Transcript return electrode

Electrosurgery
Units
ELECTROSURGICAL
UNITS (ESUs)
Jclemens (2009), electrosurgery [photograph]. Retrieved from https://commons.wikimedia.org/wiki/File:Electrosurgery.jpg
Electrosurgery: Overview
•
•
•
•
Electrical arc used in surgery (≈3000C)
Usually 1Mhz, 300W max.
Modes
– Cut
– Coagulation
– Blend
Probes:
– Monopolar
– Bipolar
ESU Summary

Modern ESUs have great flexibility, control, and feedback
measurements to control cutting and stop bleeding (coag).

Additional functions include fulguration and dessication

BMET Tests include power measurements by simple devices such
as EWH tester, or sophisticated tests by ESU Analyzers

All testing is on fixed loads that don’t resemble actively changing
tissue resistance, so surgeons may still complain

Failure modes are most likely “traumatized” parts such as power
cords, leads, and output power stages (or power supplies)
General Surgery

The electrosurgical unit (ESU) is generally used
in surgery.

The laser is less efficient, less powerful, more
costly, more bulky in the operating room

The skill of the surgeon is a major factor in the
selection of any surgical “knife.” (metal, ESU, or
laser)
ELECTROSURGICAL UNITS
•
The Electro Surgical Unit (ESU) cuts tissue by heat,
created from radio frequency (RF) currents in the
range of 100 kilohertz to 5 megahertz.
•
Surgical Functions:
•
•
•
•
•
Incisions (cutting deep into tissue)
Excisions (removing surface growths)
Coagulation (stopping blood flow)
Desiccation (drying tissues)
Fulguration (charring tissues)
Efficient, Powerful, Economical
•
ESU is the most efficient, powerful, and economical of
the thermal knives presently available.
•
It is most widely used in general surgery and in
cutaneous surgery.
•
It is capable of fast cutting through massive tissue and
of effective hemostasis (stability).
•
Adverse side effect is thermal tissue damage.
Bovie Operation 1

Transformer steps up the line voltage, which is then applied
across a spark-gap gas tube.

High-voltage AC peaks ionize gas in the spark-gap, making low
resistance and high current.

This drops the voltage and extinguishes the gas, which now
has high resistance again, so cycle starts over.

This “limit cycle” repeats at RF frequencies, where oscillation
frequency is set by series capacitor and primary coil.
Bovie Operation 2

When a return plate is used in surgery, the voltage is
taken off the primary coil shown in the figure.

The Oudin coil is a secondary coil that increases the
voltage by transformer action, so that fulguration can be
done without the return electrode.

For other surgical procedures (cutting) the patient
return plate is used.
Bovie Operation 3

During surgery, the RF current exits the relative sharp
electrode, dissipating between 50 and 400 W of power
into the tissue to make an incision.

The cutting electrode is about 0.1 mm thick and contacts
several millimeters of the tissue.

Voltage of several thousand volts sets up a line of small
sparks and raises the tissue temperature, such that the
tissue separates as the cells vaporize.
Bovie Operation 4

The electrode—tissue interface is
illustrated below.
Tissue Vaporizing

The cells themselves form capacitors with a
conductive electrolyte inside separated by a
nonconductive membrane from the interstitial
fluid (fluid between cells)

That membrane passes the RF currents into the
cell, causing it to vaporize.
Optimal Cutting Currents 1

If the voltage is high enough and is passed quickly enough through
the tissue, the thermal damage is almost imperceptible.

However, if one goes slowly or if the voltage is too low, thermal
tissue damage will result.

Surgeons might tell a BMET the cut isn’t “smooth” – they operate by
feel, and there are several “mixes” of AC current

This could mean the ESU RF power output stage has damage
Optimal Cutting Currents 2

To achieve hemostasis, a certain amount of
damage is desirable
• seals the wound, or “cauterizes”

The control of this factor is key to good surgical
technique using the ESU.

Modern ESU’s have complex RF modulation controls
Modern ESU RF Generators

An RF oscillator still forms the basis for a modern ESU, but
the AC waveform is modulated to give more control:
• Cut mode — Pure sine wave, for cutting with the least
•
•
•
coagulation (allows bleeding)
Coag mode — Pulsed sine wave, low-duty cycle, for
coagulation of bleeding tissue (stops bleeding)
Blend 1 mode — Modulated sine wave, for coagulating
as the tissue is cut.
Blend 2 mode — Modulated sine wave, for coagulating
as the tissue is cut.
Modulated RF Modes
• Coag mode—Pulsed sine wave, low-duty cycle, for
coagulating bleeding tissue.
• Blend 1 mode—Modulated sine wave, for coagulating
with cut, moderate duty cycle
• Blend
2 mode—Modulated sine wave, for coagulating
with cut, higher duty cycle
• Cut mode—Pure sine wave, for cutting with the least
coagulation.
Cut Mode

To cut tissue, the switch is usually set to
the cut position
• This connects the RF voltage to the amplifier,
which then delivers to the active electrode
1,000 to 8,000 volts peak-to-peak AC at from
100 kHz to about 2 megahertz. High-density
currents emerge from the active electrode to
do the cutting.
Return Current Dispersion 1

A blade electrode is moved through the tissue like
a knife to do the cutting.

