Electrosurgery Plasma Discharge Characteristics

Zachary Holley, UC Irvine Physics & Astronomy Senior Thesis

Abstract - Electrosurgery is used for arthroscopy, tonsillectomy, and other surgical procedures requiring cutting or coagulation. For one type of bipole electrosurgery, an electrical generator drives high frequency (~400 kHz) alternating voltage to a surgical tool with three adjacent electrodes. These electrodes make up the driving and return nodes of the bipole setup, receiving and returning the current to the generator while reducing undesired current paths in the patient. Under circumstances where cutting is desired, the potential difference between electrodes causes an electrical discharge known as a plasma. The plasma discharge is formed by heating of fluids and tissue to form microscopic vapor bubbles within which the plasma is generated. This leads to surgical incision. The type of plasma discharge during this process comes in the form of either an arc or glow, depending on the electrical characteristics of the surgical equipment. Electrical and spectroscopic studies of electrosurgical discharges are reported.

I. Introduction

Surgical procedures utilizing electrically-enhanced devices have been in use since the early 20th century when Doyen [1] used spark-gap generators to treat dermatological blemishes. Sometime in the 1990's ,[2], ArthroCare Corp. began using electrosurgical devices that applied the usage of conductive saline solutions with the presence of plasma discharges. Surgical instruments that used these plasma discharges are now being used in a variety of surgical procedures. During operation, these discharges cause significant ablation on biological tissue as a means for surgical operation [3]. The effects of plasma-enhanced processes have also been reported by Woloszko and Gilbride, [4], following the study of radio frequency (RF) current being passed through localized portions of the body with the absence of an electrical discharge. In addition to the use of spark-gap generators in the beginning of the 20th century, the application of RF current to small areas for cutting, coagulation, and dessication of tissue is credited to Cushing and Bovie [5] for the work they performed in 1928.

For the purpose of this research, discharges used in surgical procedures that produce arcs with relatively higher current densities, or glows with relatively lower

current densities and higher voltage characteristics, are considered. The electrical characteristics of an electrosurgical device using plasma-enhanced operation through high-frequency (~400 kHz) current being driven to three adjacent electrodes at the tip of the device is reported. These electrodes are arranged in a parallel pattern, where there are two identical outside surrounding the thinner central one. In addition, the spectroscopic characteristics of the plasma discharge submerged in an aqueous 0.9% saline solution is recorded, showing that the discharges contain ions and neutral molecular and atomic radicals. These plasma discharges radiate optical emissions, with the largest percentage coming from excited sodium atoms to atomic hydrogen and hydroxyl (OH) radicals. The excited species being produced during the discharge originates via joule heating of the saline solution. A vapor layer forms within the saline solution, forming small bubbles that originate where the current is highest/most concentrated. These bubbles can be seen buoyantly rising to the surface of the solution during operation of the device.

Other recent work has looked into other high-energy electrical breakdown regimes within electrolyte solutions [6], [7], including characteristics that are similar to that which is relevant to electrosurgical plasma applications. This paper will report findings on the plasma discharge regimes and characteristics found in the Gyrus G3 Generator and Gyrus jPlasmaknife. The voltage/current characteristics of the system's operation as well as the possible plasma discharges that are being employed during operation, whether it be within the arc or glow regime, are studied. These findings are supported by spectroscopic readings of the plasma and computer simulation of operation within COMSOL Multiphysics. In addition, several methods of measurement and circuit development were used in order to record these findings will be discussed. Through these processes the goal is to better understand the operational aspects of plasma-enhanced electrosurgery as well as the operation and knowledge of working in a lab environment.

II. Analytic Theory

A. Theory of Operation

The system operates on the concept of the dielectric breakdown of the medium that it operates in. For the context of this project, the medium involved included air, 0.9% isotonic saline solution, and raw chicken breast. If the voltage difference between the electrodes at the tip of the surgical tool exceeds the dielectric breakdown value of the medium, a discharge will occur. The current that passes through the circuit delivers a joule heating factor. From this we can derive the predicted time until the vapor bubble is formed and a subsequent discharge occurs.

