Angiographic Predictors of Septal Collateral Tracking During Retrograde Percutaneous Coronary Intervention for Chronic Total Occlusion: Anatomical Analysis or Rolling the Dice?

Abstract

Objectives. The aim of this study was to identify independent angiographic predictors of collateral channel (CC) tracking success, microcatheter tracking failure, and complications in chronic total occlusion (CTO) retrograde approach. We also developed a “crossability score,” comparing its predictive performance with pre-existing scores. Background. The retrograde approach was introduced for recanalization of challenging CTOs. The passage of guidewires through CCs is a key step of the procedure. Two scoring systems have been recently developed to predict CC tracking success. Methods. A total of 180 patients and 297 CCs were retrospectively analyzed in an unselected retrograde CTO population. Results. Guidewire crossing was successful in 203 collaterals (68.3%). The only independent predictor of successful CC tracking was Werner score 2. Conversely, Werner score 0, severe tortuosity (>180°), acute exit angle (<90°), and length of collateral were independently associated with tracking failure. We assigned a score to each “significant” variable to create a model that showed a greater accuracy than pre-existing scores (area under the receiver-operator characteristics curve, 0.72 vs 0.65 and 0.69). Moreover, CC length was also associated with microcatheter tracking failure and complications. Conclusions. Werner score 0, tortuosity, acute exit angle, and CC length were independently associated with CC tracking failure, whereas Werner score 2 was a predictor of crossing success. Length of CC is associated with a higher rate of microcatheter crossing failure and complications. We combined these findings into the R-ICPS score, which showed an adequate accuracy for collateral crossing prediction.

J INVASIVE CARDIOL 2022;34(4):E286-E293.

Key words: chronic total occlusion, collateral circulation, CTO score, retrograde approach

Chronic total occlusions (CTOs) are observed in 15%-25% of patients with coronary artery disease undergoing coronary angiography.^1,2 Although technically challenging and associated with higher complication rates,³ CTO percutaneous coronary intervention (PCI) may improve functional status and quality of life of patients by improving ventricular function and reducing the severity and frequency of angina episodes.^1,4-8 Moreover, recanalization may reduce the arrhythmic potential of CTO lesions, although the impact on survival is still debated.^8-13 The retrograde approach was developed as an alternative strategy for tackling particularly challenging CTO cases,^14-16 which improved the overall procedural success rates as part of the “hybrid algorithm.”^{4,14,15,17,18} The successful passage of guidewires and equipment through the collateral channels (CCs) is crucial in order to reduce collateral crossing attempts, radiation exposure, procedural time, and potential negative outcomes. Hence, collateral selection during case planning is a critical step. Several scoring systems have been developed to predict CC tracking^19,20 and procedural success in retrograde CTO-PCI.²¹ The aim of this study was to evaluate the anatomical characteristics of CCs that are predictors of CC crossing success, microcatheter tracking failure, and complications in a large series of unselected patients. Accordingly, a “crossability score” has been developed and compared with the previously published retrograde collateral channel score¹⁹ and J-channel score.²⁰

Methods

We retrospectively reviewed all consecutive patients who underwent a retrograde CTO-PCI from January 2015 to February 2019. A total of 504 elective dual-injection CTO-PCI procedures were analyzed. Before the procedure, myocardial viability with non-invasive imaging (echocardiogram, low-dose dobutamine echocardiogram, or magnetic resonance imaging) was assessed in all cases.²² Antegrade procedures, retrograde recanalization via coronary artery bypass grafting or epicardial collaterals, and cases with suboptimal visualization of collateral pathway on angiogram were excluded (Figure 1). Angiographic assessment of the collaterals was performed during dual injections in at least 2 perpendicular projections. Visibility of the CC was evaluated during selective-tip injections if coronary angiography was inadequate. We analyzed the views with the least angiographic foreshortening. Measurements were taken by means of digital calipers by 2 expert operators. The agreement between the 2 analysts was evaluated with the intraclass correlation coefficient (ICC) with a fixed cut-off >0.8.

