(Introduction) Recently considerable attention is focused on the diagnosis of atheromatous lesion using laser induced fluorescense spectra. But stronger the laser power is, more quickly the intensity of fluorescense spectra falls down (bleaching phenomenon). So we examined the relationship between the intensity of laser power and the pattern of fluorescense spectra in this study. (Methods) The fluorescense spectra was obtained from the normal intima of postmortem aorta using helium-cadmium laser (wave length; 325nm). The intensity of laser power, 9mW, 4.5mW, 1.8mW, 0.5mW, and 0.1mW were used. Constant irradiation was made and the fluorescense spectra was measured at 280msec interval. From the selected spectra, peak intensities of wave length at 390nm (I390) and 450nm (I450) were measured, and its ratio, I390/I450 (plaque index: PI), was devised. (Results) The fluorescense spectra was not recorded at laser power 0.1mW. The intensity of fluorescense spectra fell down with time, and the ratio of fluorescence intensity decrease was bigger as the laser power increased. The values of PI were almost constant through recording at each intensity power. As the laser power was bigger, the coefficient of variation (CV) was smaller. CV were 11%, 8%, 4.8%, and 4.1% at laser power 0.5mW, 1.8mW, 4.5mW, and 9mW, respectively. (Conclusion) At the strong excitation laser output, quick fall of fluorescense spectra occured. The diagnosis of atheromatous lesion, using the fluorescence intensity at one wave length seemed to be quite difficult, but the rario of two peak fluorescence intensities is a feasible index because of its less fluctuation.
