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Anesthetic Drugs: Rodents and Rabbits



Inhalation Anesthesia

Prepared by Deborah M. Mook, DVM

UPDATE: April 2006



Introduction


The general advantages to the use of inhalation agents are that the procedure is technically feasible and often preferred because it is precise, rapidly adjustable, safe, and effective especially for procedures lasting more than 1-2 hours. Postoperative recovery is rapid and less complicated than with injectable anesthetics. The major drawback to inhalation anesthesia in rabbits and rodents is the challenge (surmountable!) in establishing a patent airway from the anesthetic machine to the respiratory tract of the patient. Blind or direct visualization techniques are well-described for the intubation of rabbits and rodents and intubation is not as daunting as one might suspect for rabbits. Intubation of rodents usually requires either a tracheostomy (customarily a non-survival procedure) or specialized endotracheal tubes and laryngoscopic equipment. Gas anesthesia can also be delivered via a semi-closed mask system with a means of waste gas scavenging. Typically, 1-3% halothane and 2-4% isoflurane are adequate for anesthesia maintenance providing that system leaks are minimal.




For brief anesthesia, to permit IP injections, blood collection, nail and incisor trimming, or quick, uncomplicated surgical procedures, rodents can be placed in an induction chamber and exposed to inhalation agents delivered from a precision vaporizer or, alternatively, from cotton balls or gauze sponges in a chamber under a false floor (to prevent physical contact of the animal with the anesthetic liquid). The latter, a traditional method of administration, is less precise, safe and controllable than the former. The agents typically used are halothane or isoflurane. Diethyl 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. Delivery using a precision vaporizer 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 high vapor pressures and, if used in an induction chamber without strict volume control, may produce rapidly lethal gas concentrations.




Anesthetic Gas Characteristics

Drug Vapor Pressure1 Max. Conc.2 Metabolites
(metabolism)
MAC3
in Rats
Induction (%) Maintenance (%)
Halothane 243 32% 15-20%
(hepatic)
0.95% 1-4% 0.5-2%
Isoflurane 252

33%

0.2%

1.38%

2-6%

1-3%



  1. Vapor Pressure at 20°C (torr/mm Hg)
  2. Maximum Concentration (%) of gas at equilibrium with room air at sea level at 20°C
  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 and 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.



Volume of liquid agent/1000 ml
chamber volume
Approximate concentration of
isoflurane or halothane
0.05 ml 1%
0.1 ml 2%
0.2 ml 4%
0.3 ml 6%



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.




Reversal Agents and General Drug Metabolism and Excretion

Drug Biotransformation Excretion Reversal
Agent
Avertin 100% Hepatic Renal
Chloral hydrate 100% Hepatic Renal
Droperidol Hepatic Renal
Ether 20% Hepatic Exhalation
Fentanyl 90% Hepatic Renal Naloxone
Halothane 20% Hepatic Exhalation
Ketamine None Renal/Hepatic
Medetomidine Renal Atipamezole
Midazolam 100% Hepatic Renal
Pentobarbital 50-75% Hepatic Renal
Tiletamine None Renal/Hepatic
Xylazine 100% Hepatic 70% Renal + 30% Hepatobiliary Atipamezole, Yohimbine
Zolazepam +/- Hepatic Renal


Selected References


  • Bruch D, S Ikramuddin, J Koch, et al. 1996. Novel device for small animal anesthesia. Cont Top Lab Anim Sci 35(6): 73-4.

  • Cruz JI, et al. 1998. Observations on the use of medetomidine/ketamine and its reversal with atipamezole for chemical restraint in the mouse. Lab Animals 32: 18-22.

  • Danneman PJ, Mandrell TD. Evaluation of five agents/methods for anesthesia of neonatal rats. 1997b. Lab Anim Sci 47: 386-95.

  • Field KJ, White WJ, Lang CM. 1993. Anaesthetic effects of chloral hydrate, pentobarbitone and urethane in adult male rats. Lab Animals 27: 258-69.

  • Flecknell P.A. 1996. Anaesthesia and analgesia for rodents and rabbits. In: Handbook of Rodent and Rabbit Medicine, Laber-Laird K, Swindle MM and Flecknell PA, eds., Pergammon Press, Butterworth-Heineman, Newton, MA, pp. 219-37.

  • Flecknell, P.A. 1996. Laboratory Animal Anaesthesia: An Introduction for Research Workers and Technicians. 2nd ed., Academic Press, London, U.K.

  • Gardner DJ, Davis JA, et al. 1995. Comparison of tribromoethanol, ketamine/acetylpromazine, TelazolTM/Xylazine, pentobarbital, and methoxyflurane anesthesia in HSD:ICR mice. Lab Anim Sci 45: 199-204.

  • Huerkamp MJ. 1994. Chemical restraint of rabbits and pocket pets. Current Veterinary Therapy XII: Small Animal Practice (Kirk RW and Bonagura JD, eds.), WB Saunders Co., Philadelphia, PA.

  • Kohn DF, Wixson SK, and WJ White. 1994. Anesthesia and Analgesia in Laboratory Animals. Academic Press, Orlando, FL, 1997.
  • Libbin RM, Person P. 1979. Neonatal rat surgery: Avoiding maternal cannibalism. Science 206: 66.

  • Lipman NS, Marini RP, Erdman SE. 1990. A comparison of ketamine/xylazine and ketamine/xylazine/acepromazine anesthesia in the rabbit. Lab Anim Sci 40: 395-8.

  • Marini RP, Avison DL, Corning BF, et al. 1991. Ketamine/xylazine/butorphanol: A new anesthetic combination for rabbits. Lab Anim Sci 42: 57-62.

  • Papaioannou VE, Fox JG. 1993. Efficacy of tribromoethanol anesthesia in mice. Lab Anim Sci 43: 189-92.

  • Park CM, Clegg KE, et al. 1992. Improved techniques for neonatal rat surgery. Lab Anim Sci 42: 508-13.

  • Phifer CB, Terry LE. 1986. Use of hypothermia for general anesthesia in preweanling rodents. Physiol Behav 38: 887-90.

  • Reid WC, Carmichael KP, et al. 1999. Pathologic changes associated with use of tribromoethanol (Avertin) in the Sprague Dawley rat.Lab Anim Sci 49(6): 665-7.

  • Roughan JV, OB Ojeda and PA Flecknell. 1999. The influence of pre-anesthetic administration of buprenorphine on the anaesthetic effects of ketamine/medetomidine and pentobarbitone in rats and the consequences of repeated anesthesia. Lab Animals 33: 234-42.

  • Weiss J, Zimmermann F. 1999. Tribromoethanol (Avertin) as an anesthetic in mice [Letters to the Editor]. Lab Animals 33: 192-3.

  • Wixson, S.K. 1994. Anesthesia and analgesia for rabbits. In: Manning PJ, Ringler DH, and Newcomer CE, eds., The Biology of the Laboratory Rabbit, 2nd edition, Academic Press, Orlando, FL, pp. 87-109.

  • Zenner W, Meier G, et al. 1997. Adverse effects of tribromoethanol as used in the production of transgenic mice. Lab Animals 32: 407-13.