**5. Myocardial viability and left ventricular function**

It is in general difficult to predict which patient with stable ischemic heart disease will receive interventional or surgical revascularization in the long term, after initially being treated with optimal medical therapy (OMT). In the occluded artery trial (OAT), late opening of infarct-related arteries (IRA) post-MI in stable patients with persistent total occlusion and no severe inducible ischemia showed no difference in rates of reinfarction, death, or severe heart failure compared to OMT [39]. Nevertheless, the results of OAT in terms of CTO have to be interpreted with caution because total occlusions in this trial were subacute (3–28 days, median 8 days) and therefore did not meet the CTO definition of at least a 3-month duration. Furthermore, patients in OAT showed a relatively normal baseline LVEF of 48% and were rather asymptomatic, whereas CTOs considered for PCI should be symptomatic or have proof of ischemia and viability [40].

An ischemic burden above 12.5% favors PCI in patients with CTO undergoing pre- and postinterventional myocardial perfusion imaging, whereas subjects with mild pre-procedural ischemia (<6.25% of LV myocardium) tend to have increased ischemic burden after PCI [41]. Another magnetic resonance imaging (MRI) study significantly revealed reduction in inducible perfusion defects and improvement in segmental myocardial viability by successful CTO PCI compared to unsuccessful revascularization [42]. Furthermore, successful CTO PCI increases hyperemic and resting myocardial blood flow with enhanced regional contractility already 24 h after the procedure [43]. Patients with an infarction and a transmural involvement < 25% assessed by MRI show significant improvements in segmental wall thickening and a reduction of mean end-systolic and end-diastolic volumes after CTO PCI [44]. Finally, the diagnostic accuracy of pre-procedural contrast enhanced MRI in patients with CTO to detect myocardial infarction and to predict improvement of myocardial function after revascularization seems to be better by using a combined viability analysis rather than focusing on the widely used transmural extent of infarction [45].
