Radiofrequency ablation using injectable cooled electrode: the effects of lidocaine injection in ex vivo study

Abstract

Background

Pain control is needed during radiofrequency ablation in musculoskeletal tumor. The effect of radiofrequency ablation can be modulated by lidocaine injection.

Purpose

To evaluate the effects of injectable electrodes with intralesional lidocaine injection and compare its ablation performance with that of non-injectable electrodes in ex vivo pork sirloin.

Material and Methods

Five specimen groups were used to investigate the effects of fluid injection before radiofrequency ablation using injectable and non-injectable electrodes: three injectable electrode comparison groups with 2% lidocaine (group A); 1% lidocaine (group B); 0.9% sodium chloride (group C); injectable electrode reference group without fluid (group D); and non-injectable electrode control group (group E). The injectable and non-injectable electrodes were 17-gauge electrodes each having a 1-cm active tip. Technical parameters, ablation size, and volumes were compared between the five groups.

Results

Mean energies and currents during radiofrequency ablation were significantly lower for the four injectable electrode groups compared to group E (all P < 0.005). Two transverse diameters, vertical diameter, and volumes in the four injectable electrode groups were significantly smaller than those in group E (all P < 0.05). Among the injectable electrode groups, volumes and two transverse diameters were significantly smaller in group A than in group D (all P < 0.05).

Conclusions

A slightly smaller ablation zone is obtained when lidocaine injection is performed before radiofrequency ablation using an injectable electrode compared to a non-injectable electrode.

Keywords

Radiofrequency ablation ex vivo injectable internally cooled electrode lidocaine injections

Introduction

Radiofrequency ablation (RFA) is regarded as an effective palliative treatment for unresectable or painful bone and soft-tissue neoplasms such as those due to metastasis, as it can be used to provide local pain control (1 –6). During the RFA procedure, an anesthetic technique is required to control the pain (2,5,6). Conscious sedation is routinely induced using tramadol hydrochloride or fentanyl citrate. When performing conscious sedation, local anesthesia, e.g. a 1% or 2% lidocaine hydrochloride solution, can be additionally injected, not only in the dermis or deep subcutaneous tissues of the probe insertion site, but also in the soft tissues of the actual thermal target area.

In our clinical practice, the authors found that intralesional injection of local anesthesia into the soft-tissue mass during RFA often relieved patients’ pain during that procedure. Nevertheless, repeated injection of local anesthesia during RFA can be a cumbersome process and painful stimuli for patients. For these reasons, we try to experiment with the newly developed injectable RF electrodes (IEs; STAR injectable RF electrodes; STARmed, Goyang, Republic of Korea) that can infuse saline into the tumor constantly during ablation; this is achieved via side holes at the outer surface of the IE needle. Thus, the author and colleagues postulated that lidocaine could be injected via the IE side holes, thereby eliminating the need for an additional needle for local anesthesia and allowing pain control during RFA.

Before a clinical trial, it was necessary to assess the characteristics of IEs with intralesional lidocaine injection. Furthermore, it is important to predict its ablation performance by comparing with the existing non-injectable electrodes (NEs). Therefore, the aim of this study was to evaluate the effects of IEs with intralesional lidocaine injection and compared its ablation performance with that of NEs in ex vivo pork sirloin.

Material and Methods

This ex vivo study was approved by the Institutional Review Board of our institution. In addition, the study was supported by STARmed Co. (Goyang, Republic of Korea), who provided the required equipment.

RF system and electrodes

In this study, 17-gauge, monopolar, internally cooled IEs (STAR injectable RF electrodes; STARmed, Goyang, Republic of Korea) with a 1-cm active tip and 15-cm shaft length (Fig. 1a) were used. An IE is a perfusion electrode with two separate lumens, inner pipe for internal cooling agent, and outer pipe for infusion. The outer pipe directly connects with 12 micro holes with 0.6 mm in diameter at the distal tip at an angle of 90° to the surface in all directions; this allows fluid from the injection port to be pumped or injected into the tissue (Fig. 1b). Hand injection was performed using an injection port because an intermittent injection of lidocaine was carried out rather than continuous flow as it better represents the clinical situation in which injection is only given when the patient feels pain.

Fig. 1.

Injectable internally cooled RF electrode system. (a, b) The 12 micro holes in the distal tip. Fluid from the injection port can be pumped or injected via these side holes.

In addition, 17-gauge, monopolar, modified, internally cooled NEs (VIVA RF electrode, STARmed, Goyang, Republic of Korea), each having a 1-cm active tip, were used for comparison with the effects of the IEs under the RFA. A peristaltic pump (VIVA pump, STARmed, Goyang, Republic of Korea) was used to continuously infuse a cold saline solution (0°C) into each RF electrode lumen for cooling, so as to maintain the electrode temperature at 20–25°C. The temperature inside a cooled electrode is typically lower (5–10°). Further, an RF generator (VIVA RF generator, STARmed, Goyang, Republic of Korea) was employed using 50 W of RF power with an auto mode. The applied current, power output, and impedance were continuously monitored by the generator during the RFA and were recorded automatically using a computer program.

