2.1 Retrograde procedure in CTO score

The Retrograde procedure in CTO (RP-CTO) score study [15] encompassed 233 patients with a total of 228 consecutive CTO lesions from January 2011 to May 2015, conducted by two operators in a single center in China, of which 68.4% were lesions that failed the antegrade attempts. For each lesion using the retrograde technique, the objective was to create and verify a prediction score for successful retrograde CTO-PCI. Two-thirds of the patients were used for the derivation set, while one-third were used for the validation set. The derivation set, comprising 152 patients, was used to identify the independent factors related to retrograde CTO-PCI and develop the RP-CTO score. The validation set, consisting of 76 patients, was used to validate the score. The successful retrograde procedure was defined as achieving the endpoint, which was the restoration of TIMI flow to grade 3 in the occluded lesion and reducing the final residual stenosis to less than 30% after retrograde CTO-PCI. The retrograde approach was successful in 86.4% cases, with an 8.33% chance of coronary perforation and a 1.75% rate of cardiac tamponade in this study.

Multivariate logistic analysis found three independent retrograde procedure success variables: (1) Werner's CC0 or CC1 (score: 1); (2) diameter of distal CTO segment <1.5 mm (score: 1); and (3) b-type tortuous collateral (>180° or corkscrew CCs) (score: 1). For each observation, the assigned values were summed to obtain a total score, which was applied to categorize the lesions into four risk groups: easy (score: 0), intermediate (score: 1), difficult (score: 2), and very difficult (score: 3). The success rates for performing a retrograde approach on these four types of lesions in the derivation set were 100%, 90.9%, 69.4%, and 18.8%, respectively, with an area under the ROC curve of 0.832. The success rates in the validation group were 100%, 91.7%, 64.3%, and 0%, respectively, with an area under the ROC curve of 0.912, demonstrating good predictive ability.

2.2 Epicardial collaterals for recanalization of CTO (EPI-CTO) score

The EPI-CTO score study [16] included 103 patients with CTO lesions who had failed the antegrade attempts from January 2014 to September 2016 by a single operator in a single center in China. All subsequent procedures employed the retrograde approach. The aim was to develop a scoring system for predicting successful retrograde CTO-PCI via the epicardial CCs. CTO lesion recanalization was set as the endpoint, which was the restoration of TIMI flow to grade 3 in the occluded lesion and reducing the final residual stenosis to less than 30% after retrograde CTO-PCI through the epicardial CCs. The success rate of CTO recanalization was 76.3%, with a coronary perforation incidence rate of 9.7%, and a 1.9% rate of cardiac tamponade in this study.

Multivariate logistic analysis identified four independent factors associated with operation success, including (1) b-type tortuous collateral (>180° or corkscrew CCs) (score: 1); (2) side branch at CCs tortuosity (score: 1); (3) the CCs exit-CTO<5 mm (score: 1); and (4) the CCs diameter<0.9 mm (score: 1). Based on the total points, the operation difficulty was categorized into easy (0 point), medium difficulty (1 point), difficult (2 points), and very difficult (≥3 points) groups. The operation success rates were 100%, 86%, 27%, and 15%, respectively. In addition, this study conducted a comparison between the EPI-CTO score and the J-CTO score, RP-CTO score, and Progress-CTO score. The areas under the ROC curves of the four were 0.94, 0.67, 0.76 and 0.79, respectively (all p < 0.01), demonstrating that the EPI-CTO score was highly accurate in predicting the recanalization of CTO lesions with retrograde procedure through epicardial CCs.

In addition, a further study conducted by the researchers indicated that performing retrograde CTO-PCI via epicardial CCs carried a higher risk of coronary perforation, which was linked to renal failure, right coronary artery CTO, and EPI-CTO score ≥2 [17]. It was worth noting that the operator in this study was a high-level operator who performed more than 200 CTO-PCI cases annually. His ability to detect and promptly handle procedural complications was also high-level. Therefore, for beginners or operators with relatively fewer operations, the prevention and timely response to complications related to retrograde CTO-PCI via epicardial CCs is an essential consideration.

2.3 Retrograde collateral channel score

The CC score study [18] included 216 CTO lesions and a total of 242 CCs from a single operator in a single center in Taiwan from January 2012 to December 2015. The purpose of this study was to identify independent angiographic predictors of CCs tracking and technical success in retrograde CTO-PCI. The CC tracking success and technical success were set as the endpoints. Successful tracking of CCs was defined as the retrograde guidewire crossing the CCs and reaching the distal cap of the CTO segment. Restoring TIMI grade 3 antegrade flow and achieving a residual stenosis of 20% or less within the treated lesion after retrograde CTO-PCI was considered as technical success. The success rate for CCs tracking in this analysis was 83.5%, with a technical success rate of 81.4% and a rate of major procedural complications of 4.6%.

