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Anesthetic Drugs: Rodents and Rabbits



Comments on Specific Agents

Prepared by Deborah M. Mook, DVM

UPDATE: April 2006



Avertin (tribromoethanol)


This agent is widely used in mice for embryo transfer and vasectomy and offers the advantage of not being a federally controlled drug. It can be purchased through chemical suppliers. The drawbacks to this agent are that is cumbersome to prepare, requires special storage conditions, will decompose to toxic by-products if not stored properly, sensitizes some animals to subsequent exposure, and causes idiosyncratic deaths in about 1% of naive mice. It also has arguable inflammatory properties (Zenner, 1998; Reid, 1999; Weiss, 1999). The following information on the practical use of tribromoethanol was kindly provided by Katrina Waymire.

  • A stock of 100% avertin is prepared by mixing 10 gm of 2,2,2 tribromoethanol (Aldrich T4,840-2) and 10 ml of tert-amyl alcohol (Aldrich 24,048-6). Keep the mixture completely covered as it is light sensitive and stir on a stir plate or rotate in a tube at room temperature for 1-2 hours. Millipore filter and store in a covered container in the dark at 4°C. This 100% stock is stable in the cold for a year. If stored improperly, the agent will decompose to acidic by-products and cause severe peritonitis and high mortality. pH changes can be detected using Congo Red (turns purple if by-products present).
  • or 2.5% working stock, dilute 100% avertin 1:40 (1+39) with sterile saline or PBS (1X stock). Cover with foil. Shake gently for 30 minutes at room temperature (oily mix takes a while to go into solution). The 1X working stock is good for only 2 months and can be stored either at 4°C or room temperature. Note: With some batches of 2,2,2-tribromoethanol, the mice may become sick and die several days after being anesthetized or if the compound is dark in color then recrystallizing the 2,2,2-tribromoethanol is necessary. To recrystallize, dissolve 50 gm of 2,2,2-tribromoethanol in 500 ml of boiling petroleum ether (NOT ethyl ether!) or hexane (boiling point 69°C) on a stirring hot plate in a fume hood. Caution: These solvents are extremely flammable; exercise extreme caution. Add a full spatula of charcoal. Filter through fluted filter paper in a glass funnel preheated to 65°C into a second beaker or flask. Cool on ice to 30° C. Pour off supernatant. Break up crystals with a glass rod or metal spatula and dry thoroughly under vacuum overnight. Store at 4°C.
  • The proper dose of avertin may vary with different preparations and should be redetermined each time a new 100% stock is made. To test, inject several mice with doses ranging from 0.014 to 0.018 ml/gm body weight. The dose should be sufficient to give complete anesthesia, but it is also important to check the health and survival of the mice for 3-4 days afterward. For a 20 gm. mouse I usually start with 0.3cc IP injection and wait about 10 min for effect. If the mouse twitches when pinching feet then inject another 0.05 cc and wait 10 min more for complete anesthesia. After surgery, allow mice to recover undisturbed in a warm cage (»30°C). About 1% of mice are over-sensitized to this drug and will die following administration. If the agent is not stored properly, it will decompose creating products that cause toxic inflammatory peritonitis.



Chloral Hydrate (trichloracetaldehyde monohydrate), CIV


Chloral hydrate is a controlled drug with a prolonged onset of action leading to an extended state of pre-anesthetic delirium that may be unpleasant for the animal and it can cause gastric ulcers and adynamic ileus. At the traditional 300 mg/kg dose, cardiac output is stable, but analgesia is poor. Analgesia is adequate at 400 mg/kg, but there are physiologic dyscrasias such as hypotension and bradycardia that could be life-threatening for certain animals. To prevent adynamic ileus, working concentrations solutions should be 50 mg/ml or less. Chloral hydrate must be refrigerated. Administration of a pre-anesthetic anticholinergic agent is recommended to reduce the incidence of side effects. It should be purchased through chemical suppliers.




Fentanyl-Medetomidine


Fentanyl (Sublimaze, CII) is a potent, reversible opioid characterized by excellent analgesia, a short half-life, profound respiratory depression, and minimal cardiovascular depression. Its effects are pharmacologically reversible, but it is a controlled drug. It is not useful alone, but is given in combination with other agents. Fentanyl and medetomidine, each given in a combination of 300 mg/kg IP, provides 60 minutes of surgical anesthesia in the rat. This combination is highly fatal in mice and contraindicated. For best effect, give the medetomidine 30 minutes prior to fentanyl and keep animal in an oxygen-rich environment during induction. This combination is reversible with atipamezole (1 mg/kg IP), nalbuphine (2 mg/kg IP) or butorphanol (0.4 mg/kg IP). Fentanyl, nalbuphine and butorphanol should be purchased on an interdepartmental requisition from the Emory University Hospital Pharmacy. Medetomidine and atipamezole can be ordered through the DAR.




