Biomedical Engineering Reference
In-Depth Information
hazard reports with the use of electrosurgery in hospitals.
Another reason is that that the whole patient is electro-
active in normal mode of electrosurgery use. The RF
potentials of many body segments may easily attain some
tenths of rms volt in normal mode operation, and insu-
lation of these body segments is critical.
High frequencies have been chosen to avoid nerve
and muscle stimulation (cf. the sensitivity curve of
Fig. 4.1-31
). The output is neither constant amplitude
voltage nor constant amplitude current. The optimal
output characteristic is linked with the very variable load
resistance. Tissue resistance increases when coagulated,
fat has higher resistance than muscles and blood, and the
contact geometry is very dependent on the electrode
chosen and the way it is held by the surgeon. If a constant
amplitude current is chosen, power would be pro-
portional to load resistance, and tissue would quickly be
carbonized in high resistance situations. If constant am-
plitude voltage were chosen, power would be inversely
proportional to load resistance, and when tissue layers
around the electrode would coagulate, current would
stop flowing. Modern instrumentation therefore mea-
sures both output voltage and current, and regulates for
an isowatt characteristic.
Typical power levels in unipolar electrosurgery is about
80 W (500
U
, 200 V, 400 mA rms), in bipolar work 15 W
(100
U
, 40 V, 400 mA rms). The frequency content of
the sine wave is of course just the repetition frequency,
usually around 500 kHz. In pulsed mode the frequency
content is very broad, but most of the energy will be in
the frequency band 0.5-5 MHz. In pulsed mode the peak
voltage can reach 5000 V, so insulation in the very humid
surroundings is a problem.
The arc formed particularly in coagulation mode, is
a source both of noise and rectification. Rectification is
strongly unwanted, because low frequency signal com-
ponents may be generated which excites nerves and
muscles. In order to avoid circulating rectified currents,
the output circuit of a monopolar equipment always
contain a safety blocking capacitor (
Fig. 4.1-19
). Low
frequency voltage is generated, but it does not lead to
low frequency current flow. Even so, there may be local
low frequency current loops in multiple arc situations
(Slager et al., 1993). The resulting nerve stimulation is
a problem in certain surgical procedures.
Argon gas is sometimes used as an arc guiding
medium. The argon gas flows out of the electrode
mainly for two purposes: to facilitate and lead the
formation of an arc between the electrode and tissue
surface, and to impede oxygen in reaching the co-
agulation zone. In this mode of operation, no physical
contact is made between the metal electrode and the
tissue, the surgeon points the pen toward the tissue
and coagulation is started just as if it was a laser beam
(which it is often mixed up with). The gas jet also
blows away liquids on the tissue surface, thus facili-
tating easy surface coagulation.
Implants
The use of electrosurgery on patients with metallic im-
plants or cardiac pacemakers may pose problems. Me-
tallic implants are usually considered not to be a problem
if the form is round and not pointed (Etter et al., 1947).
The pacemaker electrode tip is a small area electrode,
where relative small currents may coagulate endocardial
tissue. The pacemaker catheter positioning should
therefore not be parallel with the electrosurgery current
density lines. This is illustrated in
Fig. 4.1-20
for a heart
pacemaker implant.
Ablation
Through catheters it is possible to destroy tissue with RF
currents in a minimal-invasive procedure. In cardiology
this is called
ablation
. Both DC and RF current has been
tried for this purpose. Choice of bipolar or monopolar
technique is important (Anfmsen et al., 1998).
Activation
C
safety
A
RF
N
Neutral plate
monitoring
Figure 4.1-19 Typical electrosurgery output circuit. Notice double plate neutral electrode for monitoring of skin contact. Safety
blocking capacitor shall prevent rectified low frequency currents in tissue.
