Acute coronary syndrome (ACS) is a clinical syndrome caused by the rupture of coronary atherosclerotic plaques, leading to complete or partial occlusive thrombosis and subsequent acute myocardial ischemia or necrosis.1 Abnormal lipid metabolism is a well-established risk factor for ACS, with low-density lipoprotein cholesterol (LDL-C) recognized as an independent contributor.2 However, despite the widespread use of intensive lipid-lowering therapies, the incidence of cardiovascular events and mortality remains high, a phenomenon termed lipid residual risk.3 Emerging evidence suggests that remnant cholesterol (RC), the cholesterol content of triglyceride-rich lipoproteins (TRLs), plays a significant role in the pathogenesis of cardiovascular disease, independent of LDL-C.4,5 TRLs include chylomicron remnants (CR), very low-density lipoprotein (VLDL), and intermediate-density lipoprotein (IDL).6 RC can be directly measured using advanced analytical techniques such as ultracentrifugation and nuclear magnetic resonance spectroscopy. However, these methods are often time-consuming and costly. Alternatively, RC can be estimated by subtracting LDL-C and high-density lipoprotein cholesterol (HDL-C) from total cholesterol (TC).7 Although this calculation provides only an approximate value, it offers the advantage of utilizing standard lipid profiles, making it more accessible for clinical and research purposes. Recent studies have shown that calculated RC levels exhibit a strong correlation with directly measured values, further validating this approach.8

Optical coherence tomography (OCT) is an intravascular imaging technique that utilizes the reflection of near-infrared light to generate high-resolution images of coronary structures.9 With a resolution of up to 10 μm, OCT enables precise evaluation and detailed characterization of atherosclerotic lesions. Recent studies have demonstrated that the optical flow ratio (OFR), derived from fundamental fluid dynamics equations without the need for medication, can accurately identify lesions that impair blood flow and detect lesion-specific ischemia. This method has shown a significant correlation with pressure wire-based fractional flow reserve (FFR).10,11 FFR measures the ratio of maximal blood flow in a stenotic artery to that in a nonobstructed artery using a pressure-sensitive coronary guidewire, with its values predictive of inducible ischemia.12 The direct association between low pre-percutaneous coronary intervention (PCI) FFR and future adverse events may reflect the integrated information about the culprit vessel, including both epicardial and microvascular conditions, as well as the global atherosclerotic burden or ischemia.13 Consequently, OFR serves as a valuable tool for evaluating the physiological characteristics of the culprit vessel and predicting patient prognosis.

Currently, there is limited evidence on the relationship between RC and culprit coronary plaque characteristics or physiological features in patients with ACS. This study aims to investigate the correlation between RC, coronary plaque characteristics, and physiological features in ACS patients using OCT, with the goal of clarifying the mechanisms by which RC contributes to the development of ACS and providing additional evidence for the prognosis assessment of these patients.