(1) R = ηl/A

(2) i = V/R = VA/ ηl

(3) j = i/A = V/ ηl

(4) P/Vol = ηj2 = ηV2/ η2l2 = V2/ ηl2

(5) ∆U = mCv ∆T + Lboil

(6) ∆U/Vol = ρ Cv ∆T + ρ Lboil = V2∆tboil/ ηl2

(7)

The derivation shows that when current (i) is passed through a resistive medium (ηl/A), heat is produced (∆U). This heat causes the vapor formation to occur. This is the predicted time until the formation of a vapor bubble where a discharge would form.

Adding the experimental parameters specific to the device used in this research (jPlasmaknife) and the conditions of operation (saline solution, generator settings, etc.) we are able to arrive at a predicted time value to achieve a formation of a vapor bubble.

∆tboil = (0.33 Ohm-m)(200x10-6 m)2/(480 V)2[(1000 kg/m3)(4186 J/kg C)(79 C) + (1000 kg/m3)(2.257x106 J/kg)]

∆tboil = 2.3x10-4 seconds to boil the saline solution.

The joule heating factor from the surgical tool heats the medium and causes vapor bubbles to form in ~230 μs. Within these bubbles the electric field is enhanced. This leads to the dielectric breakdown voltage to be reached, and a corresponding discharge to occur.

When looking at operation within the isotonic saline solution, the electrolytic properties of the solution aid in the conductive properties during operation. The generator acts as the source of the AC waveform and feeds current to the tip of the surgical tool. The tool is coated in the saline solution, where its conductive properties allow the current to easily flow. The current heats the solution, causing the vapor layer to form and thus allowing for a discharge to occur.

In the case of the raw chicken breast, the surgical tool is placed in direct contact with the surface of the flesh. When the system is engaged, the tool drives current into the

∆tboil = ηl2/V2(ρ Cv ∆T + ρ Lboil)

All values for water at STPη: Resistivity (Saline)l: LengthA: Areai: Currentj: Current densityV: VoltageP: PowerU: EnergyCv: Heat capacity Lboil: Latent heat

ρ : DensityT: Temperaturetboil: Time until boiling

point of contact between the electrodes at the tip. This allows for the joule heating to take place, and for vapor pockets to form. These pockets expand and eventually explode, which causes an incision within the flesh.

B. Computer Simulation

To visually demonstrate the electric field concentrations between the electrodes at the tip of the surgical tool with and without vapor bubbles present, simulations in COMSOL Multiphysics were conducted. These simulations show the configuration for the Gyrus jPlasmaknife and is created with its electrical properties in mind. Figure 1 shows the three electrode setup placed in a horizontal orientation. The medium of operation surrounds the tool, and the electric field strength in the area between and outside the electrodes can be seen (represented by the colors, blue being the weakest and red being the strongest).

Figure 1

In figure 1, the surgical tool is coated in the 0.9% isotonic saline solution with the presence of a vapor bubble. The electric field strength is noticeably larger within the vapor bubble (~4x106 V/m), and is high enough to induce dielectric breakdown and a subsequent discharge. There is a conversion effect of the displacement electric field that can be seen in the region surrounding the vapor bubble. The orthogonal displacement field is conserved, and as such the field strength in the remaining region between the electrodes is reduced, to compensate for the strengthened field within the vapor bubble. The other regions between the electrodes only reach a field strength of about 2.3x106 V/m, insufficient for discharge. This suggests that in order to form a plasma discharge, the existence of the vapor bubble is essential and will serve as the origin of the discharge itself. The electric field strength is also slightly enhanced at the initial point of curvature on the inside of the left and right electrodes, and this is expected to be the primary location for joule heating and vapor bubble formation. To fully understand what causes the electric field to be enhanced within the vapor bubble, we take a look at the effects of the electric displacement field in the presence of a dielectric,

D ≡ ε0E + P

Here D is the electric displacement field and is defined by the product of the relative permittivity of saline with the electric field produced by the electrodes added to the polarization density (macroscopic density of the permanent and induced electric dipole moments in the material).

