For more than half a century, coronary angiography has been the gold standard diagnostic test for the assessment of coronary artery disease, but it does not provide information about the components of the vascular wall, atherosclerotic disease burden, or the interaction of local devices with coronary artery walls. The development of intracoronary imaging techniques, such as optical coherence tomography (OCT), was conceived with the objective to overcome coronary angiography limitations. Nonetheless, up-to-date OCT is more often used in the setting of research than in routine clinical practice . In current clinical practice guidelines, OCT has limited indications for clinical applications However, many ongoing clinical trials are testing its utility for stent deployment optimization and stent follow-up . (See Fig. 1 .)
In this issue of Cardiovascular Revascularization Medicine ( CRM ), Bogale et al. address the use of OCT in the evaluation of intermediate coronary lesions. The authors conducted a retrospective analysis of patients, presenting to their center with non-ST-segment elevation myocardial infarction (NSTEMI), who underwent coronary angiography and OCT imaging. A total of 36 patients were analyzed and classified as intermediate angiographic lesions (26 patients with stenosis 40%–69%) or severe angiographic lesions (10 patients with stenosis ≥ 70%), and OCT imaging of the suspicious culprit lesion was performed during the index procedure. In the OCT measurements analysis, authors found that angiographically intermediate lesions showed a larger minimal luminal area (MLA) and less severe percentage of lumen area stenosis when compared to angiographically severe lesions (3.3 mm 2 ± 1.8 mm 2 vs. 1.6 mm 2 ± 0.6 mm 2 , p < 0.001, and 54.2% ± 11.4% vs. 70.9% ± 6.8%, p = 0.001, respectively). Authors concluded that in stabilized patients with NSTEMI and angiographically intermediate disease, OCT examination confirmed the lack of severe anatomical stenosis in most patients.
In a very appropriate manner, authors detailed the clinical data of the 26 patients in which OCT imaging was performed. Although authors proposed the use of MLA threshold of ≤ 1.9 mm 2 to determine the need of revascularization of intermediate lesions, operators seemed more prone to stenting because of morphological characteristics of the plaque (thrombus, rupture, and calcium nodules) or in case of positive cardiac biomarkers, rather than the actual MLA measurements. The report of the frequency in which OCT measurements changed physician decision-making would have been helpful, because it could be considered as an objective measure of the added value of performing the OCT assessment. An example of how OCT imaging can affect physician decision-making is the ILUMIEN I study (NCT01663896). In this study based on pre-percutaneous coronary intervention (PCI) OCT, the PCI procedure was altered in 55% of patients (57% of all stenoses) by selecting different stent lengths (shorter in 25%, longer in 43%) .
Other important limitations are the retrospective design of a single-center analysis and the small number of highly selected patients that were analyzed. Finally, the absence of evaluation of clinical endpoints and follow-up makes extrapolating these results into the clinical practice impossible.
In coronary angiography studies, the evaluation of intermediate lesions is always challenging. At present, it is widely accepted that fractional flow reserve (FFR) is the gold standard for invasive assessment of the functional significance of these lesions Despite this fact, there have been attempts to correlate OCT measurements to the functional significance of stenosis, probably because of operators’ desire to have both morphological and functional assessment at the same time.
In a systematic review that analyzes the accuracy of MLA measured by OCT to predict an FFR cutoff of ≤ 0.80, the sensitivity ranged from 70% to 93% and the specificity from 63% to 97%. There was a wide variation of MLA threshold, from 1.62 to 2.43 mm 2 . In addition, a meta-analysis (2581 patients and 2807 lesions) with a similar methodology suggests that MLA by OCT does not predict functional stenosis All these conflicting results highlight the difficulties of correlating anatomical and functional measurements.
