Comparative Intravascular Ultrasound Analysis of Ostial Disease in the Left Main versus the Right Coronary Artery

The aorto-ostial junction has high elastic fiber content with significant elastic recoil.^1-7 Ostial left main coronary artery (LMCA) stenoses have larger lumen areas with less plaque burden and more negative remodeling than non-ostial LMCA stenoses. Most ostial LMCA stenoses have been categorized as eccentric and less calcified.⁸ The RCA ostium shows a lack of arterial distensibility and excessive rigidity, presumably because it contains highly elastic rigid tissue.^1,7 Both the LMCA and RCA ostia may have similar morphologic characteristics. However, there is limited information about ostial lesions and no study has compared ostial LMCA and ostial RCA lesions.

The purpose of this study was to use intravascular ultrasound (IVUS) to: (1) evaluate the morphometry of different aortoostial disease locations; (2) assess the dominant contributor of aorto-ostial stenoses (negative remodeling versus plaque accumulation); and (3) compare morphometry between LMCA and RCA ostial stenoses.

Methods

Study population. We used IVUS to compare 45 LMCA versus 50 RCA aorto-ostial stenoses. A significant ostial stenosis was defined as LMCA or RCA ostial angiographic diameter stenosis > 50%, LMCA ostial IVUS minimal lumen area < 6 mm,^2,9 or RCA ostial IVUS minimum lumen area < 4 mm.^2,10 Exclusion criteria included severely calcified ostial lesions, lesions that could not be crossed by IVUS and preinterventional use of any percutaneous device such as the Rotablator^® (Boston Scientific Corp., Natick, Massachusetts). Coronary risk factors that were tabulated included hypertension (medicationtreated only), diabetes mellitus (known diabetes or repeated fasting blood glucose levels > 120 gm/dL), hypercholesterolemia (medication-treated or a measured level > 240 mg/dL) and current smoking (within the past 12 months).

IVUS imaging. All IVUS examinations were performed using a commercially available IVUS system (Boston Scientific) with a 40 MHz transducer. Studies were performed before any intervention and after intracoronary administration of 200 μg of nitroglycerin. The IVUS catheter was advanced 10 mm distal to the target lesion, and imaging was performed retrograde back to the aorto-ostial junction at a pullback speed of 0.5 mm/second. Ostial lesions were measured at the most proximal extent where the full circumference wall structures of the coronary ostium could be identified with the guiding catheter disengaged from the coronary ostium. Studies were recorded on 0.5 inch high-resolution s-VHS tapes for offline analysis.

IVUS analysis. Standard IVUS measurements were performed each 1 mm beginning within the ostium and continuing to the distal reference segment. Qualitative analysis was performed according to criteria of the American College of Cardiology Clinical Expert Consensus document on IVUS.¹¹ Using planimetry software (TapeMeasure, INDEC Systems, Mountain View, California), we measured external elastic membrane (EEM), stent and lumen cross-sectional area (CSA). Plaque & media (P&M) CSA was calculated as EEM minus lumen CSA (where there was no stent). Negative remodeling was defined as a remodeling index (lesion/distal reference arterial area) < 0.95. The eccentricity index wasdefined as maximum divided by minimum P&M thickness > 2.0. The maximum arc of calcium within the stenosis was measured with a protractor centered on the lumen. Statistical analysis. Statistical analysis was performed using the SPSS statistical package, version 10.0 (SPSS Inc., Chicago, Illinois). Continuous variables are presented as the mean value ± 1 standard deviation (SD) and compared with the Student’s t-test. Categorical variables are presented as frequencies and compared using chi-square statistics. A p-value < 0.05 was considered statistically significant.

Results

Clinical findings. Demographics and clinical characteristics are shown in Table 1. Patients with ostial LMCA and ostial RCA stenoses had similar risk factors and clinical presentations.

