Some vaporisers are designed to have a low internal resistance to gas flow, so that they may be used within the breathing circuit, the gas flow being driven through them by the patient's breathing. They may be used in a non-rebreathing draw-over apparatus, or as in-circuit vaporisers in a circle absorber system.
Vaporisers used in draw-over applications are usually of the simple bottle or bowl type (e.g. the Ohio 8 bottle, Goldman or Stephens), but more accurate, low-resistance draw-over vaporisers, such as the Penlon Oxford Miniature Vaporiser (OMV) and Ohmeda Draw-Over Tec vaporisers, have been manufactured.
Oxford Miniature Vaporiser
The OMV is particularly versatile, since the same vaporiser can be used to vaporise a number of agents, with only the dial scale being changed.
Some precision vaporisers (such as the Fluotec 2) have a sufficiently low internal resistance that they may be used in draw-over applications (ventilation being assisted or controlled if necessary), but the resistance of the majority of precision vaporisers is much too high for them to be used in this way.
Low-resistance vaporisers may also be used in place of precision vaporisers, if necessary. However, the inaccuracy and, frequently, restricted output of the bowl-type vaporisers makes them less satisfactory than a precision vaporiser in most situations. In addition, the inlet and outlet may need to be adapted to fit the normal 23 mm vaporiser connectors.