Multivariate logistic analysis identified that two independent factors were related to CC tracking and technical success, including (1) large CC size, this was Werner's CC2 (score: 1); (2) lack of tortuosity, CC tortuosity was defined as the presence of ≥2 high-frequency, successive curves (within 2 mm) in the context of epicardial CCs and ≥1 high-frequency curve that failed to uncoil in diastole for septal CCs (score: 1). With a ROC area of 0.800 for CC tracking and 0.752 for technical success, the CC scoring system showed excellent predictive ability. According to the total points, the difficulty of the operation was divided into easy group (3 points), medium difficulty group (2 points), difficult group (1 point), and very difficult group (0 point). The success rate of CC tracking were 95.0%, 93.8%, 77.8% and 44.4%, and the technical success rates were 91.7%, 93.8%, 75.9% and 44.4%, respectively.

In addition, the study team recently validated the CC scoring system on a larger independent cohort, covering 244 patients with 329 CCs from a single centre in Taiwan from January 2017 to December 2021, and compared its accuracy and clinical efficacy with the J-channel score [19]. Ultimately, 67.8% (223/329) of CCs tracking was successful, and technically, 92.2% (225/244) of attempts were successful. With the area under the ROC curves for the CC scoring system and J-Channel scoring system measuring 0.87 and 0.61, respectively (p < 0.01), the CC scoring system predicted the CCs tracking success in retrograde CTO-PCI with greater accuracy.

2.4 J-channel score

The J-channel scoring system study [20] included 630 patients with a total of 886 CCs from multiple centers in Japan from January 2016 to December 2016. The objective of the study was to develop a prediction scoring system for CCs guidewire crossing success. Study endpoints included CCs guidewire crossing success and retrograde technical success. Successful CCs guidewire crossing was defined as retrograde guidewire crossing through CCs from the retrograde side and reaching the CTO distal vessel segment. Retrograde technical success was defined as the restoration of TIMI flow to grade 3 in the occluded lesion and reduction of the final residual stenosis to less than 30% after retrograde CTO-PCI. The retrograde technical success rate was 84.8%, the success rate of CCs guidewire crossing was 61.2%, and the incidence of CCs perforation needing intervention was 3.7% in this study.

Two-thirds of the CCs were used for the derivation set, while one-third were used for the validation set. The derivation set was utilized to create a prediction scoring system for CCs guidewire crossing success, and the validation set was used to verify the scoring system. The study identified (1) for the septal CCs, the success rate of CCs guidewire crossing was 61.9% in the derivation set. Three independent predictors were found to be related to the success of CCs guidewire crossing, including ① small vessel size, that was, Werner's CC 0–1 (score: 2); ② reverse bend, that is, the angle of the CCs ≥90° (score: 1); and ③ continuous bends (score: 1). According to the total points, the difficulty of the operation was divided into easy group (0 points), medium difficult group (1–2 points), and difficult group (≥3 points). The success rates of CCs guidewire crossing were 92.0%, 55.3%, and 26.5% in the derivation set, respectively, and the area under the ROC curve was 0.744; the success rates in the validation set were 92.9%, 57.4%, and 16.7%, the area under the ROC curve was 0.743, indicating good predictive ability. (2) For non-septal branches, the CCs guidewire crossing success rate in the derivation group was 57.1%. Three independent factors were also found to be related to the success of CCs guidewire crossing, including ① small size (score: 3); ② reverse bend (score: 1); ③ corkscrew (score: 1). According to the total points, the difficulty of the operation was divided into easy group (0 points), medium difficult group (1–2 points), and difficult group (≥3 points). The success rates of CC guidewire crossing were 85.7%, 50.6%, and 27.3% respectively in the derivation group, and the area under the ROC curve was 0.757; the success rates in the validation group were 91.7%, 54.3%, and 19.0%, the area under the ROC curve is 0.826, suggesting good predictive ability. (3) In terms of complications, perforation of septal CC, epicardial CC, and atrioventricular groove (AVG) CC occurred at incidence rates of 1.3%, 6.4%, and 16.7%, respectively, and reverse bend was the only independent risk factors for AVG CC perforation. In addition, this study compared the ability of three guidewires to CCs crossing success: (1) For a small vessel, XTR > SION > SUOH 03; (2) For reverse bend, SUOH 03 > XTR > SION; (3) For continuous bends, SUOH 03 > SION > XTR; and (4) For corkscrew, XTR > SION > SUOH 03.

