ENDOVENOUS 1064-NM AND 1320-NM ND:YAG LASER TREATMENT OF THE PORCINE GREATER SAPHENOUS VEIN

In multiple studies, closure of the greater saphe­nous vein (GSV) through an endoluminal approach with radiofrequency (RF) or laser energy has proved to be safe and effective. These endovenous occlusion techniques are less invasive alternatives to saphenofemoral ligation and/or stripping and are typically performed under local anesthesia with patients returning to normal activities within 1 to 2 days. RF energy can be delivered through a specially-designed endovenous electrode with microprocessor control to perform controlled heating of the vessel wall, leading to vein shrinkage or occlusion by contraction of vein-wall collagen. Limiting heating to 85°C avoids boil­ing, vaporization, and carbonization of tissues. In addi­tion, heating the endothelial wall to 85°C also heats the vein media to approximately 65"C, which contracts colla­gen. Electrode-mediated RF-energy ablation of vessel walls is a self-limiting process. As tissue coagulates, im­pedance decreases markedly, limiting heat generation.

Lasers that can be used for endoluminal treatment of varicose veins target blood hemoglobin with heat, which is then transferred to vessel walls. Lasers emitting wave­lengths of 500 nm to 1064 nm have been used in this way from both inside the vessels and through the skin. Attempts have been made to optimize the absorption of laser energy by using local hemoglobin absorption peaks at 810, 940, 980, and 1064 nm. Endovenous laser treatment (EVLTT, Diomed Inc., Andover, Mass) allows delivery of laser energy directly into the blood vessel lu­men to produce endothelial and vein-wall damage with subsequent fibrosis. Lasers destroy the GSV thermally. The presumed target for absorption of laser energy is in­travascular red blood cells. However, thermal damage with resorption of the GSV has also occurred in veins emptied of blood. Therefore, direct thermal effects on the vein wall probably also occur. The extent of thermal injury to tissue depends strongly on amount of heat and length of exposure. When veins are devoid of blood, vessel walls rupture.

Using an in vitro study model, Proebstle et al pre­dicted that production of thermal gas by laser heating of blood in a 6-mm tube would result in 6 mm of thermal damage. They used a 940-nm diode laser to treat a GSV with multiple 1 -second pulses at 15 J/cm2. Results of histologic examination of the excised GSV showed thermal damage along the entire treated vein with evi­dence of perforations at the point of laser application (this damage was described as "explosive-like" photodisruption of the vein wall). As 940-nm laser energy can penetrate only 0.3 mm in blood formation of steam bubbles is the probable mechanism of action.

Initial reports have shown that use of endovenous RF energy is effective (96% occlusion at 1-3 years, <1 % in­cidence of transient paresthesia) in short-term treatment of the incompetent GSVK;" Although most patients ex­perience some degree of postoperative ecchymosis and discomfort, no other major or minor complications have been reported.

Our patients treated with EVLT show an increase in posttreatment purpura and tenderness. Most of our patients do not return to complete functional normality for 2 to 3 days-versus the 1 day of "downtime" needed after RF ClosureT (Vnus Medical, Sunnyvale, Calif.) of the GSV. As the anesthetic and access techniques for the two procedures are identical, we believe that non­specific perivascular thermal damage is the probable cause for this increased tenderness. In addition, recent studies have suggested that pulsed laser treatment, with its increased risk for vein perforation, may be re­sponsible for the increase in symptoms occurring with EVLT versus RF-energy treatment.13,16 Slow, uncon­trolled pullback of the catheter is a likely cause of ves­sel-wall overheating and perforation, as even the best surgeon may have some difficulty retracting the fiber at the exact speed needed to maintain vessel-wall heating at 85°C.

In this article, we describe a technique for treating varicose veins-using laser energy at a wavelength ap­propriate for targeting vessel walls directly and using a motorized pullback device and a diffuse-fiber delivery system to control that energy precisely. This technique should prevent damage to surrounding tissue and perforation of vessels.

Materials and Methods
Porcine GSV is remarkably similar to human GSV. In this study, fresh porcine GSVs (8-10 mm in diameter) were placed in normal saline after blood was removed with normal saline flushing. The veins were suspended in a graduated cylinder filled with normal saline. The distal end of the veins was closed with running 5-0 nylon suture, and the veins were filled with normal saline. A 550-um quartz fiber was inserted into each vein and threaded through its entire length. Position of the fiber inside the vein was determined by noting the laser's red aiming beam being emitted from the tip of the catheter. The catheter was connected to a motorized pullback device. The procedure began with starting the pullback for approximately 2 or 3 mm and then turning the laser on at various fluences. A 1320-nm, 33-Hz, 1.2-ms pulsed laser was used in a near-continuous mode at 1W, 2W, 3W, 5W, and 5.5W, and a 1064-nm, 40-Hz, 350-ns pulsed laser was used in a near-continuous mode at 5W, 15W, and 20W. The motorized pullback device was used to withdraw all laser fibers at a rate of 1 mm/s. Immediately after the veins were lasered, they were sectioned and placed in formaldehyde for histopathologic processing and evaluation. All veins were evaluated by a dermatopathologist, who was blinded to the purpose and parameters of the experiment.

Results
The extent of thermal damage to vein walls (millimeters of amorphous amphophilic material) was determined, and the vein-wall layers exhibiting thermal damage were identified. Full-thickness vein-wall damage oc­curred at 5W with the 1320-nm laser and at 20W with the 1064-nm laser.

Discussion
Optical absorption curves show that, at 810, 940, and 1064 nm, the primary absorbing chromophore in a vein is hemoglobin. When a vein is drained of blood and a laser is used at one of these wavelengths, a majority of the laser energy is transmitted through the vessel wall to heat surrounding tissue. The 1320-nm laser is ideally suited for penetrating the small amount of blood re­maining in the vessel, and its energy is much more strongly absorbed in the vessel wall by collagen. Most of the energy is concentrated in the wall for heating and shrinkage. The results of this in vitro study show that the 1320-nm Nd:YAG laser may be ideally suited for en-dovascular laser destruction of the GSV.

References

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