EDITORIAL
Think Before You Inject Understanding Electrophysiological Radiofrequency Principles and the Importance of the Local Tissue Environment David Anthony Provenzano, MD
R
adiofrequency (RF) treatment is frequently used to treat spine pain. In interventional pain medicine, monopolar RF is most commonly used. For specific anatomical areas, 1 limitation of monopolar RF is the restricted size of the created lesion.1–4 In an effort to overcome this limitation, increased interest has been shown in both cooled and bipolar RF. In bipolar RF, a passive electrode replaces the grounding pad with the goal of focusing lesioning between the 2 electrodes. In cooled RF, an internally cooled electrode is used to allow for continuous internal cooling of the tip with a perfusate.5,6 The internal perfusate serves as a heat sink and removes heat closest to the electrode, which allows for greater current deposition. In order to use RF technology effectively, practitioners must understand the electrophysiological principles and technical aspects of RF.7 The ability to lesion specific tissues, while limiting destruction to nontargeted tissues, depends on factors that influence energy delivery and local physiologic tissue characteristics. Coagulation necrosis can be described by the Bioheat equation.6 Coagulation necrosis ¼ ðheat generated local tissue interactionsÞ−heat lost In a simplified thermal RF system, 3 primary factors determine heat generation: (1) distance from the active tip, (2) RF current density, and (3) duration of application of the RF current.7 Heat losses that influence RF lesioning include (1) conduction (heat diffusion), (2) convection (circulation), and (3) lowresistance shunting. Lately, an emphasis has been placed on research relating to the local tissue environment surrounding the RF electrode.3,4,8 A detailed understanding of the influence of the local tissue environment is important. First, pain physicians often alter this local tissue environment before performing RF through the preinjection of local anesthetic. Furthermore, modulating the local tissue environment and hence affecting both electrical and thermal conductivity in certain circumstances may assist in our quest for increased coagulation. In this issue of Regional Anesthesia and Pain Medicine, Vallejo et al9 add to our understanding of RF technical principles. Vallejo et al9 used an ex vivo model to compare cooled RF and bipolar RF lesion size in multiple platform designs (ie, perpendicular to bone, parallel to bone, and muscle-only models) with alterations in the local environment. In their model, 0.5 mL of specific fluids was injected directly into the surrounding tissue. This research, and the work of others, highlights key RF technical points that should be considered. First, we need to examine bipolar RF lesions. A 3-dimensional Cartesian coordinate system describes a bipolar lesion (Fig. 1). Specific configuration parameters influence lesion development (Table 1).8,10,11 One significant parameter is the set interelectrode distance (IED). The goal is to choose an IED that results in a reproducible lesion with adequate minimum lesion height (ymin) between the cannula to ensure coagulation of the targeted structure. The IED should be set to limit hourglass lesioning (ie, diminished ablation in the vertical plane in between the electrodes), which may result in missing the targeted structure (Fig. 1). Pino et al10 and Cosman and Gonzalez11 first suggested the importance of the IED for bipolar RF, with both demonstrating that lesion size can be maximized through the expansion of the IED but only to a certain point beyond which continuous lesions are no longer achieved. Based on an egg white, no fluid preinjection model with 22-gauge 5-mm active tip cannulae, Pino et al10 suggested that continuous lesions occur when the IED is 6 mm or less. Vallejo et al9 demonstrated that an IED of 10 mm is optimal for generating around rectangular lesion. Cosman and Gonzalez11 also recommended an IED of 10 mm as a conservative selection that is not sensitive to small alterations in technical setup (eg, intertip angles and intertip offsets). Previously, our research has demonstrated that when IED
From the Pain Diagnostics and Interventional Care, Edgeworth Commons, Pittsburgh, Pennsylvania. Accepted for publication April 26, 2014. Address correspondence to: David Anthony Provenzano, MD, Pain Diagnostics and Interventional Care, Edgeworth Commons, Suite 203, 301 Ohio River Blvd, Pittsburgh, PA 15143 (e‐mail: [email protected]). This editorial was not supported by any grants or research funding by any manufacturer or third party. Dr Provenzano serves as a consultant for Kimberly-Clark. Copyright © 2014 by American Society of Regional Anesthesia and Pain Medicine ISSN: 1098-7339 DOI: 10.1097/AAP.0000000000000114
Regional Anesthesia and Pain Medicine • Volume 39, Number 4, July-August 2014
269
Copyright © 2014 American Society of Regional Anesthesia and Pain Medicine. Unauthorized reproduction of this article is prohibited.
