The hybrid approach to CTO intervention

Summary

The “hybrid” algorithm for chronic total occlusion (CTO) percutaneous coronary intervention (PCI) was developed to guide selection of the optimal CTO crossing strategy in a stepwise fashion. Dual injection should be performed in nearly all CTO PCI procedures. Four angiographic lesion characteristics are assessed (1) proximal cap ambiguity, (2) quality of the distal vessel, (3) lesion length and (4) presence of collateral vessels suitable for the retrograde approach. Wire escalation is favored for <20 mm long lesions, whereas dissection and re-entry is favored for >20 mm long lesions. The antegrade approach is favored when the proximal cap is clear, and the retrograde approach is favored for lesions with an ambiguous proximal cap and/or diffusely diseased distal vessel, provided that appropriate collateral vessels are present. Early change of crossing strategy is recommended if the initially selected crossing strategy fails.

Rationale for the hybrid algorithm

Several techniques have been developed for crossing coronary chronic total occlusions (CTOs). These techniques can be classified according to the direction of wire advancement (antegrade and retrograde) and use of the subintimal space (intraplaque wiring vs. dissection and reentry) (Figure 1) ^,^,. The CTO-ARC document recommends using the term “intraplaque” (for wire tracking within the occlusive intima-based plaque) and “extraplaque” (for wire tracking outside the plaque but still contained within the adventitial layer when describing the device course within the occluded CTO segment .

Figure 1

**Overview of the CTO crossing strategies.**

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In the antegrade approach the occlusion segment is approached from the proximal CTO cap with the intention of crossing the distal CTO cap into the distal true lumen . In the retrograde approach the occlusion segment is approached from the distal CTO cap with the intention of accessing the proximal CTO cap into the proximal true lumen .

Choosing the optimal technique for CTO crossing can be challenging and can be facilitated by an algorithmic approach. 1 The hybrid algorithm (Figure 2) was the first CTO crossing algorithm, developed in 2011 and published in 2012. Its goal has been to open the occluded vessel in the most safe, effective, and efficient way, tailoring all available crossing techniques to each specific case . The “hybrid” approach has been used in a large number of cases both in the US ^,^,^,^,^,^,^, and in Europe ^,^, with high success rates and is also useful in learning and teaching CTO PCI. Additional algorithms, such as the Asia Pacific algorithm and the EuroCTO algorithm were subsequently developed, adapting the key concepts of the hybrid algorithm to CTO PCI practices at various geographies. The hybrid algorithm continues to evolve incorporating novel equipment and techniques ^,.

Figure 2

**Overview of the updated hybrid algorithm.**

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Description of the hybrid algorithm

FOCUS BOX 1 The steps of the hybrid approach to CTO crossing:

Dual injection
Assessment of CTO characteristics
Antegrade wiring
Antegrade dissection/reentry
The retrograde approach
Change
When to stop
CTO modification when CTO crossing fails

Step 1: Dual Injection

The first and most important step of CTO PCI is to perform dual coronary injection, in nearly all cases, unless there are no contralateral collaterals. Dual injection allows good visualization of both the proximal and distal vessel, as well as the collateral circulation, allowing selection of the most suitable initial crossing technique . Dual injection also clarifies the location of the guidewire(s) during crossing attempts and can facilitate management of complications, such as perforation . In patients with ipsilateral-only collaterals, selective contrast injection in the collateral donor branch through a microcatheter can reduce contrast administration and reduce the risk of extending antegrade dissections . Since most coronary angiographies performed prior to CTO PCI do not include simultaneous injections, the CTO crossing plan should be re-assessed at the time of the procedure, after obtaining dual injections.