This displacement field satisfies Gauss's law in a dielectric:

[Separated volume charge density into free+bound charges]

[Rewritten as a function of P]

[Divergence of P yields the bound charge density, ρb]

[Divergence of D yields the free charges density]

In figure 2 where the medium of operation is air, the electric field strength, at its strongest, is only about 2-2.3x106 V/m. This field strength is not sufficient for a discharge to occur. It can be noted that without the presence of the vapor bubbles in conjunction with the saline solution, a discharge is unable to be induced.

C. Molecular Spectra

The observation of the molecular spectra during the discharge process is also measured. During the discharge process, the atoms that comprise the material that the tool is operating in are put into excited states due to the energy being transmitted from the generator. When these atoms reach their excited states, they remain there for a very short amount of time. These excited atoms release energy in the form of photons to return back

Figure 2

to their ground states. The energy and wavelength of these photons is emitted in the visible spectrum, and offers an understanding of the chemical makeup of the discharges.

III. Experimental Apparatus

A. Electrosurgery Equipment

This research focused on the operational characteristics of the Gyrus G3 Workstation (Electrical Generator) and the Gyrus jPlasmaknife. The generator's operational characteristic are the two pedals that can be seen in the image. These pedals represent the "Cut" (yellow pedal) and "Coagulate" (blue pedal) modes. The cut mode operates in a ratio of cutting and coagulation. During a surgical procedure, during incisions, the device simultaneously coagulates the surgical site in order to control the flow of bleeding and diminish the need for the assistance of irrigational tools. The ratio at which the machine does this can be changed to suit the operator's needs. These modes as well as the power output, operational mode, and Cut/Coag ratio are all specified using the digital controls. The generator has several different modes of operation depending on the necessity of the surgical

Image of Gyrus G3 Workstation from� http://www.terapeak.com/worth/acmi-gyrus-7035-3001-g3-workstation/

281598116608/

operation. The generator operates at 400 kHz, 100-480 V (depending on mode), and 21-40 W (depending on selection).

In addition to the electrical generator, the other key piece of the electrosurgical system is the surgical tool itself. This tool, sharing a similar shape to a wand, acts as the primary incision device in the system. The tip of the tool has a series of three parallel electrodes, a slim center electrode (active line), and two identically sized outer electrodes on either side of it (return line). The tool is also outfitted with an irrigation tube at the point of contact near the electrodes.

Figure 3

The electrical characteristics of the electrosurgical system are measured using a Tektronix TDS2014 Oscilloscope. Using this device the voltage and current characteristics of the circuit are measured and observed. The four channel oscilloscope allows for simultaneous voltage/current measurements of the three separate leads at the tip of the electrosurgical tool.

In order to safely measure the voltage on the oscilloscope, a voltage divider system is developed and used to step down the voltage from the surgical generator in order to coincide with the safety ratings of this particular oscilloscope. This circuit system consisted of three separate voltage divider circuits, each using a 11 M-Ohm resistor in series with a 10 k-Ohm resistor that is parallel with the oscilloscope. This allowed the voltage to be reduced by approximately 1/1000 (corresponding to a display value on the oscilloscope in the mV range) of its output value from the generator, allowing for safe observation of the voltage values while also maintaining the waveform properties that the generator provides. In order to properly introduce this voltage dividing system into the electrosurgical circuitry, the protective insulation of the surgical tool was stripped away, and the wires corresponding to each lead at the tip were then individually directed into a separate voltage divider circuit. This voltage divider is set up so that the output of each circuit can be connected via BNC cable directly to a channel on the oscilloscope.