Baseline demographic data as well as variables previously described in the J-CTO (Multicenter CTO Registry in Japan) score²³ and retrograde-specific CTO scores^19,20 were recorded. This study complied with the guiding principles of the Declaration of Helsinki. All patients signed an informed consent for both procedure and data collection/analysis for research purpose. Data were utilized in an anonymous form. Coronary CTOs were defined as coronary lesions with Thrombolysis in Myocardial Infarction (TIMI) flow grade 0 of at least 3-month duration estimated by clinical symptoms, a prior history of myocardial infarction in the target-vessel territory, or following comparison with a prior angiogram. The procedure was considered retrograde if any attempt was made to cross the lesion through a collateral vessel supplying the target vessel distal to the lesion; otherwise, the procedure was classified as antegrade. We performed biradial, radial and femoral, and bifemoral accesses in 58%, 20%, and 12% of patients, respectively. For the donor artery, we most frequently used a 6-Fr guiding catheter (64%), while 7-Fr and 8-Fr catheters were used in 26% and 10% of cases, respectively. The collaterals were classified according to Werner’s CC grade²⁴ as follows: CC0 = no continuous connection; CC1 = continuous, thread-like connections with a diameter ≤0.3 mm; and CC2 = continuous, small, side–branch-like connections with a diameter >0.4 mm. For CC tortuosity classification, we used a simple 3-type classification: (a) <180°; (b) >180°; (c) 1 corkscrew, defined as a channel making at least 1 circular bend during the whole course or a series of severe and sequential bends. The length-to-emerging point (LEP) was defined as the distance between the convergence point of the CC and the distal cap. Total distance was defined as the distance between the origin of the collateral and the distal cap of the CTO. Collateral length was defined as the difference between total distance and LEP (Figure 2A). CC exit angle was defined as the angle at the convergence point between the donor vessel (proximal to the CC origin) and the collateral (Figure 2B). CC entry angle was defined as the angle at the convergence point between the recipient vessel and the collateral (Figure 2B). Calcifications, cap morphology, and other angiographic characteristics of the target lesion were evaluated as per the definitions described by Morino et al.²³ We considered only the complications strictly related to retrograde procedures, excluding events occurring during antegrade attempts in hybrid cases. Collateral injury category encompasses both dissection and perforation of the septal branch. Severe dissection of the donor vessel was defined as a dissection type C-F according to the classification of Roger et al.²⁵Procedural CTO-PCI success was defined as successful crossing of the wire with achievement of <30% residual stenosis with final TIMI flow grade 3. CC tracking success was defined as retrograde guidewire crossing of the CC to reach the distal cap of the CTO segment. Microcatheter (MC) crossing success was defined as the successful advancement of the MC to the distal cap of the occlusion on the first attempt.

Statistical analysis. Categorical data are presented as frequencies and percentages, and compared using Chi-square or Fisher’s exact test, as appropriate. Continuous variables are expressed as mean ± standard deviation and compared with t test or Wilcoxon rank-sum after normality assessment. We studied clinical and/or angiographic factors predicting the success of CC tracking through stepwise logistic-regression analysis. After univariate analyses, variables were analyzed by a multivariate model that selected “successful collateral wiring,” “successful microcatheter crossing,” and “complications” as the positive events (P-value cut-off for entry into the model was set at <.10, except for complications for which we considered <.20 to be the cut-off). At multivariate analysis, a P-value <.05 was considered statistically significant. Multicollinearity was tested before multivariate analysis. We tried to utilize the anatomic features that showed a significant correlation with CC crossing in our multivariate logistic regression to create a new tool for prediction of collateral tracking success. Thus, we assigned 0, 0.5, 1, or 2 points to each significant variable, proportionally to their β-coefficient. For ease of use, we decided to assign 0 points and not a negative 1 to CC2. For collateral length, we found the cut-off value according to the ER criteria²⁶ and assigned 2 corresponding points to collaterals longer than this value and 0 to the other ones. The total difficulty score resulted from the sum of each rated variable. The discriminatory capacities of the new and pre-existing scores were evaluated with the receiver-operator characteristic (ROC) curve and the area under the curve (AUC) was calculated. All analyses were performed using SPSS, version 25 statistical software (SPSS, Inc).

