Application of Virtual Histological Intravascular Ultrasound in Plaque Composition Assessment of Saphenous Vein Graft Diseases
Introduction
Coronary artery bypass grafting (CABG) is a widely used surgical treatment for coronary artery disease (CAD). In China, the number of CABG procedures has exceeded 40,000 annually, with a growth rate of 10%. Saphenous vein grafts (SVGs) are commonly used in CABG due to their availability, despite their lower patency rates compared to arterial grafts. SVG patency rates decline significantly over time, from 93% at one year to 41% at ten years, primarily due to degenerative and occlusive diseases, collectively referred to as saphenous vein graft disease (SVGD). SVGD is a major cause of morbidity and mortality in CAD patients post-CABG, making the prediction, treatment, and prevention of SVGD a critical challenge.
Percutaneous coronary intervention (PCI) is the preferred revascularization strategy for SVGD, as redo-CABG is associated with higher morbidity and mortality. However, SVG-PCI is fraught with complications, the most common being slow or no-reflow, which occurs in up to 15% of cases and is linked to major adverse cardiac events (MACEs) and mortality. The no-reflow phenomenon remains poorly understood and difficult to manage, highlighting the need for better predictors of clinical outcomes in SVG-PCI patients.
Intravascular ultrasound (IVUS) has been instrumental in identifying vulnerable atherosclerotic plaques and assessing plaque characteristics. Traditional gray-scale IVUS has limitations in precisely interpreting plaque components. In recent years, virtual histology IVUS (VH-IVUS), which analyzes radiofrequency signals, has emerged as a powerful tool for quantitative assessment of plaque composition and morphology. This review explores the application of VH-IVUS in assessing plaque composition in SVGD and its potential role in predicting clinical outcomes.
Basic Principles and Advantages of VH-IVUS
IVUS involves the delivery of micro-ultrasound transducers into the coronary arteries to generate cross-sectional images of the vessel. While traditional gray-scale IVUS provides valuable insights into plaque burden and morphology, its ability to precisely identify plaque components is limited. VH-IVUS, on the other hand, uses spectral analysis of radiofrequency signals to classify atherosclerotic plaque into four types: fibrous tissue (FT), fibro-fatty (FF), necrotic core (NC), and dense calcium (DC). These components are represented in a color-coded map, enabling more accurate qualitative and quantitative analysis of plaque composition.
VH-IVUS has demonstrated high sensitivity and specificity in identifying lipid-rich necrotic cores, with reported values of 91.7% and 96.6%, respectively. Other IVUS modalities, such as iMap™ IVUS and integrated backscatter IVUS (IB-IVUS), also provide detailed plaque composition information but are less widely used than VH-IVUS. Compared to gray-scale IVUS, VH-IVUS offers superior resolution and clarity in identifying plaque components, particularly lipid-rich NC. However, it has limitations in detecting thrombus and assessing plaque composition in heavily calcified areas.
Optical coherence tomography (OCT) is another advanced imaging modality with high spatial resolution, but it struggles with signal penetration in lipid-rich or necrotic cores and is less effective in large vessels like SVGs. VH-IVUS, when combined with OCT, provides complementary information, enhancing the overall assessment of plaque characteristics.
Application of VH-IVUS in the Assessment of Plaque Composition of SVGD
SVGD is a complex and dynamic process characterized by histological and morphological changes in SVG conduits following exposure to systemic arterial pressure. Thrombosis occurs in 10% to 25% of SVG patients within one year post-surgery, and atherosclerosis develops as early as one year after CABG. Necrotic cores are typically observed 2 to 5 years post-surgery, and intraluminal hemorrhage may occur after 5 years due to NC expansion. By seven years, the histological composition of SVG plaques resembles that of native coronary arteries.
Studies using VH-IVUS have provided valuable insights into the composition of SVG plaques. For instance, Wood et al. demonstrated that SVG plaques are primarily composed of fibrous tissue (50±12%), with no significant differences between body and anastomotic lesions. Komatsu et al. reported a case where a severely stenotic SVG plaque contained 16% NC and 36% FF. Jim et al. found that plaque burden in SVG lesions was positively correlated with fibro-fatty tissue and negatively correlated with dense calcium.
In a recent study from China, VH-IVUS analysis of high-risk SVG plaques revealed that fibrous tissue was the dominant component (65.12±10.11%), followed by fibro-fatty tissue (3.80%), necrotic core (12.00%), and dense calcium (1.00%). Plaques with a burden ≥70% were associated with more fibro-fatty tissue, and graft age was positively correlated with fibro-fatty tissue area. These findings underscore the potential of VH-IVUS in characterizing SVG plaques and identifying high-risk features.
Application of VH-IVUS in the Interventional Therapy of SVGD
The no-reflow phenomenon during SVG-PCI is a significant challenge, often resulting from plaque rupture and distal micro-thrombosis. VH-IVUS has been instrumental in identifying predictors of no-reflow, such as large necrotic core areas and thin-cap fibroatheromas (TCFAs). Hong et al. confirmed that positive remodeling observed by gray-scale IVUS is a strong predictor of no-reflow, with intraluminal plaques, multi-plaque rupture, and SVG lesions independently associated with the phenomenon.
Several studies have demonstrated the relationship between VH-IVUS-defined plaque characteristics and clinical outcomes. The VIVA study showed that VH-IVUS-defined TCFA was associated with MACEs, while the PROSPECT study identified plaque burden >70%, minimum luminal area <4 mm², and TCFA as independent predictors of non-culprit lesion-related events. Kang et al. found that elevated plaque structural stress (PSS) was associated with the presence of TCFA and increased MACE risk.
In the context of SVG-PCI, limited studies have systematically investigated the relationship between plaque composition and clinical outcomes. However, based on the similarities between CAD and SVGD, it is plausible that VH-IVUS-defined plaque characteristics in SVGs could predict clinical outcomes. Further large-scale, long-term, multicenter studies are needed to validate this hypothesis.
Conclusion
VH-IVUS has emerged as a powerful tool for assessing plaque composition in SVGD, providing valuable insights into the pathophysiology of SVG lesions. By classifying plaques into fibrous tissue, fibro-fatty tissue, necrotic core, and dense calcium, VH-IVUS enables a more precise understanding of plaque characteristics and their association with clinical outcomes.
The no-reflow phenomenon remains a significant challenge in SVG-PCI, and VH-IVUS has shown promise in identifying predictors of this complication, such as large necrotic core areas and TCFAs. However, more research is needed to establish the relationship between VH-IVUS-defined plaque components and clinical outcomes in SVGD patients undergoing PCI.
The integration of VH-IVUS with other imaging modalities, such as OCT, offers a comprehensive approach to intracoronary assessment, driving advancements in intravascular imaging technology. As the field of interventional cardiology continues to evolve, VH-IVUS is poised to play a critical role in improving risk stratification, guiding interventions, and ultimately enhancing patient outcomes in SVGD.
doi.org/10.1097/CM9.0000000000000183
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