The curtailment of manufacture of methoxyflurane (Metofane) has caused a problem for researchers using this inhalant anesthetic for rodents. The alternatives, isoflurane and halothane, are available and can be used in lieu of methoxyflurane but each poses problems for the animals that were not presented with the use of methoxyflurane.

SPECIFIC AGENTS:

• Halothane:

  • Desirable effects: Halothane is easy to vaporize, and induction and recovery are rapid (1-3 minutes). It is a potent anesthetic, is non-irritant and is neither flammable nor explosive.

  • Undesirable effects: Halothane has a depressant effect on the cardiovascular system. Moderate hypotension is produced at surgical levels of anesthesia because of a reduction in cardiac output and peripheral vasodilation. A dose-dependant depression of respiration also occurs. Some hepatic metabolism of halothane occurs and marked liver microsomal enzyme induction may follow anesthesia.

  • Special comments: The desirable effects listed above make halothane an excellent agent for maintaining anesthesia in most species. It should be administered using a calibrated vaporizer since dangerously high concentrations can be attained if a simple apparatus is used. Although recovery is usually rapid, it may be considerably delayed following prolonged, deep anesthesia.

• Isoflurane:

  • Desirable effects: Isoflurane produces very rapid induction and recovery from anesthesia and the depth of anesthesia can be altered easily and rapidly. It is non-irritant, non-exploxive and non-flammable.

  • Undesirable effects: Isoflurane produces slightly more severe respiratory depression than does halothane, but slightly less depression of the cardiovascular system. Its pungent odor has been reported to cause breath-holding during induction in children, but this does not appear to be a significant problem in most species, with the exception of the rabbit.

  • Special comments: The main advantage of using isoflurane in experimental animals is that it undergoes minimal biotransformation and is almost completely eliminated in exhaled air. This suggests that there will be little effect on liver microsomal enzymes and, hence, little interference in drug metabolism or toxicology studies. This characteristic, together with the rapid induction and recovery from anesthesia, has led to the widespread adoption of isoflurane in many research establishments.

References:
   Flecknell, P. (1996). “Laboratory Animal Anesthesia: A practical Introduction for Research Workers and Technicians”, 2nd Edition, Academic Press, New York.

    Sharp, P.E. and LaRegina, M.C. (1998). “The Laboratory Rat” Ed: M. A. Suckow, CRC Press, Boca Ratan, New York.

 

INDUCTION AND MAINTENANCE CONCENTRATIONS OF INHALATION ANESTHETIC AGENTS
	Anesthetics Concentration for Induction (%)	Anesthetic Concentration for Maintenance (%)
Enflurane	3-5	1-3
Ether	10-20	4-5
Halothane	4	1-2
Isoflurane	4	1.5-3