The Stephens Anesthetic Machine
An introduction to in-circle closed circuit anesthesia
Low Flow and Closed Circuit Anesthesia
Circle systems are classified by the ratio of fresh gas flow to the minute volume uptake of the inhaled gases. If the fresh gases are provided at a rate exactly equal to the rate at which the body takes them up, then the system by definition is a closed circuit. If the flow of fresh gases is higher than the uptake, but not so high as to prevent rebreathing, the system by definition is semi-closed. If the flow of fresh gases is so high that rebreathing does not occur, the system is a non rebreathing one. Assuming an approximate oxygen consumption of 5mL/kg/min for a dog or cat, then an oxygen flow of 5mL/kg/min would result in a closed circuit. If the oxygen flow was higher than 5mL/kg/min but less than 200mL/kg/min then the system is semi closed. If the flow is 200mL/kg/min or greater then the system is non-rebreathing. The most economical way of administering inhaled anesthetic is with a closed circuit.
Vaporiser Position
Vaporisers may be positioned in-the-circuit or out-of-the-circuit. In general, vaporisers used out-of the-circuit are intricate, expensive, heavy and deliver a known concentration of anesthetic agent. Vaporisers used in-the-circuit are less expensive, lighter, and cannot be calibrated for the delivered anaesthetic concentration, as the rebreathed gases re-cycled through the vaporiser already contain anesthetic agent and can only be calibrated when used with fresh gases. Ideally, in circuit vaporisers should maintain a relatively constant temperature, not be subject to large concentration fluctuations associated with surging gas flows which occur with spontaneous respiration, and have low internal resistance. The Stephens meets these criteria.
Neither type of vaporiser is absolutely superior to the other and each has certain advantages and disadvantages. Neither type is more difficult to use than the other. The most important factor in the use of either, is to understand the effects of the fresh gas flow (oxygen) on the inspired concentration of the inhaled anaesthetic and the effects of intermittent positive pressure ventilation on the inhaled anesthetic concentration.
It is important to understand and differentiate between the anesthetic concentration delivered from the vaporiser and the actual inspired concentration. The inspired concentration is the concentration being inspired by the patient from the inspiratory limb of the circuit. Sometimes these two concentrations are similar and sometimes they are very different. In the out-of-circuit vaporiser with oxygen fresh gas flows of 2L/min, the inspired concentration will be close to the dial setting. With low fresh gas flows of 2-500cc/min, the inspired concentration will be substantially less than the dial setting. With in-circuit vaporisers, the inspired concentration will be lower with high oxygen fresh gas flows than with lower fresh gas flows.
Guidelines
There are many variations in techniques for using inhalant anesthetics; however, the most common method in the dog is to administer an anticholinergic drug such as atropine with a tranquilliser or a narcotic. After a few minutes, anesthesia is induced with an intravenous thiobarbiturate such as thiopental or thiamylal. Following this the animal is then intubated and inhalant anesthesia begun. The following guidelines for using the Stephens Anesthetic machine are based on using the above technique. As with any anesthetic technique dosages, flows and concentrations need to be varied from any guidelines or recommendations depending on the responses of each individual patient. If other techniques are used then these guidelines need to be modified for the particular circumstance. If a healthy animal is to be induced and maintained with only an inhaled anesthetic then the induction and maintenance concentrations will need to be higher than those in these guidelines. On the other hand if the animal is very ill then a much lower concentration may be needed or the patient may not be able to tolerate the chosen agent at all. Oxygen flows used commonly, vary greatly. Those listed here are not the only ones which can be used but are just suggestions for a very economical approach, however they too need to be modified depending on the circumstances of a particular patient.
Regardless of the oxygen flow chosen it is of the utmost importance that the operator understands the effects of the oxygen flow on the inspired anaesthetic concentration and the oxygen requirements of the patient.
These guidelines are for healthy dogs given a moderately sedative pre-anesthetic and which are going to be anesthetised with halothane or isoflurane.
Our thanks to Professor Alan M. Klide, University of Pennsylvania for help in the preparation of these additional notes. |
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