Methoxyflurane (Metofane®) is no longer being manufactured, is no longer available for purchase from DAR, and has been relegated to the annals of anesthesiology as a historical footnote along with other obsolete anesthetic agents such as cyclopropane, chloroform and the like. Consequently, this erstwhile staple of rodent inhalation anesthesia induction and short-term anesthesia requires a replacement.

Alternatives

Potential options for methoxyflurane can be grouped into (a) other inhalation agents or (b) injectable agents. Here we will deal with inhalation agent alternatives; those wishing to substitute injectable agents should consult with a member of the veterinary staff.

Inhalation Agent Substitutes

There are two recommended inhalation agents for replacement of methoxyflurane: halothane and isoflurane. Either can be purchased from the Emory University Hospital Pharmacy. Ether is not recommended by the veterinary staff as it is explosive and flammable with a pungent, unpleasant odor and requires specific IACUC and Chemical Safety approval. Both isoflurane and halothane can be delivered with a precision vaporizer or in the more traditional, less precise and less controlled manner of exposure using an induction chamber (i.e., bell jar, dessicator, etc.). The former method requires an anesthesia machine and considerable up front investment, but offers the advantages of precision, rapid adjustment, safety, effectiveness, and, in the long run, conserves anesthetic agent and becomes cost effective. Investigators interested in purchasing anesthesia machines for rodent use should contact the veterinary staff for advice. It is important to remember that isoflurane and halothane have much higher vapor pressures than methoxyflurane and, if used in an induction chamber without strict volume control, may produce rapidly lethal gas concentrations. The characteristics of halothane, isoflurane and methoxyflurane are given in the table below.

Anesthetic Gas Characteristics

1. Vapor Pressure at 20oC (torr/mm Hg)
   
   

2. Maximum Concentration (%) of gas at equilibrium with room air at sea level at 20oC

3. MAC = minimum alveolar concentration (minimum concentration to maintain anesthesia in 50% of patients (indicator of potency), values given are for the rat. Generally, anesthetic maintenance requires 1.5-2.0 times MAC.

Halothane (Fluothane)

This halogenated halocarbon that is a cardiac and respiratory depressant with fast induction and recovery. It is less irritating to the upper airways than other agents, but has poorer analgesia and muscle relaxation qualities and sensitizes the heart to catecholamines. Halothane has been shown to be mutagenic and hepatotoxic with other untoward effects usually related to metabolic products of the gas (including toxic by-products such as bromides and free). If halothane is used in a bell jar, gas exposure can be prevented or reduced to safe levels of exposure by using it only in a fume hood. Market availability of halothane is diminishing.

Isoflurane (Aerrane, Forane). This agent is not metabolized into toxic by-products (but still should be used in a fume hood if administered in an induction chamber), has fast induction and recovery, does not sensitive the heart to catecholamine induced arrhythmias, and maintains good cardiac output. It is the preferred agent for inhalation anesthesia, but its major drawback is a higher cost relative to halothane.

Practical Use in An Induction Chamber

As both halothane and isoflurane have similar vapor pressures (see table above), their use is described interchangeably (as “gas” or “agent”) in the ensuing protocol. Special consideration should be given to keeping animals isolated from agent in the liquid phase which can be irritating to the skin and eyes. Owing to the high volatility of these agents, the lid should be kept on the induction chamber constantly or the volume of gas will be rapidly exhausted.

For induction, a concentration of 2-4% concentration of gas is normally adequate. To use either gas accurately, the induction chamber volume must be known precisely. After determining the chamber volume (it is recommended to record this permanently somewhere easily retrievable), add 0.1-0.2 ml of gas (in liquid form from the bottle) for each liter of chamber capacity. This can be done by applying the gas in liquid phase from its bottle to a cotton ball below the false floor of the container. For small containers, a piece of cotton can be enclosed in a histology tissue cassette and the agent may be poured or applied onto the cotton in the cassette Use of 0.2 ml liquid agent per 1000 ml chamber volume will give about a 4% concentration of gas. In the experience of the DAR veterinary staff, using nine naïve ICR mice (5 males & 4 females; 2 months of age) introduced to the chamber sequentially after the introduction of isoflurane (0.2 ml/L chamber volume), recumbency was obtained in 57 +/- 21 seconds. However, for rapid and effective induction, the agent had to be replenished in the chamber approximately every 3 mice.

Induction Chambers

Any number of apparati from simple jars with screw-top lids, dessication chambers, bell jars, or specific inhalation chambers (i.e., Inhalation Narcosis Chamber, Harvard Apparatus, #59-6717, 1-800-272-2775, $132.50) may be used for anesthesia induction.

IACUC Protocol Considerations. Remember that changes in anesthesia procedures (i.e., from methoxyflurane to another agent) related to an approved IACUC protocol require a request for protocol modification (http://www.emory.edu/IACUC/forms.htm).

• 1999 National AALAS Meeting - Seminar 25 notes
• Mouse Parvovirus Update - (11/15/1999)
• MPV Quarantine Lifted - (11/15/1999)
• September 22, 1999 Update
• Mice of Black/6 Lineage and MPV
• August 27, 1999 Update
• SDAV ALERT - (2/11/2000)
• Methoxyflurane - No longer being manufactured (1/17/2000)
• Mouse Parvovirus - Quarantine Lifted (3/17/2000)
• Rat SDAV at Rollins - (5/01/2000)
• Emory Rat Pinworm Eradication Campaign - (6/14/2000)