Injectable agents

There are many available injectable anaesthetic agents. These include:

  • Ketamine (in combination with a sedative)
  • Alphaxalone
  • Hypnorm (with midazolam or diazepam)
  • Etomidate (with fentanyl)
  • Propofol
  • Pentobarbital – produces light anaesthesia, at higher doses surgical anaesthesia is produced but the safety margin is very narrow. A further disadvantage is that full recovery from this anaesthetic could take several hours.

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The most commonly used anaesthetic are:

  • Ketamine (combined with medetomidine) – Produces surgical anaesthesia in many species, including all small rodents, and can be partially reversed with atipamezole to speed recovery.
  • Hypnorm (combined with midazolam) – Produces surgical anaesthesia in rodents and rabbits, and can be partially reversed with buprenorphine or butorphanol
  • Propofol – produces surgical anaesthesia for 5-10 mins in most species when given intravenously.

Local anaesthesia

There a several routes of administration we can choose from.

  • Intramuscular (IM) – It takes about 5-10 minutes for anaesthesia to develop after intramuscular administration. Can cause pain on administration in small rodents as the muscle mass is so small relative to the volume to be injected. Some agents are irritant and can cause muscle damage ( e.g. Ketamine ).
  • Intravenous (IV) – Ideal as the dose can be adjusted to be appropriate for the individual animal, and to produce the depth of anaesthesia required. Unfortunately intravenous injection is technically demanding in small rodents.
  • Intraperitoneal (IP) – This is one of the most widely used means of giving injectable anaesthetics to rodents, but in 5-10% of cases not all of the injected anaesthetic is delivered intraperitoneally. If a large proportion is delivered into the fat or intestines, then an inadequate anaesthetic effect will be produced. Not all anaesthetics are suitable for administration by this route
  • Subcutaneous (SC) – It takes 5 – 15 minutes for anaesthesia to develop after subcutaneous administration. It may be less stressful and produce more consistent results than use of the intramuscular or intraperitoneal routes. Not all anaesthetics are suitable for administration by this route.

What dose rate should we use?

The dose of anaesthetic needed by individual animals varies, so the best way to give these drugs is by intravenous injection.

Since this produces a very rapid effect, the dose given can be adjusted to meet the requirements of the individual animal.

This is practicable in larger species, but in small rodents intravenous administration of drugs is more difficult. In these small animals the anaesthetic is usually given as a fixed-dose by intraperitoneal, subcutaneous or intramuscular injection.

It is therefore important to check that the dose used is appropriate for the particular strain, age and sex of the animal used, by anaesthetising only one or two animals initially.

The dose can then be adjusted before anaesthetising larger numbers.

Duration of anaesthesia

Most of the commonly used injectable anaesthetics will produce a period of medium to deep surgical anaesthesia.

The depth and duration of anaesthesia depends upon the dose given, but giving high doses can cause dangerous depression of the respiratory and cardiovascular system. It will often take 60 – 180 minutes for animals to recover fully from the anaesthetic.

If we give an anaesthetic intravenously, we can use shorter-acting agents and recovery can be complete in 20-30 minutes.

Can recovery be speeded up?

If intravenous injection is not suitable, recovery from some anaesthetic combinations can be speeded up by partial reversal of their effects by administering an antagonist.
For example in rodents, the recovery time from Ketamine / Medetomidine ( or Ketamine / Xylazine ) can be greatly reduced by giving Atipamezole ( an antagonist to Medetomidine and Xylazine ).

Alternatively, rapid recovery can be achieved by using an inhalational anaesthetic agent instead of an injectable one.

  Mice Rats Guinea Pigs Rabbits
Alphaxalone 10 IV 10 IV 40 IM, IP 9-12 IM
Fentanyl/Fluanisone (Hypnorm)+ diazepam 0.4 + IM 0.6 + IM 1.0 + IM 0.3ml IM + IM
Ketamine/medetomidine + 0.5 IP + 0.25-0.5 IP + 0.5 IP SC + SC

Updated on 12th May 2020

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