Comparison Between Optical Coherence Tomography and Intravascular Ultrasound for Intermediate Left Main Coronary Artery Lesions

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.