Ablation protocol

All studies were performed using four excised fatless pork sirloins. Before RFA, the pork tissue was kept at room temperature for >2 h to equilibrate. For each specimen, the electrode tip was inserted into the sirloin to a depth of 3 cm; the electrode was then fixed using a long flexible clamp to prevent movement during the experiment.

To check the accuracy of the tissue temperature measurement, an additional thermocouple was inserted into the sirloin at a fixed distance of 5 mm from the RF electrode. The temperature sensing point was located at the center of the 1-cm active tip and the sensor was fixed to the electrode using a plastic jig to allow correct positioning in each case. Ablation was performed using IEs and NEs with a RF power of 50 W for 5 min.

Five specimen groups (Table 1) were used to investigate the effect of fluid injection before RFA, having different fluid compositions and using IEs or NE: three IE comparison groups with 2% lidocaine (Huons, Republic of Korea) (group A) and 1% lidocaine (Huons, Republic of Korea) (group B) in a hydrochloride solution, along with 0.9% sodium chloride (normal saline [N/S]; Choongwae, Republic of Korea) (group C); an IE reference group without fluid (group D); and an NE control group (group E).

Table 1.

Five specimen groups.

	Modality	Fluid injected before RFA
Group A (n = 20)	IE	2% lidocaine with hydrochloride solution
Group B (n = 20)	IE	1% lidocaine with hydrochloride solution
Group C (n = 20)	IE	Normal saline
Group D (n = 20) (IE reference)	IE	None
Group E (n = 20) (NE control)	NE	None

IE: injectable internally cooled electrode; NE: non-injectable internally cooled electrode; RFA: radiofrequency ablation.

A 30-mL syringe was connected to the RF electrode through the injection port so that the fluid could be into the 12 side holes of the distal tip. Then, 1 mL of fluid was forcefully injected into the center of the tissue through the IEs after filling the dead space (0.36 mL) of electrode via a manual method. We performed RFA immediate after lidocaine or normal saline injection; then, we sliced the pork sirloin and cut it into 100 blocks (20 blocks for each specimen group).

Ablation zone size measurement

After making the sirloin blocks after each RFA, we measured the ablation diameter and fluid distribution area within 5 min. Sirloin blocks containing each RFA zone were dissected along the longitudinal plane passing through the probe axes; they were then cut transversely and perpendicular to the longitudinal plane at the center of the ablation zone (transverse plane). As the white central area of the RF-induced ablation zone has previously been shown to correspond to the zone of coagulation necrosis, two observers measured the maximum vertical diameter (DV) along the electrode and the transverse diameter (DT1) perpendicular to the DV using Image J software. The second transverse diameter of the ablation zone (DT2) was measured in the plane perpendicular to that passing through the probe axes (Fig. 2). The volumes of the ablation zones were evaluated by approximating the lesion to a sphere using the formula: ablation-zone volume = π (DV × DT1 × DT2)/6 (7).

Fig. 2.

Measurement of the ablation zone. (a) On longitudinal plane, we measured vertical diameter (DV) along the electrode. (b) On transverse plane, we measured transverse diameter (DT1) perpendicular to DV and another transverse diameter (DT2) in the plane perpendicular to that passing through probe axes.

Statistical analysis

The normal distributions of the three diameters (DT1, DT2, and DV) and the volumes of the ablation zones, along with the technical parameters, were tested for the five specimen groups using the Kolmogorov–Smirnov test. The mean initial tissue temperature and mean initial tissue impedance before fluid injection, along with the mean tissue temperature, mean current flow, and mean tissue impedance during RFA, were analyzed using a non-parametric Kruskal–Wallis test with post-hoc testing. The energy, three diameters (DT1, DT2, and DV), maximum temperature, and ablation-zone volume were assessed using one-way analysis of variance (ANOVA) and post-hoc tests with Bonferroni correction. The statistical significance was set to P < 0.05. All statistical analyses were performed using SPSS software (version 21.0; SPSS Inc., Chicago, IL, USA) and MedCalc 17.0 (MedCalc Software, Mariakerke, Belgium).