Editorial
Regional Anesthesia and Pain Medicine • Volume 39, Number 4, July-August 2014
FIGURE 1. A schematic diagram demonstrating important lesion parameters for a bipolar configuration. x is maximum lesion length; ymax, maximum height; ymin: minimum height; z, maximum depth; IED, set IED. The yellow dot represents a nerve that is not being treated secondary to incomplete (hourglass) lesioning in the middle of the lesion in the y axis.
increases beyond 13 mm, hourglass lesioning significantly increases.8 In these more recent ex vivo studies, longer active tips and larger cannulae were examined.8,11 Another bipolar technical parameter, which was studied by Vallejo et al,9 was the influence of RF lesion time. In this current study, increasing the RF lesion time from 90 seconds to 150 seconds did not significantly increase final lesion size. In some cases, final lesion size was decreased, although not significantly, with increasing lesion time. It is hard to explain how lesion size decreases with increasing time, but this may have to do with differences in tissue specimens and lesion variability. Ideally, the influence of lesion time should be measured in the same specimen and set up with lesion size recordings taken at certain time points. This would limit the influence of specimen variability. At first, one may think that these results are in contrast to previously published research, which has demonstrated that increasing lesion time is advantageous and leads to the improvement in lesion form and size.10,11 Pino et al10 demonstrated the optimal lesioning time to be between 120 and 150 seconds. Cosman and Gonzalez11 also demonstrated that lesion time is a very important consideration, especially when a larger IED is chosen. When one looks closely at the results of Vallejo et al9 for the largest IED setting of 15 mm, increasing lesion time did improve the volume of coagulation in the region in between electrodes. In addition, extending lesion time increased the midpoint temperature as demonstrated in the thermal camera images published by Vallejo et al.9 This is of critical importance because hourglass lesioning could lead to missing the targeted nerve. One way to reduce this risk is to increase lesion time. Future work is required to define optimal time settings for bipolar RF. The authors also examined the influence of fluid preinjection on lesion development with multiple platform designs for bipolar and cooled RF. For bipolar RF, the influence of fluid preinjection on volume was dependent on multiple variables including the set IED, RF cannula configuration, and needle positioning relative to bone. The authors should be congratulated for examining a
270
model containing bone that attempts to replicate the bonemuscle interface seen clinically. As shown in previous research, under certain conditions and with specific compositions, the preinjection of fluid limits hourglass lesioning through the enhancement of ymin growth (Fig. 1).8 When evaluating any study that examines fluid preinjection, it is important to investigate multiple variables including the volume of injection, composition of the fluid, and the method of delivery. The method of fluid delivery will determine the final position of the preinjected fluid. For example, when performing bipolar RF, the goal should be to incorporate the fluid between the RF cannulae. Vallejo et al9 examined 0.5-mL fluid preinjection that was delivered via a separate needle and syringe. Typically, when injecting fluids prior to RF, the fluid will exit the distal portion of the cannula. Therefore, in order for the fluid to saturate the lesion target zone for bipolar RF, the open portion of the active tip should be positioned toward the middle portion of the lesion, and injection should occur throughout the entire needle track in an effort to limit the fluid from mainly distributing at the distal end of the cannulae. Previous research has suggested that injection technique is important.8 TABLE 1. Bipolar Configuration Parameters Involved in Lesion Development Technical Parameter Active tip dimensions—size and length Fluid preinjection composition Fluid preinjection technique Fluid preinjection volume IED Lesion time Tip configurations—offset, angle, skew Tip temperature
© 2014 American Society of Regional Anesthesia and Pain Medicine
Copyright © 2014 American Society of Regional Anesthesia and Pain Medicine. Unauthorized reproduction of this article is prohibited.