Medetomidine-Ketamine Mixtures


Medetomidine is an alpha-2 receptor agonist with sedative and analgesic properties similar to xylazine, but with fewer undesirable side effects. It provides excellent anesthesia in combination with ketamine. It will still produce hypotension, bradycardia, respiratory depression, diuresis and glycosuria and is often fatal if re-dosed during the same procedure. However, it is completely and safely reversed with atipamezole (1 mg/kg IM, SC, IV or IP). Ketamine is a neuroleptanalgesic drug popularly used in rodents. It has little visceral analgesic properties alone. Ketamine doses, when used in combination with medetomidine, may need to be reduced for male rodents by a factor of 40-50%. Ketamine should be purchased on an interdepartmental requisition from the Emory University Hospital Pharmacy. Medetomidine and atipamezole can be ordered through the DAR.




Pentobarbital Sodium (Nembutal), CII


This sedative-hypnotic agent has little analgesic action at sub-anesthetic doses. It is a strong inducer of hepatic microsomal enzymes and a profound respiratory depressant at anesthetic dosages for many species. The veterinary formulation (100 ml) contains 65 mg/ml in 10% ethanol and the human formulation (20 ml) has 50 mg/ml in 0.678 mg/ml propylene glycol. Rapid injection of propylene glycol may cause intravascular hemolysis, hypotension, apnea, cardiac arrythmias, bradycardia. The analgesic effect of pentobarbital can be enhanced by balanced anesthesia with opioids, ketamine (pentobarbital 20 mg/kg + ketamine 60 mg/kg in rats) or narcotic agonist-antagonist drugs. Pentobarbital should be should be purchased on an interdepartmental requisition from the Emory University Hospital Pharmacy and should be diluted 1:9 with sterile water and used at a working concentration of 5.0-6.5 mg/ml to prevent chemical irritant peritonitis and to permit accurate dosing. Recovery times are often very lengthy and hypothermia, respiratory depression, and hypotension are potential complications. In guinea pigs, hamsters and rabbits, pentobarbital can be highly lethal.




Urethane


This agent is readily soluble in water and used as a 10-20% solution. It provides long-lasting anesthesia with minimal cardiovascular or respiratory depression, but urethane is mutagenic and carcinogenic. Consequently, it is considered a chemical hazard and is used only for non-survival procedures. It may be purchased through chemical suppliers.




Xylazine (Rompun)-Ketamine (Ketaset), CIII


The combination of ketamine and xylazine has been popular for the anesthesia of a wide variety of species, with the effective dose varying widely among species and between strains of the same species. While there may be genetic variability in response to anesthesia within rodent species, as a general rule, surgical anesthesia can be maintained for 45-60 minutes with a single injection of xylazine and ketamine IP. For rabbits undergoing minimally invasive diagnostic procedures requiring immobilization, surgical procedures of moderate intensity (i.e. wound suturing, tissue biopsies) lasting less than 30-45 minutes, or for induction to permit intubation prior to the administration of gas anesthesics the combination of 5 mg/kg xylazine with 35 mg/kg ketamine SC or IM is effective. However, for procedures with intense sympathetic stimulation (i.e. laparotomy) or for anesthesia lasting 60-90 minutes, 0.1 mg/kg butorphanol tartrate (TorbutrolR, Aveco Co., Inc., Fort Dodge, IA) or 0.75 mg/kg acepromazine maleate (PromaceR, Aveco Co. Inc., Fort Dodge, IA) should be given at the time of anesthesia induction with xylazine and ketamine. If it is necessary to further extend anesthesia, incremental doses of one-fourth to one-half the original ketamine dose can be given. Xylazine can be partly reversed with 0.2 mg/kg yohimbine IV. Mixtures of ketamine and xylazine are not stable and often have changes in potency detectable within 7 days of mixing. Ketamine should be purchased on an interdepartmental requisition from the Emory University Hospital Pharmacy. Xylazine and yohimbine can be ordered through the DAR.




Administration Routes


Interscapular SC injections are preferred for both rabbits and rodents providing that the administered agent is not excessively irritating. Because of the stress of restraint, relatively low muscle mass, risk of accidental sciatic nerve injection, inflammation, and prolonged recovery times, intramuscular (IM) injection of anesthetics to rodents is not recommended. Intraperitoneal (IP) injection is better tolerated than IM and is recommended if SC injection is not feasible. Injections by the IP route should be given lateral to the umbilicus in order to avoid injection of the cecum. Properly restrained rabbits will tolerate IM injections into the semimembranosus, semitendinosus, and epaxial muscles.




Needle Sizes and Sites and Recommended Volumes for Injection

Species Subcutaneous Intramuscular Intraperitoneal Intravenous

Mouse

Scruff, 2-3 ml, 20G

Not recommended. Can use quadriceps or caudal thigh, 0.05 ml, 23G

2-3 ml, 21G

Lateral tail vein, 0.2 ml, 25G

Rat

Scruff, back, 5-10 ml, 20G

Not recommended. Can use quadriceps or caudal thigh, 0.3 ml, 21G

5-10 ml, 21G

Lateral tail vein, 0.5 ml, 23G

Hamster

Scruff, 3-4 ml, 20G

Not recommended.  Can use quadriceps or caudal thigh, 0.1ml, 21G

3-4 ml, 21G

Femoral or jugular vein (cut down), 0.3 ml, 25G

Guinea Pig

Scruff, back, 5-10 ml, 20G

Not recommended.  Can use quadriceps or caudal thigh, 0.3 ml, 21G

10-15 ml, 21G

Ear vein, saphenous vein, 0.5 ml, 23G

Rabbit

Scruff, flank, 30-50 ml, 18G

Quadriceps, caudal thigh or lumbar muscles, 0.5-1 ml, 20G

50-100 ml, 20G

Marginal ear vein, 1-5 ml (slow bolus) or CRI, 21G

Adapted from Flecknell, 1987 (Table 3.4)