Results

Technical parameters

The mean initial tissue temperatures measured 5 mm from the electrode tip area were 19.7°C ± 1.1 (range = 18–22°C). During the RFA, the mean tissue impedance was maintained without sudden or rapid increase or decrease during the ablation period. Further, as regards the mean energy and mean current flow during RFA, these values were significantly lower in the four IE groups compared to those for the NE control group (group E) (all P < 0.005). Among the IE groups, the mean energy was higher in group D compared with group A (P = 0.003); group A exhibited the lowest applied RF energy. No significant differences in the mean impedance values during RFA were noted for the IE and NE groups (all P > 0.005). During RFA, the mean tissue temperatures measured 5 mm from the electrode tip area were lower in the three IE groups of A, B, and C compared to that of group E (all P < 0.001). The average mean tissue temperature was 62.5 ± 4.9°C in the IE groups and 68.8 ± 3.7°C in the NE group. The maximum tissue temperature measured 5 mm from the electrode tip was lower in group A than in group E (P < 0.001) (Table 2, Fig. 3).

Fig. 3.

Comparison graphs of technical parameters and ablation zones during RFA for five specimen groups.Group A: 2% lidocaine injection using IE; Group B: 1% lidocaine injection using IE; Group C: N/S injection using IE; Group D: IE reference without fluid injection; Group E: NE control without fluid injection.

Table 2.

Comparison of technical parameters among five ablation groups using post-hoc test.

Variables	Group A	Group B	Group C	Group D	Group E	Comparison group	Post-hocP value
Energy (Kcal)	0.45 ± 0.09	0.50 ± 0.09	0.53 ± 0.07	0.54 ± 0.05	0.72 ± 0.07	A vs. E, B vs. E, C vs. E, D vs. EA vs. D	<0.001^= 0.03^
Mean current (mA)	0.60 ± 0.04	0.61 ± 0.04	0.62 ± 0.03	0.61 ± 0.02	0.66 ± 0.03	A vs. E, B vs. E, C vs. E, D vs. E	<0.005^†
Mean impedance (Ω)	52.50 ± 4.16	53.25 ± 4.39	52.55 ± 4.94	56.15 ± 3.87	55.4 ± 2.96	All	>0.005^†
Mean temperature (°C)	59 ± 4.27	63 ± 4.93	63 ± 4.09	65 ± 4.67	69 ± 3.73	A vs. E, B vs. E, C vs. E	<0.001^†
Maximum temperature (°C)	73.15 ± 5.50	78.0 ± 8.10	78.65 ± 7.44	78.75 ± 8.19	83.50 ± 5.83	A vs. E	<0.001^†

Data are presented as mean ± standard deviation.

*P < 0.05 was regarded as statistically significant after Bonferroni correction.

^†P < 0.005 was regarded as statistically significant after Bonferroni correction.

Group A: 2% lidocaine injection using IE; Group B: 1% lidocaine injection using IE; Group C: N/S injection using IE; Group D: IE reference without fluid injection; Group E: NE control without fluid injection.

Ablation zone

As regards the ablation zone, the diameters (DV, DT1, and DT2) and volumes in the four IE groups were significantly smaller than those of the NE control group (all P < 0.05). Among the IE groups, the volume, DT1, and DT2 of the ablation zone were significantly smaller in group A compared to group D (all P < 0.05). Further, DT2 was significantly smaller in group A compared to group C (P < 0.001); however, no significant difference was observed for the DT1, DV, and volume values. No significant differences were found between the DV values among the four IE groups (P > 0.05) (Table 3, Fig. 3).

Table 3.

Comparison of ablation zones among five ablation groups using post-hoc test.

Variables	Group A	Group B	Group C	Group D	Group E	Comparison group	Post-hocP value
DV (cm)	1.61 ± 0.08	1.60 ± 0.10	1.62 ± 0.11	1.64 ± 0.10	1.79 ± 0.13	A vs. E, B vs. E, C vs. E, D vs. E	<0.001
DT1 (cm)	1.27 ± 0.08	1.29 ± 0.08	1.31± 0.07	1.35 ± 0.07	1.46 ± 0.10	A vs. E, B vs. E, C vs. E, D vs. EA vs. D	<0.001= 0.019
DT2 (cm)	1.44 ± 0.09	1.51 ± 0.08	1.52± 0.09	1.57 ± 0.10	1.67 ± 0.13	A vs. E, B vs. E, C vs. E, D vs. EA vs. DA vs. C	<0.05<0.001<0.001
Volume (mL)	1.54 ± 0.18	1.63 ± 0.19	1.69± 0.19	1.83 ± 0.22	2.30 ± 0.32	A vs. E, B vs. E, C vs. E, D vs. EA vs. D	<0.001= 0.01

P < 0.05 was considered statistically significant after Bonferroni correction.

Discussion

The intention of the current ex vivo study was to evaluate the effects of intralesional lidocaine injection using IE before RFA in soft tissue, because predicting the ablation performance using IE before clinical application is important. If the ablation performance cannot be predicted, thermal injury to non-targeted areas including neurovascular structure may occur, or conversely insufficient ablation margin can be created (8). Here, we found that injection of a 1% or 2% lidocaine solution before RFA via an IE creates a smaller ablation zone with the appropriate temperature at which coagulation necrosis occurs. Therefore, thermal injury can be avoided to critical structures around the ablation zone using an IE with intralesional lidocaine injection with our study protocol.

