**9. Specialized CTO recanalization techniques**

As described above, CTO remains one of the most difficult subsets in interventional treatment of CAD patients and is generally considered to be challenging during a revascularization approach because of high procedural complexity. With the introduction of innovative catheter-based devices and the development of standardized treatment algorithms, CTO PCI has been increasingly performed with high success and low complication rates. At this, a thorough assessment of specific lesion-related factors and the use of a systematic step-up interventional strategy contribute to lower periprocedural comorbidities with better post-procedural outcome [98, 174].

Currently, there are three major CTO crossing techniques: (1) antegrade wire escalation (AWE), (2) antegrade dissection re-entry (ADR), and (3) retrograde procedures including retrograde wire escalation (RWE) and retrograde dissection re-entry (RDR).

### **9.1. Antegrade techniques**

#### *9.1.1. Antegrade wire escalation*

AWE is the most widely used CTO crossing technique and is appropriate for short occlusions or extended ones where a remaining microchannel is expected [174, 175]. However, AWE was found to be unlikely successful in complex lesions [176].

Classical antegrade techniques are single wire-based starting with a soft hydrophilic wire seeking for microchannels, followed by gradual escalation to harder and stiffer wires [98]. Higher penetrating force is needed in more fibrous and calcified lesions, and nonhydrophilic wires represent a good alternative for loose tissue or intimal plaque tracking [98]. However, strong angulated lesions with evidence of bridging collaterals usually exhibit a higher risk of perforation, and the use of stiffer wires with a higher tip load and penetration force in these cases should be cautious [174]. Furthermore, gradually increasing wire tip load with the goal of finding the safest wire has the potential to decrease risk of perforation [98].

When performing AWE, the guidewire is advanced to the occlusion point, advanced across the lesion, and followed by the microcatheter that adds support and increases penetration power, allows wire exchange or wire reshaping, and finally maintains position once the lesion is crossed to place an extra support wire for balloon dilatation and stenting [69]. In case of subintimal positioning, the wire is guided back into the true lumen by different techniques or withdrawn and redirected if it leaves the target vessel [174].

Parallel wire techniques facilitate re-entry of the true lumen by leaving the first wire in the subintimal space to seal the false track and act as a marker. Continued manipulation of this wire close to the distal cap should be avoided as it can cause subintimal hematoma that compresses the distal true lumen and complicate re-entry. A second penetrating wire is therefore introduced using a microcatheter, and an attempt is made at redirection into the true lumen. Double lumen microcatheters contain both a monorail and an OTW port and are ideally suited for parallel wiring techniques.

Seesaw wiring involves simultaneous use of two microcatheters and wires and has the advantage of avoiding the need for complex exchange of OTW microcatheters. Also, wires can be reshaped and their roles switched promptly.

#### *9.1.2. Antegrade dissection re-entry*

ADR techniques make intentional use of a dissection plane in the subintimal space for crossing CTOs. This concept was first introduced by Antonio Colombo who originally advanced a knuckled guidewire through the subintimal space until it spontaneously re-entered into the distal true lumen (subintimal tracking and re-entry technique) [177]. However, high restenosis and reocclusion rates are found in extensive subintimally stented lesions [165]. Therefore, ADR should not be enforced as a first-line technique. The mini-STAR was presented as bail-out technique and includes limited subintimal tracking distances [178] associated with improved outcomes [179]. Dedicated subintimal tracking and re-entry devices such as the CrossBoss catheter and Stingray balloon allow controlled re-entry into the distal true lumen from the subintimal space [180, 181].