Step 2: Assessment of CTO characteristics

Detailed review of the coronary angiogram (and coronary computed tomography angiography, if available ^, forms the basis for crossing strategy selection. Four angiographic parameters are assessed: (a) the morphology of the proximal cap; (b) the length and morphology (tortuosity, calcification, course ambiguity) of the occlusion; (c) the vessel size, quality and presence of bifurcations of the distal vessel; and (d) the location and suitability of collateral channels or bypass grafts for retrograde access (Figure 3) :

Figure 3

**Review of the coronary angiogram for planning CTO percutaneous coronary intervention.**

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FOCUS BOX 2 The key components of angiographic review to guide CTO PCI:

Proximal cap (clear or ambiguous location, tapered or blunt, side branches, calcification)
Occlusion length and morphology (tortuosity, calcification, ambiguity of its course)
Distal vessel (size, bifurcations, tortuosity, calcification)
Potential retrograde options for reaching the distal true lumen, such as collaterals (type, size, tortuosity, entry and exit angle, distance between exit and distal cap) or aortocoronary bypass grafts

1. Proximal cap location and morphology

Clearly identifying the entry point to the occlusion is critical for crossing strategy selection, as advancing guidewires and microcatheters through an ambiguous proximal cap can cause perforation. Performing multiple angiographic projections, reviewing prior angiograms, performing intravascular ultrasound (if there is a side branch next the proximal cap) and contrast injection through a microcatheter located just proximal to the occlusion can help clarify the location of the proximal cap (Figure 4). Additional approaches to proximal cap ambiguity include dissection/reentry strategy proximal to the occlusion (“move the cap” techniques ) and retrograde crossing.

Figure 4

**Antegrade and retrograde approaches to CTOs with ambiguous proximal cap.**

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An ambiguous proximal cap increases the complexity of the procedure and decreases the likelihood of success . A favorable proximal cap is one that is tapered, as opposed to blunt, and has no bridging collaterals or major side branches that could make engagement of the CTO segment difficult using traditional wire escalation techniques. A particularly challenging anatomic subset is that of flush aorto-ostial occlusions, which often require use of a primary retrograde approach.

2. Lesion length and morphology

Longer occlusions are often harder to cross. CTO lesion length is dichotomized into <20mm and ≥20mm long . Dual injection or coronary computed tomography angiography is needed for accurately assessing the occlusion length, which is usually overestimated when using single coronary injection.

Antegrade or retrograde crossing of short (<20 mm) CTOs is usually done with antegrade or retrograde wiring, whereas in long (≥20 mm) CTOs, dissection/re-entry techniques are often needed, especially for very long and tortuous occlusion segments. Lesion length <20 mm has been associated with faster CTO crossing in the J-CTO (Multicenter CTO Registry in Japan) registry .

3. Distal vessel

A distal vessel of large caliber (>2.0 mm) that fills well, does not have significant disease and is free from calcification and major branches makes CTO recanalization easier . Conversely, small, diffusely diseased, tortuous and calcified distal vessels are more challenging to recanalize, especially following subintimal guidewire entry. Some distal vessels are small due to hypoperfusion, leading to negative remodeling and often increase in size after recanalization . Distal CTO caps of native coronary artery CTOs in prior CABG patients are often calcified and resistant to guidewire penetration, as they have been exposed to systemic arterial pressure. Distal vessel calcification may hinder wire re-entry in case of subintimal guidewire entry.

4. Collateral circulation

Retrograde access to the distal vessel can be obtained via septal collaterals, epicardial collaterals (called non-septal in the CTO-ARC document ), or (patent or occluded) aortocoronary bypass grafts. Saphenous vein grafts ^,^, and septal collaterals are preferred over epicardial collaterals, as perforation of the latter is more likely to cause tamponade ^,^,. Retrograde CTO PCI via left internal mammary grafts is sometimes feasible but carries high risk of complications . Optimal collateral vessels for retrograde CTO PCI :

Originate from a healthy (or repaired) donor vessel.
Can be easily accessed with wires and microcatheters.
Have minimal tortuosity .
Have large caliber. The size of the collaterals is often assessed using the Werner classification (CC0: no continuous connection; CC 1: threadlike connection; CC2: side branch-like connection) . Crossing invisible septal collateral channels is often possible with the surfing technique, letting the wire find the path of least resistance .
Are not the only (or dominant) source of flow to the CTO segment (which places the patient at risk for intraprocedural ischemia during crossing of the collateral).
Have favorable entry and exit angle, facilitating entry and exit of the guidewire.
Enter the CTO vessel distal to the distal cap.

