Surgeons, New York, New York.
Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr Banerjee reports institutional grants from Boston Scientific, Abbott Vascular, and Chiesi; consulting fees from AstraZeneca. Dr Shishebhor reports personal fees from Medtronic, Abbott Vascular, Boston Scientific, Terumo, Philips. Dr Mustapha reports consulting and teaching income from Bard Peripheral Vascular, CSI, Medtronic, Terumo Medical, and Philips. Dr Armstrong reports consultant income from Abbott Vascular, Boston Scientific, Medtronic, and Philips. Dr Ansari reports no conflicts of interest. Dr Rundback reports compensation from VIVA Physicians (board member); consultant income from Bayer, Daichii-Sankyo, Philips, Toray, and Vesper; advisory board for Abbott Vascular, Boston Scientific, Medtronic; speakers’ bureau income from Abbott Vascular, Bard, Cook Medical, CSI, Gore, Medtronic; institutional grant support: Site PI for Medtronic, NIH, Biologic, Cordis, PQ Bypass, Limbo, Surmodics, Intact Vascular, SAVAL, PLX-CLI03, Boston Scientific; Site Co-PI for Ekos, AV INPACT; National PI for EXIMO; stock options for EXIMO. Dr Fisher reports advisory board income from CSI, Medtronic, Philips, Abbott Vascular, Bard, Cordis; research grants from Philips, Terumo. Dr Peña reports speaker honoraria from Bard, Cook Medical, Penumbra, BTG, WL Gore, Medtronic, Abbott Vascular; consultant income from Boston Scientific; Avanos; investor in Brightwater, Cagent Medical. Dr Brilakis reports consulting/speaker honoraria from Abbott Vascular, Boston Scientific, CSI, Elsevier, GE Healthcare, Medicure, InfraRedX, and Medtronic; research support from Regeneron and Siemens; board of directors for Cardiovascular Innovations Foundation; board of trustees for Society of Cardiovascular Angiography and Interventions. Dr Lee reports honoraria from CSI, Cook Medical; consultant income from Bard. Dr Parikh reports advisory board income from Abbott Vascular, Boston Scientific, Medtronic, CSI, Philips; consultant honoraria from Asahi Intecc, Terumo, Meril Lifesciences, Siemens, HeartFlow, Abiomed; research grant support (Site PI) for Shockwave Medical, Surmodics, TriReme Medical; institutional research grants from the NIH.
Manuscript submitted December 27, 2018 and accepted January 7, 2019.
Address for correspondence: Subhash Banerjee, MD, Dallas VA Medical Center, 4500 S. Lancaster Road (111a), Dallas, TX 75216. Email: email@example.com