#### **9.2. Retrograde techniques**

Currently, there are three major CTO crossing techniques: (1) antegrade wire escalation (AWE), (2) antegrade dissection re-entry (ADR), and (3) retrograde procedures including ret-

AWE is the most widely used CTO crossing technique and is appropriate for short occlusions or extended ones where a remaining microchannel is expected [174, 175]. However, AWE was

Classical antegrade techniques are single wire-based starting with a soft hydrophilic wire seeking for microchannels, followed by gradual escalation to harder and stiffer wires [98]. Higher penetrating force is needed in more fibrous and calcified lesions, and nonhydrophilic wires represent a good alternative for loose tissue or intimal plaque tracking [98]. However, strong angulated lesions with evidence of bridging collaterals usually exhibit a higher risk of perforation, and the use of stiffer wires with a higher tip load and penetration force in these cases should be cautious [174]. Furthermore, gradually increasing wire tip load with the goal of finding the safest wire has the potential to decrease risk of perforation

When performing AWE, the guidewire is advanced to the occlusion point, advanced across the lesion, and followed by the microcatheter that adds support and increases penetration power, allows wire exchange or wire reshaping, and finally maintains position once the lesion is crossed to place an extra support wire for balloon dilatation and stenting [69]. In case of subintimal positioning, the wire is guided back into the true lumen by different techniques or

Parallel wire techniques facilitate re-entry of the true lumen by leaving the first wire in the subintimal space to seal the false track and act as a marker. Continued manipulation of this wire close to the distal cap should be avoided as it can cause subintimal hematoma that compresses the distal true lumen and complicate re-entry. A second penetrating wire is therefore introduced using a microcatheter, and an attempt is made at redirection into the true lumen. Double lumen microcatheters contain both a monorail and an OTW port and are ideally

Seesaw wiring involves simultaneous use of two microcatheters and wires and has the advantage of avoiding the need for complex exchange of OTW microcatheters. Also, wires can be

ADR techniques make intentional use of a dissection plane in the subintimal space for crossing CTOs. This concept was first introduced by Antonio Colombo who originally advanced a knuckled guidewire through the subintimal space until it spontaneously re-entered into

rograde wire escalation (RWE) and retrograde dissection re-entry (RDR).

found to be unlikely successful in complex lesions [176].

withdrawn and redirected if it leaves the target vessel [174].

suited for parallel wiring techniques.

*9.1.2. Antegrade dissection re-entry*

reshaped and their roles switched promptly.

**9.1. Antegrade techniques**

58 Interventional Cardiology

[98].

*9.1.1. Antegrade wire escalation*

As complexity rises, advanced techniques are needed to improve procedural success. The retrograde approach has the ability to significantly increase success rates, particularly in challenging lesions (**Table 2**) and has become a widely used strategy for CTO PCI during recent years [182, 183]. Retrograde crossing of the CTO against the direction of blood flow is easier due to the softer, often tapered, and less ambiguous distal cap [15]. These properties in contrast to proximal cap morphology during an antegrade approach facilitate entering the CTO body with the retrograde guidewire. Additional advantages of the retrograde approach are found in the presence of ostial occlusions, unfavorable proximal cap (blunt stump, side branch), ambiguity of the occluded segment, poor distal target or distal bifurcation [184], and good interventional collaterals in post-CABG patients and in failed antegrade cases.

Retrograde CTO PCI can be performed via several collateral pathways including transseptal collaterals [185, 186], epicardial collaterals, and SVG [187]. Intraseptal collaterals are nonepicardial vessels, representing a safe route for CTO PCI with a lower risk of rupture, pericardial effusion, and tamponade [188]. The use of microcatheters seems to dramatically reduce injury to septal channels during a transseptal retrograde approach [189] and also increases the availability of additional routes through tortuous epicardial collaterals [190]. Previously, the CART technique with its retrograde approach was limited to the transseptal pathway in


After failed antegrade attempt

**Table 2.** Anatomical features favoring the retrograde approach.

nearly 80% and resulted in more balloon dilatations of the septal channels and a higher perforation rate [55, 191].

An in-hospital analysis of procedural and long-term outcomes from the European multicenter ERCTO registry demonstrated increased numbers of safe and successful retrograde procedures with good long-term outcomes [192]. However, the retrograde approach also seems to be independently associated with increased risk of periprocedural complications [193]. IVUS, as described above, can serve as a useful tool for the detection of procedure-related vessel damage and subintimal wire tracking to help guide retrograde CTO PCI [55].
