It is often desirable to mount several vaporisers in series on the same machine, so that different anaesthetics may be administered when desired. However, this arrangement presents the hazard that, if more than one vaporiser can be switched on at a time, the patient may be exposed to a relative overdose of anaesthetic agent. In addition, the downstream vaporiser will become contaminated with the anaesthetic contained in the upstream vaporiser.
A number of precautions may be taken to prevent such accidents:
A single dismountable vaporiser may be fitted; this is removed from the machine and replaced when another agent is to be used. This has the added advantage that only one vaporiser for an infrequently used agent need be purchased for use on several machines. However, there is always the possibility of a vaporiser being dropped and, in many vaporisers, liquid anaesthetic may drain into the gas outlet and be driven into the patient circuit if the vaporiser is tipped or inverted while it is being carried. Newer vaporisers, such as the 'Tec 5 series and Dräger 2000, are designed to prevent this from happening.
The vaporisers may be arranged so that the vaporiser to be used is connected to the circuit using flexible hose and 23 mm adapters, and disconnected when not in use. This is often the most convenient method on pole-mount anaesthesia machines.
A switch on the back bar may be used to direct gas flow through only one vaporiser at a time, e.g. the Fraser Harlake Selectatec back bar and the Vapour changeover switch used with Dräger 19.1 vaporisers.
Two Tec 4 vaporisers mounted on a Fraser Harlake selector back bar. The vaporiser in use is selected by the switch located between the vaporisers on the back bar.
A mechanical locking system may be used that only allows one vaporiser to be switched on, e.g. Ohio selector manifolds and Dräger 19.3 vaporisers.
The Ohio triple selector manifold allows the left, centre or right vaporiser to be used. Slots in the selector (arrowed) line up with flanges on the vaporiser control dials.
A mechanical inter-connector may be used to prevent more than one vaporiser being switched on at a time, e.g. the Ohmeda Selectatec system and Dräger 19.2 vaporisers.
||Ohmeda Selectatec vaporisers incorporate pins in the manifold that are linked to the control dial.|
||When one vaporiser is in use, the pins extend and prevent other vaporisers from being switched on.|
||The Dräger Interlock 1 system for Vapour 19.2 vaporisers features a rotating bar on the manifold with teeth that fit into a cut-out on the back of the control dial.|
If none of the above is possible, mounting the vaporiser for the most volatile agent downstream will prevent release of high concentrations of a volatile agent owing to contamination of a vaporiser designed for an agent with a low saturated vapour pressure. Vaporisers should be arranged in the order:
This will, of course, do nothing to prevent the patient being inadvertently exposed to more than one anaesthetic at a time.
Low-resistance vaporisers have a low internal resistance to gas flow and are designed to be used within the breathing circuit, either in draw over apparatus or as in-circuit vaporisers in the circle absorber system.