Annals of Vascular Diseases Advance Published Date: December 1, 2016
©2016 Annals of Vascular Diseases
doi:10.3400/avd.oa.16-00120
Original Article
Morphological Comparison of Blood Vessels that were Heated with a Radiofrequency Device or a 1470-nm Laser and a Radial 2Ring Fiber Takashi Yamamoto1 and Masahiro Sakata2
Introduction: Radio waves and lasers can be used as heat sources during endovenous thermal ablation (EVTA) for saphenous vein insufficiency. A morphological comparison of veins that had been treated with EVTA was performed between those treated with an endovenous closure system (a radiofrequency [RF] system) and those treated with a Radial 2Ring fiber connected to a 1470-nm laser generator (2R). Methods: The experiment was conducted in a system that reproduces the physiological conditions found in the saphenous veins during EVTA. The 2R experiment was performed at two different power levels, 60 J/cm (2R-60) and 90 J/cm (2R-90). The heated vessels were morphologically examined in detail, and the detected morphological changes were classified into three groups: low-temperature changes (LTC), mid-temperature changes (MTC), and high-temperature changes (HTC). The thickness of the layers exhibiting each type of change was measured. Results: In the 2R groups, HTC, MTC, and LTC were observed from the superficial to deep layers. In the 2R-60 group, the layers exhibiting LTC, MTC, and HTC were 17 ± 3.2, 42 ± 10.5, and 190 ± 14.6 µm thick, respectively. In the 2R-90 group, these layers were 14 ± 4.0, 105 ± 64.2, and 363 ± 71.3 µm thick, respectively. In the RF group, only LTC were observed (thickness: 251 ± 72.6 µm). Conclusions: The RF device was able to heat the target vessels more efficiently than the laser device. (This article is a translation of Jpn J Phlebol 2015; 26: 23–8.)
Keywords: varicose veins, laser, radiofrequency, endovenous thermal ablation
1 2
Ochanomizu Vascular & Vein Clinic, Tokyo, Japan Sakata Vein Clinic, Osaka, Osaka, Japan
Received: November 5, 2016; Accepted: November 5, 2016 Corresponding author: Takashi Yamamoto. Ochanomizu Vascular & Vein Clinic, Fujimi 2-15-8-401, Chiyoda, Tokyo 102-0071, Japan Tel: +81-90-8522-7947 E-mail:
[email protected] This article is a translation of Jpn J Phlebol 2015; 26: 23–8.
Introduction Endovenous laser ablation (EVLA) and radiofrequency ablation (RFA) are the most commonly employed techniques for ablating incompetent saphenous veins.1,2) In Japan, conducting EVLA with a 980-nm laser generator and a bare fiber (ELVeS, biolitec, Bonn, Germany) has been covered by the governmental social insurance system since January 2011, whereas performing EVLA with a 1470-nm laser generator and a Radial 2Ring fiber (ELVeS, biolitec) (2R) has been covered since May 2014, and RFA (Venefit, Medtronic, Minneapolis, MN, USA) has been covered since June 2014. All of these devices utilize heat to denature vessel walls, but the heating systems used differ between EVLA and RFA. The light produced by the laser generator passes through the fiber, is emitted from the tip of the fiber, and heats the vessel wall, whereas the radio waves produced by the RFA device are transformed into heat at the heating coil mounted at the far end of the catheter. EVTA therapy is considered to be a less invasive alternative to saphenous stripping, but postoperative pain was reported to be a significant side effect after EVLA performed with a 980-nm laser generator and a bare fiber.3,4) Several improvements have since been made in the heating devices used in such procedures, resulting in some reports indicating that performing EVLA with a 1470-nm laser and a radial-emitting fiber, or RFA causes less pain.5–8) In order to investigate the reason why performing EVLA at a higher wavelength with a radialemitting fiber causes less pain, the influence of the laser wavelength and laser fiber on the mechanism of action used to heat the vessel wall was researched by investigating heated vessels morphologically.9) In the latter study, the morphological changes were classified into four types (i.e., swelling, vacuolization, carbonization, and tissue loss), and it was shown that each type arises at a specific temperature (i.e., 70°C, 120°C, 300°C, and higher, respectively). In addition, the 2R was found to heat the target vessel homogenously at a lower maximum temperature, which is considered to be the reason it causes less pain.
