Page 22 - Clinical Manual of Small Animal Endosurgery

10 Clinical Manual of Small Animal Endosurgery

Fig. 1.10 Typical fibre-optic light guide.

Light guide cable

Light guide cables (Fig. 1.10) transmit the light to the endoscope, and
generally consist of thousands of small fibres (from 30 µm to hundreds
of micrometres in diameter) surrounded by a protective jacket. For rigid
endoscopy (rather than flexible endoscopy), the light guide is a separate
unit, equipped with metal ends, which are inserted into the endoscope
at one end, and introduced into the light source at the other. They can
be fitted with adapters for endoscopes from different manufacturers, and
are available in various diameters for use with the different-diameter
endoscopes available. Smaller endoscopes require smaller cables, thus
preventing overheating. In fact, although fibre-optic light is defined as
‘cold’ light, significant heat is generated. This can pose a hazard to the
patient, especially when the cable is laid on the skin or the end of the
scope or light guide is allowed to rest on the tissues. Incidents of drapes
igniting have also been reported.
When an insufficient amount of light transmission is noticed, the cable
and connections need to be checked for cleanliness and/or damage. Light
guide cables are delicate, and need to be handled with care. If any fibres
are broken the ability of the cable to transmit light is reduced; single
broken fibres appear as black dots when the light is projected on a white
surface. When more than 20% of the fibres are damaged the cable needs
replacing. Another type of degradation is discoloration, when changes
in the colour of the light transmitted will be noticed.
Less commonly, liquid-filled light guides are used, which are less prone
to damage by mechanical means but are more expensive and less tolerant
of heat. Light guides are commonly sterilised in the autoclave.