Fig. 5.1
With increasing lesion complexity there is a decreasing applicability for antegrade wire escalation based strategies and an increasing need for strategies utilizing blunt dissection. AWE Antegrade Wire Escalation, DRE Dissection-Re-Entry
Lesion Length
CTO length is a predictor of both case efficiency and chance of inadvertent sub-intimal wire passage [2]. Whilst it is possible to treat longer occlusions by AWE, increasing CTO length functions as a surrogate marker of procedural efficiency, with longer occlusions resulting in longer case times, higher inadvertent rates of sub-intimal wire passage and lower procedural success rates [5]. There is no biological explanation for a binary cut-off in length when deciding what strategy to employ and it may be reasonable for other anatomical features to influence this, not least the feasibility of other strategies.
Lesion length is both a measure of complexity and procedural duration (efficiency), with longer lesions (>20 mm) requiring a longer time to wire and being associated with both a higher chance of inadvertent sub-intimal wire passage and vessel perforation [6]. The argument has been made, therefore, for the prospective use of blunt dissection techniques in longer lesions, with >20 mm having been chosen as the cut-off [7]. It may be, that in the argument for aiming for a more efficient procedure, the development of newer guide wires will result in longer occlusions being treatable efficiently by AWE.
Proximal Cap Anatomy
For a procedure to be successful from an antegrade perspective, the proximal cap needs to be defined, either angiographically, or by adjunctive imaging (CT coronary angiography or intravascular ultrasound). The morphology of the proximal cap can be described as tapered, blunt or ambiguous (Fig. 5.2), where ambiguity is defined as a lack of clarity as to the subsequen vessel course.
Fig. 5.2
The proximal cap is a key determinant of procedural strategy and be described as tapered, blunt or ambiguous
The proximal cap of the occlusion, submitted to diastolic pressure, is usually the area of the most adverse vessel remodeling [8], with higher concentrations of both fibrous and calcific tissue being present. A blunt proximal cap is more likely to be resistant to penetration than a tapered cap, with a higher implied need for coronary guidewires with a high penetration force. There is also a relationship between how “blunt” the proximal cap is and lesion chronicity [9]. If a proximal cap is described as “ambiguous”, the term describes a lack of clarity of the subsequent vessel course. The most common causes of “ambiguity” in the proximal CTO cap are either the presence of a (at least moderate sized) side branch or multiple bridging collaterals. Proximal caps associated with significant adverse vessel remodeling and or calcium are more likely to require the early use of highly penetrative wires to advance beyond the proximal cap of the occlusion. The combination characteristics, therefore, of both “bluntness” and ambiguity suggest a requirement for a high penetration wire and a lack of certainty over where it should be directed. It is for this reason that the presence of an ambiguous cap usually implies that a retrograde strategy is best employed and believed to be safer than an antegrade approach. For an AWE strategy to be employed, any ambiguity of the proximal cap must be clarified.
In-CTO Tortuosity (>45°)
Tortuosity within the CTO segment is associated with both a higher risk of inadvertent sub-intimal wire passage and vessel perforation [6]. This is especially the case when associated with calcium, where the higher resistance to forward progress often necessitates the use of highly penetrative wires, with a consequently higher risk of vessel exit at points of curvature. The presence of in-CTO tortuosity is both a predictor of lesion complexity [5] and a marker of the peri-procedural need to switch strategy [10, 11].
Presence of Intra-CTO Calcium
The presence of calcium indicates a higher need for penetrative wires and a highly supportive interventional set-up. Where the calcium is very severe, it may not be possible to penetrate with the current generation of wires and blunt dissection strategies may need to be employed to circumnavigate extreme areas of calcification.
Previous Procedural Failure
As a marker of failure this captures factors associated with lesion complexity, which may be additive, rather than individually predictive, of lesion complexity. In addition factors related to the initial procedure may create difficulties for subsequent procedures, such as the creation of sub-intimal dissection planes.
Other Factors
So, whilst increasing complexity may predict a decreasing chance of success by AWE, there are specific anatomic considerations that must also be taken into account when deciding the initial choice of strategy:
Degree of Disease in the “Distal Landing Zone”
The “quality” or degree of plaque burden, of the vessel beyond the distal cap can be referred to as the “distal landing zone”. This term largely relates to the feasibility of re-entry procedures, which have negotiated the CTO segment by blunt dissection and aim to “re-enter” the lumen of the distal vasculature from the sub-intimal space. A highly diseased distal landing zone may, however, adversely affect the chances of any antegrade strategy and may favour, if extreme, a primary retrograde procedure.
Presence of “Interventional Collaterals”
Whilst not directly affecting the feasibility of AWE, the presence of collaterals that can be crossed by interventional equipment (and thus deemed “interventional”) will affect the pre-procedure planning in selecting the most appropriate strategy [12].
Basic Principles of CTO PCI
The fundamental tenets of CTO PCI address the differences between CTO PCI and “conventional” PCI and set the groundwork for a successful procedure regardless of strategy employed.
Planning
Ad hoc CTO PCI is not recommended, given the critical role planning plays in assessing the anatomical features discussed above. Off-line analysis facilitates detailed analysis of collateral channel pathways enabling a more considered evaluation of the risk/benefit ratio [12].