The ongoing FORZA trial (FFR or OCT Guidance to RevasculariZe Intermediate Coronary Stenosis Using Angioplasty, NCT01824030) is a randomized clinical trial that compares the use of guidance with FFR versus guidance with OCT for treatment of intermediate lesions. The results of this trial will help to properly situate OCT’s role in this clinical context
In the January/February issue of CRM , Secco et al. analyzed the use of OCT to the assessment of neointimal tissue coverage of third-generation sirolimus-eluting stents (SESs). In this single-center and prospective study, authors enrolled 16 patients presenting with ST-segment elevation myocardial infarction (STEMI) and multivessel disease who were candidates for a staged procedure. The culprit lesion of the STEMI was treated with at least one third-generation SES and was assessed by OCT imaging during the second procedure, which was deferred to 30, 80, or 180 days depending on risk features of the patient. A total of 3060 struts were analyzed, and the frequency of uncovered struts was 19.6% at 30 days, 1.3% at 90 days, and 1.8% at 180 days (p < 0.001). Cross sections with > 1 uncovered struts were significantly reduced during follow-up (51.3%, 6.5%, and 5.7%, p < 0.001, respectively). Meanwhile, cross sections containing thrombus were 6.2% at 30 days. Conversely, no thrombus was detected at follow-up. Authors concluded that new third-generation SESs promote early and persistent strut coverage with low peri-strut thrombus. Finally, they hypothesize that the safety of the tested stent could allow a potential reduction of the regimen of dual antiplatelet therapy (DAPT).
It has to be noted that this study presents important limitations that must be considered by the reader. First, authors enrolled a limited number of highly selected patients (about 17% of all STEMI treated in their center), and thereby external validity is compromised. Second, routine manual thrombectomy and glycoprotein IIb/IIIa inhibitors use in all cases do not clearly represent the actual clinical practice and clinical practice guidelines recommendations; these factors probably could have affected the results. Regarding OCT imaging, the lack of OCT assessment at baseline precludes the analysis of relevant OCT variables, such as baseline incomplete stent apposition, late-acquired malapposition, and late-persistent malapposition.
Intravascular imaging has been crucial in the understanding of the vascular response to device implantation. Because of OCT high spatial resolution (10 microns), visualizing extraordinary coronary artery images similar to histology is possible. Delayed arterial healing and poor endothelialization have been strongly related to late and very late stent thrombosis in pathology studies For these reasons, serial OCT analysis has become an extremely helpful tool for the assessment of arterial healing and stent endothelialization after implantation, allowing the follow-up of stent strut coverage, strut apposition, and precise measurement of neointimal proliferation. However, it also has to be noted that OCT has some limitations, such as the inability to visualize the thin endothelium and low penetration depth. Also some reports suggest that the precise nature of tissue coverage has not been demonstrated
Device technology has developed many strategies to overcome the delayed arterial healing, such as better antiproliferative drugs, more biocompatible polymer, better scaffold design, and improvement in implantation techniques. Clearly most of the efforts to improve stents endothelialization and reduce delayed arterial healing are intended to shorten DAPT duration. International clinical practice guidelines recommend at least 6 months of DAPT following drug-eluting stent (DES) implantation, and shorter DAPT duration may be considered after DES implantation in patients at high bleeding risk
Secco et al. suggest that OCT assessment of neointimal coverage of third-generation SES struts can provide security of a shorter DAPT regimen (1–3 months) because of the low ratio of uncovered/total stent struts. Since the evidence is very limited, it is difficult to offer any recommendation regarding the use of OCT as guidance to tailor DAPT at the moment. In select cases, such as patients with recent or prior bleeding or requiring surgery or needing concomitant oral anticoagulation, OCT follow-up of the stent can supply additional information to inform this difficult clinical decision. Moreover, it has to be stated that in stented patients the benefit of DAPT is also attributable to prevention of adverse events consequent to plaque rupture at sites remote from the stented segment, which is more frequently seen in acute coronary syndrome patients .
For stent follow-up, experts consider it appropriate to use OCT for the comparison of different stent platforms, assuming that a more uniform strut coverage could improve the late outcomes .
OCT actual and future indications in clinical practice
Recommendations for OCT use in clinical practice are shown in Table 1 . The clinical application in which most experts agree that OCT could be recommended is the assessment of mechanisms of failure of stents and scaffolds
