Recent discussions in the office of APCF have highlighted some concerns about anaesthetics, in particular the OLD stand-by agents: chloral hydrate and avertin. I emphasise the word old, as both chloral hydrate and avertin are outdated.

Firstly, Chloral hydrate: this agent needs to be justified to the AEC and the AEC must give approval for its use. Approval must be based on scientific rationale and not on cost, availability or past experience/preferences of the user.

Chloral hydrate has traditionally been justified because it lasts approximately two hours and is thought to have minimal impact on the cardiovascular system. This is considered a good feature of this agent.

However there is no analgesia with chloral hydrate and the depth of anaesthesia is variable and the quality of anesthesia questionable. These adverse factors impact on the welfare of the animal.

Chloral hydrate’s use in recovery surgery is unacceptable because it causes peritonitis and paralytic ileus if it is too concentrated.

Finally the justification that chloral hydrate has minimal effect on the cardiovascular system is flawed it is a significant respiratory depressant.

PIs are advised to re-consider their anaesthetic choice away from the use of chloral hydrate

Further reading:

Baxter, M. G., Murphy, K. L., Taylor, P. M., & Wolfensohn, S. E. (2009). Chloral Hydrate Is Not Acceptable for Anesthesia or Euthanasia of Small Animals. Anesthesiology, 111(1), 209.

Beland, F. A., Schmitt, T. C., Fullerton, N. F., & Young, J. F. (1998). Metabolism of chloral hydrate in mice and rats after single and multiple doses. Journal of Toxicology and Environmental Health. Part A, 54(3), 209–226.

Field, K. J., White, W. J., & Lang, C. M. (1993). Anaesthetic effects of chloral hydrate, pentobarbitone and urethane in adult male rats. Laboratory Animals, 27(3), 258–269.

Silverman, J., & Muir, W. W. 3rd. (1993). A review of laboratory animal anesthesia with chloral hydrate and chloralose. Laboratory Animal Science, 43(3), 210–216.

Secondly, Avertin which has not been commercially available for many years has been replaced tribromoethanol (TBE). TBE mimics Avertin’s mode of action, but TBE is a non-approved (FDA) and non-pharmaceutical chemical. PIs are using TBE because it is cheap, readily available and easily prepared for a non-pharmaceutical grade chemical.

TBE is still used in the anaesthesia of mice for embryo transfers. There are many reports that describe the safe and effective use of this drug provided PIs strictly adhere to the precautions necessary for preparation and storage.

However, there are many reports that describe complications to the use of TBE and this is resulting in questions about the use of TBE in animal models being asked by many reviewers of papers and editors of journals. PIs are being asked to justify the use of TBE (and other non-pharmaceutical reagents) on scientific grounds because of the fear that the use of unapproved reagents may have unforeseen consequences on the animal causing it distress and suffering during and/or after an experimental procedure.

These concerns need to be addressed by the PI and considered by the AEC before any protocol is approved.

PIs are generally reluctant to modify an existing anaesthetic regimen because it adds another variable to the protocol and therefore can affect research parameters making outcome comparisons difficult. However, it is a requirement of humane animal care and use to use less harmful pharmaceutical-grade drugs when available. The use of TBE needs to be justified to the AEC and the AEC must give approval for its use. Approval must be based on scientific rationale and not on cost, availability or experience/preference of the user.

Therefore, the PI should think about other anaesthetic regimens that might be just as good and safer. The most common examples include; isoflurane inhalant, ketamine/xylazine injectable or ketamine and other injectable drug combinations. The PI needs to be prepared to conduct a pilot study and comparison of key outcomes under different anesthetic regimens in order to provide definitive answers which in turn will aid the AEC in making a decision about a particular anaesthetic regimen. The use of a non-pharmaceutical-grade reagent can only be justified and approved when scientific necessity dictates that it must be used.

Further reading:

Anestidou, L. (2003). A Change is in Order. Lab Animal, 32(2), 20.

Gardner DJ, Davis JA, Weina PJ, Theune B. Comparison of tribromoethanol, ketamine/acetylpromazine, Telazol/xylazine, pentobarbital, and methoxyflurane anesthesia in HSD:ICR mice. Lab Anim Sci. 1995 Apr;45(2):199-204.

Lieggi C, Artwohl J, Leszczynski J, Rodriguez N, Fickbohm B, Fortman J. Efficacy and safety of stored and newly prepared tribromoethanol in ICR mice. Contemp Top Lab Anim Sci. 2005 Jan;44(1):17-22.

Lieggi C, Fortman J, Kleps R, Sethi V, Anderson J, Brown C, Artwohl J. An evaluation of preparation methods and storage conditions of tribromoethanol. Contemp Top Lab Anim Sci. 2005 Jan;44(1):11-6.

Meyer, R. E., & Fish, R. E. (2005). A review of tribromoethanol anesthesia for production of genetically engineered mice and rats. Lab Animal, 34(10), 47–52.

Papaioannou VE, Fox JG. Efficacy of tribromoethanol anesthesia in mice. Lab Anim Sci. 1993 Apr;43(2):189-92.

Reid, W. C., Carmichael, K. P., Srinivas, S., & Bryant, J. L. (1999). Pathologic changes associated with use of tribromoethanol (avertin) in the Sprague Dawley rat. Laboratory Animal Science, 49(6), 665–667.

Silverman J, Schucker AE, Anestidou L, Villarreal Acosta Y. Anesthetics in GEM: does TBE make the grade? Lab Anim (NY). 2003 Feb;32(2):19-21.

Weiss J, Zimmermann F. Tribromoethanol (Avertin) as an anaesthetic in mice. Lab Anim. 1999 Apr;33(2):192-3

Zeller, W., Meier, G., Burki, K., & Panoussis, B. (1998). Adverse effects of tribromoethanol as used in the production of transgenic mice. Laboratory Animals, 32(4), 407–413.