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Visnyk N'l'UU KP1 Seriia Radiolekhnika tiadioaparatobuduummia, "2020, Iss. 83, pp. 55—61
УДК 616-72:616-71:004.891.3:616.12-008.318.1
Comparison of Monopolar and Bipolar Cox-Maze Ablation Based on Clinical Data and Mathematical Modeling
Stasiuk Y. P.1, Sychyk M. M.1'2, Maksymenko V. B.1'2, Kravchuk В. B?
1 National Technical University of Ukraine 'Igor Sikorsky Kyiv Polytechnic Institute", Kyiv, Ukraine 2Amosov National Institute of Cardiovascular Surgery, Kyiv, Ukraine
E-mail: marina.eychik&gmaiLcom
Purpose: Today radiofrequency heart, ablation is gold standard for the radical surgical treatment, of different, types of heart, rhythm disturbances. The purpose of the research is a comparative analysis of monopolar and bipolar electrodes for ablation during open heart, surgery (Cox-Maze ablation). Methods: The analysis is made based on clinical data and mathematical modeling. By the use of system of electro-anatomical mapping of radiofrequency ablation zones a three-dimensional model of left, atrium is created. Then a potential map, which represents the amplitude of myocardium activity, is imposed on this model. The amplitude of myocardium activity, the width and depth of the electro-thermal destruction zone and the ablation line continuity are main parameters of clinical data analysis. For mathematical modeling the COMSOL Mult.iphysics 5.4 software is used. Two variation of mathematical model for monopolar and bipolar ablation are created. Main analysis parameters of mathematical modeling are: diagrams of thermal field distribution, size of myocardial tissue destruction, and duration of the ablation procedure. Results: Both monopolar and bipolar ablation can be used for Cox-Maze procedure. But. potential map and mathematical modeling show that, with monopolar ablation the destruction zone has a hemispherical shape and scar line is not. uniform along the depth of a heated myocardial tissue. It. can lead to a recovery of pathological signals conduction from the pulmonary veins to the atrium. Uneven distribution of thermal fields with a clearly-defined maximum increases the risk of evaporation and microexplosions. At. the same time the duration of monopolar ablation is significantly longer. Conclusion: It. is shown that, bipolar ablation has advantages in pulmonary veins isolation. This type of instruments allows creation of effective and safe uniform thermal transmural destruction with only one application of radiofrequency current, energy.
Key words: heart.: arrhythmia: radiofrequency ablation: monopolar electrode: bipolar electrode: mathematical modeling
DOI: 10.20535/RADAP. 2020.83.55-61
Introduction
Statistics researches provide tendency of 0.5% of total population suffering from one of the most common arrhythmias atrial fibrillation [4]. In search of ways for treating this heart rhythm disturbance Dr. James L. Cox in 1987 proposed the so-called Cox-Maze procedure [2]. It is used in cases when riiedi-camental treatment isn't effective. In the classic version. it is "cut and saw" procedure. It is performed to create series of scars on heart that do not pass electrical impulses [6]. Tims, it is possible to create the only one possible way for electrical impulse to pass along heart wall in correct order. As shown by the practice of using the Cox-Maze procedure, its effectiveness is about 90%. However, due to the complexity of execution and duration it has not been widely used among cardiac surgeons [12].
The development of electro surgery has greatly simplified the radical treatment of cardiac arrhythmias. Today there is no need for performing surgical incisions of the heart wall for Cox-Maze procedure. Necessary SCSI'S CcUl be obtained by thermal destruction of the corresponding heart areas. In world practice. radiofrequency ablation (RFA) and cryoablation have become the most widespread among the possible sources of energy for heart ablation procedure. Microwave and ultrasound also can be used to treat heart rhythm disturbances but they are not so popular [8]. As for Ukraine, only RFA is widely used in arrhythmias treatment. Radiofrequency ablation significantly reduces the time of Cox-Maze procedure, reduces the time of patient postoperative recovery and can be performed minimal invasion by catheters or laparoscopically. The combination of these advantages
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Stasiuk Y. P., Sychyk M. M., Maksymonko V. B., Kravchuk B. B.
over a classic Cox-Maze has led to the widespread use of RFA in clinical practice.