Results

A total of 180 consecutive patients were enrolled in our study. The demographic and angiographic data are summarized in Table 1. Mean age was 62.22 ± 9.36 years, 81% were male and 29% of patients had diabetes. Previous coronary artery bypass grafting was documented in 11% and previous PCI was performed in 56%. CTOs most commonly involved the right coronary artery (69%). The target CTO was located in the left anterior descending artery and left circumflex artery in 19% and 12% of cases, respectively. Successful CC crossing was achieved in 203 collaterals (68.3%). In 112 patients (62.2%), 2 or more collaterals were attempted. Microchannel tracking was successful in 178 collaterals (87.6%) successfully crossed by wire. In a small number of patients (<10) a gentle balloon dilation using a small balloon (1.25-1.5 mm) eased the passage of the MC in a second attempt. Procedural success was achieved in 128 procedures (71.1%). The procedural success rate was high (88.6%) in the group of patients who had successful guidewire crossing via CC. Median procedure time was 140 minutes (interquartile range [IQR], 116.75-176.25 minutes). Median patient air kerma dose and contrast volume were 4071.14 mGy (IQR, 2695.10-5890.65 mGy) and 300 mL (IQR, 237.5-400 mL), respectively. Wire and MC types used are described in Figure 4 and Figure 5.

The main findings can be summarized as follows: (1) CC0 (OR, 0.33; 95% CI, 0.17-0.61; P<.01) and CC2 (OR, 3.19; 95% CI, 1.3-7.3; P<.01) were found to be strong predictors of CC crossing failure and success, respectively (Table 2 and Figure 3). (2) Length of collateral had a negative impact on CC crossing success (OR, 0.98; 95% CI, 0.97-0.99; P<.01) for each 1 mm increase, as reported in Table 2 and depicted in Figure 3. (3) CC crossing was significantly affected by severe tortuosity (OR, 0.55; 95% CI, 0.32-0.95; P=.03) and acute exit angle of collateral (OR, 0.44; 95% CI, 0.25-0.75; P<.01) (Table 2 and Figure 3). (4) Length of collateral (OR, 0.98; 95% CI, 0.97-0.99; P=.04) was also a predictor of MC tracking failure (Table 3). The Corsair Pro MC was associated with a greater tracking success rate, which persisted after adjustment (OR, 2.5; 95% CI, 1.4-4.3; P<.01) (Figure 5). (5)

The incidence of complications was low (Table 5). Length of collateral was the only predictor of complications on multivariate analysis (Table 4). (6) ROC curves for predictive the value of pre-existing scores created for assessment of CC tracking success showed AUCs of 0.65 and 0.69 for the retrograde collateral channel score and the J-channel score, respectively (Figure 6). The new score that resulted from our analysis ranges from 0 (very simple crossing) to 4.5 (very difficult crossing). This model performs quite well on ROC curve analysis for successful crossing (the area under the curve is 0.728 ± 0.030), as shown in Figure 7. We termed the new score the R-ICPS score.

Discussion

Collateral channel crossing still represents an important issue in retrograde CTO approach and has been the topic of recent publications. The presence of “interventional collateral” is deemed pivotal for procedural success, albeit the assessment of suitability relies prevalently on previous operator’s experience without performing a proper logistic regression to quantify the potential impact of angiographic variables on procedural outcomes.^15,27,28 ­Consistent with previous publications, in our study, Werner score is associated with successful crossing with CC0 independently associated with failure.^19-21,29,30 Even though an “invisible” (CC0) collateral can occasionally be used successfully,³¹ it has been demonstrated that the size of the channel is one of the major determinants of CC wiring success.^21,29,30 Although the profile and flexibility of polymeric guidewires and MCs have been optimized over the past few years, a larger collateral is still less prone to friction during wire tracking and equipment advancement.

Tortuosity has a significant impact on collateral tracking. This finding is in line with previously published reports,^19,20,31 but so far, the definition of tortuosity has been either complex or non-uniform in previous literature. Therefore, we tried to simplify the assessment of CC tortuosity. Surprisingly, a tortuosity >180°, but not a corkscrew anatomy, affected the tracking success. This finding is probably related to the fact that corkscrew anatomy is often a characteristic of epicardial collaterals and is less frequent in septal branches. Moreover, we found a significant association between Werner score 2 and corkscrew anatomy (P<.001).

