Medical-Engineering Background of Endovenous Laser Ablation

Abstract

Endovenous laser abalation (EVLA) is one of advanced intravascular treatments for varicose veins, however laser-tissue interaction in the blood is still controversial. As laser physics is fundamental for EVLA, physicians’ precise comprehension would contribute to the safe and effective procedure with patients’ satisfaction. Continuous wave (CW) lasers were applied in initial clinical experiences in EVLA, because they had been already used in the body surface irradiation. After selective photothermolysis had been proposed, pulse wave (PW) lasers were introduced to minimize the heat diffusion to the surrounding tissue and penetrate sufficiently to the target with minimal energy. Thus, concerns have been raised about the mechanism of action and the extent of mechanical damage that may be produced by PW lasers. This paper discussed the basic laser- tissue interaction in the blood and the current understanding of ablation mechanism. It is essential that popular wavelengths (980–1470 nm) in Japan applied to EVLA are theoretically absorbed to hemoglobin and water. In addition, laser oscillation mode also determines the irradiant power or peak power, which indicates the speed of laser action. Irradiance for CW or fluence for PW is an index of energy density (W/cm²) and we must know the irradiant area for their calculation. Nevertheless, as EVLA is a blind procedure, irradiant area cannot be measured and standardization of energy density is complicated. Linear endovenous energy density and endovenous fluence equivalent are proposed from this background, disregarding the diameter of veins or venous deformity by varicosity. The possible mechanisms of action are suggested by steam bubble, direct contact or heat diffusion. According to selective photothermolysis, PW laser seems to be more advantageous than CW by ablating within the thermal relaxation time, cleaning off the coagulum from the fiber tip by shock wave and non-thermal insult (photomechanical action) on the vein wall. In addition, the fourth mechanism could be proposed by laser penetration through bubbles, generated by waterselective wavelength laser. Fiber design is also crucial for preventing blood coagulation and carbonization. Jacket type and radial emitting fiber are underdeveloped for this purpose. In conclusion, as blood is one of biological tissue including hemoglobin and water, endovenous laser emission is totally different from emission in the air. Further investigation is necessary for optimization of EVLA and developing the future system.