Acquiring accurate readings of the voltage and current waveforms that are produced from the generator is an essential step when isolating the actual discharge characteristics at the tip of the surgical tool. After obtaining an accurate and consistent waveform for the voltage of all three leads inside the surgical device, the current came next.

In order to measure the current in the circuit, an attempt to insert a "current-identifying" resistor was made. Using this method, a resistor of a known resistance is inserted into the circuit onto one of the leads. The "high" connection of the resistor (that is, the reading before passing through the resistor) is sent to one channel of an oscilloscope, and the "low" connection (reading that occurs after the resistor) is sent to another channel.

Image of Tektronix TDS2014 from http://www.tek.com/oscilloscope/tbs1000-digital-storage-oscilloscope

Figure 4

Using Ohm's Law, the difference in voltages between these two connections can be measured, and knowing the resistance, a current measurement can easily be obtained. However, after several attempts to acquire an accurate and even plausible current estimate, it was clear that this method was not able to work. Due to the lack of knowledge of the internal circuitry of the electrosurgical generator, and to a consistent 60 Hz pickup from extraneous sources, the current reading was often sporadic, hovering at values that did not seem to meet initial theoretical expectations. These values were often a factor of ten higher than were originally expected. At this point it was clear that another method would need to be used.

With the assistance of Dr. Heinrich Boehmer, the idea to use what is known as a Pearson Probe came to fruition. A Pearson Probe is a device that operates similar to an inductor. A coil of wire is wrapped around a ferromagnetic iron core. The probe includes two terminals which are connected to a BNC output at the base of the monitor. This allows the monitor to be connected easily to the oscilloscope, a high-impedance (1 megohm) input. A diagram of what the monitor looks like is shown below.

From http://www.pearsonelectronics.com/current-monitor-instruction-guide

The current to be monitored are from the leads of the electrosurgical tool. When the electrosurgical generator is operated, a current flows through this lead and induces a voltage in the current monitor, due to Faraday's Law. This voltage is then translated to the display of the oscilloscope. What can now be seen is an accurate waveform of the current that is flowing through the circuit.

In order to quantify the current that is being seen on the oscilloscope, as it is just displaying a voltage, Pearson stamps each of their current monitors with a conversion factor. This conversion factor is in the form of Volts/Amp. That is, for a certain number of volts (as seen on the oscilloscope), there is a corresponding current value. For this project, the model that is being used is the Model 410. This model, according to the Pearson

Figure 5

literature, states that the conversion is 0.1. This means that there are 0.1 Volts for every Amp, or 100 mVolts/Amp. However, it is required to ensure that the accuracy of the probe is reliable for the frequency ranges that the generator is operating in. In order to complete this, a calibration test needs to be performed.

The calibration test was a simple setup using a waveform generator. A BNC cable was connected directly from the waveform generator to 50 Ohms and an oscilloscope. This allowed for reliable measurement of the current in the circuit using Ohm's Law. The Pearson monitor was then inserted into this circuit, enclosing the line coming from the waveform generator, and was connected to a second channel on the oscilloscope. This calibration test did not include a 50 Ohm terminator for the Pearson probe connection. Acquiring data over a wide frequency range (40 KHz-20 MHz), the accuracy of the Pearson monitor's conversion value can be seen over this range. To do this, 10 data points were collected. The current in the circuit was obtained through Ohm's Law on the main line from the waveform generator using the displayed voltage and the known 50 Ohm resistor. The Pearson voltage was then applied to this current and converted into a mV/A value and then compared to the stamped value on the monitor itself (which is 100 mV/A). Below you will see a table displaying the values obtained in each of the 10 frequency levels, and two graphs representing the experimental conversion factor that the Pearson monitor operated in within those specific frequencies. One graph shows the relationship at lower frequencies, and the other shows that at higher frequencies.