More favorable collateral circulation characteristics make it easier to use retrograde techniques, both as the initial strategy or as an early crossover strategy. What constitutes an “interventional” collateral (i.e. a collateral that can be wired during a retrograde approach) depends on the experience and skills of the operator. In-depth understanding of the collateral circulation is important during antegrade crossing attempts, because dissection re-entry techniques and the formation of subintimal hematomas may compromise ipsilateral or bridging collaterals, leading to poor visualization of the distal vessel at the re-entry zone and occasionally ischemia.

Step 3: Antegrade Wiring

Antegrade wiring (also called antegrade wire escalation) is the most widely used CTO crossing technique ^,^,^,. Various guidewires are advanced in the antegrade direction (original direction of blood flow). Guidewire choice depends on the CTO characteristics. If there is a tapered proximal cap, a polymer-jacketed, low penetration force, tapered guidewire is used initially, with subsequent escalation to intermediate and high penetration force guidewires, if needed. If there is a blunt proximal cap, antegrade wiring is usually started with an intermediate penetration force polymer-jacketed guidewire, or a composite core guidewire. Stiff, high penetration force guidewires may be required in highly resistant proximal caps or when areas of resistance are encountered within the body of the occlusion. After crossing 1-2 mm through the proximal cap, de-escalation to less penetrating guidewires should follow before navigating through the CTO segment to minimize the risk of perforation .

Step 4: Antegrade Dissection and Re-Entry

For long lesions approached in the antegrade direction, upfront use of a dissection/re-entry strategy is often recommended, especially in the presence of severe tortuosity or calcification. Dissection can be achieved either by advancing a “knuckle” formed at the tip of a polymer jacketed guidewire (such as the Gladius Mongo, Fielder XT, Bandit, Fighter, or Pilot 200) or by using the CrossBoss catheter (the CrossBoss catheter is currently mainly used for in-stent CTOs) . Antegrade dissection minimizes the risk for perforation (by the blunt guidewire loop or by the CrossBoss catheter tip) and allows for rapid crossing of long occlusion segments. Reentry into the distal true lumen is currently performed using the Stingray system in most cases, although novel wire-based techniques (such as antegrade fenestration and re-entry ) and novel dual lumen microcatheters (such as the ReCross that has two over-the-wire lumens) can be useful in selected cases. Guidewire advancement distal to the distal cap should be minimized to reduce the risk of subintimal hematoma formation that can hinder reentry into the distal true lumen.

Step 5: The retrograde approach

The retrograde technique differs from the antegrade approach in that the occlusion is approached from the distal vessel with guidewire advancement against the original direction of blood flow ^,. A guidewire is advanced into the artery distal to the occlusion through a collateral channel or through a bypass graft, followed by placement of a microcatheter at the distal CTO cap. Retrograde CTO crossing is then attempted either with retrograde wiring (usually for short occlusions, especially when the distal cap is tapered ) or using retrograde dissection/reentry techniques, most commonly the reverse controlled antegrade and retrograde tracking (reverse CART) technique. The retrograde wire can also facilitate antegrade wiring, acting as marker of the distal true lumen (“just marker” technique).

The retrograde approach is critical for achieving high success rates, especially in more complex occlusions, ^, however it is also associated with an increased risk of complications, such as myocardial infarction ^,^,^,, perforation and donor vessel injury ^,. Hence, approaching a CTO in the antegrade direction first is preferred in most cases, if feasible.

