The use of different laser wavelengths has evolved over time. The progression of technology has moved steadily along the wavelength spectrum. The initial lasers comprised the lower end of the wavelength spectrum (810, 940, 980, and 1064 nm), represented by the hemoglobin-specific laser wavelengths (HSLWs) [15] (Fig. 8.1). The name is derived from the target chromophore, which is hemoglobin. In vitro studies have demonstrated that as the hemoglobin within the red blood cell absorbs the energy from the laser, this results in a combination of heat and steam bubble formation [7]. Endothelial destruction ensues resulting in a thrombotic occlusion.

Of note, the mechanism of action does not require direct contact between laser fiber and the vein wall. With regard to the treatment itself, this has implications. Initially, the thought was that vein wall contact was necessary in order to effect a technically successful ablation. Therefore, direct pressure was applied during the procedure traditionally, and this resulted in increased pain and bruising. In vitro studies identified a direct correlation between fiber/vein wall contact, vein wall perforation, and pain and bruising [7]. As a result compression is no longer a part of the standard procedure. Moreover, pulsed energy lasers have fallen out of favor for similar reasons , given that the bursts of increased thermal injury may have resulted in increased vein wall perforations , manifested by increased pain and bruising. Therefore, continuous energy lasers are now used predominantly [12, 16, 17].

Progressing further with laser technology, longer wavelength lasers were found to have a greater affinity for water as the chromophore (1320, 1470, and 1510 nm) (Fig. 8.1). These are known as the water-specific laser wavelengths (WSLWs) . Water, as compared to hemoglobin, functions more efficiently as a chromophore resulting in a 40-fold improvement in energy absorption when the comparison is made. The mechanism of action is thought to be more selective as compared to the HSLW, because water within the vein wall is targeted directly, resulting in direct damage to the intima [8]. Consequently, lower power settings are required in order to achieve the same effective LEED [6, 18, 19].

The data supporting the varying laser wavelengths have evolved over time, and the general progression has been from the lower wavelengths (HSLWs) to the higher wavelengths (WSLWs). As an example, one of the original randomized studies compared the 810 and 980 nm lasers, both HSLWs, using the same LEED. Fifty-one patients underwent treatment, and technical success for the treatment did not differ between the two groups (one treatment failure per group); however, pain and bruising were less in the 980 nm group [6]. Continuing with the progression, longer wavelength lasers underwent pairwise comparisons. Proebstle et al. compared the 940 nm HSLW laser to the 1320 nm WSLW laser. Care was taken to maintain the same LEED across the comparison groups, approximately 60 J/cm for the 940 nm group (power = 30 W) and 60 J/cm for the 1320 nm group (power 8 W) [20]. Corroborating the increased specificity for the vein wall when using the WSLW laser, the amount of pain and bruising was less in the 1320 nm group while maintaining the same level of treatment efficacy. This relationship held true even when the same patient underwent treatment in one leg with the 810 nm HSLW laser while undergoing treatment in the other leg with the 1320 nm WSLW laser [15]. Use of the 1470 nm WSLW laser has also contributed to the trend of maintaining treatment efficacy while further decreasing post-procedural symptomatology. Shutze et al. evaluated the treatment of 1439 veins, where the 1470 nm laser was used in 295 procedures and the 810 nm laser was used in 1144 procedures [21]. Pain and bruising as well as quality of life scores were improved in the 1470 nm cohort. Moreover, the incidence of endothermal heat-induced thrombosis (EHIT) was diminished in the 1470 nm group as compared to the 810 nm group (2.4 vs 6.0%, P = 0.0122). In a three-way comparison of the 810, 980, and 1470 nm fibers, the pain and bruising scores improved progressively with each successive increase in wavelength while maintaining equal efficacy [22]. An in vitro adjunct was performed as part of the same study, and thermal injury depths were found to be less in the 1470 nm laser as compared to the 810 nm laser, which is consistent to the results that are identified clinically.

