Given the considerable amount of progress made by this subspecialty in recent years, there is a need for an updated document that includes data from recent clinical trials and registries, information on novel devices and techniques, and an up-to-date revision on the training requirements to approach CTO PCI. Since , there have been considerable changes in practice, in parallel with the publication of a plethora of novel data, dramatically changing the landscape of CTO percutaneous coronary intervention PCI.
Non-intralesional ipsilateral bridging collaterals may provide antegrade flow to the vessel beyond the occlusion and give a false impression of a functional incomplete occlusion. This should be distinguished from flow within the occluded segment by a careful frame-by-frame assessment in different angiographic projections. They have the capacity to preserve myocardial function but will not prevent ischaemia during exercise due to a limited capacity to increase blood flow 9.
The collateral supply provides a perfusion pressure in the range of mmHg at the occluded territory, a pressure that leads to the functional reduction of distal vessel size, which then leads to the underestimation of the vessel dimensions during a recanalisation procedure The presence of collaterals does not predict viability, as they also develop in patients with prior myocardial infarction and large akinetic territories, i. Moreover, the presence of a well-developed network of collaterals is not protective towards ischaemic insults, as even in such a patient population revascularisation might provide a survival benefit compared with medical therapy The angiographic assessment has been refined beyond the classic Rentrop classification by introduction of the grading of the collateral connection size 13 , which is also helpful to select the appropriate guidewires and techniques for collateral crossing.
A more detailed analysis of collateral supply led to the introduction of a collateral scoring system for the suitability for retrograde transcollateral interventions Viable myocardium subtended by a CTO is generally ischaemic, regardless of the degree of collateralisation, as has been shown in fractional flow reserve FFR studies 15 , CTO recanalisation aims to improve myocardial perfusion of the corresponding ischaemic territory This in turn has beneficial effects at multiple levels attendant on CTO success.
First, successful CTO PCI relieves ischaemia 18 , 19 , which has been shown to be associated with a decrease in severity and frequency of angina, as well as improved functional status and better quality of life 19 , Second, an untreated CTO is associated with incomplete revascularisation 21 , 22 , which in turn has been associated with persistent left ventricular dysfunction at follow-up As such, CTO recanalisation allows complete revascularisation which might lead to improvement in left ventricular function In a cohort of patients with implantable cardioverter-defibrillator, CTO subjects experienced a higher incidence of appropriate shocks for malignant ventricular arrhythmias, as compared to patients without CTO Moreover, ventricular arrhythmias can be observed even in the absence of myocardial scar, thus possibly being related to an ischaemic phenomenon Since there are only observational data 12 suggesting a mortality benefit for CTO recanalisation, consideration for intervention should be in order to improve symptoms and quality of life.