Frequency (KHz)

Main Line Voltage (Into 50 Ohms) (V) Current (mA)

Pearson Voltage (Open) (mV)

Pearson Voltage/Amp (mV/A)

40 0.6 11 1.36 123100 1.4 27.7 2.88 103.97200 2.72 54.5 5.44 100400 5.84 116.8 11.3 96.71000 13 260 26.4 1012000 13 260 26.4 1016000 3.6 72 8.16 11310000 1.68 33.6 4.32 128

20000 2.16 43.2 7.6

175

Figure 6

0 500 1000 1500 2000 25000

20

40

60

80

100

120

140123 mV/A103.97 mV/A

100 mV/A 96.7 mV/A 101 mV/A101 mV/A

Pearson Probe Calibration (Lower Frequency)

Frequency (KHz)

Pear

son

Volta

ge/A

mp

(mV/

A)

0 5000 10000 15000 20000 250000

20406080

100120140160180200

96.7 mV/A

101 mV/A101 mV/A

113 mV/A128 mV/A

175 mV/A

Pearson Probe Calibration (Higher Frequency)

Frequency (KHz)

Pear

son

Volta

ge/A

mp

(mV/

A)

As can be seen from the graph in figure 8, at frequencies above 2 MHz, the Pearson monitor begins to significantly lose accuracy, showing conversion values up to 75% higher than what the model states at 20 MHz. In figure 7, at frequencies below 40 KHz, a similar effect seems to occur. The frequency range of around 100 KHz to about 2 MHz shows the most consistency. In the context of this project, where the electrosurgical generator operates in a frequency range of 200-400 KHz, the Pearson monitor that was used was calibrated at a 96.7 mV/A sensitivity rating.

Figure 7

Figure 8

IV. Measurements and Analysis

A. Voltage Measurements

In order to understand the voltage characteristics when the system is in operation, voltage measurements were conducted in the "Cut" mode at a duty cycle of 80% "Cut" 20% "Coag". The power setting for these measurements was established at 40 Watts. The actual measurement of the voltage trace of each of the three leads was obtained in the startup phase only, which is the time period of operation before any actual discharges occur.

When conducting the measurements, the protective insulation of the jPlasmaknife was removed to expose three wires within, each connected to a different lead at the tip of the surgical tool. The voltage divider circuits were implanted into the surgical circuit using these wires in order to effectively reduce the output voltage of the generator to be within the defined operational safety parameters of the oscilloscope.

Figure 9 shows an image of the voltage traces of each of the three leads displayed on the oscilloscope. The GREEN trace corresponds to the left return lead, the PINK trace corresponds to the center active lead, and the BLUE trace corresponds to the right return lead. The y-axis of the screen measures the peak-to-peak voltage of each of the leads. It is split into 8 divisions, with each division indicating 100 mV for the green and blue traces, and 500 mV for the pink trace. The x-axis of the screen measures time. There are also 8 divisions, each measuring 1 microsecond.

Figure 9

From this trace, we can see that the active lead of the tool is approximately 5 times the voltage of that of the left lead and eighteen 18 times that of the right lead. The voltage differences between the leads may give insight into what path the current is flowing within this experimental configuration and duty cycle. The current path may differ between operational modes and return path. The behavior exhibited in this trace was consistent throughout subsequent tests. The blue trace was the most erratic trace observed. It often remained at a voltage of below 100 mV for each test and as can be seen the waveform trace itself is very coarse.

The phase differs between the center lead and left return lead (green) by 180 degrees, indicating a reverse polarization between the two. The phase difference relationship with the center lead and the right return lead (blue) is roughly 135 degrees, a strange result and one that this research has been unable to understand. This occurrence can be the product of an error or malfunction in the system with respect to this specific lead or a deeper convolution of design. Further tests on different surgical tools with similar configurations could prove to expose its existence.

B. Current Measurements

In order to obtain further knowledge into the discharge characteristics of the system, current measurements were obtained. These current measurements all occurred with the same operational parameters as that of the voltage measurements. The calibrated Pearson probe was used by inserting the wire connecting the jPlasmaknife to the generator through its center. By reading the voltage display on the oscilloscope from the probe, the current of the circuit is realized.