It is well known that it is easier to navigate through septal CCs from left anterior descending (LAD) to the right coronary artery (RCA) compared with from the RCA to LAD, because of the sharper angle at the end of an RCA collateral coupled with their tortuosity. Nonetheless, the role of “exit” and “entry” angles in CC crossing and procedural success is still subject of debate.^19,20,28 This discrepancy may also be related to the different chosen thresholds (45° or 90°) and to non-uniform techniques utilized for angle evaluation in previous publications. In this study, an acute exit but not an acute entry angle was associated with lower collateral crossing success. Of note, in a recent study, Zhong et al³² reported that acute exit and entry angles are also predictors of MC tracking failure after wire tracking success.

To the best of our knowledge, collateral length is a parameter that has never been considered quantitatively in previous studies or previous specific scores. In this analysis, per-millimeter increase in collateral length was found to have a proportional negative impact on success rate. A shorter collateral should be preferred because it may provide less resistance and reduce difficulties in advancement of materials from donor to occluded artery. ­Accordingly, length had an impact both on guidewire and microcatheter crossing success in our study. It is noteworthy that in a study by McEntergart,²⁸ “non-procedural” collaterals tended to be longer. We also found a relationship between collateral length and complications. Even though collateral injury was not infrequent (14.4%), the rate of cardiac tamponade was low in our dataset, with only 3 cases (1.6 %) requiring pericardiocentesis (Table 5). The low rate of adverse events in our series may be related to the exclusion of epicardial collaterals that are associated with a higher risk of complication in comparison with septal branches.^20,22,33-35 It is noteworthy that we did not include data prior to 2015 in order to gain insight into the impact of novel techniques and materials coupled with extensive operator experience, thus avoiding any potential bias related to the availability of new devices such as dedicated guidewires and channel dilators, or to the operators’ learning curves. In this series, both CTO expert (>50 CTO-PCIs per year)³⁶ and non-expert operators performed the procedures^37,38 and this could explain the relatively low percentage of successful retrograde procedures. However, we did not find any significant difference between the 2 classes of operator regarding CC crossing success, which was the main topic of this paper (P=.38). The high rate of procedural success after successful CC crossing highlights the importance of this step during the procedure.

As concerns scores, the most frequently used scoring system for CTO, the J-CTO score, was developed to grade the difficulty in crossing a CTO within 30 minutes, as well as the overall success rate.²³ However, this score was derived from and validated by a dataset consisting mainly of antegrade procedures²³ and its use in retrograde or hybrid procedures may be less useful.^19,30,39 The predictive value of pre-existing scores created for assessment of CC tracking success was poor in our population, but useful to propose a new equipped and efficient tool for prediction of collateral tracking success, the R-ICPS score. This model needs to be validated on other series and extended to a larger number of subjects.

Study limitations. This is a single-center  retrospective study, which may result in a potential bias in patient selection and interventional strategy. Nevertheless, we included all of the consecutive patients at our center to allow a certain degree of generalizability of results. Angiographic analyses were performed by 2 expert CTO operators and not by a core laboratory. Random measurement errors and the small volume of cases are 2 considerable limiting factors. An external and larger sample size will be needed in the future to validate our findings. Both surfing and tip-injection technique were evaluated, although a differentiation of the 2 techniques for crossing success and complication would have been worthwhile. We excluded epicardial collaterals both for our limited sample size and for substantial anatomical differences compared with septal branches. Moreover, we restricted the main analysis of the study (prediction of CC crossing) to anatomical features in order to achieve a certain degree of results standardization, allowing future external validation in different populations.

Operator skills and experience have a critical impact on the final outcomes. The experience cannot be measured objectively, although the number of cases per year is considered a good indicator. The utilization of some materials appears limited because of their delayed availability over time (ie, Suoh 03; Asahi Intecc). We did not consider the impact of septal dilation using a small balloon on MC tracking success because this technique was performed in a small percentage of patients.

Conclusion

Appropriate collateral selection is essential for retrograde procedure success. In this study, we investigated the angiographic features associated with successful collateral crossing in a recent CTO dataset. Werner score 0, severe tortuosity (>180°), acute exit angle (<90°), and length of collateral were independently associated with collateral tracking failure, whereas Werner score 2 was a predictor of crossing success and we combined these variables to create a score with a better predictive performance than previous specific scores. Length of collateral is also an angiographic feature associated with MC crossing failure. These results may be helpful for operators in the upfront selection of CC during procedural planning of the most challenging CTO cases scheduled to be performed via a hybrid and/or retrograde approach.

