Biomedical Engineering Reference
In-Depth Information
muscle contractions, muscle relaxants are given. The
shock elicits a seizure not very different from a grand mal
epileptic attack, the seizure is to last longer than 25
seconds. The effect is presumably due to the enormous
synchronized activity of the whole CNS. The treatment
is usually repeated several times within a few weeks span.
The treatment is often followed by a loss of memory for
recent events,
Figure 4.1-18 illustrates the monopolar circuit. The
monopolar coagulation electrode is often in the form of
a sphere.
Thewaveformusedismoreorlesspuresinusoidalin
cut mode, to highly pulsed with a crest factor of 10 or
more for the spread coagulation mode. In spread co-
agulation tissue contact is not critical, the current is
passed to the tissue mostly by fulguration (electric arc).
The electromagnetic noise generated may be severe
over a large frequency spectrum, and this causes trou-
ble for medical instrumentation connected to the same
patient.
Electrosurgery is based on the heat effect of the cur-
rent, and this is proportional to the square of the current
density (and the electric field) and tissue conductivity.
The power volume density W v is falling extremely
rapidly with distance from the electrode, as shown by
the equation for a voltage driven half sphere electrode
at the surface of a half infinite homogeneous medium.
With constant amplitude current the power volume
density is:
and the therapeutic effect
is not
permanent.
4.1.13 Electrosurgery
High frequency (also called radio frequency, RF) cur-
rent is used to cut or coagulate tissue. The method
must not be confused with electrocautery. In electro-
surgery the current is passing the tissue, with heat
development in the tissue and cold electrodes (dia-
thermy). With electrocautery the current is passed
through a wire and not through tissue, and the wire is
accordingly heated. Bipolar forceps are used for mi-
crosurgery, they represent a dipole current source in
the tissue. A unipolar (in the field of surgery called
monopolar) circuit is used in general surgery. The
neutral electrode is a large flexible plate covered with
sticky hydrogel for direct fixation to the skin. The
neutral plate is often split into two, and a small current
is passed between the two plates via the skin and
tissue. Impedance is measured, and if this impedance is
outside pre-set or memory set limits, the apparatus will
warn of poor and dangerous plate contact.
The active electrode may be handheld, or endoscopic:
long and thin types either flexible or rigid.
i 2
4p 2 s r 4
W v ¼
ð half sphere Þ
(4.1.12)
Tissue destruction therefore occurs only in the very
vicinity of the electrode. Power dissipation is linked with
conductance, not admittance, because the reactive part
just stores the energy and sends it back later in the AC
cycle. Heat is also linked with the rms values of voltage
and current, ordinary instruments reading average values
cannot be used. Because heat is so current density de-
pendent, the effect is larger the smaller the cross sec-
tional area of an electrode, or at a tissue zone
constriction. This is an important reason for the many
A
ELECTRO-
SURGERY
N
Figure 4.1-18 Monopolar electrosurgery.
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