Regional Anesthesia and Pain Medicine • Volume 39, Number 4, July-August 2014
Interestingly, when small volumes of fluid were preinjected prior to cooled RF, lesion size was not altered. This raises the question: Is cooled RF different than monopolar and bipolar RF and not influenced by the preinjection of fluid? Additional research is required to answer this question. In the fields of cardiology and radiology, fluid preinjection has been studied with cooled RF technology, and in these models, specific fluids have been shown to significantly influence final lesion size.12–15 Goldberg et al12 demonstrated that preinjection of sodium chloride (NaCl) solution before RF ablation increases energy deposition, tissue heating, and lesion size. However, Goldberg et al12 also revealed that there is a limit to size enhancement. When the NaCl concentration is increased beyond a point (>5.0%), reduced lesion temperatures occur, which negatively affect lesion size. When examining the influence of NaCl, it is important to study multiple concentrations. In this study, NaCl concentrations of 0.9% and 7.3% were examined. Bruners et al13 also studied cooled RF technology with fluid preinjection and found that the preinjection of hetastarch increased lesion size, but preinjection with NaCl was not found to be beneficial over the no-fluid condition. As Vallejo et al9 point out, we cannot extrapolate the results from these studies to interventional pain medicine equipment. The RF systems and equipment setups are different and often involve impedance controlled multiple lesion cycles, larger electrodes, and increased power outputs. In an effort to increase understanding of modulating the local tissue environment, research must be specific to interventional pain medicine equipment. As with all research, more questions are raised. Would a larger volume of injection, a different fluid composition (ie, NaCl concentration >0.9% and
Think Before You Inject Understanding Electrophysiological Radiofrequency Principles and the Importance of the Local Tissue Environment David Anthony Provenzano, MD
R
adiofrequency (RF) treatment is frequently used to treat spine pain. In interventional pain medicine, monopolar RF is most commonly used. For specific anatomical areas, 1 limitation of monopolar RF is the restricted size of the created lesion.1–4 In an effort to overcome this limitation, increased interest has been shown in both cooled and bipolar RF. In bipolar RF, a passive electrode replaces the grounding pad with the goal of focusing lesioning between the 2 electrodes. In cooled RF, an internally cooled electrode is used to allow for continuous internal cooling of the tip with a perfusate.5,6 The internal perfusate serves as a heat sink and removes heat closest to the electrode, which allows for greater current deposition. In order to use RF technology effectively, practitioners must understand the electrophysiological principles and technical aspects of RF.7 The ability to lesion specific tissues, while limiting destruction to nontargeted tissues, depends on factors that influence energy delivery and local physiologic tissue characteristics. Coagulation necrosis can be described by the Bioheat equation.6 Coagulation necrosis ¼ ðheat generated local tissue interactionsÞ−heat lost In a simplified thermal RF system, 3 primary factors determine heat generation: (1) distance from the active tip, (2) RF current density, and (3) duration of application of the RF current.7 Heat losses that influence RF lesioning include (1) conduction (heat diffusion), (2) convection (circulation), and (3) lowresistance shunting. Lately, an emphasis has been placed on research relating to the local tissue