Myocardium tissue ablation with nanosecond, ultraviolet pulses: in-vitro study with porcine myocardium tissues

To investigate the optimum irradiation conditions for the transmyocardial laser revascularization (TMLR), ablation characteristics have been explored in vitro with porcine myocardium tissues. With a nanosecond optical parametric oscillator (OPO), the ablation depth and the thickness of thermally damaged tissue were measured at a constant peak intensity or fluence in the ultraviolet spectral region of 230 - 400 nm. It was found that at a peak intensity of 80 MW/cm², the ablation depth steeply increased for < 300 nm, while the thickness of thermally damaged tissue decreased with decreasing the wavelength. To understand the wavelength dependence of the ablation characteristics, we measured the optical properties of the tissue. This showed that the total attenuation coefficient largely increased with decreasing the wavelength for < 300 nm. Therefore, it is considered that at the shorter wavelengths the optical energy density deposited in the tissue would be high enough to ablate the whole region of the light-penetrated tissue. However, the wavelength dependence might be changed at some higher intensities or fluences. But our experiment using the 3rd and 4th of a Q-switched Nd:YAG laser, the shorter wavelength (266 nm) still gave the deeper ablation for up to 2-5 J/cm². The influence of photomechanical effects on the ablation mechanism is also discussed.