Due to the high popularity of a radiofreqnency ablation. there is a wide range of equipment for this procedure on the market. In general, it can be divided into radiofrequency current generators and electrodes for electro-thermal destruction. In turn, all electrodes can be divided into monopolar and bipolar. Monopolar electrodes for RFA are designed to carry out point destruction of small sizes. Also they can be used for linear destruction. clS ct combination of several tens of point ablations. Bipolar electrodes allow to instantly create large linear electro-thermal destruction.
1 The main objective of the article
The main objective of this article is to compare the possibilities of monopolar and bipolar ablation for a Cox-Maze procedure. The comparison is carried out both on the basis of clinical material (the potential map of myocardium tissue activity is used) and mathematical modeling of radiofrequency ablation during open-heart surgery.
2 Materials and Methods
2.1 Comparison of monopolar and bipolar Cox-Maze ablation based on clinical data
Comparison is performed based on analysis of RFA procedures results for bipolar and monopolar methods. Clinical data was collected in the Department of Arrhythmias (Aniosov National Institute of cardiovascular surgery. Ukraine. Ivyiv) with the use of the electro-anatomical mapping system of radiofrequency ablation zones (Ensite Velosity. Abbott. USA). This system allows to build a three-dimensional model of the cardiac chamber and to impose a potential map on it. That map gives quantitative values of the myocardium activity amplitude. It clearly shows in color-coding (Figure 1) areas of post-ablation scars that are formed in the postoperative period. Purple color in this map corresponds to the amplitude of heart tissue activity-above 0.5 niV. In clinical practice it is a characteristic of the active tissue. The scale of amplitude from 0.1 to 0.5 niV corresponds to the heart scar tissue. The width of the ablation zone and the line continuity are evaluated for complete isolation of the pulmonary veins from the atrium. Recovery of myocardial activity in the destruction area is the main canse of recurrence of arrhythmia after RFA [10].
Fig. 1. The color coding of the heart tissue activity
The left atrium potential map of patient A is shown on Figure 2. He had RF isolation of the pulmonary veins by bipolar clamps (line 1) and the atrial lines by a monopolar electrode (line 2).
The potential map of patient B is shown on Figure 3. In this case RF pulmonary vein isolation was performed only by a monopolar electrode (line 3).
2.2 Comparison of monopolar and bipolar Cox-Maze ablation based on mathematical model
2.2.1 Main equation of the mathematical model
To compare monopolar and bipolar ablation during open heart surgery a mathematical model of this procedure is created. To simulate the resistive heating of biological tissue. COMSOL Mnltiphysics 5.4 software is used. Model is created based on combination of Bioheat Transfer and Electric Current modules. The main equation of the proposed model, as in its analogues [12, 14]. is the heat transfer in biological tissue equation [1]:
( OT \
pcVdt) = v•kVT+q+Qm-Qp, (1)
where p is the density (kg/m3), c is the specific heat
(J/(kg^K)), k is the thermal conductivity (W/(m^K)), q
3
is the cooling by blood perfusion (W/m3), Qm is the
3
In the proposed mathematical model the energy of the RF current is the heating source. A current frequency for RFA procedure is in the range of 3001000 kHz. Taking into account current frequency, the small volume of the influence zone and fact that myocardium is a tissue with high water content, only the resistive component of the total impedance of biological tissue can be taken into account [3]. Tims, the heating source can be described by the equation:
q = a\E\2, (2)
where \E \ is the magnitude of the electric field vector (V/m) and a is the electrical conductivity (S/m).