In addition to research being applied to laser wavelength in order to improve the specificity of treatment to the vein wall, mechanical factors have also been manipulated in order to enhance the specificity of treatment to the vein wall. This has been best exemplified by the modifications to the fiber tip. The original laser fibers were bare-tip fibers. Along with the original procedural methodology to apply manual compression to the site of treatment, the factors worked in concert to promote contact between the fiber tip and the vein wall. This resulted in an increased incidence of vein wall perforation and increased post-procedural pain and bruising [7]. Consequently, jacket-tip fibers were developed, and they come in various iterations, including ceramic and metallic types. The jacket-tip functions to act as a mechanical barrier preventing direct contact between the laser fiber and the vein wall, and some jacket-tip fibers are configured to reduce the energy density supplied by diverting and dispersing the laser energy. As a physical barrier, the metallic jacket tip results in a 0.010 inch physical barrier between the laser fiber and the vein wall. Moreover, the configuration of the jacket tip results in a 15° divergence of the emitted laser light, increasing the effective diameter of emitted energy from 600 to 905 μm [23] (Fig. 8.2). With regard to dispersing the laser-emitted energy and taking this concept to its fullest extent, a radial-emitting fiber was created that results in 360° dispersion by deflecting the laser energy orthogonally out of the side of the catheter [24].

The theoretical advantages behind the evolution in fiber-tip design have translated to improvements in clinical outcomes. One of the initial studies to evaluate fiber-tip design in a randomized fashion compared patients treated using a 980 nm laser using either a bare-tip or a jacket-tip fiber [23]. The LEED was normalized to 100 J/cm in both groups, and technical success was achieved in all treated patients. The pain scores improved significantly in the jacket-tip group as compared to the bare-tip group. The relationship was further evaluated using the 1470 nm WSLW laser. Three different kinds of fiber tips were compared, including a bare tip, jacket tip, and radial-emitting tip [24]. Once again, treatment efficacy was found to be equivalent between all groups. Pain scores improved successively when transitioning from the bare tip to the jacket tip and then to the radial-emitting tip.

More recently, investigators have attempted to tease out the variety of wavelength/fiber-tip combinations and their relative importance in affecting procedural outcomes [22]. The study was a combination of an in vitro as well as a clinical analysis comparing the 810, 980, and 1470 nm fibers using bare-tip or jacket-tip fibers . As alluded to previously, the pain and bruising scores improved in direct correlation with increasing wavelength, and this was corroborated by the in vitro data. When evaluating bare-tip versus jacket-tip fibers, the depth of thermal injury was worse in the bare-tip group as compared to the jacket-tip group in the 810 nm group (1.05 mm ± 0.34 mm vs 0.36 mm ± 0.26 mm; P < 0.0005) and in the 1470 nm group (0.71 mm ± 0.31 mm vs 0.20 mm ± 0.16 mm; P < 0.0005). Moreover, a multivariate analysis was performed to compare all variables. Both the in vitro data and the clinical data demonstrated that the fiber type was a more dominant variable, as compared to the laser wavelength, in improving outcomes. The results were additive, with the best outcomes obtained in the 1470 nm laser with the use of a jacket-tip fiber [22].

Once the procedural equipment is in place, there are myriad procedural variables that can be manipulated to alter safety and efficacy outcomes. Moreover, the variables are often dependent on the type of laser being used, and this will be delineated further below. Briefly, the procedural variations are related to the pullback time and to the power settings, both of which interact to produce the linear endovenous energy density (LEED) .

Initial modes of treatment were performed using a pulse mode, and over time this has evolved to a continuous mode due to decreased pain and bruising [12]. With further collective experience, the pullback rate increased from 3 mm/s to 1 cm every 3–5 s. The pullback rates in general would equate to a range of 12–20 cm/min. Furthermore, the accuracy and reproducibility of the pullback rate have improved over time; the original sheaths did not exhibit markers as compared to the newer sheaths that are available today, which exhibit distance markers. With regard to the technique , this has implications because the user performs a continuous pullback gauging the 1 cm markers on the catheter relative to the amount of energy used, and this has been more accurate than gauging the timing of the pullback. This technique is also less dependent on the power setting, which can vary significantly based on the laser wavelength being used.