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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_605_Библиотеки_им_академика_М_И_Перельмана.pdf
241Classical (Open) Surgery
conversion of colloidal systems from sol to gel state (see Section 6.3:
Ultrasound Dissection). This effect can be seen while boiling an egg. In
medical common speech the term coagulation is often used as a synonym
for hemostasis by high-frequency current applications incorporating a series
of thermal effects like devitalization, coagulation, and desiccation. The
thermal coagulation effect must not be mixed up with the synonym for
hemostasis. If the tissue temperature is raised to above 60˚C a transition in
the cell structure takes place. This effect can be characterized by a change
in tissue color, by the formation of derivatives of collagen, and by tissue
contraction. Tissue contraction can be used to seal bleedings from minor
blood vessels
[7].
6.2.4 Thermal Desiccation
Thermal desiccation refers to the event of the drying out of a cell, also
known as dehydration. At temperatures of up to 100˚C intra- and extracellular water dehydrates causing cells to dry out and shrink. The contraction
of the tissue in addition to the glue effect, caused by the dehydration in
conjunction with the derivatives of collagen formed during tissue coagulation, leads to effective sealing of blood vessels to a diameter of 0.5 mm. For
the hemostasis of larger sized vessels, additional mechanical compression,
e.g., by high-frequency forceps, is necessary. Two issues during highfrequency application can be associated with thermal desiccation. The glue
effect not only is useful to seal vessels, but can also cause tissue to adhere to
the active electrode. Tearing away of the adhering electrode might do further damage to the target tissue. Furthermore, the high electrical resistance
of dehydrated tissue acts as an isolator between the active electrode and the
target tissue and can hinder further ther mal effects, such as the ignition of
an electric arc necessary to cut tissue. Both mentioned cases can lead to
severe problems during high-frequency applications.
6.2.5 Thermal High-Temperature Effects
Thermal effects that are present at temperatures above 100˚C are classified
in the group of thermal high-temperature effects (
temperature effects are essential for cutting tissue as tissue can only be
separated when tissue temperature is raised above 100˚C. To heat tissue
above the boiling point of water, the tissue has to be dry and high power
density is required. This can be achieved either with high-frequency
current with a peak voltage of above 200 V generating electric arcs
Fig. 6.27). High-
242 Biomedical Engineering in Gastrointestinal Surgery
between the active electrode and the target tissue or by the use of laser
sources.
carbonization and vaporization, showing the different thermal effect
zones for a monopolar active electrode. The localization of the thermal
effect zones start at the active electrode: vaporization zone, carbonization
zone; followed by the low-temperature effects starting from the highest to
lowest temperature effect as described above (
Fig. 6.27B gives an overview of the high-temperature effects
Fig. 6.27).
6.2.6 Carbonization
Carbonization is the first of two high-temperature effects and is defined
as partial oxidation of tissue hydrocarbon compounds if the temperature
exceeds 200˚C and the tissue is within an oxygen-containing atmosphere.
As temperatures above 100˚C cannot be achieved by endogenous heat,
exogenous heat in the form of electric arcs or laser has to be inducted
into the desiccated tissue. Tissue carbonization produces smoke that limits
the visual sight to the target area. Furthermore, inflammable gases can be
present in the smoke, which mixed with oxygen can lead to fires or
explosions if electric arcs or lasers are ignited. The effect of carbonization
is undesired but has to be passed to achieve the cutting effect during
tissue vaporization when temperatures of above 500˚C are reached
[7].
6.2.7 Vaporization
The desired tissue effect for cutting tissue by high-frequency current or laser
is achieved when temperatures of above 500˚C are reached within an atmosphere containing oxygen. These temperatures are sufficient to evaporate the
tissue structures. The downside of the vaporization of tissue is the smoke
created with the risk of explosion or fire breakout when sparks are present.
6.2.8 Principles of Electrosurgery
Electrosurgery uses radio-frequency, respectively, high-frequency alternating current, to achieve thermal effects within biological tissue as
described in the previous section. The alternating current within the
active electrode is used to activate oscillating movements of ions within
cells and therefore raises the intracellular temperature due to intracellular
frictional forces. The heat input is used for the modification or destruction of tissue, leading to ablation, hemostasis, and cutting effects.
