Practical Gastrointestinal Endoscopy

THERAPEUTIC COLONOSCOPY 175
layer to steam. Because it is low voltage, however, cut current is
less able to traverse desiccated tissue and to heat deeply. ‘Autocut’
intelligent electrosurgical units automatically adjust power
output to match the resistance of the tissue being heated in order
to produce a predictable rate of transection.
Coagulating current has intermittent higher-voltage spikes with intervening
‘off periods’, which last for about 80% of the time (Fig.
7.9). The higher voltage allows a deeper spread of current flow
across desiccating tissue, whereas the off periods reduce (except
at high power settings) the tendency for gas ionization, sparking
and local tissue destruction.
Blended current combines both waveforms (Fig. 7.10), some units
providing the ability to select blends with relatively greater ‘cut’
than ‘coag’ characteristics. The differences between the various
makes of electrosurgical unit suggest that the output characteristics
are more complex than this brief summary suggests, some
appearing to provide more effective hemostasis than others.
When changing from one unit to another it is therefore essential
to be cautious, to start with low power settings. If possible, try out
the unit on a small lesion or the periphery of a larger one, rather
than entering the ‘big-time’ unrehearsed and then regretting it.
DESICCATION
Coagulating current—
intermittent high-voltage pulses
can pass desiccated tissue.
Fig. 7.10 Blended current combines
thecharacteristics of both
cutting and coagulating currents.
Current density
Tissue heats because of its high electrical resistance, typically
around 100 ohms, although resistance varies according to the
particular tissue (fat conducts poorly and so heats little); water
loss (desiccation) during heating increases resistance and the
drying tissue is also mechanically harder to transect. If electric
current is allowed to spread out and flow through a large area
of tissue, the overall resistance and heating effect falls (Fig. 7.11).
To obtain effective electrocoagulation, the flow of current must
be restricted through the smallest possible area of tissue—the
principle of ‘current density’ (Fig. 7.12). This principle is basic to
all forms of electrosurgery and explains why no noticeable heat
is generated at the broad area of skin contact with the patient
‘return plate’, whereas intense heat occurs in the closed snare
loop (Fig. 7.13). Even a relatively small area of contact between the
buttock or thigh and patient plate is adequate, and extra moisture
or electrode jelly is unnecessary at the power used for endoscopic
polypectomy.
The essential in polypectomy is to coagulate the core of the polyp stalk,
with its plexus of arteries and veins, before transection. Closing
the snare loop both stops the blood flow (‘coaptation’) and tends
to concentrate the current to flow through and heat-coagulate the
core (Fig. 7.14). The tightness of the loop is critical, since the area
through which the current is concentrated (current density) decreases
as the square of snare closure (πr 2 ), thus causing a square
law relationship between snare closure and increasing current
Fig. 7.11 Current flows more
easily through larger areas of
tissue resistance and so produces
little heat.
Fig. 7.12 Current density results
from constricting tissue and
greatly increases heating.