The retrograde approach can be used either upfront (primary retrograde) or after a failed antegrade crossing attempt ^,^,^,^,^,^,^,. Factors that favor a primary retrograde approach include flush aorto-ostial or branch ostial occlusion, ambiguous proximal cap, distal cap at a bifurcation, poor distal vessel quality, good interventional collaterals, and also heavy calcification and chronic kidney disease (as the retrograde approach can often be performed using smaller amounts of contrast).

Step 6: Dynamic change in procedural strategies

Alternating between different crossing strategies enhances the success, safety, and efficiency of CTO PCI. If the initial or subsequent crossing strategy does not achieve progress, small changes (such as modifying the guidewire tip angulation or changing guidewire) or more significant changes (such as converting from antegrade wiring to antegrade dissection/reentry or the retrograde approach) should be made, based on pre-procedural planning and the evolution of the procedure ^,. The operator should avoid getting “stuck in a failure mode”, in which excessive time, radiation and contrast are expended with little or no progress being made while repeatedly attempting the same technique. This may preclude the use of alternative strategies and increase the risk of complications . Optimal application of the hybrid algorithm requires expertise with all crossing strategies, to minimize impediments to making a change.

Only approximately 50-60% of CTOs are successfully crossed with the initially selected crossing strategy ^,^,^,. The timing and choice of subsequent crossing strategies depends on lesion characteristics, challenges encountered with the original technique, equipment availability and operator expertise ^,^,.

Step 7: When to stop

Reasons to stop a CTO-PCI attempt include:

occurrence of a complication
high radiation dose (usually >5 Gray air kerma dose in the absence of lesion crossing or substantial progress)
large contrast volume administration (>3.7x the estimated creatinine clearance)
exhaustion of crossing options, and
patient or physician fatigue .

Continuous assessment of risk vs. benefit should guide decision-making and choice of strategy during various stages of the procedure. On many occasions, it may be best to fail rather than pursue highly aggressive strategies that may lead to serious complications .

Step 8: CTO modification

CTO modification (often called subintimal plaque modification or “investment procedure”) can sometimes be used when CTO crossing attempts fail (for example due to failure to reenter into the distal true lumen or very distal guidewire reentry). CTO modification is performed by extraplaque balloon angioplasty with or without subintimal tracking and re-entry (STAR). CTO modification can facilitate subsequent CTO crossing attempts by modifying the plaque and creating dissection planes ^,. The optimal interval for repeat crossing attempts is unclear at present but most operators wait for 6-8 weeks to allow healing of the dissections.

Conclusion

In summary, a systematic, algorithmic approach to CTO crossing that relies on dual coronary injection and careful angiographic review to select the initial and subsequent crossing strategies along with prompt change of approach when failing to achieve progress can help improve the success and efficiency and reduce the risk of complications of CTO PCI.

Personal perspective

Emmanouil Brilakis

The hybrid algorithm systematized the approach to CTO crossing using a logical step-by-step approach. It also streamlined training in CTO PCI at various practice settings and career stages ^,^,^,^,^,^,^,. Procedural success at centers experienced in hybrid CTO PCI is close to 90% with approximately 3% risk for major complications .

Successful application of the hybrid approach requires expertise in various CTO equipment and techniques. A common training pathway is to start with antegrade wire escalation, followed by antegrade dissection/reentry, retrograde via saphenous vein grafts and septal collaterals, and finally retrograde via epicardial collaterals (Figure 5) .

Figure 5

**The 4 stages of learning CTO PCI.**

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Achieving 100% success is unlikely to be achieved with current equipment and techniques without a significant increase in the incidence of complications . The operator should continually assess the potential risks and benefits of implementing a new technique vs. continuing with the currently utilized technique. Determining the optimal duration for each step of the procedure depends heavily on operator experience.

There is an ongoing international collaboration for unifying the key principles of the hybrid and newer CTO crossing algorithms into a global algorithm that will further advance performance of CTO PCI and training in CTO PCI around the world.

Additional resources

See the Video on the chapter content

Video

The hybrid approach to CTO intervention

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Check out the 100+ hybrid case recordings available online at https://www.ctomanual.org/