HOLMIUM-YAG LASER ABLATION OF VASCULAR TISSUE

The ablation of atherosclerotic lesions without collateral thermal or shock wave damage is thought to be a key element for successful laser angioplasty. This study evaluated the effectiveness of pulsed holmium:YAG laser (2.1 μm wavelength) for this application. Fresh normal tissue (n = 139) and arteriosclerotic canine arteries (n = 21) as well as formalin‐preserved normal canine (n = 31) and atherosclerotic human arteries (n = 177) were irradiated under saline via a 600 μm diameter fiber placed perpendicular to the intimal surface with 0–10 gm of force. The laser was operated in the free running mode (FRM; 250 μsec pulsewidth, 5 Hz, 30–7,100 mJ/mm2) and in the Q‐switched mode (QSM; 200 nsec pulsewidth, 6 Hz, 30–1,100 mJ/mm2). Following the experiments, the samples were prepared for histologic and morphomet‐ric analysis. Ablation thresholds in the FRM were 60 and 180 mJ/mm2 in fresh and preserved canine tissue, respectively. Ablation thresholds in the QSM for fresh and preserved canine tissues were 75 and 180 mJ/mm2, respectively. Thresholds for human atherosclerotic tissue were dependent on the amount of calcification. In the QSM and FRM, there were no samples that could not be penetrated at 1,100 mJ/mm2 and above. Histologic examination of the FRM samples revealed confined columns of tissue ablation, with approximately 55–250 μm and 70–140 μm zones of thermal effect being apparent in the fresh and formalin‐preserved samples, respectively. The QSM samples (fresh and preserved) revealed minimal thermal effect (0–20 μm) to the in‐tima and media (smooth muscle cells) between 300 and 1,100 mJ/ mm2, whereas thermal effect in the adventitia (collagen) was 64–172 pm with the same energy fluence. We conclude (1) that the thermal effect can be optimized by shortening the pulsewidth (Q switching at 200 nsec) or by limiting the total energy delivered and (2) that controlled ablation of both normal and atherosclerotic vascular tissue can be accomplished with the holmium:YAG laser. Copyright © 1990 Wiley‐Liss, Inc., A Wiley Company