243Classical (Open) Surgery
6.2.9 Physical Theories of Electrosurgery
Biological tissue acts in several different ways if an electrical current is
introduced within. Some of the effects can be used for medical diagnosis
or therapy. This chapter explains the physical principles of the effects
involved in electrosurgical applications. The main physical effects occurring when an electrical current is introduced into living tissue are the
galvanic, faradic, and thermal effects. Depending on the polarity of the current and on the frequency for alternating currents, different physical
effects take place. As for electrosurgery, the thermal effect is most relevant. Other effects, such as the galvanic and faradic effects, are mentioned
to give the reader the understanding about the advantages of highfrequency current over other electrical current forms.
Direct current or low-frequency current that is introduced into tissue
leads to an ionic displacement, which is called galvanic effect. This effect
has no relevance for high-frequency applications. Nevertheless, this effect
can be used to introduce medication into specific areas of the human
body; this therapy is known as iontophoresis
The faradic effect takes place when low-frequency alternating current
with frequencies from 1 to 20,000 Hz is applied to biological tissue. This
type of current brings anions and cations to oscillate in synchrony to the
applied current leading to action potentials within the cells. These action
potentials are responsible for a depolarization of muscles and nerves,
resulting in muscle fasciculation that is unpleasant and dangerous for the
patient. The maximum muscle stimulation level lies between 10 and
100 Hz, which makes an electric shock from a standard electric outlet
voltage of 230 V at 50 Hz dangerous. The faradic effect is used for electric
stimulation therapy in patients with muscular paralysis. For highfrequency application these frequency bands must be avoided at all times.
The thermal effect makes use of alternating currents of more than
300 kHz. Frequencies of 300 kHz or more are considered as highfrequency, given the name high- or radio-frequency application. The
pulses of currents with such a high-frequency are able to evade the galvanic
and the faradic effect. The applied current stimulates the ions within the
cells to oscillate. Electric energy is converted into oscillation movement of
the cell particles which is mechanical energy. The frictional forces between
the oscillating particles, especially large proteins, convert the mechanical
energy into thermal energy which leads to an increase in intracellular
temperature. The conversion of electrical energy to thermal energy via
mechanical energy takes place extremely quickly and without losses.
[8].
244 Biomedical Engineering in Gastrointestinal Surgery
According to the Joules Law the heat Q produced due to the current
flow within an electrical resistor is proportional to the converted energy
P in a certain time period Δt and represents the fundamental principle of
electrosurgical procedures (Formula
6.1).
Q 5 P 3 Δt 5 I
2
3 R 3 Δt 5
U23 Δt
R
; (6.1)
Q: heat,
P: converted energy,
Δt: activation time,
I: current,
R: resistance,
U: voltage.
The Joules Law can be converted to a formula that incorporates all
important electrosurgical parameters—current density j, specific tissue
resistance ρ, tissue volume V, and activation duration Δt.
2
Q 5 j
3 ρ 3 V 3 Δt; (6.2)
Q: heat,
j: current density,
ρ: specific tissue resistance,
V: tissue volume,
Δt: activation duration.
The current density is an essential electrical variable in regards to cutting tissue with a high-frequency current. The current density j is defined
as the current I flowing through a defined area A at a certain point in
time t. The current density and therefore the cutting behavior can be
affected by changing either the affected area or the current flow through
this area. Only when a cur rent density of 16 A/cm
trosurgery be conducted efficiently. As formula
2
is present, can elec-
(6.2) shows the heat input
into the tissue is proportional to the square of the current density.
Fig. 6.28 depicts the effect of the current density to the temperature rise
when the same current flow is applied through different tissue areas.
The temperature rise in
current densities ranging from 0.01 to 1 A/cm
Fig. 6.28 ranges from 0.004˚C to 40˚C, due to
2
.
Further more, two important variables for electrosurgery are the
specific electrical resistance ρ of the tissue and the volume V involved.
Biological tissue can be treated as an ohmic resistor. Values of specific
resistance vary strongly. Muscle tissue and tissue well supplied by blood
Classical (Open) Surgery
Figure 6.28 Effect of current density on rise in tissue temperature—the temperature
rise is proportional to the square of the current density. In the pictured diagram
a constant current is applied to three differently sized areas. The areas are variable in
size with a factor of 100 ranging from 25 cm
middle area and 0.25 cm
a temperature rise from 0.004°C to 40°C, which is a factor of 100
2
in the uppermost area. The reduction in area leads to
2
in the lowermost area to 1 cm2in the
2
. From MITI.
