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Letters to the Editor |
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José M. Burdío, PhD,
,
Ana Navarro, MD,*,
Paloma Ros, MSc,,
Antonio Güemes, MD,*,
Ramón Sousa, MD,*,
Eloy Tejero, MD,* and
Ricardo Lozano, MD*
Department of Surgery A, Hospital Clínico Universitario, Lozano Blesa, Zaragoza, Spain*
Ernest Lluch 1, bloque E, 3°, 3a, 17600 Figueres (Girona), Spain
Department of Electric Engineering and Communications, University of Zaragoza, Spain. e-mail: fburdio@comll.es
Editor:
We have read with interest the article by Dr Lobo and colleagues in the January 2004 issue of Radiology (1). The authors tried to obtain the best concentration and volume for an NaCl solution prior to treatment with radiofrequency (RF) ablation. They used stepwise systematic increases in either the volume or the concentration of NaCl solution in agar phantoms. The effect of one variable (either concentration or volume) on temperature was studied with the other variable held constant.
We have also studied the effect of concentration of an NaCl solution, especially with continuous infusion through the tissue. At first we used 0.9% NaCl solution (2), but we finally used 20% NaCl solution (3). According to our results, we do not completely agree with the statement, "increased electrical conductivity has competing effects on RF ablation: It enables increased energy deposition and greater heating, but it also increases the energy required to heat a given volume of tissue," which appears in the article of Dr Lobo and colleagues on page 181. We think that increased conductivity does not increase the total energy required to heat a given volume of tissue but improves the deposition of energy through the tissue because it enables less voltage to be used, and therefore, less arcing and charring occurs.
In fact, with higher NaCl concentration of the solution (3), we used less total energy per volume of tissue than with lower NaCl concentration with a similar RF ablation approach (2). To our knowledge, the only limitation of this approach is that it requires a circuit and a generator to supply a higher current.
If a more concentrated NaCl solution is used, the conductivity of the tissue (
) is raised, and the impedance (R) is reduced, as is indicated by the following equation: R = (1/)(l/s), where l is the length and s is the surface through which the current flows.
At least when manual control of power is used, one actually selects the voltage (V) to be delivered by the generator, and the current (I) depends on the encountered impedance (R), as stated by Ohm’s law (V = RI). Therefore, to keep the voltage constant, the current must increase with the same proportion by which the impedance decreases. We also know that the power (P) and, hence, the resistive heating are equal to the square of the current multiplied by the impedance—P = R(I2). Then, the deposited power increases with a more concentrated NaCl solution if the generator does not shut off with decreasing impedance.
REFERENCES
Department of Radiology, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, MA 02215. e-mail: sgoldber@caregroup.harvard.edu
We have carefully reviewed the letter by Dr Burdío and colleagues with regard to our article (1) and wish to issue the following response.
We feel that our statement, "increased electrical conductivity has competing effects on RF ablation: It enables increased energy deposition and greater heating, but it also increases the energy required to heat a given volume of tissue," is well supported by the data in the article. The data of table 1 clearly validate the first half of the sentence, as increased NaCl concentration leads to greater heating to a maximum. Table 2 demonstrates that this occurs with increased energy deposition. Additionally, it must be pointed out that given that temperatures decrease under generator-constant conditions, by definition, a greater amount of energy is required to heat the given volume of tissue. We have further documented this latter assertion in our recent poster presentation at the 2003 RSNA scientific assembly, in which we documented greater energy requirements to achieve an equivalent 50°C isotherm for increased conductivity.
We strongly disagree with the alternative hypothesis presented by Dr Burdío and colleagues, namely that "increased conductivity does not increase the total energy required to heat a given volume of tissue but improves the deposition of energy through the tissue because it enables less voltage to be used, and therefore, less arcing and charring occurs." Our findings hold true for both generator-constant conditions and conditions for which no tissue boiling was noted. As such, the concept of arcing and charring is irrelevant to the biophysical principles being discussed. Dr Burdío and colleagues also assert that less voltage is used. However, for the generator-constant regions we described, by definition, the voltage was held constant at the generator maximum. Hence, at this constant voltage, the lower temperatures seen at increased conductivity can only result from the fact that increased conductivity increases the energy required to heat the volume of tissue. Increased amounts of energy are required but are unavailable to heat the tissue.
It must also be pointed out that the calculation of energy per volume of tissue ablated does not necessarily translate to the amount of energy necessary to heat at a given distance from the electrode. Furthermore, the fact that Dr Burdío and colleagues required less energy to achieve ablation states nothing about the temperatures that were generated throughout the tissue volume. Indeed, it is quite plausible that they achieved coagulation at different NaCl concentrations at different temperatures, ranging anywhere from 50°C to 100°C.
It must be pointed out that there are several other potential mechanisms for the variance in results seen between our setup and that of Dr Burdío and colleagues. These have to do with the fact that they used solutions that were allowed to diffuse through the tissue. As such, issues such as improved thermal conductivity and the direct coagulative effects of NaCl on the tissue are alternate possibilities that could have enabled a reduction in the amount of heat or energy necessary to destroy a given volume of tissue. It must be further pointed out that the system of Dr Burdío and colleagues was that of a bipolar and not a monopolar electrode.
It is important to mention that our experiments involve a two-compartment model with regions of very different electric conductivity. We are in the process of studying and submitting a manuscript that documents how this two-compartment model dramatically alters the electric field and therefore the heating characteristics in complex ways, which should override the physics arguments presented.
REFERENCES
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