Affiliations and Disclosures

From the Institut Cardiovasculaire Paris, Ramsay Générale de Santé, Massy France.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr Garot is the Medical Director and a shareholder in CERC. Dr Lefèvre reports proctoring fees from Terumo, Boston, and Abbott; honoraria from Boston Scientific, Terumo, Abbott, and Edwards Lifesciences; support for attending meetings and/or travel from Abbott and Medtronic; participation on a data safety monitoring board or advisory board for Abbott. The remaining authors report no conflicts of interest regarding the content herein.

Manuscript accepted June 28, 2021.

Address for correspondence: Anna Maria Ioppolo, MD, Institut Cardiovasculaire Paris, Hôpital Privé Jacques Cartier, 6 avenue du Noyer Lambert, 91300 Massy, France. Email: ioppoloannamaria@gmail.com

References

1. Tomasello SD, Boukhris M, Giubilato S, et al. Management strategies in patients affected by chronic total occlusions: results from the Italian Registry of Chronic Total Occlusions. Eur Heart J. 2015;36(45):3189-3198. Epub 2015 Sep 2. doi: 10.1093/eurheartj/ehv450

2. Fefer B, Knudtson ML, Cheema AN, et al. Current perspectives on coronary chronic total occlusions: the Canadian Multicenter Chronic Total Occlusions Registry. J Am Coll Cardiol. 2012;59(11):991-997. doi: 10.1016/j.jacc.2011.12.007

3. Megaly M, Ali A, Saad M, et al. Outcomes with retrograde versus antegrade chronic total occlusion revascularization. Catheter Cardiovasc Interv. 2020;96(5):1037-1043. Epub 2019 Nov 28. doi: 10.1002/ccd.28616

4. Galassi AR, Sianos G, Werner GS, et al. Retrograde recanalization of chronic total occlusions in Europe: procedural, in-hospital, and long-term outcomes from the Multicenter ERCTO Registry. J Am Coll Cardiol. 2015;65(22):2388-400. doi: 10.1016/j.jacc.2015.03.566

5. Stone GW, Reifart NJ, Moussa I, et al. Percutaneous recanalization of chronically occluded coronary arteries: a consensus document: part II. Circulation. 2005;112(16):2530-2537. doi: 10.1161/CIRCULATIONAHA.105.583716

6. Wijeysundera HC, Norris C, Fefer P, et al. Relationship between initial treatment strategy and quality of life in patients with coronary chronic total occlusions. EuroIntervention. 2014;9(10):1165-1172. doi: 10.4244/EIJV9I10A197

7. Werner GS, Martin-Yuste V, Hildick-Smith D, et al. A randomized multicentre trial to compare revascularization with optimal medical therapy for the treatment of chronic total coronary occlusions. Eur Heart J. 2018;39(26):2484-2493. doi: 10.1093/eurheartj/ehy220

8. Christakopoulos GE, Christopoulos G, Carlino M, et al. Meta-analysis of clinical outcomes of patients who underwent percutaneous coronary interventions for chronic total occlusions. Am J Cardiol. 2015;115(10):1367-1375. Epub 2015 Feb 18. doi: 10.1016/j.amjcard.2015.02.038

9. Joyal D, Afilalo J, Rinfret S. Effectiveness of recanalization of chronic total occlusions: a systematic review and meta-analysis. Am Heart J. 2010;160(1):179-187. doi: 10.1016/j.ahj.2010.04.015

10. Lee PH, Lee SW, Park HS, et al. Successful recanalization of native coronary chronic total occlusion is not associated with improved long-term survival. JACC Cardiovasc Interv. 2016;9(6):530-538. Epub 2016 Mar 2. doi: 10.1016/j.jcin.2015.11.016

11. Lee SW, Lee PH, Ahn JM, et al. Randomized trial evaluating percutaneous coronary intervention for the treatment of chronic total occlusion. Circulation. 2019;139(14):1674-1683. doi: 10.1161/CIRCULATIONAHA.118.031313

