Laser-irradiation-induced relaxation of blood vessels in vivo.

The response of blood vessels to laser irradiation in vivo was studied in the dorsal skin flap glass window chamber model of hamsters. The vasodilatory response of venules was critically dependent on the wavelength of irradiating laser. Relaxation was not produced in arterioles, although it was tried repeatedly. Vessels were irradiated with the 514.5 nm single line argon laser with irradiances from 1 to 10 W/cm2 on a 1.2 mm-diameter spot. Irradiation of venules with 2.2 W/cm2 and 4.25 W/cm2 produced reversible relaxation. Venules relaxed initially and after the interruption of irradiation returned to their original diameter. At higher irradiances (8.5 W/cm2) an irreversible relaxation was observed. At irradiances of 10 W/cm2 and above initial relaxation was accompanied with constriction, focal coaguli, and hemostasis. Irradiation with the argon-pumped dye laser at 595 nm did not produce any significant relaxation.