Some controversies in endovenous laser ablation of varicose veins addressed by optical–thermal mathematical modeling

Abstract

Minimally invasive treatment of varicose veins by endovenous laser ablation (EVLA) becomes more and more popular. However, despite significant research efforts performed during the last years, there is still a lack of agreement regarding EVLA mechanisms and therapeutic strategies. The aim of this article is to address some of these controversies by utilizing optical-thermal mathematical modeling. Our model combines Mordon's light absorption-based optical-thermal model with the thermal consequences of the thin carbonized blood layer on the laser fiber tip that is heated up to temperatures of around 1,000 °C due to the absorption of about 45% of the laser light. Computations were made in MATLAB. Laser wavelengths included were 810, 840, 940, 980, 1,064, 1,320, 1,470, and 1,950 nm. We addressed (a) the effect of direct light absorption by the vein wall on temperature behavior, comparing computations by using normal and zero wall absorption; (b) the prediction of the influence of wavelength on the temperature behavior; (c) the effect of the hot carbonized blood layer surrounding the fiber tip on temperature behavior, comparing wall temperatures from using a hot fiber tip and one kept at room temperature; (d) the effect of blood emptying the vein, simulated by reducing the inside vein diameter from 3 down to 0.8 mm; (e) the contribution of absorbed light energy to the increase in total energy at the inner vein wall in the time period where the highest inner wall temperature was reached; (f) the effect of laser power and pullback velocity on wall temperature of a 2-mm inner diameter vein, at a power/velocity ratio of 30 J/cm at 1,470 nm; (g) a comparison of model outcomes and clinical findings of EVLA procedures at 810 nm, 11 W, and 1.25 mm/s, and 1,470 nm, 6 W, and 1 mm/s, respectively. Interestingly, our model predicts that the dominating mechanism for heating up the vein wall is not direct absorption of the laser light by the vein wall but, rather, heat flow to the vein wall and its subsequent temperature increase from two independent heat sources. The first is the exceedingly hot carbonized layer covering the fiber tip; the second is the hot blood surrounding the fiber tip, heated up by direct absorption of the laser light. Both mechanisms are about equally effective for all laser wavelengths. Therefore, our model concurs the finding of Vuylsteke and Mordon (Ann Vasc Surg 26:424-433, 2012) of more circumferential vein wall injury in veins (nearly) devoid of blood, but it does not support their proposed explanation of direct light absorption by the vein wall. Furthermore, EVLA appears to be a more efficient therapy by the combination of higher laser power and faster pullback velocity than by the inverse combination. Our findings suggest that 1,470 nm achieves the highest EVLA efficacy compared to the shorter wavelengths at all vein diameters considered. However, 1,950 nm of EVLA is more efficacious than 1,470 nm albeit only at very small inner vein diameters (smaller than about 1 mm, i.e., veins quite devoid of bloo...

Ins and outs of endovenous laser ablation: afterthoughts

“…This is in the spirit of two other proposed mechanisms that may unlikely dominate the efficacy of EVLA despite suggested importance, i.e., direct laser light absorption by the vein wall [26] and direct contact between fiber tip and vein wall [28]. Concerning the former, comparing zero with normal vein wall absorption in our model simulations gave similar computed wall temperatures, implying a limited importance of direct vein wall light absorption [11]. Concerning the latter, the small vein wall injury line of about 0.6 mm width relative to the vein wall circumference of about 10 to 60 mm (diameters between 3 and 20 mm) was suggested inadequate for vein obliteration [8].…”

Moving heat source in a confined channel: Heat transfer and boiling in endovenous laser ablation of varicose veins

“…Although it would have been preferred to test blood and water at wavelengths where the absorption coefficients of HbO 2 and H 2 O are more alike, the wavelength of 980 nm used in EVLA was applied. We emphasize that the selection of wavelength for EVLA treatment has been subject of much debate; see Poluektova et al [28]. It will be shown that the most striking difference occurring in the experiments with these fluids is related nevertheless to boiling dynamics at the fiber tip as a result of the blood chemical composition.…”

“…The carbonized layer absorbs 55% of the energy provided at the fiber tip ( [15]). The extent of the channel volume directly heated by laser light emanating from the fiber tip is primarily determined by radiation absorption in the fluid; see Poluektova et al [28]. Heat conduction from the fiber tip, if it is carbonized and hot, is a second important mechanism of heat transfer to the fluid.…”

“…Heat conduction from the fiber tip, if it is carbonized and hot, is a second important mechanism of heat transfer to the fluid. Heating of the fluid close to the tip by conduction adds to the heating by radiation and causes the temperature rise to be more localized near the fiber tip (Poluektova et al [28]). As a result, the carbonized layer promotes boiling.…”

“…Consequently, the Grashof numbers are similar (7 and 6). Calculations have already shown [28] that two heat sources contribute about equally to the increase of the temperature on top of the tube, representative of the blood vessel wall in EVLA: the exceedingly hot fiber tip, the hot blood surrounding the fiber tip, heated up by direct absorption of the emitted laser light out of the tip.…”