The Stephens Anesthetic Machine
Soda Lime Canisters
Soda Lime Canisters are used to absorb the exhaled carbon dioxide from the animal’s lungs. They are designed to produce minimum channelling of the expired respiratory gases through the soda lime contained within each canister. The carbon dioxide chemically reacts with the soda lime producing heat and water.
Two canisters are placed in the expiratory limb of the anesthetic apparatus, thus any resistance caused by the soda lime occurs in the passive expiratory phase of the respiration and there is little resistance during inspiration. During the absorption of carbon dioxide by the soda lime, heat is produced and this is used as an indicator of absorptive efficiency. (See Fig. 2)
The soda lime canister nearest to the expiratory limb of the circle is always the first canister to become exhausted (i.e., lower canister). When the soda lime in the lower canister is exhausted, the canister is cold to the touch and may be removed temporarily from the circuit and refilled with fresh soda lime. Each canister lasts approximately 7.5 hours with a 25kg dog.
A Simple Method to Ensure Fresh Working Soda Lime
Change your lower canister every Monday or second Monday morning-this will be governed by duration, size of patient and number of anesthetics for a week.
When the lower canister is exhausted, it should be removed and refilled with fresh soda lime. It should then be placed in the upper position, and the top canister be placed nearest to the animal (in the lower position). By doing this it is assured there is always one soda lime canister containing fresh soda lime within the circle at all times. When the anesthetic apparatus is to be stored for any length of time it is advisable to place a cork firmly in the "Y" piece of the apparatus, thus sealing the system and preventing the slow decomposition of the soda lime by the carbon dioxide contained in the atmosphere. An approximate estimation of the time to exhaust one canister is 7.5 hours per each 260gm canister with a 25kg/55lb dog.
Unidirectional Valves
The unidirectional valves are manufactured in one unit embodying two one-way valves and provision for an inlet for fresh gases. Fresh gas enters the system through the inlet nipple and fills the rebreathing bag. The unidirectional valves have a tendency to make a "clicking" noise during operating which is an advantage to the veterinarian who may be working alone. The noise produces an auditory indication of the animals' respiratory rate. The valve assembly is readily dismantled for cleaning by unscrewing the valve covers.
Flowmeter
The apparatus is supplied with a specially constructed "rotameter" type oxygen flow meter. This flow meter has a maximum measurable output of 600cc per minute. We suggest the use of a 4 litre bag as it has less resistance than some smaller bags. The new latex bags of 2 litres or less have acceptable low resistance. When using the apparatus for very small animals whose metabolic requirements for oxygen are small, make sure oxygen is set at 5mL/kg body weight for oxygen entering the system to prevent the uneconomical wastage of anesthetic vapour. Wastage will occur with the loss of vaporised anaesthetic into the atmosphere following over-filling of the rebreathing bag, if too high an oxygen inflow is used. If excess gas flows into the system it can easily be removed by opening the blow off /pop off valve situated near the lower canister, and squeezing the excess gas from the rebreathing bag. The rebreathing bag should always contain sufficient gas so that the maximum inspiratory effort made by the animal will not completely empty the bag. However, it should not be so full of gas that a maximum expiration by the patient will over-distend the bag.
Related Information
Introduction to in-circle closed circuit Anesthesia
Low flow and closed circuit Anesthesia for veterinarians
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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