The accurate measurement of lumen dimensions is important for assessing the severity of coronary stenoses. In the study comparing the lumen measurement obtained ex vivo in human coronary arteries using intravascular ultrasound, OCT and histomorphometry, and in vivo in patients using intravascular ultrasound and OCT with and without balloon occlusion, both intravascular ultrasound and OCT overestimated the lumen area compared with histomorphometry (mean difference 0.8 mm² for OCT and 1.3 mm² for intravascular ultrasound) [3]. The lumen dimensions in vivo obtained using intravascular ultrasound were larger than those obtained using OCT (mean difference 1.67 mm² for intravascular ultrasound relative to OCT with balloon occlusion and 1.11 mm² relative to OCT without balloon occlusion) [3]. OCT has a moderate diagnostic efficiency in identifying hemodynamically severe coronary stenoses with fractional flow reserve (FFR) ≤ 0.80 measured with pressure wire (sensitivity = 82%, specificity = 63%) [4]. The optimal cutoff value associated with FFR ≤ 0.80 was 1.95 mm² of minimal lumen area [4]. Like intravascular ultrasound, the assessment of OCT is not specific for identifying severe stenoses, thus limiting the positive predictive value [4]. Table 14.2 summarizes the cutoff values of OCT-derived minimal lumen area that correspond to functionally significant stenosis. Recently, an OCT study reported that there was a moderate correlation between OCT-derived FFR measurements using computational fluid dynamics algorithm and direct FFR measurements using pressure wire (r = 0.72, p < 0.001) in patients with intermediate coronary stenosis in the left anterior descending coronary artery, as represented in Fig. 14.1 [8]. This OCT approach without use of pressure wire may be useful for evaluating the simultaneous functional and anatomic severity of coronary stenosis [8]. However, further studies are required to establish its feasibility and effectiveness. In contrast to determination of functionally significant severity of coronary artery, the OCT examination is reliably sensitive and specific for characterizing different types of atherosclerotic plaques: fibrous , fibrocalcific , and lipid-rich plaques (Fig. 14.2) [9, 10]. Morphological features detected by OCT were associated with the occurrence of post-interventional complications. The presence of thin-cap fibroatheroma identified by OCT was a predictor of post-PCI myocardial infarction [11]. Figure 14.3 represents a typical case that showed post-PCI myocardial infarction in patient treated with elective stent implantation.

The optimal OCT criteria for stent deployment have not been established yet. In the CLI-OPCI (Centro per la Lotta contro l’Infarto-Optimisation of Percutaneous Coronary Intervention) study, the reference lumen narrowing had to be greater than 4 mm², and the stent-lumen distance, namely, malapposed distance, had to be less than 200 μm for optimal stenting [12]. However, this study was retrospective, and the decision as to whether to perform further actions if the OCT criteria were not satisfied was left at the operator’s discretion [12]. In the multicenter, randomized DOCTORS (Does Optical Coherence Tomography Optimize Results of Stenting) study [13], the guidelines for the procedural strategy incorporating OCT information were as follows: (1) additional balloon overdilations were to be performed in case of stent underexpansion (the ratio of in-stent minimal lumen area to reference lumen area was ≤ 80%), (2) management of malapposition or edge dissection was at the operator’s discretion, and (3) additional stent implantations were to be performed to rectify incomplete lesion coverage. These methods of stent optimization led to a larger minimum lumen area compared with immediate post-stenting and subsequently improved the functional outcome assessed by FFR after PCI [13]. Table 14.3 summarizes the considerations for stent optimization using OCT.

The CLI-OPCI study firstly evaluated 1-year clinical outcomes in matched patients between angiographic guidance alone and angiographic plus OCT guidance. The use of OCT was associated with a lower risk of cardiac death or myocardial infarction (odds ratio = 0.49, p = 0.037) [12]. This observational study suggested the potential usefulness of OCT-guided PCI compared to conventional therapy. The ILUMIEN I (Observational Study of OCT in Patients Undergoing FFR and PCI) was a prospective, nonrandomized, observational study of PCI procedural practice in a total of 418 patients (with 467 stenoses) undergoing intra-procedural pre- and post-PCI FFR and OCT [14]. Based on pre-PCI OCT findings, the procedure was altered in 57% of all stenoses by selecting different stent lengths, and further stent optimization based on post-PCI OCT findings was done in 27% of all stenoses using additional post-dilation or implantation of new stents [14]. With the decreases of stent malapposition, underexpansion, and edge dissection, the change in treatment strategy appeared to be associated with reduced rates of periprocedural myocardial infarction [14]. Although intriguing, these results need confirmation in randomized controlled trial to firmly establish the clinical benefit of OCT-guided PCI.

Several benefits including adequate stent expansion, improved strut coverage, or FFR after PCI were also noted in patients receiving OCT-guided PCI. According to the OCT substudy of the thrombectomy versus PCI alone (TOTAL) trial, OCT-guided primary PCI for ST segment elevation myocardial infarction was associated with a larger final stent minimum lumen diameter compared to angiographic guidance (2.99 ± 0.48 mm versus 2.79 ± 0.47 mm, p < 0.0001) [15]. Although this study was statistically underpowered to detect a difference in clinical outcomes in OCT-guided patients, these findings suggested that OCT had the potential to improve clinical outcomes in patients undergoing PCI [15]. The ILUMIEN II study retrospectively compared OCT guidance with intravascular ultrasound guidance in propensity scores matched population and demonstrated that stent expansion was comparable between OCT- and intravascular ultrasound-guided patients [16]. Recently, the ILUMIEN III randomized trial tested whether or not OCT-based stent sizing strategy would result in a minimum stent area similar to or better than that achieved with intravascular ultrasound guidance and better than that achieved with angiography guidance alone [17]. In this trial, stent diameter was determined according to measurements of the external elastic lamina in the proximal and distal reference segments, and stent length was determined as the distance from distal to proximal reference site using the OCT automated lumen detection feature [17]. After stent implantation, high-pressure or larger noncompliant balloon inflation was performed to achieve a minimum stent area of at least 90% in both the proximal and distal halves of the stent relative to the closest reference segment [17]. Regarding minimum stent area, OCT guidance was non-inferior to intravascular ultrasound guidance, but not superior. OCT guidance was also not superior to angiography guidance [17]. Accordingly, these data warrant a large-scale randomized trial to establish whether or not OCT guidance results in superior clinical outcomes to angiography guidance [17].