DIFFERENTIAL VASCULAR-RESPONSE TO LASER PHOTOTHERMOLYSIS

Individual blood vessels in the chick chorioallantoic membrane were se lectively coagulated through photothermolysis, using pulsed laser irra diation at 585 nm. Pulse durations were chosen to be 0.45 ms and 10 ms , which correspond to the thermal relaxation times in blood vessels of 30 mu m and 150 mu m diameter, respectively. The short pulses, at a l ight fluence F = 3 Jcm(-2), caused permanent occlusion of vessels of 4 0 mu m diameter or less, whereas larger caliber vessels (60 - 120 mu m ) required F = 4 - 5 Jcm(-2). The long-duration pulses, at F = 7 Jcm(- 2), caused coagulation of the larger diameter vessels; the small-calib er vessels and capillaries showed resistance to photothermolysis and r equired multiple exposures to achieve coagulation. The fluence versus diameter (F versus d) relationship for coagulation was calculated for the two pulse durations. The energy deposited in a cylindrical absorbe r of diameter d by an optical field, incident perpendicular to the ves sel, was expressed analytically and compared with the energy required to coagulate a blood vessel of the same lumen diameter. When thermal d iffusion is incorporated into the model, our findings can be accounted for quantitatively. This information will be of use for improving the laser treatment of port wine stains and other vasculopathies. A surpr ising observation was that arterioles were damaged at lower incident e nergy densities than venules having the same lumen diameter, despite t he fact that absorbance in oxygen-rich and oxygen-poor blood is the sa me at 585 nm.