Acute biologic response to excimer versus thermal laser angioplasty in experimental atherosclerosis. Vascular injury and platelet accumulation after balloon angioplasty are two potentially important triggers of the process of restenosis that may be minimized by the use of laser energy to ablate atherosclerotic plaque. The type of laser most suitable to achieve these goals remains unknown. Accordingly, angiographic and histologic studies and quantitative platelet deposition analysis were performed on 27 atherosclerotic rabbit iliac arteries randomized to treatment with excimer laser or thermal laser angioplasty. Excimer laser angioplasty was achieved with 35 to 40 mJ/mm2 of 308 nm xenon chloride irradiation delivered through a 4.5F catheter made of 13 concentrically arranged 200 microns fiber optics, at a repetition rate of 25 to 30 Hz and a pulse duration of 135 ns; thermal laser angioplasty was achieved with a 1.7 mm metal probe heated with 10 W of continuous wave argon laser energy. The baseline and post-laser luminal diameters of excimer laser-treated vessels (0.92 +/- 0.28 and 1.56 +/- 0.48 mm, respectively) were similar to those observed in thermal laser-treated vessels (1.05 +/- 0.44 and 1.61 +/- 0.41 mm, respectively). Perforation occurred in 4 (29%) of 14 thermal laser-treated arteries and in 0 of 13 excimer laser-treated arteries (p = 0.04); spasm was observed in only 1 thermal laser-treated vessel. On the basis of a quantitative histologic grading scheme (damage scores of 0 to 4), greater degrees of injury were measured in thermal versus excimer laser-treated vessels (2.4 +/- 1.0 versus 1.3 +/- 0.4, p = 0.009). 