Rodent anaesthetics – make the right choice

Rodents are often anaesthetised for minor procedures, routine surgery (e.g. castration and ovariohysterectomy) and, from time to time, for more invasive surgery such as a tumour removal

Posted: 26 June 2018

Rodent anaesthetics – make the right choice

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Anaesthesia of rodents follows the same principles as that for dogs, cats and rabbits, though the mortality rate is often higher, mainly as a result of the difficulties associated with;

  • Compromised health
  • Failure to recognise illness
  • Unfamiliarity with the species 
  • Poor anaesthetic technique and monitoring
  • Failure to provide adequate supportive care

Inhalant anesthetics – they’re ideal

For most rodents, the safest anesthetics to use are inhalants such as isofluorane and sevofluorane. They’re ideal as they produce rapid onset loss of consciousness with minimal side effects.

Induction of rodents is usually achieved in an induction chamber and requires a precision vaporiser for safe and accurate delivery. It’s also essential to monitor them closely at all times as an overly deep plane of anaesthesia can rapidly lead to fatalities if the concentration is too high.

Two key advantages of these agents is that they’re not metabolised and therefore have little or no toxic effects. They are also relatively insoluble in blood, and therefore easily expelled during respiration ensuring a quick recovery.

Risk of hypothermia

Animals under anaesthesia are always at increased risk of hypothermia, and this is particularly so in small rodents. Loss of body heat can be significant especially if they’re clipped and washed to prepare for surgery.

Failure to properly regulate the core temperature can significantly compromise the patient, and can be the difference between the success or failure of a procedure.

Species specific considerations

In general, smaller animals have higher metabolic rates and frequently require higher doses of anaesthetics and analgesics at more frequent intervals to achieve the desired effect. Monitoring depth of anaesthesia is essential throughout.

Anaesthetic monitoring of small rodents

This is essential, and is achieved by using all of the following;

  • Digital pinch reflexes
  • Observation of respiratory pattern
  • Checking mucous membrane colour and capillary refill time
  • Monitoring muscle tone and responsiveness to limb and jaw movement
  • Rectal temperature – heat loss in small mammals can be rapid and life threatening. Always insulate your patient using bubble wrap. Provide additional heat with hot water bottles (surgical gloves with warm water work very well) or infrared heaters.
  • Heart rate – monitor via ECG or pulse oximeter

Using injectables – some key principles

  • Injectable anaesthetics – these are often best given by the intraperitoneal route.
  • Injectable analgesics and reversal agents – these are mostly given subcutaneously.
  • Intramuscular injections - generally to be avoided due to the small muscle mass available.
  • Dilution – by diluting drugs in sterile water or saline solution you’ll find it easier to accurately measure the volume for injection. It may also make some drugs less irritating when injected.

Species variations – using anaesthetics in different rodents

1. Mice

Isofluorane is recommended as the first choice of anaesthetic in mice. It should be delivered at a known percentage (1-3% for maintenance; up to 5% for induction) in oxygen from a precision vaporizer.

Surgical anaesthesia can be achieved with the following combination  – fentanyl (0.05g/kg) + midazolam (5mg/kg) + medetomidine (0.5mg/kg) given IP

2. Rats

Rat anaesthesia and analgesia considerations are similar to those described for the mouse. Ketamine combinations in mice are more likely to provide adequate surgical anaesthesia and so may not require supplementary isofluorane.

A typical combination for light surgical anaesthesia can include the following – ketamine (75-100mg/kg) + midazolam (4-5mg/kg) both mixed in the same syringe and given IP

3. Hamsters

Hamster anaesthesia is similar to rat and mouse anaesthesia, though some anaesthetic doses differ.

4. Guinea Pigs

Guinea pigs can be difficult to anaesthetise so ensure you plan and prepare carefully. They may be anaesthetised by facemask when using isofluorane and sevofluorane. Endotracheal intubation can be attempted, but usually requires specialised training, skill and experience.

Monitoring - the toe-pinch withdrawal reflex is less reliable as an indicator of surgical anaesthesia in this species. The ear pinch response is helpful to monitor depth of anaesthesia.

A typical combination of injectable anaesthesia might include ketamine (40mg/kg) and medetomidine (0.25-0.5mg/kg).

Intraperitoneal administration works well, however avoid the large caecum. Avoid intramuscular injections as they may result in self-mutilation due to irritation. Intravenous injection is difficult.

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