12. Gutiérrez-Chico JL, Louvard Y. DECISION-CTO: A “negative” clinical trial? Really? Cardiol J. 2017;24(3):231-233. Epub 2017 Apr 18. doi: 10.5603/CJ.a2017.0049

13. Behnes M, Mashayekhi K, Kuche P, et al. Prognostic impact of coronary chronic total occlusion on recurrences of ventricular tachyarrhythmias and ICD therapies. Clin Res Cardiol. 2021;110(2):281-291. Epub 2020 Nov 4. doi: 10.1007/s00392-020-01758-y

14. Tsuchikane E, Yamane M, Mutoh M, et al. Japanese multicenter registry evaluating the retrograde approach for chronic coronary total occlusion. Catheter Cardiovasc Interv. 2013;82(5):E654-E661. Epub 2013 Jul 30. doi: 10.1002/ccd.24823

15. Brilakis ES, Grantham JA, Rinfret S, et al. A percutaneous treatment algorithm for crossing coronary chronic total occlusions. JACC Cardiovasc Interv. 2012;5(4):367-379. doi: 10.1016/j.jcin.2012.02.006

16. Christopoulos G, Menon RV, Karmpaliotis D, et al. The efficacy and safety of the “hybrid” approach to coronary chronic total occlusions: insights from a contemporary multicenter us registry and comparison with prior studies. J Invasive Cardiol. 2014;26(9):427-432.

17. Karmpaliotis D, Michael TT, Brilakis ES, et al. Retrograde coronary chronic total occlusion revascularization: procedural and in-hospital outcomes from a multicenter registry in the United States. JACC Cardiovasc Interv. 2012;5(12):1273-1279. doi: 10.1016/j.jcin.2012.06.025

18. Maeremans J, Walsh S, Knaapen P, et al. The hybrid algorithm for treating chronic total occlusions in Europe: the RECHARGE registry. J Am Coll Cardiol. 2016;68(18):1958-1970. doi: 10.1016/j.jacc.2016.08.034

19. Huang CC, Lee CK, Meng SW, et al. Collateral channel size and tortuosity predict retrograde percutaneous coronary intervention success for chronic total occlusion. Circ Cardiovasc Interv. 2018;11(1):e005124. doi: 10.1161/CIRCINTERVENTIONS.117.005124

20. Nagamatsu W, Tsuchikane E, Oikawa Y, et al. Successful guidewire crossing via collateral channel at retrograde percutaneous coronary intervention for chronic total occlusion: the J-Channel score. EuroIntervention. 2020;15(18):e1624-e1632. doi: 10.4244/EIJ-D-18-00993

21. Chai WL, Agyekum F, Zhang B, et al. Clinical prediction score for successful retrograde procedure in chronic total occlusion percutaneous coronary intervention. Cardiology. 2016;134(3):331-339. Epub 2016 Mar 18. doi: 10.1159/000444181

22. Galassi AR, Werner GS, Boukhris M, et al. Percutaneous recanalisation of chronic total occlusions: 2019 consensus document from the EuroCTO Club. EuroIntervention. 2019;15(2):198-208. doi: 10.4244/EIJ-D-18-00826

23. Morino Y, Abe M, Morimoto T, et al. Predicting successful guidewire crossing through chronic total occlusion of native coronary lesions within 30 minutes: the J-CTO (Multicenter CTO Registry in Japan) score as a difficulty grading and time assessment tool. JACC Cardiovasc Interv. 2011;4(2):213-221. doi: 10.1016/j.jcin.2010.09.024

24. Werner GS, Ferrari M, Heinke S, et al. Angiographic assessment of collateral connections in comparison with invasively determined collateral function in chronic coronary occlusions. Circulation. 2003;107(15):1972-1977. Epub 2003 Mar 24. doi: 10.1161/01.CIR.0000061953.72662.3A

25. Rogers JH, Lasala JM. Coronary artery dissection and perforation complicating percutaneous coronary intervention. J Invasive Cardiol. 2004;16(9):493-499.