Before evaluating the intralesional fluid injection, we assessed the ablation performance of IEs, compared to NEs. It was found that the mean volume and transverse diameters of the ablation zone decreased significantly for each IE case compared to the NE control. Before discussing the differences in the electrical conductivities, differences in mean energy, current, and temperature were observed between the IE and NE groups; mean energy and mean current flow during RFA were significantly lower in the IE groups compared to those in the NE control group, thus leading to a decrease in the size of the coagulation zone.

The ablation protocol in this study was designed with reference to the moving-shot technique (9). In brief, the technique involved continuous movement of the ablation needle over 5 min at a low power (50W). The moving-shot technique is commonly used in RFA of thyroid nodules, as it is effective for avoiding damage to the surrounding structures while producing successful clinical outcomes (9,10). Because musculoskeletal tumors are usually oval-shaped, with critical structures near the tumors, a moving-shot technique is also suitable for soft-tissue tumor ablation (9,11,12). In a previous study on musculoskeletal tumor ablation (11), the moving-shot technique yielded good results for locoregional control of malignant soft-tissue tumors. Therefore, our ex vivo study was designed with the moving-shot technique for soft-tissue tumor ablation. In our study, it seems that raising mean energy and current in the IE groups compared to the NE control group could be a possible solution for achieving the same performance of both two groups.

Many studies in the liver have reported that intralesional fluid injection before or during RFA, typically N/S, improves the electrical and thermal conductivity, yielding increased energy deposition, tissue heating, and ablation volume (13 –17). In contrast, in our study, we observed that there was a tendency towards a decrease in the size and volume of the ablation zone in the IE fluid injection group compared with the IE reference group when relatively lower power is employed; the largest reduction was observed in the 2% lidocaine IE group. We assumed that differences were caused by the different distributions of electrical conductivities created around the electrode by the different protocols of injected liquid. In ex vivo studies (17,18), diverse results can be obtained when different kinds of electrodes and different kinds and concentrations of pre-injected fluid are used under various experimental conditions. However, the present study could not find any comparable research using IEs with lidocaine under the parameters of this study. The only similar reports are several experimental studies that have reported evaluations of the effects of an interstitial fluid infusion containing lidocaine using monopolar or internally cooled electrodes (19,20). For example, Vollejo et al. have investigated the effect of 0.5 mL of various kinds of fluid (0.9% and 7.3% saline, 2% lidocaine with 0.6% NaCl, bupivacaine, etc.) injected into chicken muscle, concluding that any fluid injection has a lesser effect on the ablation size when cooled RF is applied (20). Our study has shown that, despite the ion-containing fluid, the administration of relatively small volumes of 1% or 2% lidocaine may potentially minimize the issue of fluid pre-injection.

This study has several limitations. First, explanted pork sirloin may not have the same characteristics as the tissue encountered in clinical practice. Further, explanted pork sirloin is a normal muscle rather than tumor tissue. Second, ex vivo results cannot be used as a precise model of an in vivo scenario, in which microperfusion and macroperfusion will have an impact on both the ablation and cooling. Third, the ablation area can be affected by the direction of the meat grains; therefore, measurement errors can occur. However, most of the cases examined in this study exhibited a uniformly shaped ablation area. Furthermore, the pre-injected fluid could spread with less resistance in the direction of the grains and the unwanted portion could be ablated. Attempts were made to match all organizational pork blocks equally. It was confirmed that pre-injected fluid was primarily located around the ablation area, so there is no concern of burning occurring in unexpected areas. Fourth, the force of the manual fluid injection through the 30-mL syringe may have differed in each case; these differences may have affected the extent of the solution dispersion. However, manual bolus injection is usually performed in a clinical scenario and the fluid infusion rate should, therefore, be validated in further studies. Lastly, the degree of pain reduction due to the lidocaine injection could not be evaluated, because this was an ex vivo study. That is, the conditions of this study differed significantly from a clinical setting, where the degree of pain reduction can be evaluated directly based on the patient’s responses during the RFA procedure. Thus, further clinical study will be required to evaluate the patient pain control achieved using lidocaine injection via an IE before or during RFA.

In conclusion, it was found that intralesional lidocaine injections administered via IEs before RFA could be a feasible method. A slightly smaller ablation zone was obtained when lidocaine injection using an IE was performed.

Footnotes

Acknowledgements

The authors thank Dong Un Kim and Jung Hyuk Zu for the technical assistance for the radiofrequency ablation experiments. The study was supported by STARmed Co. (Goyang, Korea), which provided the required equipment.

Declaration of conflicting interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: JHB has been a consultant of STARmed, the RF medical company, since 2017.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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