Angiographic findings. Minimal lumen diameter (p = 0.2) and reference vessel diameter (p = 0.9) were similar in both ostial lesion locations (Table 1); and percent diameter stenosis was not different (p = 0.24) between the two lesion locations. Angiographic diameter stenosis correlated poorly with the IVUS minimal lumen area in ostial LMCA (r = 0.148, p = 0.33) and ostial RCA lesions (r = 0.14, p = 0.34) (Figure 1). Sixty percent (27/45) of ostial LMCA lesions had an angiographic diameter stenosis > 50%, and 20% (9/45) of ostial LMCA lesions had an angiographic diameter stenosis > 50% and an IVUS minimal lumen area > 6 mm². Seventy-two percent (36/50) of ostial RCA lesions had an angiographic diameter stenosis > 50%, and 32% (16/50) of ostial RCA lesions also had an IVUS minimal lumen area > 4 mm².

Comparisons between LMCA and RCA. The overall length of the LMCA was 8.62 ± 4.58 mm. Both ostial LMCA and ostial RCA lesions were short, although the ostial LMCA lesions tended to be shorter than ostial RCA lesions (p = 0.061). EEM area was larger in the LMCA both at the ostium (p = 0.001) and in the distal reference (p = 0.001), while minimal lumen area (p < 0.0001) and lesion and distal reference plaque areas (p = 0.085 and p < 0.0001, respectively) were smaller in the ostial RCA lesions. Both ostial locations had modest amounts of calcium (45^º in ostial LMCAand 53^º in ostial RCA lesions, p = 0.5); and both ostial locations had modest plaque burdens (60% in ostial LMCA and 64% in ostial RCA lesions, p = 0.14). Of note, both ostial locations had a marked frequency of negative remodeling: 38/45 (84%) in ostial LMCA versus 43/50 (86%) ostial RCA lesions (p = 1.0, Figure 2, Table 2). However, the remodeling index did not correlate with lesion length (LMCA: r = 0.014, p = 0.925 and RCA: r = 0.109, p = 0.45), and the remodeling index correlated poorly with the overall length of the LMCA (r = 0.259, p = 0.094).

Discussion

This study compared patterns of ostial disease between LMCA and RCA locations. The main findings are as follows: The morphology of ostial lesions is similar in the RCA and LMCA locations; both are associated with: (1) short lesion length; (2) higher frequency of negative remodeling; and (3) poor correlation between angiographic diameter stenosis and IVUS minimal lumen area.

Pathologic studies have shown that ostial lesions are frequently calcified, fibrotic and sclerotic.^1,12 Previous clinical studies have suggested a high elastic fiber content within ostial lesions,⁷ and the poor outcome after balloon angioplasty was attributed to significant elastic recoil.^2,14 Consequently, stent implantation has become the preferred approach to treat aorto-ostial lesions.^3,16–18 The aortic elastic muscle fibers extend to the LMCA and sometimes even into the proximal left anterior descending artery. The frequency with which these fibers reach the left main bifurcation may depend on left main length and, therefore, have an impact on stenosis development.¹³ To date, there have been limited data about negative remodeling in aorto-ostial lesions; however, negative remodeling has been reported at the other locations within the coronary tree.^19,20 In comparison to ostial lesions, positive remodeling is frequently observed in non-ostial culprit lesions with unstable coronary syndromeand plaque rupture.^21–23 Drug-eluting stent implantation in aorto-ostial lesions achieved excellent results with lower angiographic restenosis and clinical outcomes and a similar rate of target lesion revascularization compared to the shaft lesion.^24–27

It has been suggested that structural differences may exist between the ostial LMCA and the ostial RCA. Ostial LMCA stenoses are more common in women, mostly eccentric and have a larger lumen area, less plaque burden, less calcification and more negative remodeling than non-ostial LMCA stenoses.⁸ As compared with LMCA ostial stenting, RCA ostial stenting is associated with a higher restenosis rate and a higher incidence of target lesion revascularization. RCA stent areas are smaller than LMCA stent areas, either due to differences in vessel size or arterial distensibility and excessive rigidity of the aorto-ostial RCA junction.⁷ Size differences are supported by the results of the present study.

Study Limitations

This study was a small retrospective analysis and may therefore not have sufficient power to detect differences in subgroups. We did not include severely calcified lesions. Grayscale IVUS has a limited ability to assess plaque composition. Conclusion IVUS morphometry is remarkably similar in ostial LMCA and RCA lesions. Negative remodeling is the dominant contributor to lumen compromise in both locations.

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