Local Anesthesia


Specific agents can be given by various routes, topical, infiltrative, regional, retrobulbar, intra-articular, subsynovial, epidural/subarachnoid, and refrigeration/hypothermia, to block nerve depolarization and conduction. Effective use of local anesthesia is useful in MAC reduction - the outcome can be a safer, cheaper anesthetic prep. For surgical purposes, local anesthetics are generally ineffective when applied topically either pre- or post-operatively. Penetration is usually poor through intact skin, the acidic environment of inflammed tissue neutralizes the drug, and the agent may be removed from the site by the grooming patient. Infiltrative or line blocks of a surgical incision site are the most common applications in research. A 0.5% lidocaine (Xylocaine) block of a surgical site will provide < 90 min. of local anesthesia. To avoid cardiac arrhythmias, do not exceed a total dose of 7 mg/kg. For over 3 hours of analgesia, use 0.25% bupivicaine (Marcaine) in the same way, but do not exceed a total local dose of 8 mg/kg. Local anesthetic infusions can be used to facilitate anesthesia when a plane induced by injection is not quite adequate and additional injections may prove dangerous. Topical hypothermic sprays are useful for providing local anesthesia for tail snips of young rodents.




Hypothermia


Hypothermia is favored as a neonatal rodent anesthetic because of low mortality, ease of use, safety, and cost. Newborn rodents are functionally poikilothermic and, with a relatively small body mass, are amenable to rapid core cooling that causes Arefrigeration analgesia by blockage of nerve conduction. Neural conduction is diminished by 75% when tissue is cooled below 20°C and complete neural blockage occurs at about 9°C. To induce hypothermia, pups are either (1) placed in a latex sleeve and immersed up to the neck in crushed ice and water (2°C-3°C) or (2) placed in a paper-lined tube and packed in dry ice. The former method requires a 3-4 minute induction time (2-3 minutes to unconsciousness and 3-5 minutes to complete blockage of neural transmission). The latter method may require 15 minutes to obtain a surgical plane of anesthesia. Analgesia for hypothermia induced by these methods lasts approximately 10 minutes. Simply placing conscious animals in a cold room or on an ice pack are unacceptable as induction may take 30-45 minutes. The anesthetic state may be prolonged by placing the hypothermic pup on an ice pack (3°C-4°C). Studies have shown that rodent pups will maintain a core body temperature of approximately 5°C when kept on an ice pack for a maximum of 15 minutes. Illumination of the surgical field should be fiber optic in nature, because incandescent bulbs may cause inadvertent and uncontrollable warming. Pups should be recovered and slowly rewarmed in an incubator at 33°C or in a warm nest. Complete recovery typically requires 30-60 minutes.




Selected References


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

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

  • Danneman liJ, 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, lientobarbitone and urethane in adult male rats. Lab Animals 27: 258-69.

  • Flecknell li.A. 1996. Anaesthesia and analgesia for rodents and rabbits. In: Handbook of Rodent and Rabbit Medicine, Laber-Laird K, Swindle MM and Flecknell liA, eds., liergammon liress, Butterworth-Heineman, Newton, MA, lili. 219-37.

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

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

  • Huerkamli MJ. 1994. Chemical restraint of rabbits and liocket liets. Current Veterinary Theraliy XII: Small Animal liractice (Kirk RW and Bonagura JD, eds.), WB Saunders Co., lihiladellihia, liA.

  • Kohn DF, Wixson SK, and WJ White. 1994. Anesthesia and Analgesia in Laboratory Animals. Academic liress, Orlando, FL, 1997.

  • Libbin RM, lierson li. 1979. Neonatal rat surgery: Avoiding maternal cannibalism. Science 206: 66.

  • Liliman NS, Marini Rli, Erdman SE. 1990. A comliarison of ketamine/xylazine and ketamine/xylazine/aceliromazine anesthesia in the rabbit. Lab Anim Sci 40: 395-8.

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

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

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

  • lihifer CB, Terry LE. 1986. Use of hyliothermia for general anesthesia in lireweanling rodents. lihysiol Behav 38: 887-90.

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

  • Roughan JV, OB Ojeda and liA Flecknell. 1999. The influence of lire-anesthetic administration of bulirenorlihine on the anaesthetic effects of ketamine/medetomidine and lientobarbitone in rats and the consequences of relieated 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 liJ, Ringler DH, and Newcomer CE, eds., The Biology of the Laboratory Rabbit, 2nd edition, Academic liress, Orlando, FL, lili. 87-109.

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