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Annals of Vascular Diseases Advance Published Date: December 1, 2016 Yamamoto T and Sakata M
On the other hand, RFA catheters are equipped with thermal sensors on their tips, which make it possible for them to precisely control the temperature in an automatic manner. Therefore, it is presumed that the morphological changes induced in heated blood vessels after RFA would be limited to swelling and partial vacuolization. In this study, the morphological changes induced in heated vein walls by the 2R and RFA were investigated using an apparatus that can replicate the circumstances found in the great saphenous vein (GSV) during EVTA procedures.
Materials and Methods Incompetent trunks of the GSV that were removed during stripping operations performed with the invaginated stripping method were utilized in this study. Before the stripping operations, ultrasonography was conducted to measure the diameter of the GSV at 7 cm distal to the saphenofemoral junction with the patient in the sitting position, in order to exclude GSV with diameters smaller than 4 mm or larger than 10 mm. The removed veins were subsequently turned inside out, that is, they were returned to their physiological state; preserved in a refrigerator at 5°C; and used within 3 days. The veins were cut into sections of 15 cm in length, after carefully eliminating those with large branches or that displayed significant diameter changes or varicosity. Venous blood was collected from healthy volunteers and mixed with ethylenediaminetetraacetic acid in order to prevent coagulation. The patients provided informed consent for the use of their GSV in this study. Volunteers provided informed consent for the use of their blood. The experimental apparatus consisted of acrylic pipes, rubber plugs, needles, and 7F sheaths. The GSVs were passed through the pipes, stretched tight, and fixed to the sheaths, which were fitted through rubber plugs on both ends of the pipes. Both ends of the GSV, which were covered by the sheaths, were ligated so as to prevent leakage. The main advantage of this apparatus was that it has separate channels for the inside and outside of the GSV; a sheath and a needle were inserted through a rubber plug, the sheath was connected to the GSV while the lumen of the needle opened into the space inside the pipe but outside the GSV. After fixing the GSV to the sheath, physiological saline was poured into the outer GSV space through the needle, with the lock of the sheath opened to a sufficient extent to compress the GSV from outside. Then, the needle lumen was locked, and the blood was poured into the GSV through the sheath while the lock of the sheath at the other end of the GSV was open. So, the blood did not expand the GSV but passed through it and filled the remaining space. Subsequently, a laser fiber or 2
Fig. 1 T he experimental system consisted of acrylic pipes, rubber plugs, needles, and 7F sheaths.
RFA catheter was inserted into the GSV through the sheath, and then the heating procedure was carried out (Fig. 1). A 1470-nm diode laser generator (Leonardo, biolitec), radial-emitting fibers (ELVeS Radial 2Ring fiber, biolitec), a radiofrequency generator (ClosureRFG generator, Medtronic), and catheters (ClosureFast catheter, Medtronic) were utilized in this experiment. The laser power was set at 6 or 9W, and the fibers were moved at a rate of 1 mm/s. This means that the linear endovenous energy density was set at 60 J/cm or 90 J/cm. The RFA was performed at 120°C for 20 seconds, that is, the temperature of the heating unit of the catheter reached 120°C within 4–6 seconds, and this temperature was maintained for 20 seconds. Three settings were employed in this experiment; laser power: 60 J/sec (2R-60), laser power: 90 J/sec (2R-90), and RFA (RF). In all, 10 GSVs were ablated under each setting and compared histologically. The ablated GSV segments were fixed in 10% formalin and cross sectioned at the center of the ablated segments. The sections were stained with azan trichrome stain prior to being submitted for histological evaluation. The resultant histological changes were classified into four groups: low-temperature changes (LTC), such as swelling of the smooth muscle or elastic fibers; midtemperature changes (MTC), such as the fusion or vacuolization of elastic fibers; and high-temperature changes (HTC), such as tissue carbonization (Fig. 2).9) Three measurement points were chosen on each cross-sectioned vessel, as follows: The first point was located in the middle of the section, and the second and the third points were located equidistant from the first point and the edge of the section on either side. At each point, the thickness of the layers exhibiting each type of thermal change was measured. Then, mean thickness values were calculated for each layer and compared. The observations and measurements were conducted by the corresponding author, whereas the experiment was carried out by all authors.