llopiiiiiHiiiiH MoiioiiojiHpiioï Ta 6iiiojiapiioï Cox-Mazoafjjiauiï lia ociioui Kjiiiii'iimx .aairnx Ta MaioMaTH'iiioro Mo^ojiiouaiiiiM
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Fig. 2. The electro-anatoriiical mapping of the radiofreqnency ablation zones in left atrium by bipolar clamps: 1 line of pulmonary vein isolation. 2 atrial lines (mitral valve isthmus line. LA roof line. LA floor line)
Fig. 3. The electro-anatomical mapping of the radiofreqnency ablation zones in left atrium by a monopolar electrode: a) 3 line of pulmonary vein isolation: b) the recovery of conduction of pathological signals from the vein to the atrium; c) the formation of conditions for the generation of arrhythmia substrate in the middle of
the destruction zone, around scar
The radiofreqnency ablation procedure during open-heart surgery is carried out under the condition of cardiopulmonary bypass. That's why there is no cooling by the blood flow in heart chambers. However, it is appropriate to take into account cooling from the blood flow of small vessels and capillaries of the heart wall. It is indicated as Qp [ ]. At the same time the influence of the environment is present. It can be indicated as Qa. The amount of heat released result of metabolic processes during the duration of application up to 60 s is negligible [5]. Therefore it is not taken into account in the model. With all refinements the final version of the equation for heart RFA modeling is:
( S )
PC\^T = V^kVT+a\E |2-Qp-Qa.
(3)
2.2.2 Geometry and thermoelectric characteristics of mathematical model components
Two versions of the mathematical model geometry for simulating monopolar and bipolar ablation are considered (Fig. 4). In both cases, there is an element of myocardial tissue surrounded by ambient air flow at a speed of 0.3m/s. For Cox-Maze RFA the affected area is in the left atrium near the month of pulmonary veins. In this zone thickness of heart walls is approximately 1-4 mm [16]. An average value of 3 mm is chosen for modeling. In the case of bipolar ablation modeling, the thickness of myocardial tissue element is twice as large, since the clamp branches are applied on both sides of pulmonary veins. [9.11].
As a tool for monopolar ablation. a classical electrode with a diameter of 7F and a length of
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CiacioK iO. 11., Ch'ihk M. M., MaKCHMoiiKo B. B., Kpaii'iyK B. B.
4 mrii is chosen (Fig. 4a). To simulate a bipolar clamp, two electrodes with a length of 7 cm and a width of 3 mm located in acrylic plastic branches are considered (Fig. 4b). The electrode material is platimim-iridium alloy. The thermoelectric characteristics of all elements of the mathematical model are presented in Table 1
(b)
Fig. 4. Geometry of mathematical models for monopolar (a) and bipolar (b) ablation, where 1 is the myocardium tissue. 2 is the ambient air. 3 is the monopolar electrode. 4 is clamps of the bipolar electrode
2.2.3 Initial and boundary conditions of the mathematical model
The initial temperature for an element - myocardial tissue, as well as blood of small capillaries in heart wall is set at 36.6°C, for electrodes - 20°C. In the mathematical model, it is believed that the electrical conductivity of myocardial tissue increases linearly
°
Environmental cooling is simulated by a laminar
°
speed of 0.3m/s. Cooling by the blood flow of small capillaries is realized by a boundary condition Biological Tissue. The perfusion rate is 0.0012 rii/s and is considered uniform over the entire surface of the heart.
To simulate monopolar ablation it is believed that the lower surface of the myocardial tissue element acts as a passive electrode. Therefore, a boundary condition Ground is applied to it. When modeling bipolar ablation. the upper jaw is considered active, the lower one passive.
Calculation of the mathematical model is carried out for a current frequency of 500 kHz. with maximum application duration of 60 s. The potential of the radio-frequency current is applied to surfaces of the active electrode and in both cases is set to 20 V. The size
of destruction is estimated by the area of heart tissue
°
area of irreversible thermal destruction [7]. The critical
°
to a sharp evaporation of fluid and ruptures of a heart wall as a result of micro explosions [17].
3 R-Gsults
Comparisons of line 1 and lines 2.3 on Figure 2 and Figure 3 shows that single-phase RFA with bipolar clamps (1) provides a uniform in width and depth destruction line around pulmonary veins. In clinical efficiency it is equivalent to creating a complete blockade for conduction of pathological signals from veins to the atrium and vice versa. Lines 2 and 3 are more chaotic and scattered in space. That is due to they are a set of ablation points, each with some differences in the electrical parameters. Tims, they create a nonuniform zone of destruction. It is confirmed by the presence of more colors on the scale of the myocardial tissue activity amplitude.