245
have low resistance values in the range of 160300 Ω. Tissue with low
fluid content as bone, cartilage, and fat have high specific resistance values
ranging from 500 to 1000 Ω.
The thermal distribution within the tissue in a given time is complex
as a series of combinations of thermal conductivity, convection, perfusion
of blood, and metabolic heat production have to be included in the consideration. Pennes introduced a simplified bioheat model in 1948 which
describes the heat transfer within living tissue including the blood transfusion and metabolic heat production for a small defined control volume
(Formula
6.3). His consideration was initially developed for a heat transfer
observation of a human forearm. The bioheat equation formulated in
1948 is considered the standard and most current literature is derived
from it due to its simplifying assumptions
ρm
Δh 1 ρc
f
@T
@t
5 P 1 Q
m
1 λ
1r@
[9].
@T
r
@r
@r
ρbðT 2 TbÞ; (6.3)
1 w
bcb
ρ: density of medium,
m
: mass which changes phase,
f
Δh: phase change enthalpy,
c: specific heat capacity,
T: tissue temperature,
t: activation time,
P: externally applied power density,
246 Biomedical Engineering in Gastrointestinal Surgery
Qm: basal metabolism,
λ: thermal conductivity,
r: distance from active electrode,
w: blood perfusion,
b: values of blood.
The Pennes bioheat equation includes heat gain mechanisms in the
form of metabolic heat and externally induced power and heat loss
mechanisms in the form of blood perfusion and heat transfer. The first
term of the equation on the left side depicts the amount of water or tissue
that runs through a phase change from liquid to gaseous. The second
term on the left is the rate of temperature increase in the control volume.
The first term on the right side represents the externally induced power
density, the second term is the heat produced by metabolism, the third
term characterizes the heat transfer from and to the control volume, and
the last term represents the thermal effect of the blood perfusion.
The equation is valid for application duration shorter than approximately 30 s. For electrosurgical applications the metabolic heat, heat transfer, and perfusion can be neglected. According to the bioheat equation the
surgeon has three means of controlling the effect of the electrosurgical
application which are the contact surface between the active electrode and
the tissue, the induced current density which is dependent on the contact
surface and the introduced current flow, and the activation time. Still today,
the equation is the base for more sophisticated applications
[10].
6.2.10 Electrosurgical Techniques
Electrosurgical applications make use of the physical theory explained in
the previous chapter to achieve the desired thermal effects. To avoid the
faradic effect and the related adverse effects, frequencies higher than
300 kHz are used. Temperature of up to 100˚C can be reached with the
implementation of endogenous energy. For tissue cutting purposes higher
temperatures have to be achieved. This is only possible if exogenous
energy is applied to the tissue by electric arcs. Electric arcs can be generated with a minimum peak voltage of 200 V independent from the flowing current. The major advantage of electrosurgical cutting over
mechanically cutting tissue with a scalpel is the hemostasis effect at the
edge of the incision which minimizes bleeding.
Electrosurgical applications can be conducted in two different forms—
monopolar and bipolar techniques are state-of-the-art. The setup for
Classical (Open) Surgery
247
both procedures is the same—an active electrode and a neutral electrode
have to be conductively interconnected with the human body, closing the
electric circuit to the high-frequency voltage source, the electrosurgical
unit (ESU). The current flow always follows the same route from the
generator to the active electrode into the target tissue and back to
the generator through adjacent tissue, departing the human body through
the neutral electrode. The difference between the two application forms
lies within the size and arrangement of the two electrodes. Tissue cutting
is only possible by means of the monopolar technique.
6.2.11 Monopolar Technique
Monopolar application is by far the most commonly used form, incorporating an active electrode with an extremely small surface area compared
to the neutral electrode which has a huge surface area. The small surface
area of the active electrode in conjunction with high current generates
very high current densities at the target tissue (
rent density is important for adequate thermal input to achieve the
desired coagulation or cutting effects. The current flows through the
body to the large surface return electrode which is placed on the skin of
the patient. Due to the large surface area the current density is considerably smaller and the thermal effect is negligible.
Nonetheless, severe burns at the contact site of the neutral electrode
may occur due to current leakage
(
Fig. 6.29B) with divided electrodes and active electrode monitoring lower
the risk of electrical damages but do not completely eliminate them
Fig. 6.29A). The high cur-
[11]. Modern electrode designs
[12].
