Composition of coronary atherosclerotic plaque in the intima and media affects intravascular ultrasound measurements of intimal thickness.

OBJECTIVES

The purpose of this study was to evaluate how intravascular ultrasound-determined thickness and reflectivity of the inner echogenic layer of coronary artery plaque is affected by changes in collagen, elastin, proteoglycan, calcium and lipid content in the intima and media.

BACKGROUND

Coronary artery plaque often results in disruption of the internal elastic lamina and in increased collagen in the media as well as increased intimal lipid, calcium and proteoglycan content. How these factors affect intravascular ultrasound-derived measurements of intimal thickness are unknown.

METHODS

Twenty-one coronary artery segments from 10 patients with varying degrees of coronary artery disease were obtained fresh and unfixed at autopsy. Intravascular ultrasound (30-MHz, 3.5F catheter) was performed at regions in the vessel where a three-layered region was evident. Quantitative measurements of inner echogenic and deeper echolucent layer thickness were determined as well as estimates of ultrasound reflectivity and compared with histologic measurements of intimal and medial thickness and collagen, proteoglycan, lipid and calcium content.

RESULTS

In plaque > 300 microns thick and equivalent histologic levels of collagen, inner echogenic layer reflectivity was greater in regions with a greater amount of calcium and proteoglycan and lesser amounts of lipid. Inner echogenic layer thickness correlated well with intimal thickness but had the best correlation with intimal and medial thickness when medial collagen content was increased (r = 0.89). The median observed difference between intimal and inner echogenic thickness was 160 (p < 0.05 compared with zero), whereas the median observed difference between inner echogenic thickness and intimal plus medial thickness when medial collagen content was increased was 60 (not significantly different from zero).

CONCLUSIONS

The inner echogenic layer of human coronary artery plaque may represent both intima and media. The echolucent layer by ultrasound may represent only the basal section of the media that does not contain high collagen content.