Anaesthesia may produce nausea and vomiting so some animals should be fasted before surgery. Dogs, cats, non-human primates and ferrets vomit. Pigs sometimes vomit. Withholding food for 8h is usually sufficient to prevent vomiting. Sheep and other ruminants regurgitate and withholding food may not reduce this.
Vomiting or regurgitation during the onset or recovery from anaesthesia can have serious consequences, as the protective coughing reflexes that are present when an animal is fully conscious may have been lost. This can result in material being inhaled into the lungs, causing severe pneumonia.
There is no need to withhold food and water from rodents, rabbits and guinea pigs as these animals do not vomit. Withholding food can be detrimental in these species1.
Acclimatising the animal
To be sure that animals are fit to be anaesthetised, they should be given time to become acclimatised to the animal unit (for 7-10 days). Reducing stress also makes anaesthesia safer for the animal.
Before anaesthetising animals, they should be examined and checked that they are in good health. Most laboratory animals should be in good health, but it is important to assess this systematically.
Animals should also be weighed, ideally on several occasions, for example at daily intervals, so that you can assess that they are fit to be anaesthetised.
Anaesthesia is stressful and can cause an animal to reduce its food and water consumption when it recovers. Continuing to weigh them will help ensure that they are recovering well from the anaesthetic.
The technique where you anaesthetise a localised area by injecting anaesthetic (e.g. Lidocaine) into the tissues is called local anaesthesia. Larger areas can be anaesthetised by blocking specific nerve trunks or by administration epidurally or intrathecally (into the CSF) but these are advanced techniques requiring specialised training.
Other than the use of topical anaesthesia for procedures such as venepuncture, these techniques are most frequently used in larger species such as sheep.
Although localised areas of anaesthesia are produced when using local anaesthesia, the animal is still fully conscious, so either physical restraint is needed, or the animal should be trained to accept the procedure or should be sedated.
General anaesthesia is considered to have three components:
- Loss of sensation
- Loss of consciousness
- Production of muscle relaxation.
This is sometimes known as the Triad of Anaesthesia. These components can be provided by administration of several different agents in combination, or use of a single anaesthetic agent that produces all of these effects. The anaesthetic agents used can be given either by injection or by inhalation.
Provided that appropriate equipment is available, volatile anaesthetics offer many advantages when anaesthetising animals:
- They are simple to administer
- Induction (onset) of anaesthesia is usually smooth and rapid.
- It is easy to change the depth of anaesthesia
- Recovery is usually rapid (within 10-15 minutes) and uneventful
- The equipment needed for their safe use is relatively expensive
- It is usually possible to anaesthetise only one animal at a time
- Waste anaesthetic gases must be removed for health and safety reasons
Advantages of injectable anaesthesia:
- Multiple animals can be anaesthetised at the same time (provided you can monitor them effectively).
- Injectable anaesthetics do not require the use of specialist equipment, but it is important to be able to deliver oxygen as many of these agents cause respiratory depression.
- Once the anaesthetic has been administered the dose cannot be changed.
- There is individual variability in the response to a given dose of anaesthetic.
- Since the dose of anaesthetic needed by individual animals varies, the best way to give these agents is by intravenous injection. This is the preferred route for administering injectable anaesthetics as it allows the dose to be adjusted to match the particular animal’s requirements. Unfortunately, it can be technically difficult to use this route in small rodents, so most injectable anaesthetics are given by intraperitoneal or subcutaneous injection in these species.