environment surrounding the RF electrode.3,4,8 A detailed understanding of the influence of the local tissue environment is important. First, pain physicians often alter this local tissue environment before performing RF through the preinjection of local anesthetic. Furthermore, modulating the local tissue environment and hence affecting both electrical and thermal conductivity in certain circumstances may assist in our quest for increased coagulation. In this issue of Regional Anesthesia and Pain Medicine, Vallejo et al9 add to our understanding of RF technical principles. Vallejo et al9 used an ex vivo model to compare cooled RF and bipolar RF lesion size in multiple platform designs (ie, perpendicular to bone, parallel to bone, and muscle-only models) with alterations in the local environment. In their model, 0.5 mL of specific fluids was injected directly into the surrounding tissue. This research, and the work of others, highlights key RF technical points that should be considered. First, we need to examine bipolar RF lesions. A 3-dimensional Cartesian coordinate system describes a bipolar lesion (Fig. 1). Specific configuration parameters influence lesion development (Table 1).8,10,11 One significant parameter is the set interelectrode distance (IED). The goal is to choose an IED that results in a reproducible lesion with adequate minimum lesion height (ymin) between the cannula to ensure coagulation of the targeted structure. The IED should be set to limit hourglass lesioning (ie, diminished ablation in the vertical plane in between the electrodes), which may result in missing the targeted structure (Fig. 1). Pino et al10 and Cosman and Gonzalez11 first suggested the importance of the IED for bipolar RF, with both demonstrating that lesion size can be maximized through the expansion of the IED but only to a certain point beyond which continuous lesions are no longer achieved. Based on an egg white, no fluid preinjection model with 22-gauge 5-mm active tip cannulae, Pino et al10 suggested that continuous lesions occur when the IED is 6 mm or less. Vallejo et al9 demonstrated that an IED of 10 mm is optimal for generating around rectangular lesion. Cosman and Gonzalez11 also recommended an IED of 10 mm as a conservative selection that is not sensitive to small alterations in technical setup (eg, intertip angles and intertip offsets). Previously, our research has demonstrated that when IED
From the Pain Diagnostics and Interventional Care, Edgeworth Commons, Pittsburgh, Pennsylvania. Accepted for publication April 26, 2014. Address correspondence to: David Anthony Provenzano, MD, Pain Diagnostics and Interventional Care, Edgeworth Commons, Suite 203, 301 Ohio River Blvd, Pittsburgh, PA 15143 (e‐mail: [email protected]). This editorial was not supported by any grants or research funding by any manufacturer or third party. Dr Provenzano serves as a consultant for Kimberly-Clark. Copyright © 2014 by American Society of Regional Anesthesia and Pain Medicine ISSN: 1098-7339 DOI: 10.1097/AAP.0000000000000114
Regional Anesthesia and Pain Medicine • Volume 39, Number 4, July-August 2014
269
Copyright © 2014 American Society of Regional Anesthesia and Pain Medicine. Unauthorized reproduction of this article is prohibited.
Editorial
Regional Anesthesia and Pain Medicine • Volume 39, Number 4, July-August 2014
FIGURE 1. A schematic diagram demonstrating important lesion parameters for a bipolar configuration. x is maximum lesion length; ymax, maximum height; ymin: minimum height; z, maximum depth; IED, set IED. The yellow dot represents a nerve that is not being treated secondary to incomplete (hourglass) lesioning in the middle of the lesion in the y axis.
