The Stephens Anesthetic Machine
Universal Vapouriser
Designed to be versatile, the Stephens vaporiser is very compact and is readily adapted for use with any volatile anaesthetic agent, i.e., halothane, methoxyflurane and isoflurane.
The main control on the vaporiser provides a smooth increase in concentration between OFF and ON. In addition to this, there is a metal sleeve on the inner circular port, which extends into the glass anaesthetic jar. By raising or lowering this sleeve, the vapour concentration delivered may be decreased or increased to suit the clinical requirements. It is normally only used with mask induction for animals of 2 kg or less.
In contrast to most in circuit vaporisers, an exceptionally wide and constant bore is maintained throughout the mechanism, regardless of the concentration setting on the main control. This is a unique feature, and is achieved by the use of a special port design in the vaporiser head. The bore is equivalent to a diameter of 7/8" (22mm). Back pressure and turbulence are thus kept to an absolute minimum.
The bowl of the vaporiser enables the anesthetist to quickly check the levels of anesthetic agent remaining in the vaporiser. The priming volume required is smaller than used in most other vaporisers, e.g., 8-12 mls of halothane or isoflurane and 50-60 mls of methoxyflurane. These priming (filling) volumes should never be exceeded.
A gauze wick can also be fitted to this vaporiser. The gauze wick is used to attain a greater rate of vaporisation with the poorly volatile anesthetic agent methoxyflurane. This wick should never be used with halothane or isoflurane.
As the anesthetic vaporiser is located within the circuit, the mass of anesthetic delivered to the animal (at a particular setting of the vaporiser) changes with the variations in the minute volume of respiration. If the depth of anesthesia increases, the decrease in minute volume which results is followed by a decrease in the rate of vaporisation of the anesthetic. Conversely, the rate of vaporisation of the anesthetic increases as the depth of anesthesia decreases. Because changes in the depth of anesthesia are automatically followed by compensating changes in the mass of anesthetic delivered to the animal, the system is to some extent self-regulating.
In those cases in which intermittent positive pressure ventilation is used, the vaporiser setting should be reduced. The flow of oxygen into the system should be increased to slightly above the metabolic requirements of the animal. This higher oxygen inflow ensures that the rebreathing bag will slowly overfill and that periodic emptying of the excess gas will tend to prevent the possibility of a build-up of concentration of anesthetic within the system, due to hyperventilation.
The adjustable metal sleeve is normally used in the UP position for maintenance of anesthesia. It may be advantageous to lower the sleeve for induction of very small animals in cold climates or when anesthetising cats and other lightweight animals to enable a higher concentration to be achieved during induction. Once this is accomplished, the sleeve is returned to the UP position. The sleeve gives the vaporiser great versatility in a variety of different climates with a number of different anesthetic agents.
The vaporiser locks in the OFF position. To turn it on pull the knob out and turn clockwise.
Related Information
Introduction to in-circle closed circuit Anesthesia
Low flow and closed circuit Anesthesia for vetrinarians
Stephens FAQ
Frequently Asked Questions
Parts Diagram Of The Stephens
A detailed Diagram showing the components of the Stephens Anesthetic Machine |
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