Significant coronary disease of the left main coronary artery (LMCA) is found in 4%-5% of all coronary angiography procedures. It is a particularly important site, since it supplies up to 75% of myocardial blood supply, and damage at this level thus implies a large amount of left ventricular myocardium at risk, with a mortality rate close to 40% at 3 years, if revascularisation is not performed.
Anatomically, it has a number of particularities compared to the rest of the coronary arteries, such as its larger diameter (5±0.5 mm) and variable length (10.5±5.3 mm), a composition, particularly at the aorto-ostial level, more similar to the aorta than to the coronary arteries, and in up to 20-30% of the population there is also a division between the anterior descending artery (LAD) and the circumflex artery (LCx) of a third branch called the ramus intermedius or bisector branch.
Classically, it has been determined that a significant angiographic stenosis should reach at least 50% of the vessel diameter by visual estimation, which corresponds to 75% of the vessel area. However, angiography has a number of limitations inherent to the technique and location of stenosis, and other techniques are therefore available for evaluation. Intracoronary ultrasound (ICUS or IVUS) deserves, together with the pressure guidewire, special consideration in determining the severity assessment (anatomical and functional) of lesions in this location.
Several ICUV studies have attempted to find a minimum luminal area (MLA) as the cut-off point, ranging from 4.5-7.5 mm2, to decide whether to perform revascularisation or not. However, the most commonly used cut-off value is 6 mm2 for various reasons. First, it is correlated with functionally significant values using pressure guidewire. Second, the linear law is applied (assuming the fractal nature of the vasculature and a cut-off value of 3 mm2 for the LMCA branches). Finally, it has been validated by the prospective LITRE study with clinical results at 2 years of follow-up. Other studies in Asian population have proposed lower cut-off values (4.5 mm2). However, this population has different body size and therefore smaller LMCA size, the study has lower sensitivity (1/4 of patients with area \>4.5 mm2 had positive pressure guidewire), and clinical validation is not presented unlike the LITRE study.
In addition to its value in diagnosis, use of ICUSE allows for optimisation of percutaneous coronary intervention (PCI) if necessary, with decreased events as compared to angiography. Therefore, current clinical practice guidelines consider the use of IVUS to stratify the severity of all LMCA lesions as an indication IIa B. In turn, it has been proposed to integrate the use of ICUS and pressure guidewire in the assessment of doubtful LMCA lesions. Thus, in ambiguous lesions of the LMCA, a MLA \>6 mm2 would indicate no revascularisation, a MLA \<4.5-5 mm2 would indicate revascularisation, and MLA values between 4.5-5 and 6 mm2 would make it advisable to use FRF/iFR to decide.
Optical coherence tomography (OCT) is another intracoronary imaging modality, with greater resolution and significant differences from ICUS. It is an expanding technique. However, its usefulness in LMCA is somewhat more limited, mainly due to the difficult technique of complete filling with contrast and the native area of the ostial segments. Another disadvantage of its use in LMCA is its limited penetration depth (2-3 mm) compared to ICUS (4-8 mm), and since the LMCA usually has diameters of 3.5-4.5 mm, inadequate assessment may occur. In addition, no MLA cut-off point with OCT has been demonstrated for the management of LMCA lesions. On the other hand, because of the differences in imaging with both techniques, the thresholds established as cut-off points in IVUS cannot be extrapolated to OCT. There are, however, some correlation studies between ICUS and OCT, both in vivo and in vitro, but not specifically in LMCA. In all these studies, it has been shown that ICUS consistently overestimates OCT measurement by ≈10%, the latter being the closest to the real value. The underuse of this technique in the LCMA is justified by the potential technical problems already mentioned and the lack of a validated MLA cut-off point at this level. The potential prognostic implication of finding, even in patients with functionally nonsignificant lesions, vulnerable plaques or thin-cap fibroatheromas (TCFAs) in OCT has recently been highlighted. The objective is to compare the minimal luminal area by ICUS and OCT of angiographically intermediate LCMA lesions and to assess the prognostic value of TCFA assessed by OCT.
