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BACKGROUND: In vitro experiments have shown that holmium laser energy can effectively ablate even calcified plaque in human arterial vessels. Because high-energy densities from holmium lasers can easily be transmitted through quartz fibers, this solid-state laser has been suggested as an alternative intraluminal treatment of atherosclerotic plaque. METHODS AND RESULTS: To develop an intimal plaque, 35 New Zealand White rabbits underwent electrical stimulation of their right carotid artery for 28 days. Subsequently, in 25 rabbits, holmium laser angioplasty (wavelength, 2.12 microns; pulse duration, 150 microseconds; energy density, 350 mJ/mm2) was performed. To study the morphological results, the vessels were excised after 7, 14, 28, and 42 days. Cross sections were analyzed in regard to laser-specific injury. Staining of alpha-actin was used to identify smooth muscle cells (SMCs). After bromodeoxyuridine labeling, the extent of proliferation (number of cells undergoing DNA synthesis) was determined by using a monoclonal antibody. Holmium laser ablation resulted in an initial decrease of the numbers of intimal cell layers in the early group (7 days after treatment: 5 +/- 1 cell layers with 76 +/- 39 microns; control: 13 +/- 3 cell layers with 144 +/- 44 microns). Quantification of SMCs undergoing DNA synthesis in the intima (control: 51 +/- 19 cells/mm2) showed a significant increase of labeled cells after 7 (216 +/- 74 cells/mm2, p = 0.003) and 14 days (281 +/- 139 cells/mm2, p = 0.011). Integrity of the internal elastic lamina was disrupted in all animals after intervention. Seven and 14 days after treatment, a considerable reduction of medial cell nuclei was found in 10 of 12 animals. SMC proliferation in the medial layer was increased within the first 2 weeks after laser ablation (168 +/- 113 cells/mm2; control: 8 +/- 4 cells/mm2; p = 0.023). Six weeks after holmium laser angioplasty, SMC proliferation had returned to control levels in the intima and remained increased in the medial layer. This proliferative response resulted in a significant increase of intimal thickening within 6 weeks after laser ablation (30 +/- 6 cell layers, 375 +/- 97 microns resp.; p = 0.001 each). CONCLUSIONS: Holmium laser treatment leads to considerable vessel wall injury and results in SMC proliferation in the intimal and medial layer with a maximum of proliferative activity within the first 2 weeks. Subsequently, this results in considerable intimal and medial hyperplasia within 6 weeks after treatment.
Hassenstein et al. (Sun,) studied this question.