**2. Basics of chronic total occlusions**

### **2.1. Definition**

A "true" total occlusion is defined as a coronary lesion with thrombolysis in myocardial infarction (TIMI) flow grade 0. In order to be classified as "chronic," the occlusion needs to be present for at least 3 months. It is difficult in clinical practice to determine the period of time for which a total occlusion has been present. The age of the occlusion is usually specified by detailed assessment of medical history and cardiovascular symptoms over the past 3 months [7–9]. Despite using contemporary criteria for CTO, Fefer et al. reported determined CTO duration in only 46% of cases, whereas another recent survey showed a known occlusion duration in 61% of CTO cases, with the undetermined duration of CTO as a predictor of procedural failure and major adverse cardiac events (MACE) [1, 10].

#### **2.2. Prevalence and clinical features**

In a recent report from the Canadian multicenter CTO registry, about 15% of patients without previous coronary artery bypass graft (CABG) surgery or known CAD and about 18% of patients with clinically significant CAD show at least one CTO on coronary angiogram [9]. In these registries, only 40% had a prior history of myocardial infarction (twice as high as without CTO), and more than 50% of CTO patients showed normal LV ejection fraction [11]. Furthermore, 64% of these patients underwent medical therapy, 26% were referred to CABG (with 88% successfully bypassing CTO), and only 10% underwent PCI of the CTO [1, 9]. In this study, only 5% of patients with a CTO were asymptomatic and it was in general difficult to attribute symptoms to the CTO in symptomatic multi-vessel disease (MVD) cases. Interestingly, recanalization of an occluded left anterior descending artery (LAD) rather than PCI of an occluded right coronary artery (RCA) results in greater increase of left ventricular function and more beneficial autonomic nervous system parameters with a potential antiarrhythmic effect [12].

Patients with CAD and CTO are mostly men, tend to be older, and usually have a higher cardiac risk profile. Interestingly, peripheral artery disease was found to be the strongest clinical predictor for the presence of a CTO [5]. In comparison to men, females with CTO tend to have less vessel disease, are usually older, have a higher frequency of hypertension and diabetes, and smoke less, but overall sex has no influence on CTO PCI failure [9, 13].

#### **2.3. Spatial distribution of CTO**

increasing selection as a treatment option [6]. However, discrepant CTO PCI quantity and success rates exist among catheterization laboratories [1] and may be explained by individual skills among operators, lesion assessment, and the absence of consensual treatment strategies. Recently, CTO PCI has become more predictable as a consequence of dedicated tools, stan-

Contemporary PCI strategies with dedicated devices significantly improved procedural success, and the introduction of drug-eluting stents (DES) led to better long-term patency with preservation of left ventricular (LV) function. Still, there is little systematic evidence that postprocedural outcomes have relevantly changed, although much retrospective data suggest

A "true" total occlusion is defined as a coronary lesion with thrombolysis in myocardial infarction (TIMI) flow grade 0. In order to be classified as "chronic," the occlusion needs to be present for at least 3 months. It is difficult in clinical practice to determine the period of time for which a total occlusion has been present. The age of the occlusion is usually specified by detailed assessment of medical history and cardiovascular symptoms over the past 3 months [7–9]. Despite using contemporary criteria for CTO, Fefer et al. reported determined CTO duration in only 46% of cases, whereas another recent survey showed a known occlusion duration in 61% of CTO cases, with the undetermined duration of CTO as a predictor of

dardized procedural techniques, and continuous educational programs.

procedural failure and major adverse cardiac events (MACE) [1, 10].

CTO PCI as favorable.

**2.1. Definition**

**2. Basics of chronic total occlusions**

**Figure 1.** Stenosis versus chronic total occlusion.

44 Interventional Cardiology

Few prospective surveys and a report from the National Heart, Lung, and Blood Institute (NHLBI) Dynamic Registry show CTO located in the RCA in over 50% of the cases [1, 14]. These figures are consistent with the Canadian multicenter CTO registry report, where, in most of the cases, CTO was found in the RCA (47%), 20% in the LAD, 16% in the left circumflex (LCX), and 17% in multiple locations [1, 9]. In a recent post-mortem analysis in CTO with and without CABG, CTO was most frequently located in the RCA (57.9%), followed by the LAD (22.1%) and LCX (20.0%), mainly located in the proximal segment (68.4%) of the vessel [15].

Garcia et al. examined the clinical and angiographic characteristics as well as clinical outcomes of >1300 consecutive CTO PCIs prospectively and retrospectively in multiple centers in the US. The study showed that proximal lesions were more common, and these patients had a higher prevalence of adverse comorbidities, mostly heart failure with reduced left ventricular ejection fraction (LVEF). Furthermore, proximal lesions had more adverse angiographic features (including proximal cap ambiguity, side branch at proximal cap, blunt or no stump, and moderate or severe calcification) but had more interventional collaterals and showed a higher angiographic complexity, resulting in longer and more complex procedures. The retrograde approach was used in half of the cases involving proximal CTO lesions and was successful in one-third of these cases. Surprisingly, procedural success and complication rates were similar to mid- and distal lesions [16].