The thermal field distribution diagrams for monoarid bipolar ablation are shown on Figure 5. It can be concluded, that the proper transmural destruction
is achieved in both cases. Also the heating does not
°
Time-30 s Surface: Temperature <degC) Contour: Temperature (degC)
I
(a)
(b)
Fig. 5. Diagrams of thermal field spreading in time: monopolar electrode (a), bipolar electrode (b)
The nse of a monopolar electrode in one application causes destruction up to 5 mm long (Fig. 5a). It should be noted, that the destruction zone has a hemispherical
llopiiiiiHiiiiH MoiioiiojiHpiioï Ta fiiiiojiapiioï Cox-Mazoafijiauiï 11a ociioui K.;iiiii>mnx .aairnx Ta MaTOMai'H'iiioro Mo^ojiiouaiiiiM 59
Tafwi. 1 Thorriio-oloctrical parameters of model materials
Material Density, p (kg/m3) Electrical conductivity, a (S/m) Thermal conductivity, k (W/(m-K)) Specific heat, c (J/(kg-K))
Left atrium 1200 0.225 0.531 3600
Platinum-iridium 21500 4-106 71 132
Acrylic plastic 1190 10-6 0.18 1470
Aii- 1.225 0 0.028 1006
Blood 1000 0.667 0.543 4180
shape arid it is not uniform along the depth of a heated tissue. A heating maximum is observed at depths of up to 1 mm from the tip of the electrode.
By the use of bipolar clamp, one application with duration up to 30 s is enough to create uniform in depth and thickness thermal destruction along the entire length of the pulmonary vein (Fig. 5b). The heating zone is uniform throughout the thickness of the heart wall. There are no pronounced maxima, as with monopolar ablation. Also, there is practically no temperature gradient. Only at the edges of the sample can a slightly lower heating temperature be observed. That is duo to the cooling by ambient air flow.
4 Discussion
Based on clinical data (Fig. 2 and Fig. 3) it can be said that monopolar ablation scar line isn't uniform in width and depth. It is a common canse of the recovery of pathological signals conduction from the vein to the atrium. Also it can lead to the formation of conditions for the generation of arrhythmia substrate in the middle of the destruction zone, around some scar. Therefore, for the formation of linear destruction along the entire length of pulmonary veins month dozens of ablation points must be applied.
With hemispherical shape of destruction for monopolar ablation, to ensure proper destruction transmnrality, the surgeon must overlap points of thermal damage by at least 30% (Fig. 6). The improper implementation of monopolar ablation is shown on Figure 6a. The destruction points practically do not overlap. The zone between two points of destruction which is heated to less than 50°C is marked in black. Here it's possible that over time electrical conductivity will be restored and arrhythmia as well. The minimum permissible overlap of destruction points (30%) is shown on Figure 6b. In this case, the tissue between
two points of destruction that is not heated to more
°
ficant. Therefore, the risk of restoration of electrical conductivity is considered not significant.
Width of the destruction, obtained by mathematical modeling of monopolar RFA is about 5 mm. The diameters of pulmonary veins are individually for each patient and average is 15,6±3,2mm [15]. A total length
±
tli a minimum overlap of destruction points in 30% it is necessary to carry out about 20 points of monopolar ablation. Application duration for each RFA point is about 30 s with power 30-40 W to ensure effective linear destruction. This means that it will take at least 10 minutes to isolate one pulmonary vein. That is why a total duration of the RFA procedure, taking into account the time for electrode positioning, will be about 25-30 minutes.
Time=30 s Surface: Temperature (degC) Contour: Temperature (degC)
-2 0 2 4 6
Length(mm)
(a)
Time=30 s Surface: Temperature (degC) Contour: Temperature (degC)
(b)
Fig. 6. Diagrams of destruction points overlapping: no overlaying (a), with the minimum allowed overlapping of 30% (b)
Bipolar RFA that simultaneously clamps the entire diameter of the pulmonary vein between branches lasts 30 seconds for one application. In most cases is sufficient for complete isolation of pulmonary veins.
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Стасюк Ю. П., Сичик M. М., Макоцм<шко В. В., Кравчук В. В.
