Chronic total occlusions (CTOs) present one of the most challenging obstacles to interventional cardiologists in the current era. Technical difficulties associated with percutaneous coronary intervention (PCI) of CTOs are reflected by lower success rates of the procedure (approximately 60–80%) compared to other lesion subsets.^1–3 Furthermore, there are substantial risks associated with this procedure. Recanalization of a CTO may be complicated by extensive coronary dissection or perforation.⁴ Radiation skin injuries during long complicated CTO procedures are not uncommon.⁵ Further, the use of excessive intra-arterial contrast, usually required in this procedure, carries a moderate risk of contrast- induced nephropathy.⁶ The latter is associated with a more adverse clinical outcome.^6,7 Nevertheless, successful revascularization of CTOs is linked with improved symptomatic as well as prognostic outcomes,^1,3,4,8 which has rekindled interest in this area.
Currently, a CTO is defined as a complete occlusion (thrombolysis in myocardial infarction [TIMI] grades 0 and 1 flow) for at least 3 months or more. Several studies have reported clinical and angiographic predictors of technical success rates for CTO interventions.^1,9,10 We are proposing a novel classification system for CTOs addressing the relationship between the degree of technical difficulty (based on angiographic findings) and the potential risk of the procedure. The purpose of this classification is to provide interventional cardiologists with an objective tool to make a balanced, informed decision prior to attempting an antegrade PCI approach to treat CTOs.
This system is based on the technical difficulty of recanalizing the vessel (Grade A) and the risk of adverse outcomes (Grade B). The greater the technical difficulties of the procedure, the higher the “A” ranking, while a higher “B” classification is associated with an incremental risk in attempting to reopen the artery. The final classification is specified as an “AB” grading.

Classification “A” grading: Technical challenges of recanalizing CTOs
“A0” grade reflects a relatively higher possibility of technical success.
The presence of the following features places the lesion at an “A0” grade:
• A stem of patent vessel longer than 10 mm;
• No branching vessels originating within 5 mm of the occlusion;
• Tapering configuration or a visible track;
• No evidence of calcification on fluoroscopy;
The absence of any single feature places the lesion at the next higher grade.

“A1” grade reflects a relatively lower possibility of reopening with a guidewire and subsequent angioplasty.
Presence of any one of the following features places the lesion at an “A1” grade:
• A stem of patent vessel shorter than 10 mm proximal to the occlusion;
• Presence of branching vessels originating within 5 mm of the occlusion;
• Blunt configuration of the leading edge with no visible track;
• Evidence of some calcification on fluoroscopy.

“A2” grade indicates the highest technical difficulty in tackling CTOs, and rates of successful PCI are likely to be very low.
The presence of any one of the following features places the lesion at an “A2” grade:
• Ostial total occlusion;
• Total occlusion precisely at a bifurcation point;
• Significant calcification on fluoroscopy;
• Long lesion as evidenced by retrograde filling of the distal segment.

Classification “B” grading: Risk associated with attempts at reopening

“B0” grade denotes a relatively low risk of adverse outcomes and is characterized by the presence of any of the following features:
• Absence of bridging collateral vessels;
• Absence of an aneurysmal appearance at the point of occlusion or in close proximity;
• Presence of good distal vasculature as seen by retrograde filling via collaterals from other arteries.

On the contrary, the absence of the above-mentioned features (“B1” Grade) signify a high risk of adverse outcomes.
The final classification is a combination of Grades “A” and “B”, thus taking into consideration the practical complexities and the risks associated with attempts to reopen an occluded artery.

Grade “AB” classification overview
• A0 B0 – High probability of reopening, with low risk;
• A1 B0 – Lower possibility of reopening, but the risk is low;
• A2 B0 – Significantly difficult lesion to reopen, but with low risk;
• A0 B1 – High probability of reopening, but at the cost of high risk;
• A1 B1 – Lower probability of reopening, with high risk;
• A2 B1 – Technically the hardest, with high risk.

Discussion

We present a simple CTO classification system, taking into account the technical difficulties and the risks associated with attempting to treat a CTO. This classification is partly based on findings of several studies that have shown angiographic predictors of successful PCI of CTOs.^1,9,10
These include factors such as length of the occlusion, blunt stump of the occlusion, presence of bridging collaterals and coronary calcification. Transluminal coronary calcification, in particular, represents an important impediment to successful recanalization of CTOs,¹⁰ and thus represents an integral part of the proposed classification which requires adequate evaluation prior to attempting this procedure.
Until recently, there has been little evidence of potential benefit of recanalizing a CTO which, coupled with the technical complexities, may explain the disparity between the frequency of occurrence and the actual attempts of PCI in CTOs.¹¹ A number of studies have now shown that successful CTO revascularization not only provides relief of symptomatic ischemia, but may prolong survival as well.^1,3,4,8
The use of this classification system in planning an optimal strategy, together with recent advances in technologies for CTO revascularization^{12– 15} such as laser guidewire, optical coherence reflectometry, blunt microdissection catheter and drug-eluting stents, may aid in overcoming the formidable challenges in PCI of CTOs.

Conclusion
We are proposing a simple classification system that is based on the technical difficulties and the risks associated with attempting an antegrade approach of PCI of CTOs, which will hopefully provide interventional cardiologist a balanced approach in attempting this complex procedure whilst considering the associated potential consumption of resources, time and radiation exposure.