Current Challenges Operator Experience
Ten to 20 years ago most of the reports of CTO results included 'well selected casesÔÇÖ and used a more 
liberal de­nition including recent occlusions so that the success rates of >60 % are overestimated.10,15,16Even experienced non-CTO operators, as the SYNTAX participants, still today achieve success rates of about 50 % only, a figure not far from CABG either.17 The classical predictors of failure until the late 1990s included no stump, occlusion at side branch, orthograde collaterals (caput medusae) and occlusion age >3 months. Severe calcifications, marked tortuosity or long occlusions were considered undoable. Material and strategies, namely retrograde approach, were recently renewed predominantly by a few Japanese pioneers. Unfortunately, most centres still do not restrict the recanalisations of CTO to a few selected operators, which results in insufficient experience (<30/year) and less favourable outcomes. Only high-volume CTO operators will be able to deal with difficult morphology and achieve success rates >80 % even in unselected cases. Since 2006 the EuroCTO club has been collecting clinical and procedural data of all consecutive CTO procedures - and in an online registry (www.ercto.org) since 2008. During these seven years data of more than 14,000 CTO procedures by more than 35 ERCTO operators were collected. The overall success rate increased from 75 to 85 %, in 2012 reaching 91 % in the hands of the most experienced who enrolled >100 procedures annually (see Figure 1). The result of the EuroCTO club online registry depicts a clear relation of caseload and success even in highly experienced CTO operators (see Figure 1). Furthermore in this survey a retrograde approach was chosen in 12 % of cases with a success rate of 65 %, underscoring that this is a good strategy after antegrade failure.
Operators With and Without Retrograde Experience
This European registry more recently depicted that the success rate of the less experienced operators who do not apply the retrograde approach is about 80 % as opposed to 90 % of those who do.18 Similarly in 2009 a US retrospective analysis of 636 consecutive CTO procedures (overall success 69 %) confirmed that less experienced operators who did not adopt the novel retrograde strategy had a lower success rate than those who did (59 versus 75 %).19
Morphology
An analysis of the registry data 2008-2010 comprising 1,914 consecutive patients of then 16 centres revealed as independent predictors of antegrade failure: blunt stump, occlusion length >20 millimetres (mm), severe calcification and previously failed attempt (76 versus 83 %).20 Noteworthy is that the success rate of a second attempt was 85 % if a different operator had failed before (n=636) and 69 % if it was the same operator who failed before (n=260).21In the Japanese J-CTO registry from 12 enrolling centres 498 patients with 528 CTOs the success rate was 88.6 % for first attempt and 65.8 % for retry cases, which confirms the European experience.22 Other independent predictors of lower success rates in this registry were calcification, bending, blunt stump and occlusion length >20 mm. The retrograde approach was chosen in 26 % of cases, and was successful without trying antegrade first in 79 %, and after futile antegrade attempt in 68-74 %, similar to European results.
In summary, complex morphology, that until recently was deemed impossible to negotiate is barely influencing success rates nowadays. According to contemporary experience the most important factor inversely influencing success independent from the approach is severe calcification. No doubt other factors like tortuosity, absence of stump, length of occlusion and failed recanalisation will cumber the procedure, but are by no means a reason to deny an attempt by experienced operators.20,22 There is only one exception to this rule, chronically occluded vessels with no visible distal target should not be addressed at all because the 'blind penetration bears an unpredictable risk of perforation and life-threatening tamponade.
Crucial Steps to Success
The most important prerequisite is operator and team experience in CTO procedures. The operator should be skilful, trained specifically in CTO intervention, be patient, persistent and yet cautious. Every operator should select patients according to their level of expertise or seek for expert backup or refer those beyond their reach to CTO masters. stepping up quite early (Confianza 9 Pro or Miracle 6g) and then step down to a softer wire again. The three fundamental elements of wire handling are rotating, pushing and pulling the wire. It is important to feel the resistance at the wire tip, when pulling the wire, since a wrong channel often exerts much higher resistance than the correct lumen. Any wire has the tendency to follow the outer part of the vessel curve, which can often cause the tip to exit the lumen. It is therefore preferable to direct the wire-curve towards the inner part of the vessel bend.
Patient Preparation
It is mandatory to carefully review the film for best views, occlusion entry and exit, and possible mistakes of previous unsuccessful attempts. The patients glomerular filtration rate (GFR) (millilitre/minute [ml/min]) should be considered as a limit to dye consumption that should not exceed 4-8 times the nominal number of GFR in ml.23 An activated clotting time (ACT) measurement has to be accomplished every 30-40 minutes (min) to keep the level above 250 seconds (sec) and all lines need to be flushed every 10-20 min to avoid clotting and embolisation that is life-threatening if it occurs at the contralateral access. Since wire exits are not uncommon during the process of recanalisation IIb/IIIa inhibitors should be avoided.