The waveforms on the oscilloscope for the current measurements correspond to the voltage of the center lead (pink trace) and the voltage from the Pearson probe (blue trace). The per-division scales are 500 mV and 200 mV for the center lead voltage and Pearson voltage, respectively.

Figure 9

Using these two waveform traces, some characteristics of the current can be seen. The spikes of the blue traces in each of the images are the discharges occurring during the operation of the system. In figure 10, the amplitude and rate of these discharges varies noticeably. The largest current amplitude in this trace was ~6.2 Amps. In figure 11, only two discharges occurred in this time segment of the waveform. The largest current amplitude in this trace was ~3.3 Amps. A voltage droop during the discharges can also be seen in both of the traces. The voltage drops between 25-33% in the existence of the discharge.

Observing the amplitude of the blue traces to achieve current:

Largest spike: ~600 mV → 6.2 Amps

Largest spike: ~330 mV → 4.2 Amps

Figure 10

Figure 11

The discharge current amplitude and rate of formation seems to vary with each test. This is most likely a result of the inconsistencies of the experimental parameters with each test. The saline coating on the tip of the surgical tool changes its area of coverage as the saline boils off. When operating on the chicken breast, the composition of the tissue changes with each subsequent test as the proteins are denatured. Debris collecting between the electrodes was also a prevalent occurrence, which had an impact on the performance of the tool. The generator itself would sometimes cease to operate if the debris obstruction between the leads was large enough.

V. Conclusion and Further Study

Plasma formation occurs on timescales of approximately 100 microseconds. The voltage and phase differences between the electrodes at the tip of the surgical tool during the startup phase differ between path (center-left, center-right). The current amplitude of the discharges were between 3.3-6.2 Amps, seeing a corresponding 25-33% voltage reduction in the circuit. The discharge rate of the plasma is intermittent in addition to producing inconsistent current amplitudes.

As for the specific type of plasma that the system is utilizing in operation, further study and observation is needed. Using an oscilloscope with longer data stream collection capabilities, a more in-depth analysis of the discharges can be conducted. When looking at the time period of the discharges (<2 microseconds) and studying the behavior of the voltage, further understanding and data regarding the nature of the plasma can be collected. This can lead to potential advancements in isolating which plasma types may be more beneficial for different surgical procedures.

References

[1] Intracranial tumors,” Surg. Gynecol. Obstet., vol. 47, pp. 751–784, 1928.E. Doyen, “Sur la destruction des tumeurs cancereuses accessibles,”Arch. Elect. Med. Physiol., vol. 17, pp. 791–795, 1909.

[2] Stalder, Kenneth R., Donald F. McMillen, and Jean Woloszko. "Electrosurgical plasmas." Journal of Physics D: Applied Physics 38.11 (2005): 1728.

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[4] G. K. Feld, “Evolution of diagnostic and interventional cardiac electro-physiology: A brief historical review,” Amer. J. Cardiol., vol. 84, no. 9A, pp. 115R–124R, Nov. 1999.

[5] H. Cushing and W. T. Bovie, “Electrosurgery as an aid to the removal of intracranial tumors,” Surg. Gynecol. Obstet., vol. 47, pp. 751–784, 1928.

[6] V. D. Volovik, G. F. Popov, and A. L. Shkilev, “Dissipative processes in nanosecond electric discharges in liquid electrolytic solutions,” Sov.Tech. Phys. Lett., vol. 13, no. 9, pp. 448–450, Sept. 1987.

[7] V. G. Zhekul and G. B. Rakovskii, “Theory of electrical discharge formation in a conducting liquid,” Sov. Phys. Tech. Phys., vol. 28, no. 1, pp. 4–8, Jan. 1983.

[8] Pearce, J. A. (1986). Electrosurgery.