Endotracheal tubes

As the primary and secondary coils of a transformer are in close proximity over several square inches area, a considerable capacitor action exists and a potential of up to 250 volts occurs between either, or both, secondary coil outlets and earth. A patient lying on cotton fabrics and antistatic rubber on a metal table on an antistatic floor can be considered wellkarthed even without a diathermy pad or spilled liquids. All electrically illuminated endoscopes employ the metal sheath as one low-voltage conductor to convey power to the bulb and one outlet of the transformer is applied to this, completing a hazardous circuit when the endoscope is inserted. Oesophagoscopes and bronchoscopes are particularly dangerous since they probe in close relation to the heart. I note, with regret, that the latest endoscopes, illuminated by a fibre optic light source, have a flexible metal sheath around the glass fibres, thereby introducing an unnecessary hazard. The capacitor action of a transformer can be completely removed by an earthed layer of metal foil wound between the primary and secondary coils and it is fair to say that most electro-medical apparatus has this safeguard. However, it is difEcult to check the presence and effectiveness ofthis foil, and safer by far to insist on batteries for endoscopes, for batteries are completely safe. It is always tempting to save money by using mains electricity instead of expensive batteries, but if high quality mains equipment is purchased and suitably checked and serviced, the ultimate cost will be greater, the dangers to staff and patients greater, the electronic interference more severe, and the cables more numerous than if batteries are used wherever possible.