Results The laser-heated (2R-60 or 2R-90) veins tended to look slightly blackish, whereas the veins subjected to RFA
Annals of Vascular Diseases Advance Published Date: December 1, 2016 Morphological Comparison between the Effects of RFA and 1470-nm Lasers
(a)
(a)
(b)
(c) (b) Fig. 3 M acroscopically, the surfaces of the vessels heated with the 2R device looked slightly blackish (a) and (b). The vessels heated with the RF device became flesh-colored (c). RF: radiofrequency
(a) (c)
Fig. 2 (a) The low-temperature changes included the swelling of smooth muscle and/or elastic fibers (× 200, azan trichrome stain). (b) The mid-temperature changes included the fusion or vacuolization of elastic fibers (× 200, azan trichrome stain). (c) The high-temperature changes included tissue carbonization (× 200, azan trichrome stain).
turned cream-colored (Fig. 3). During the morphological examinations, HTC, MTC, and LTC (in this order) were observed from the intima to media in the 2R-60 and 2R-90 groups (Fig. 4), and the thicknesses of the layers exhibiting each type of change were as follows: HTC: 17 ± 3.2 µm (mean ± standard deviation), MTC: 42 ± 10.5 µm and LTC: 190 ± 14.6 µm in the 2R-60 group; HTC: 14 ± 4.0 µm, MTC: 105 ± 64.2 µm, and LTC: 363 ± 71.3 µm in the 2R-90 group (Table 1). The pattern of the changes was essentially the same in the 2R-60 and 2R-90 groups, but the MTC and LTC layers were thicker in the 2R-90 group. Only LTC were observed (thickness: 251 ± 72.6 µm) in the RF group (Fig. 5) (Table 1).
Discussion It was demonstrated that human cells are irreversibly degenerated if they are heated to about ≥43°C, and it was
(b)
Fig. 4 In the 2R-60 (a) and 2R-90 (b) groups, a very thin layer of high-temperature changes was seen in the intima. In the 2R-90 group, a thick layer of mid-temperature changes was seen from the intima to the media, and a thick layer of low-temperature changes was present above the media (× 100, azan trichrome stain).
also shown that the heating time needed to degenerate cells decreases at an exponential rate as the temperature rises.10) Moritz et al. reported that the required time to irreversibly degenerate the epidermal cells of porcine skin in vivo was 6 hours at 44°C, 4 minutes at 50°C, 5 seconds at 60°C, and a really short period of time at 70°C.
