Estimation of a minimum laser power with wavelengths of 1.47, 1.56, and 1.68 μm for efficient obliteration of varicose veins

Experiments modelling endovenous laser obliteration (EVLO) are performed. As a result, laser radiation powers P_c at which collagen denaturation, tissue necrosis, and vasa vasorum destruction occur throughout the entire venous-wall thickness and, at the same time, the surrounding tissues are not subjected to unnecessary heating, are found. The main criterion for determining P_c is the achievement of 100% denaturation of venous-wall proteins, confirmed by morphological and calorimetric analysis. The P_c values for laser wavelengths of 1.47, 1.56, and 1.68 mm are found to be 6.0 ± 0.2, 5.0 ± 0.2, and 6.0 ± 0.2 W, respectively. It is established for all wavelengths in use that the temperature of the external venous-wall surface reaches 91 ± 2 °C at the corresponding power P_c. We relate the dependence of P_c on the radiation wavelength to the formation of a coagulum on the optical fibre tip moving through a blood-filled vessel. The achievement of temperature necessary for coagulum formation is determined by the simultaneously occurring processes of energy absorption and its dissipation in the form of heat. These processes become more intense with an increase in the absorption coefficient of the medium. A mechanism is proposed to explain the relationship between the P_c value and laser wavelength, based on the influence of the absorption coefficient of medium (blood) on the temperature near the fibre tip.