2.5 R-ICPS score

The objective of the R-ICPS scoring system study [21] was to determine the angiographic factors that predicted the success of CCs tracking, failure of microcatheter tracking, and complications in retrograde CTO-PCI. The study included 297 CCs in 180 CTO patients from a single center in France from January 2015 to February 2019. The study endpoints included CCs tracking success and procedural CTO-PCI success. Successful CCs guidewire crossing was defined as retrograde guidewire crossing through CCs from the retrograde side and reaching the CTO distal vessel segment. Successful microcatheter crossing was achieved by advancing the catheter to the distal cap of the occlusion on the first attempt. Retrograde technical success was defined as the restoration of TIMI flow to grade 3 in the occluded lesion and reduction of the final residual stenosis to less than 30% after retrograde CTO-PCI. Achievement of successful CCs crossing was 68.3%, microchannel tracking success was achieved in 87.6%, and the success rate of retrograde technique was 71.1% in this study.

Multivariate logistic analysis identified five independent factors related to CCs tracking success or failure, including (1) Werner's CC0 (score: 1); (2) Werner's CC2 (score: 0); (3) severe tortuosity (>180°) (score: 1); (4) acute exit angle (<90°) (score: 0.5); (5) The length of the CCs > 78 mm (score: 2). According to the total score from 0 to 4.5, the difficulty of the operation ranged from easy to very difficult, and the procedural success also decreased. In addition, this study compared the R-ICPS score with the CC score and J-channel score. The areas under the ROC curves of the three were 0.728, 0.657 and 0.698, respectively, which suggested that the R-ICPS score showed greater accuracy in predicting CCs tracking success. In terms of complications, the incidence of collateral injury was 14.4%, and the incidence of cardiac tamponade was 1.6%. Further analysis showed that CCs length was the only independent factor associated with microcatheter tracking failure and complications. This study also compared the ability of four guidewires to CCs tracking success: Sion Black > Fielder FC > Suoh 03 > Sion and the ability of five microcatheters to CCs tracking success: Corsair Pro > Caravel > Turnpike > Corsair > Finecross.

2.6 Age, connections, and channel score

The objective of the Age, connections, and channel scoring system study [22] was to create a new Chinese scoring system for accurately predicting the successful CCs traverse via retrograde technique. The study included a total of 458 CCs in 309 CTO patients in a single center in China from June 2015 to December 2021. The study endpoints included successful retrograde collateral traverse and procedural success. Successful retrograde CCs traverse was defined as the guidewire successfully reached recipient vessels through the CCs. Retrograde technical success was defined as the restoration of TIMI flow to grade 3 in the occluded lesion and reducing the final residual stenosis to less than 30% without complications after retrograde CTO-PCI. During the period from June 2015 to December 2019, a total of 208 CTO patients with 348 CCs were included in the training cohort; from January 2020 to December 2021, the validation cohort consisting of 101 CTO patients with 115 CCs was established in the study.

Multivariate logistic analysis identified three independent factors associated with retrograde CCs traverse success: (1) age; (2) connections between CCs and recipient vessels; (3) CCs tortuosity. The overall score was obtained by summing the scores associated with each predictor, and the success rate of retrograde CCs traverse was projected on the basis of the total score. According to the total points, the difficulty of the operation was divided into easy, moderate difficulty, and difficult groups. The success rates of retrograde collateral traverse in the training group were 81.5%, 44.8%, and 16.5%, respectively. The area under the ROC curve was 0.826, indicating good predictive ability.

2.7 Retrograde procedure in CTO from the ERCTO registry (ERCTO RP-CTO) score

The ERCTO RP-CTO score study [23] aimed to determine the factors that predicted the success of retrograde CTO-PCI. The study encompassed a total of 2364 CTO patients from 58 centers in Europe, with 57 operators, from January 2018 to December 2019. Among them, an initial retrograde approach was used in 1953 cases (82.6%), whereas a primary antegrade strategy was changed to retrograde in 411 cases (17.4%). The primary outcome of the study was procedural success, which was defined as achieving a residual stenosis of less than 10% and restoring TIMI flow grade 3 antegrade flow after retrograde CTO-PCI. The success of the procedure was noticed in 1820 cases (77.0%) and was more likely to occur following a first retrograde effort compared to converting from a primary antegrade strategy.

Following a multivariable analysis, it was found that an absence of lesion calcification, a higher degree of distal vessel opacification, little or no proximal target vessel tortuosity, Werner's CC1 or CC2, and the top tertile of operator volume were factors that significantly increased the likelihood of achieving procedure success. The total score was calculated by adding the scores for each predictor, and the anticipated risk corresponding to the entire score was the success rate of retrograde CTO-PCI. For example, a patient with slight calcification, clear distal opacification, moderate tortuosity, Werner's CC1, and operated on by a highly experienced cardiologist will obtain a score of around 215 points, which corresponds to an estimated likelihood of success of 82%.