increases beyond 13 mm, hourglass lesioning significantly increases.8 In these more recent ex vivo studies, longer active tips and larger cannulae were examined.8,11 Another bipolar technical parameter, which was studied by Vallejo et al,9 was the influence of RF lesion time. In this current study, increasing the RF lesion time from 90 seconds to 150 seconds did not significantly increase final lesion size. In some cases, final lesion size was decreased, although not significantly, with increasing lesion time. It is hard to explain how lesion size decreases with increasing time, but this may have to do with differences in tissue specimens and lesion variability. Ideally, the influence of lesion time should be measured in the same specimen and set up with lesion size recordings taken at certain time points. This would limit the influence of specimen variability. At first, one may think that these results are in contrast to previously published research, which has demonstrated that increasing lesion time is advantageous and leads to the improvement in lesion form and size.10,11 Pino et al10 demonstrated the optimal lesioning time to be between 120 and 150 seconds. Cosman and Gonzalez11 also demonstrated that lesion time is a very important consideration, especially when a larger IED is chosen. When one looks closely at the results of Vallejo et al9 for the largest IED setting of 15 mm, increasing lesion time did improve the volume of coagulation in the region in between electrodes. In addition, extending lesion time increased the midpoint temperature as demonstrated in the thermal camera images published by Vallejo et al.9 This is of critical importance because hourglass lesioning could lead to missing the targeted nerve. One way to reduce this risk is to increase lesion time. Future work is required to define optimal time settings for bipolar RF. The authors also examined the influence of fluid preinjection on lesion development with multiple platform designs for bipolar and cooled RF. For bipolar RF, the influence of fluid preinjection on volume was dependent on multiple variables including the set IED, RF cannula configuration, and needle positioning relative to bone. The authors should be congratulated for examining a
270
model containing bone that attempts to replicate the bonemuscle interface seen clinically. As shown in previous research, under certain conditions and with specific compositions, the preinjection of fluid limits hourglass lesioning through the enhancement of ymin growth (Fig. 1).8 When evaluating any study that examines fluid preinjection, it is important to investigate multiple variables including the volume of injection, composition of the fluid, and the method of delivery. The method of fluid delivery will determine the final position of the preinjected fluid. For example, when performing bipolar RF, the goal should be to incorporate the fluid between the RF cannulae. Vallejo et al9 examined 0.5-mL fluid preinjection that was delivered via a separate needle and syringe. Typically, when injecting fluids prior to RF, the fluid will exit the distal portion of the cannula. Therefore, in order for the fluid to saturate the lesion target zone for bipolar RF, the open portion of the active tip should be positioned toward the middle portion of the lesion, and injection should occur throughout the entire needle track in an effort to limit the fluid from mainly distributing at the distal end of the cannulae. Previous research has suggested that injection technique is important.8 TABLE 1. Bipolar Configuration Parameters Involved in Lesion Development Technical Parameter Active tip dimensions—size and length Fluid preinjection composition Fluid preinjection technique Fluid preinjection volume IED Lesion time Tip configurations—offset, angle, skew Tip temperature
© 2014 American Society of Regional Anesthesia and Pain Medicine
Copyright © 2014 American Society of Regional Anesthesia and Pain Medicine. Unauthorized reproduction of this article is prohibited.
Regional Anesthesia and Pain Medicine • Volume 39, Number 4, July-August 2014
Interestingly, when small volumes of fluid were preinjected prior to cooled RF, lesion size was not altered. This raises the question: Is cooled RF different than monopolar and bipolar RF and not influenced by the preinjection of fluid? Additional research is required to answer this question. In the fields of cardiology and radiology, fluid preinjection has been studied with cooled RF technology, and in these models, specific fluids have been shown to significantly influence final lesion size.12–15 Goldberg et al12 demonstrated that preinjection of sodium chloride (NaCl) solution before RF ablation increases energy deposition, tissue heating, and lesion size. However, Goldberg et al12 also revealed that there is a limit to size enhancement. When the NaCl concentration is increased beyond a point (>5.0%), reduced lesion temperatures occur, which negatively affect lesion size. When examining the influence of NaCl, it is important to study multiple concentrations. In this study, NaCl concentrations of 0.9% and 7.3% were examined. Bruners et al13 also studied cooled RF technology with fluid preinjection and found that the preinjection of hetastarch increased lesion size, but preinjection with NaCl was not found to be beneficial over the no-fluid condition. As Vallejo et al9 point out, we cannot extrapolate the results from these studies to interventional pain medicine equipment. The RF systems and equipment setups are different and often involve impedance controlled multiple lesion cycles, larger electrodes, and increased power outputs. In an effort to increase understanding of modulating the local tissue environment, research must be specific to interventional pain medicine equipment. As with all research, more questions are raised. Would a larger volume of injection, a different fluid composition (ie, NaCl concentration >0.9% and