26. Unal I. Defining an optimal cut-point value in roc analysis: an alternative approach. Comput Math Methods Med. 2017;2017:3762651.Epub 2017 May 31. doi: 10.1155/2017/3762651

27. Christopoulos G, Kandzari DE, Yeh RW, et al. Development and validation of a novel scoring system for predicting technical success of chronic total occlusion percutaneous coronary interventions: the PROGRESS CTO (Prospective Global Registry for the Study of Chronic Total Occlusion Intervention) score. JACC Cardiovasc Interv. 2016;9(1):1-9. doi: 10.1016/j.jcin.2015.09.022

28. McEntegart MB, Badar AA, Ahmad FA, et al. The collateral circulation of coronary chronic total occlusions. EuroIntervention. 2016;11(14):e1596-e1603. doi: 10.4244/EIJV11I14A310.\

29. Rathore S, Katoh O, Matsuo H, et al. Retrograde percutaneous recanalization of chronic total occlusion of the coronary arteries: procedural outcomes and predictors of success in contemporary practice. Circ Cardiovasc Interv. 2009;2(2):124-132. Epub 2009 Feb 20. doi: 10.1161/CIRCINTERVENTIONS.108.838862

30. Galassi AR, Boukhris M, Azzarelli S, Castaing M, Marzà F, Tomasello SD. Percutaneous coronary revascularization for chronic total occlusions: a novel predictive score of technical failure using advanced technologies. JACC Cardiovasc Interv. 2016;9(9):911-922. Epub 2016 Apr 13. doi: 10.1016/j.jcin.2016.01.036

31. Joyal D, Thompson CA, Grantham JA, Buller CE, Rinfret S. The retrograde technique for recanalization of chronic total occlusions: a step-by-step approach. JACC Cardiovasc Interv. 2012;5(1):1-11. doi: 10.1016/j.jcin.2011.10.011

32. Zhong X, Ge L, Ma J, et al. Microcatheter collateral channel tracking failure in retrograde percutaneous coronary intervention for chronic total occlusion: incidence, predictors, and management. EuroIntervention. 2019;15(3):e253-e260. doi: 10.4244/EIJ-D-18-01003

33. Danek BA, Karatasakis A, Tajti P, et al. Incidence, treatment, and outcomes of coronary perforation during chronic total occlusion percutaneous coronary intervention. Am J Cardiol. 2017;120(8):1285-1292 Epub 2017 Jul 24. doi: 10.1016/j.amjcard.2017.07.010

34. Ma JY, Qian JY, Ge L, et al. Retrograde approach for the recanalization of coronary chronic total occlusion: collateral selection and collateral related complication. Chin Med J (Engl). 2013;126(6):1086-1091.

35. Benincasa S, Azzalini L, Carlino M, et al. Outcomes of the retrograde approach through epicardial versus non-epicardial collaterals in chronic total occlusion percutaneous coronary intervention. Cardiovasc Revasc Med. 2017;18(6):393-398. Epub 2017 Mar 14. doi: 10.1016/j.carrev.2017.03.013

36. Suzuki Y, Tsuchikane E, Katohh O, et al. Outcomes of percutaneous coronary interventions for chronic total occlusion performed by highly experienced Japanese specialists: the first report from the Japanese CTO-PCI Expert Registry. JACC Cardiovasc Interv.  2017;10(21):2144-2154. Epub 2017 Oct 18. doi: 10.1016/j.jcin.2017.06.024

37. Brilakis ES, Banerjee S, Karmpaliotis D, et al. Procedural outcomes of chronic total occlusion percutaneous coronary intervention: a report from the NCDR (National Cardiovascular Data Registry). JACC Cardiovasc Interv. 2015;8(2):245-253. doi: 10.1016/j.jcin.2014.08.014

38. Shammas NW, Shammas GA, Robken J, et al. The learning curve in treating coronary chronic total occlusion early in the experience of an operator at a tertiary medical center: the role of the hybrid approach. Cardiovasc Revasc Med. 2016;17(1):15-18. Epub 2015 Oct 30. doi: 10.1016/j.carrev.2015.09.004

39. Nombela-Franco L, Urena M, Jerez-Valero M, et al. Validation of the J-chronic total occlusion score for chronic total occlusion percutaneous coronary intervention in an independent contemporary cohort. Circ Cardiovasc Interv. 2013;6(6):635-643. Epub 2013 Nov 19. doi: 10.1161/CIRCINTERVENTIONS.113.000447