Conclusion
Clinical data and mathematical modeling results are presented showing, that effective transmural destruction can be achieved with both monopolar and bipolar instruments for open-heart ablation. However, bipolar ablation has some advantages for pulmonary veins isolation. First of all. this is a quick and effective method. It allows creation of continuous linear destruction along the entire line of pulmonary veins month. At the same time, as shown by mathematical modeling, when using a bipolar electrode, a uniform thermal destruction is observed. The temperature gradient is minimal, unlike in monopolar ablation. In this case, it is enough to perform only one application of radiofrequency current energy to create a block of conductivity. That reduces the duration of the procedure to several minutes. The limitation of bipolar ablation method is the convenience of its using only for the isolation of pulmonary veins on the heart surface. A monopolar electrode for RFA is the method of choice in case of linear ablation in heart chambers cavity.
References
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[2] Cox .1. L., Schuessler R. B., D:Agostino H. .1. Jr. Stone C. M., Chang B. C., Cain M. E., Corr P. B., Boineau .1. P. (1991) The surgical treatment of atrial librillation. 111. Development of a delinitive surgical procedure. The ■Journal of Thoracic and Cardiovascular Surgery, Vol. 101, Iss. 4, pp. 569-583. D01:10.1016/S0022-5223(19)36684-X.
[3] Doss .1. (1982) Calculation of electric Holds in conductive media. Medical Physics, Vol. 9, Iss. 4, pp. 566 573. DU1:10.1118/1.595107.
[4] Fedorova O. (2013) Vsemyrnaia orhanyzatsyia zdravookhranonyia podtverzhdaet hlobalnuiu spydomyiu fybrylliatsyy predserdyi [World Health Organization conllrms global epidemic of atrial librillation]. Ukrainskyi medychnyi zhumal [Ukrainian medical journal/. [In Russian].
[5] Labonte S. (1994) Numerical model for radio-frequency ablation of the endocardium and its experimental validation. IEEE Transactions on Biomedical Engineering, Vol.41, Iss. 2, pp. 108 115. DOl: 10.1109/10.284921.
[6] Lawrance C., Henn M., Damiano .Ir R. (2015) Surgical ablation for atrial librillation: techniques, indications, and results. Curr Opin Cardiol, Vol. 30, Iss. 1, pp. 58-64. DOl: 10.1097/HCO.0000000000000125.
[7] Perez .1., Oonzalez-Suarez A., D:avila A., Borjano E. (2018) RF-onorgisod intracoronary guidewire to enhance bipolar ablation of the interventricular septum: in-silico feasibility study. International .Journal of Hyperthermia, Vol. 34, Iss. 8, pp. 1202-1212. DOLIO.1080/02656736.2018.1425487.
[8] Saint L., Lawrance C., Okada Sh., Kazui T., Robertson .1., et al. (2013) Performance of a Novel Bipolar/Monopolar Radiofrequency Ablation Device on the Beating Heart in an Acute Porcine Model. Innovations (Phila), Vol. 8, Iss. 4, pp. 276-283. D01:10.1097/lM1.0b013o3182a77f2b.
[9] Schutt D., Borjano E., Haemmerich D. (2009) Elfect of electrode thermal conductivity in cardiac radiofrequency catheter ablation: a computational modeling study. Int .1 Hyperther. International .Journal of Hyperthermia, Vol. '25, Iss. 2, pp. 99-107. DOLIO.1080/02656730802563051.
[10] Siebermair .1., Neumann В., Risch F., Riesinger L., Vonderlin N., et al. (2019) High-density Mapping Oui-dod Pulmonary Vein Isolation for Treatment of Atrial Fibrillation - Two-year clinical outcome of a single center experience. Scientific Reports, Vol. 9, Article number: 8830. D01:10.1038/s41598-019-45115-0.
[11] Sychyk M. (2017) Catheter radiofrequency ablation of cardiac arrhythmogenic zones of improved efficiency and safety. PhD Thesis. Faculty of Biomedical Engineering National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Kyiv, Ukraine.
[12] Tungjitkusolmun S. (2000) Finite element modeling of radio-frequency cardiac and hepatic ablation. PhD Thesis, University of Wisconsin.
[13] Voeller R., Zierer A., Lall Sh., Sakamoto Sh., Schuessler R., et al. (2010) Efficacy of a novel bipolar radiofrequency ablation device on the beating heart for atrial librillation ablation: A long-term porcine study. The .Journal of Thoracic and Cardiovascular Surgery, Vol. 140, Iss. 1, pp. 203-208. D01:10.1016/j.jtcvs.2009.06.034.
