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Intravascular ultrasound can be used to assess borderline lesions seen on coronary angiography. A 50 year old woman with angina and objective evidence of ischaemia in the anterior wall underwent multiple coronary angiograms following recurrent admissions with angina. These failed to demonstrate any specific lesion in the left anterior descending artery that might guide an intravascular ultrasound examination. Examination of the left anterior descending artery using intravascular ultrasound showed significant obstruction of the ostium by eccentric plaque, which was unsuspected on angiography. The ostium of the left anterior descending artery contained extensive eccentric plaque reducing the lumen to minimal diameter of 1.9 mm with a cross sectional area of 3.7 mm2. The plaque cross sectional area measured 5.6 mm2 which corresponds to a stenosis of 61% of the total area bounded by the media. The maximal diameter of the ostium measured 4 mm and the percentage stenosis was 53%. The plaque extended proximally into the left main coronary artery but did not significantly obstruct the lumen. The left anterior descending artery ostial lesion was fairly short in length and the remainder of the proximal, mid and distal segments of the left anterior descending artery appeared free of significant atheroma.
Pathological studies show that atherosclerotic lesions have a distinct spatial distribution around coronary bifurcation. In the case of the left main bifurcation, lesions were frequently found on the outer walls of the proximal branches particularly the left anterior descending artery, where wall shear stress was low. The flow divider and inner walls of the arterial branches were found to be relatively free of atherosclerosis. In fact this was exactly the pattern of distribution of atherosclerosis in our patient with eccentric plaque limited to the outer sector of the left anterior descending artery opposite the branching point of the circumflex artery.
Coronary angiography has limitations in assessing the three dimensional spatial arrangement of coronary bifurcations and hence short ostial lesions at these sites may be missed on angiography. Intravascular ultrasound is the technique of choice for detecting such lesions in patients with recurrent angina and objective evidence of ischaemia, in whom angiography may not reveal any obstructive lesion.