Use of Nonprecision Vaporizers
Nonprecision vaporizers were originally designed for and used extensively for administration of low–vapor pressure liquid inhalant anesthetics, such as methoxyflurane, that are no longer available. These vaporizers are classified as variable-bypass, flow-over and non–temperature compensated. Because of their low resistance to gas flow, these vaporizers are located in the breathing circuit (vaporizer-in-circuit [VIC]).
Two examples of nonprecision vaporizers are the Ohio No. 8 vaporizer (Figure 4-26) and the Stephens vaporizer, both of which consist of a glass jar containing a wick and a control dial. The wick absorbs anesthetic contained in the jar and increases the surface area available for vaporization, thus increasing saturation of the carrier gas with anesthetic.
The imprecise control of anesthetic output characteristic of these vaporizers is adequate for methoxyflurane, which has a low vapor pressure and will achieve a maximum of only 4% concentration even if the vaporizer is fully open. This control is inadequate for more volatile anesthetics such as isoflurane, sevoflurane, or halothane, however, which can achieve very high concentrations and produce a dangerously excessive anesthetic depth.
Nonprecision vaporizers are sometimes used to deliver halothane or isoflurane. Ohio No. 8 vaporizers (nonprecision) can be used with halothane or isoflurane, but the wick should be removed and no more than 100 mL of anesthetic should be put into the vaporizer. For the first 2 minutes, a setting of 6 to “fully open” should be used, and thereafter a setting of 2 to 6 is usually adequate. If a Stephens vaporizer (nonprecision) is used to deliver halothane or isoflurane at low oxygen flow rates, the metal sleeve should be fully retracted and the anesthetic should be filled only to the anesthetic level line. A vaporizer setting of 4 is used for the first 2 minutes, and for the maintenance period a setting between “off” and 4 is usually adequate.
This practice of using nonprecision vaporizers to deliver isoflurane or halothane has become very uncommon in recent years, however, and will not be commonly encountered by the veterinary anesthetist.
In a nonprecision vaporizer, the concentration of the anesthetic delivered to the patient is unknown because the vaporizer control dial settings (“0” through “10” for the Ohio, and “Off” to “On” with the distance between divided into eighths for the Stevens) are unrelated to specific anesthetic percentages. The anesthetist varies the relative amount of anesthetic delivered to the patient by turning the control dial, based on the patient’s depth of anesthesia. When these vaporizers are used, the exact output at any given setting varies, is not known, and is influenced by a number of factors discussed in the next section.
Nonprecision vaporizers are located in the breathing circuit (VIC) and do not compensate for changes in temperature, carrier gas flow rates, or back pressure. These factors have several serious implications that must be understood by the anesthetist using this type of vaporizer, so that appropriate adjustments can be made in the control dial position to compensate.
At any given setting the vaporizer will deliver a greater concentration of anesthetic if located in a warm room than if located in a cold room. In contrast, vaporizer output will decrease in response to the drop in temperature that occurs at high carrier gas flow rates. Therefore any change in temperature of the liquid anesthetic will change vaporizer output and consequently patient anesthetic depth.
The amount of anesthetic delivered to the patient will increase if the patient’s ventilation or carrier gas flow rate is increased because either of these situations will increase the flow of gas through the anesthetic chamber, which in turn will increase vaporization.
Any buildup of pressure within the breathing circuit (as may occur when the patient is bagged, the O2 flush valve is activated, or a ventilator is used) may result in increased concentration of anesthetic being delivered to the patient and may result in excessive anesthetic depth. Manual or mechanical ventilation is therefore more challenging when used with nonprecision vaporizers. The anesthetist must ensure that the vaporizer setting is greatly reduced (or in some cases turned off) when bagging the animal or when using a ventilator. In contrast, when using a precision out-of-circuit vaporizer [VOC], it is not normally necessary to turn the vaporizer off when bagging the patient because of the back-pressure compensation.
The use of a nonprecision vaporizer to deliver a high–vapor pressure anesthetic such as isoflurane results in less precise control over anesthetic depth than does use of a standard precision vaporizer. Close monitoring of the patient is essential, particularly during the first 5 minutes of anesthesia, when patient depth increases rapidly.