Visibility
Occlusive plaque renders the vessel course, in the absence of significant adventitial calcium, invisible. Whilst its course may be inferred, from either previous angiographic films or predictive anatomy, this is inaccurate and difficult to employ within highly mobile coronary vessels. Most CTOs are supplied by the contralateral coronary circulation, so the distal vasculature beyond the occlusion, when viewed by an antegrade injection, is either invisible or only faintly visible. Visualizing the distal coronary bed, via a second guide or diagnostic catheter, is a critical way of ensuring any progress is within the vessel structure. Dual injections also offer an invaluable way of assessing the contralateral collateral circulation, assessing the feasibility of a potential retrograde approach, accurately assessing CTO length and the size and location of the distal target vessel, evaluating whether there is a significant bifurcation at the distal cap, and thus for deciding on the optimal CTO PCI strategy. It is not infrequent to reveal patent microchannels within the CTO segment that were invisible with single catheter injection. The reliance on single catheter visualization for CTO PCI not only reduces distal vessel visibility, rarely adding useful information on wire progress, but also runs the risk of proximal contrast-induced dissection. Dual injection is best performed at low magnification, with prolonged imaging exposure, and without table panning, to allow for optimal delineation of the CTO segment and collateral vessel location and course. The donor vessel (vessel that supplies the territory distal to the CTO) is injected first, followed by injection of the occluded vessel. Bridging collaterals are important to recognize and avoid during both antegrade and retrograde crossing attempts to minimize the risk of vessel perforation, especially when advancing microcatheters and balloons, as their course is invariably outside the CTO artery structure. It is recommended that dual catheter injections are undertaken with interventional guides in both CTO and donor vessel. This will permit a rapid change in strategy if required. However, for operators who are not proficient with the retrograde approach, a diagnostic catheter can be used for contra-lateral guidance of the antegrade work.
Back-up Support
There is a high need for back-up support within CTO procedures. This is most commonly increased passively by larger French guide catheters, but can also be addressed successfully by “high-support” catheters. Additionally the back up support provided to CTO crossing (either by wires or microcatheters/balloons) can be increased by several methods. For more details, please refer to Chap. 12.
Use of “Over-the-Wire” Equipment
As CTO procedures use wires for highly selective purposes, there is a high need to either exchange wires or reshape wire tips. The use therefore, of over-the-wire (OTW) equipment enables wire exchange or reshaping without loss of position. OTW equipment will also help increase back-up support for wires, with the weight required to deflect the tip of the wire increasing as the OTW equipment is moved closer to the wire tip. Either OTW balloons or microcatheters can be used; however, OTW balloons are usually less adapted, with balloon tips generally too stiff to deliver co-axial force within tortuous coronary arteries. Moreover, they are more easily kinkable after guidewire removal, in such situation leading to the incapacity to advance a wire though the catheter. Finally, the marker is not at the tip; true position of the distal balloon tip may be uncertain. For all those reasons, we strongly recommend the use of a microcatheter. Such microcatheters are discussed in Chap. 3.
Wire Selection
There have been considerable advances in wire technology, which have enabled more complex CTOs to be treated. The engineering characteristics of the wire can now be translated with a high degree of precision into clinical characteristics and should inform the operator’s choice of wire. While a myriad of choices are available, a more circumscribed choice facilitates a greater understanding of wire handling characteristics, with associated efficiency and economic benefits. A choice of four specific CTO wires will cover most anatomy and can be divided up accordingly:
Tapered Polymer-Coated Wires
These are employed to access fine, difficult to visualize angiographically, channels. The polymer coating increases wire lubricity, allowing it to negotiate plaque-dense environments, which otherwise exposed coil wires would be unable to. The low gram weight of the wire, combined with the distal polymer sleeve, means they are rarely associated with vessel exit or inadvertent dissection, as such they are often chosen as the first wire of choice [1]. Examples are the Fielder XT, Fielder XT-A or Fielder XT-R.
Medium Weight Wires
There is considerably more diversity within this subset, with variations dependent on tip load, tip coating and torque transmission. Choices are informed on differential ability to transmit torque and the lubricity of either the sleeve or the tip of the wire. Medium gram weight polymer wires are a reliable step-up wire, where low gram weight polymer wires have failed to progress. Again, the presence of the polymer sleeve makes spontaneous vessel exit unlikely at the cost of decreased tactile feel. Examples are the Pilot 150 or 200.
High Gram Weight Wires
These highly specified wires are designed to penetrate dense, occlusive plaque and are often tapered at the tip to increase the amount of penetration force that can be applied. The trade-off for the high penetration force is a relative lack of tactile feel which restricts their use to well defined anatomy. Commonly used wires in that family are the Confianza Pro 12, or Progress 200 T.
Medium Weight Highly Toqueble Wires
This is the Gaia wire family. These wires have a unique internal design and tapered tip that turn them extremely stiff and resistant when torqueing is applied to the wire, but their tip will still deflect when pushed forward. With the Gaia wires, a torqueing device should be used, with minimal rotations, limited to 90° on each direction. The wire is pushed in the desired direction. When the tip deflects, the body of the wire enters into a sinusoidal conformation; at this point, the wire is pulled back and its tip redirected. This combination of push and turn is most likely to be successful with this wire. Those tapered wires can exit the vessel structure; therefore, care should be applied when using these wires in ambiguous CTO segments.
The Proximal Cap & How to Assess It
As part of any antegrade procedure, assessment of the proximal cap of the occlusion is perhaps the most important procedural element. Histological data informs us that it is the site of the highest plaque density, with more evidence of calcification and adverse remodeling [8]. Any attempt to wire the distal true lumen of an occlusion may be hampered by early sub-intimal wire passage, as a consequence of eccentric proximal calcification and a failure to appreciate the 3D anatomy of the cap. The proximal cap should be visualized in several (at least 3) orthogonal planes, without radiographic panning. If this is insufficient to define the proximal cap, a selective injection of contrast can be made, via a microcatheter placed just proximal to, but not in, the cap.
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