Knife high-density currents disperse throughout
the conductive fluids of the body and return at a
low-current density to the patient return
electrode to complete the circuit back to the ESU.
• Return electrode has uniform contact to
avoid burns due to current concentrations
Return Current Dispersion 2
•
The return electrode is large
in area and gelled to keep
the skin resistance low and
the region cool.
Ground Isolation to Avoid Burns


The RF circuit is isolated from ground in newer ESUs
So if the patient’s body comes in contact with ground
(through a metal operating table, for example), there
should be no current flow (or burns)

In some older machines, there is no ground isolation, so
a patient touching grounded table could mean burns

But stray capacitance can still lead to some ground
current even in modern equipment

A tingle can result from touching active ESU patient
Cut Mode is Smoothest

In the cut mode, the ESU continuously delivers its
highest average power.

Thus, at every instant as the blade is moved along,
the tissue receives the same treatment. This results
in a smooth cut with no jagged edges.

Complaints that cut isn’t smooth could mean
modulation or power output problems (or operator
error in setting)
Coag Mode is Slower, Ragged

In the coag mode, the average power
delivered to the tissue is reduced from cut
mode, as AC pulses are “on” for only 15%
to 20% of the time instead of 100%
• A blunt (ball-tipped) electrode may also
be touched to the tissue
• The larger surface gives less-dense
current and doesn’t vaporize cells
Coag Mode

Automatically pulsing the AC voltage on 20% and off
80% slows the cutting process, and allows the heat to
propagate into the tissue to form the coagulum.

The depth of coagulation also depends on how long the
electrode contacts the tissue, because tissue damage is
caused by heat propagating into the tissue.

The edge of the cut will tend to be ragged, and some
browning of the tissue will be visible
Blend Modes 1 and 2

Blend modes 1 and 2 are used to cut and seal “bleeders”
simultaneously.

Blend 1: The pulsed AC ON time to OFF time ratio is
25/75 (25% duty cycle) so cuts more than Coag, but seals
blood better.

Blend 2: The pulsed AC ON time to OFF time ratio is
50/50 (50% duty cycle), so cuts half the time with less
“blood sealing” than Blend 1, but smoother cut.
Fulguration

This word means “lightning,” and this is exactly what the
fulguration spark is.

The air between the body and a sharp ESU pencil ionizes
when the electric field intensity exceeds 3000 kV/m, like
lightning

This ionized “plasma” chars the unwanted tissue

A return electrode is not needed at the high voltage, but
does improve current flow
Dessication

If the ESU needle electrode is introduced into a mass,
such as a vascular tumor, the currents will inject power
that raises the fluids to above 100 C, vaporizing and
dehydrating the lesion.

Since lipids and proteins require more than 500 C to
decompose, the surgeon has a mechanism to control
dehydration.

He or she keeps the temperature below 500 C so as to
not decompose the tissue while dehydrating it.
Minimum Testing:



Lights & alarms
Electrode monitoring
Arc on wet soap


Coagulation, Cut & Blend modes
Footswitch and handpiece operation
Surgical Techniques, Problems

The use of an ESU is very dependent on the surgeon preferences, so
one surgeon may see an ESU “fault” where another doesn’t.

The use of the ESU is a refined surgical skill, developed by practice.

ESU machines from different manufacturers have different waveform
shaping, amplitudes, duty cycles, and crest factors

A new surgeon or new ESU machine may see a machine problem that
really just requires re-training to the new machine (new settings)

The BMET must have an ESU tester to prove all modes are OK
ESU Performance Testing 1

Also, calibration of the power levels is done into a test load of fixed
resistance, even with sophisticated testers

But the actual tissue resistance depends upon its type as well as
the electrode contact area pressure against the tissue.

Some high-end ESU machines use active feedback to measure
resistance and adapt power output

All of these factors influence how much power actually gets into the
tissue.
ESU Performance Testing 2

The energy then getting into the tissue depends on the
duration of contact, which is his “fine control”

The effect of the RF current on the tissue cannot be
controlled completely from the ESU machine

It is controlled by the surgeon who has experience in the
procedures required and with the specific ESU being used
Patient Lead Classifications




Monoterminal — An ESU with one wire for patient contact.
Bi-terminal — An ESU with two wires for patient contact.
Active electrode — The electrode that delivers treatment to the surgical
field.
Patient electrode — The large surface area return electrode.

Monopolar electrode — An active electrode that uses a patient
electrode to complete the circuit.

Bipolar electrode—Two electrodes in close proximity and of
approximately the same size that are arranged so that the current tends
to be confined to a small region between the two electrodes (used for
precise coagulation, such as removing skin growths).
Lead Resistance-to-Ground Check

The resistance of both leads to ground should exceed
several megohms, as a check for ground isolation.

This will insure that any alternative path from the
patient to ground would not complete the circuit, to
prevent burn injuries at the point of patient-to-ground
contact.

Someone may set the return plate on a radiator, or it
may make contact with a grounded bed or operating
table.
Split Return Pad with Monitor

Serious burns can result from Patient Return Electrodes which
make poor body contact, or none at all.

One solution is to use two “split” return pads, and monitor the
resistance between the pads.

The ESU will show a fault and not deliver current If the
resistance doesn’t match the expected skin resistance – due to
poor contact of either pad (or shorted pads on a radiator!)

The Cost: Every safety “interlock” gives another mode of
instrument failure.
Common Problems:






Patient Burns
Broken cables
Faulty foot switch or hand piece
Blown power transistors
Adjustments (REM etc.)
Fires
Safety:

Safety is extremely important;



High power spark in O2 rich
environment
Possible high current density for patient
Improper use
Electrical safety methods:



Electrical Isolation
Return electrode resistance
REM electrode