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Пор1вняння монополярно!" та бшоляр-hoï Cox-Maze абляци на основ! клшь чних даних та математичного моделю-вання
Стасюк Ю. П., Сичик M. М., Максшитко В. В., Кравчук Б. Б.
Мета: Сьогодш радючастотпа абляц!я серця е золотим стандартом для радикального xipyprinnoro л1-куваппя р1зпих тишв порушепь ритму серця. Метою досл1джеппя був пор1впялышй апал!з мопополярпих та бшолярпих електрод!в для абляци шд час операцп па в!дкритому серц! (Cox-Maze абляц!я).
Матер1али i методи: Апал1з проводпвся па основ! клнпчпих дапих та за результатами математичного мо-делюваш1я. За допомогою системи електроапатом1чпого картографувашш зон радючастотшн абляци була створена трпвпшрпа модель л!вого передсердя. Потам па
Пор1вняння монополярно! та бшолярно! Сох-Магеабляцй на основ! клшчннх даних та математичного моделювання
61
цю модель було накладено карту потенц1ал1в, яка представляв амшптуду електрично! активноста мюкарда. Основними параметрами для анал!зу шншчних даних були: амшптуда електрично! активноста мюкарда; ширина та глибина зони абляци; безперервшсть лшп абляци. Для математичного моделювання було використа-но програмне забезпечення COMSOL Multiphysics 5.4. Створено дв! Bapian,ii математпчно! модел! для монополярно! та бшолярно! абляци. Основними параметрами анал!зу результатав математичного моделювання були: д!аграми поширення терм!чнпх пол!в; розм!ри зони де-струкцн; трпвалкть процедури абляци.
Результати: Для Cox-Maze абляци можпа викори-стовувати як монополярш, так i бшолярш електроди. Однак карта потенц!ал1в та математпчне моделювання показали, що при монополяршй абляци зона деструкци мае нашвсферпчну форму, а лшя рубця не р!вном!рна по глпбпш зони виливу. Це може призвести до в!днов-лення патолопчних сигнал ¡в в!д легеневих вен до пере-дсердя. Нер1вном1рний розподш терм!чних полш з ч!тко вираженим максимумом зб1лынуе ризик випаровувапня та мшровибух1в. У той же час тривалють монополярно! абляци зпачпо б!лыпа.
Висновок: В представлешй робот! показано, що 6i-полярна абляц!я мае переваги при ¡золяци легеневих вен. Цей тип електрод!в дозволяв створити ефективну та безпечну р!вном!рну терм!чну трапсмуральпу дестру-кц!ю лише з одним застосуванням енерги радючастотно-го струму.
Ключовг слова: серце; аритм!я; радючастотпа абля-ц!я; монополярний електрод; бшолярний електрод; математпчне моделювання
Сравнение монополярной и биполярной Сох-Maze абляции на основе клинических данных и математического моделирования
Стасюк Ю. П., Сычик М. М., Максименко В. Б., Кравчук Б. Б.
Данная работа посвящена сравнительному анализу монополярного и биполярного методов радиочастотной изоляции легочных вен при абляции на открытом сердце. Для исследования использовались клинические данные и математическое моделирование. С использованием системы электроанатомического картирования (Ensite Velosity, Abbott, USA) построено трехмерную модель предсердия пациента и наложено на нее карту потенциалов. Проанализированы амплитуда электрической активности ткани миокарда, а также ширина и непрерывность линии абляции. В среде COMSOL Multiphysics разработано математическую модель процедуры радиочастотной абляции (РЧА) сердца. Проанализированы особенности распространения термических полей при монополярной и биполярной РЧА. Показано, что биполярная абляция имеет преимущества при изоляции легочных вен. Она позволяет осуществить эффективную, безопасную, однородную, трансмураль-ную электротермическую деструкцию стенки сердца при одной аппликации радиочастотного тока.
Ключевые слова: сердце; аритмия; радиочастотная абляция; монополярный электрод; биполярный электрод; математическое моделирование
