Endovenous laser ablation (EVLA) is successfully used to treat varicose veins. However, the exact working mechanism is still not fully identified and the clinical procedure is not yet standardized. Mathematical modeling of EVLA could strongly improve our understanding of the influence of the various EVLA processes. The aim of this study is to combine Mordon's optical-thermal model with the presence of a strongly absorbing carbonized blood layer on the fiber tip. The model anatomy includes a cylindrically symmetric blood vessel surrounded by an infinite homogenous perivenous tissue. The optical fiber is located in the center of the vessel and is withdrawn with a pullback velocity. The fiber tip includes a small layer of strongly absorbing material, representing the layer of carbonized blood, which absorbs 45% of the emitted laser power. Heat transfer due to boiling bubbles is taken into account by increasing the heat conduction coefficient by a factor of 200 for temperatures above 95 °C. The temperature distribution in the blood, vessel wall, and surrounding medium is calculated from a numerical solution of the bioheat equation. The simulations were performed in MATLAB™ and validated with the aid of an analytical solution. The simulations showed, first, that laser wavelength did virtually not influence the simulated temperature profiles in blood and vessel wall, and, second, that temperatures of the carbonized blood layer varied slightly, from 952 to 1,104 °C. Our improved mathematical optical-thermal EVLA model confirmed previous predictions and experimental outcomes that laser wavelength is not an important EVLA parameter and that the fiber tip reaches exceedingly high temperatures.

Endovenous laser ablation (EVLA) produces boiling bubbles emerging from pores within the hot fiber tip and traveling over a distal length of about 20 mm before condensing. This evaporation-condensation mechanism makes the vein act like a heat pipe, where very efficient heat transport maintains a constant temperature, the saturation temperature of 100°C, over the volume where these non-condensing bubbles exist. During EVLA the above-mentioned observations indicate that a venous cylindrical volume with a length of about 20 mm is kept at 100°C. Pullback velocities of a few mm/s then cause at least the upper part of the treated vein wall to remain close to 100°C for a time sufficient to cause irreversible injury. In conclusion, we propose that the mechanism of action of boiling bubbles during EVLA is an efficient heat-pipe resembling way of heating of the vein wall.

The results of our experiments show that wavelength on its own has not been demonstrated to be an important parameter to influence the temperature profile.

I take issue with a point made by the authors twice within their Discussion. Their assertion that clopidogrel reduces the failure rate of prosthetic grafts to approximate that of venous grafts is unfounded. This comparison simply cannot be made in their study in a statistically valid fashion. Patients in the CASPAR trial were not randomized to conduit; hence, it is highly likely that those patients who received venous vs prosthetic grafts differed significantly in key clinical and anatomic variables. The authors did not share the relevant characteristics of the two conduit subgroups in the report. Multiple high quality randomized trials have demonstrated the clear superiority of venous over prosthetic grafts for LEB, even in the above-knee position. 3 The implication that clopidogrel treatment might alter this established paradigm is not substantiated by their trial, and I think highly speculative.In comparing the data from CASPAR and bypass versus angioplasty in severe ischemia of the leg (BASIL) 4 to that from North American studies such as Project or Ex-Vivo vein graft Engineering via Transfection (PREVENT) III, 5 I am struck by two persistent observations. Despite the fact that two-thirds of the patients in the CASPAR trial presented with symptoms of critical ischemia, only 25% of the LEBs performed in this study were at a tibial or pedal level. In BASIL roughly one-third of the grafts performed were infrapopliteal, whereas in PREVENT III the proportion was reversed (two-thirds were tibial/pedal). Therefore, I question if the results from these trials are relevant to patients requiring infrapopliteal bypass surgery, which appears more common in current surgical practice on this side of the Atlantic, particularly in the endovascular era. Finally, all of these trials continue to demonstrate inadequate medical therapy in LEB patients, specifically the low utilization of statins, which were associated with a beneficial effect in the CASPAR trial, as well as in PREVENT III. 6 Perhaps an intensive-dose statin trial may still be of relevance in the LEB population? Michael S. Conte, MD University of California San Francisco San Francisco, CalifREFERENCES 1. Belch JJF, Dormandy J, the CASPAR Writing Committee, Biasi BM, Cairols M, Diehm C, et al. Results of the randomized, placebocontrolled clopidogrel and acetylsalicylic acid in bypass surgery for peripheral arterial disease (CASPAR) trial. J Vasc Surg 2010;52:825-33. 2. Jackson MR, Johnson WC, Williford WO, Valentine RJ, Clagett GP. The effect of anticoagulation therapy and graft selection on the ischemic consequences of femoropopliteal bypass graft occlusion: results from a multicenter randomized clinical trial. J Vasc Surg 2002;35:292-8. 3. Twine CP, McLain AD. Graft type for femoro-popliteal bypass surgery. Cochrane Database Syst Rev 2010;12:CD001487. 4. Bradbury AW, Adam DJ, Bell J, Forbes JF, Fowkes FG, Gillespie I, et al. Bypass versus Angioplasty in Severe Ischaemia of the Leg (BASIL) trial: analysis of amputation free and overall survival by treatment receiv...