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Annals of Vascular Diseases Advance Published Date: December 1, 2016 Yamamoto T and Sakata M
Fig. 5 In the RF group, neither high-temperature changes nor mid-temperature changes was observed. The only thermal changes noted in this group were low-temperature changes, which were seen from the intima to the media (× 100, azan trichrome stain). RF: radiofrequency Table 1 T he thickness of the layers exhibiting each type of thermal change
2R-60 2R-90 RF
HTC, µm
MTC, µm
LTC, µm
17 ± 3.2 14 ± 4.0 0
42 ± 10.5 105 ± 64.2 0
190 ± 14.6 363 ± 71.3 251 ± 72.6
All values indicate the mean ± standard deviation. HTC: high-temperature changes; LTC: low-temperature changes; MTC: mid-temperature changes; RF: radiofrequency
Therefore, to burn blood vessel walls properly, it is reasonable to suppose that a temperature of 70°C is required during EVLA, in which the heating device is moved continuously, and a temperature of around 58°C is sufficient during RFA, in which the catheter stays in the same place for >15 seconds. Theoretically, it can be assumed that temperatures higher than the abovementioned values cause undesirable damage to the surrounding tissue, and temperatures below those values will lead to the persistence of vital endothelium cells, resulting in recanalization of the target vessel. However, practically only some of the heat is conducted to the vessel walls because of the cooling effect of the blood, which lies between the heating device and the vessel wall; therefore, in clinical procedures, excessive heat is employed to ensure that sufficient heat is transmitted to the vessel wall, and an abundant amount of TLA fluid is injected around the vessel to avoid thermal damage to the surrounding tissue. We previously investigated the influence of different laser fibers and wavelengths on the mechanism of action of EVLA. In the latter study, radial-emitting fibers, such as the Radial 2Ring, were shown to disperse light around the fiber and to help prevent unwanted heat dissipation as a result. Furthermore, such fibers enable the uniform heating of the vessel wall, especially if they are used with 4
a laser of a water-specific wavelength, such as 1470 nm.9) However, it was also mentioned that vessel vacuolization was associated with a temperature of >120°C, and carbonized tissue although it was only found in the intimal layer and was associated with temperatures up to 200–300°C. Such temperatures are much higher than the required temperature. The meaning of the water-specific wavelength is frequently misunderstood, that is, some people tend to think that it does not apply to blood, but water is the main constituent of blood. Consequently, energy from a laser with a water-specific wavelength is inevitably absorbed by blood, turning it into carbon. Moreover, as carbon absorbs laser light intensely, resulting in a temperature increase,11) the precise control of temperature during EVLA procedures is difficult. One possible solution is to reduce the power of the laser generator in order to decrease the temperature of the vessel wall. It is also possible to reduce the power of the laser without impairing its therapeutic effect when a radialemitting fiber and a 1470-nm laser are employed because this combination is considered to be capable of heating the vessel wall effectively while causing less disturbance in the blood.1,6) In this experiment, two different laser power levels (i.e., 60 J/cm and 90 J/cm) were employed; however, as mentioned above the results were essentially the same from the perspective of the type of morphological changes induced. The thicknesses of the layers exhibiting each type of change differed between the two groups, but even in the 2R-60 group the temperature of the inner surface of the vessel wall rose up to 300°C. On the other hand, the ClosureFast catheter is equipped with a thermal sensor, which enables it to control the temperature of the target region precisely at 120°C. It is predicted that the heated temperature of the vessel wall will be lower than that of the catheter due to the cooling effect of blood. Regarding the morphological changes observed in this study, only swelling was seen in the RF group, but no regions of vacuolization were identified. This indicated that the temperature of the heated section of the vessel wall was lower than 120°C. Consequently, taking these results together, the RFA device seemed to produce better heating outcomes. However, it is conceivable that dead cells do not necessarily change their morphological appearance; in other words, a lack of morphological changes does not denote cell survival. A reliable method for confirming the area affected by cell death is to perform an experiment in vivo and wait for a certain period of time before conducting observations. In this study, GSVs that were collected during stripping operations were used, and this fact hinders accurate evaluations of the viability of the heated cells. Consequently, this study demonstrated the relationships between the heating devices used and the types
Annals of Vascular Diseases Advance Published Date: December 1, 2016 Morphological Comparison between the Effects of RFA and 1470-nm Lasers
of morphological changes they induced, but it is inadequate for discussing the clinical merits and demerits of these approaches, such as the extent of thermal damage caused to the surrounding tissue.
Conclusion The morphological changes induced in the vessel wall were compared between the 2R and RFA groups. It was morphologically demonstrated that the heating temperature produced by the RFA device was lower than that induced by the 2R device thanks to the latter device’s temperature control system, which maintains the temperature of the catheter at exactly 120°C.
Disclosure Statement The first author and co-authors have no conflicts of interest.
Author Contributions Study conception: all authors Data collection: all authors Analysis: TY Investigation: TY Writing: TY Funding acquisition: all authors Critical review and revision: all authors Final approval of the article: all authors Accountability for all aspects of the work: all authors
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