Figure 6.29 Monopolar high-frequency technique for therapeutic heat introduction
into the target tissue. (A) The technique incorporates an extremely small surface area
active electrode compared to the neutral electrode which has a huge surface area. The
current flows through the body to the large surface neutral electrode, which is placed
on the skin of the patient. The desired coagulation or cutting effects are realized at the
active electrode due to the present high current density leading to the necessary high
temperatures; (B) state-of-the-art design of a return electrode. All from MITI.
248
Biomedical Engineering in Gastrointestinal Surgery
6.2.12 Electrosurgical Coagulation and Desiccation (Hemostasis)
Electrosurgical hemostasis is achieved by the means of applying a highfrequency current into the target tissue. The current is dispersed divergently within the tissue and the current density decreases with the distance
to the contact surface. As the tissue heating is proportional to the square
of the current density, higher temperatures are reached in proximity to the
contact surface. In close proximity to the contact surface temperatures of
up to 100˚C are reached, vaporizing intra- and extracellular water. The
hemostatic effect strives forward until the tissue loses its electrical conductivity due to dehydration and the formation of vapor covering the tissue.
As long as a peak voltage of 200 V is not reached, and the tissue is strongly
dehydrated, no further coagulation and desiccation is possible.
6.2.12.1 Impedance-Controlled Electrocoagulation
The effect of “self-insulation” during electrocoagulation can be significantly reduced by a computer-controlled output of the electrical energy.
By measuring continuously the tissue impedance/resistance at the tip of
the instrument (bipolar electrodes), the pulsed energy output can be
adapted in a way to avoid early carbonization
specifically designed instruments is available (
The combination of pressure and pulsed energy enables a far higher
sealing effect than conventional electrocoagulation. Arterial vessels can be
[13]. Today, a wide range of
Fig. 6.30).
Figure 6.30 An impedance-controlled sealing dissection device. Note the gray lever
in the middle. If it is turned counter clockwise, a knife in the tip is pushed forward
dissecting the tissue after sealing. From MITI.
Classical (Open) Surgery
249
reliably occluded up to a diameter of 57 mm. In most devices, a knife is
integrated. It can be activated after the sealing procedure to sever the tissue.
6.2.12.2 Argon Plasma Coagulation
A new contactless technology for (superficial) hemostasis was introduced
into surgery in the 1990s: argon plasma coagulation (APC)
[14].The
device is a monopolar coagulator which conducts radio-frequency current
to the tissue in a jet of argon gas
characteristic blue color (gray in print versions) (
[15]. The ionized gas (plasma) gains a
Fig. 6.31). However,
Figure 6.31 (A) The principle of APC. A jet stream of argon is blown against the
tissue surface providing an electrical bridge. (B) Even though the application angle
is well below 90 degrees, the plasma hits the tissue precisely (PJ, plasma jet; Ar,
argon; HF, high frequency; NE, neutral electrode; d, distance between tip of the electrode and tissue surface; U, voltage). All from Erbe Elektromedizin GmbH.
250
Biomedical Engineering in Gastrointestinal Surgery
Figure 6.32 (A) The APC generator and gas supply unit; (B) the plasma hits the liver
surface. The sharp gas blow displaces blood and improves the coagulation effect.
All from Erbe Elektromedizin GmbH.
argon is more than just a vehicle for the current, since it also blows away
the blood covering the tissue, which improves the efficacy (
Fig. 6.32) [15].
Currently, APC becomes increasingly popular in interventional endoscopy, since it is ideally suited to stop diffuse bleeding of surfaces, such
as telangiectasias
[16].
6.2.13 Electrosurgical Cutting
To overcome the nonconductive dehydrated tissue and the thin vapor
layer an electrical arc has to be ignited to cut tissue. An ohmic current
flow is avoided due to the nonconductive vapor layer. Only when peak
voltages above 200 V are present is a dielectric breakdown between the
biological tissue and the electrode reached and an electric arc is ignited.
Electric arcs are necessary to cut tissue, as the temperature within cells
has to be raised above 100˚C which is not possible through endogenous
heating. The temperature of electrical arcs is assumed to reach about
1300˚C. The arcs are striking the tissue in small spots with only
1020 μm in diameter leading to a very high current density (
Fig. 6.28).