Influence of laser wavelength and pulse duration on gas bubble formation in blood filled glass capillaries

Background and Objectives: Hypervascular skin lesions (HVSL) are treated with medical lasers characterized by a variety of parameters such as wavelengthl, pulse duration tp, and radiant exposure E that can be adjusted for different pathology and blood vessel size. Treatment parameters have been optimized assuming constant optical properties of blood during laser photocoagulation. However, recent studies suggest that this assumption may not always be true. Our objective was to quantify thermally induced changes in blood that occur during irradiation using standard laser parameters. Study Design/Materials and Methods: Glass capillary tubes (diameter D ¼ 100, 200, and 337 mm) filled with fresh or hemolyzed rabbit blood were irradiated once at l ¼ 585, 595, or 600 nm, tp ¼ 1.5 milliseconds; and also atl ¼ 585 nm, tp ¼ 0.45 milliseconds. E was increased until blood ablation caused formation of permanent gas bubbles. In a corroborative study, human blood was heated at 508C and absorbance spectra were measured as a function of time. Results: Threshold radiant exposure, Ethresh, for gas bubble formation was found not to depend on l, which might be surprising in view of the 10-fold lower absorption coefficient at 600 nm as compared to 585 nm. The spectroscopic study revealed heat-induced changes in blood constituent composition of hemoglobins (Hb) from initially 100% oxyhemoglobin (HbO2) to deoxyhemoglobin (HHb) and, ultimately, methemoglobin (metHb) as the major constituent. Model calculations of Ethresh(l,D) based on changing constituent blood composition during heating with milliseconds lasers were found to correlate with experimental results. Conclusions: For laser treatment of HVSL it appears thatl is of secondary importance and that the choice of tp is a more important factor. Lasers Surg. Med. 36:281–288, 2005. 2005 Wiley-Liss, Inc.