It is difficult to anaesthetise guinea pigs safely: these animals are easily stressed, orotracheal intubation is not practical, and there is wide interindividual variability in the response to anaesthetics. Stress or pain can cause anorexia, which, in its turn, leads to ketosis and death. This makes it essential to manipulate guinea pigs very gently and use post-operative analgesics and assisted nutrition, if needed.
Numerous precautions, listed below, can be used to minimise anaesthetic risks.
Vitamin C should be administered prior to the operation. Guinea pigs do not need to undergo a period of pre-operative fasting (these animals do not vomit), although a brief fast of 4-6 hours can be observed if wished, in particular to avoid finding residual food in the mouth in the case that dental surgery is planned.
Injectable anaesthetic drugs must be dosed carefully; for this reason it is important to weigh the guinea pig precisely.
During the anaesthesia and throughout the whole period of emergence, heated mattresses must be used in order to prevent the animal from developing hypothermia. The hair should be shaved as little as possible and the surgical field washed with tepid antiseptic solutions, avoiding alcohol-based ones. The animal should always be rehydrated with warmed fluids. The guinea pig should be kept warm and monitored carefully until recovery of consciousness.
Oxygen should always be administered during anaesthesia, even when an injectable anaesthesia is used. Face masks must be transparent in order to allow observation of the animal’s complexion. Guinea pigs, like rabbits, depend on obligate nasal respiration, so it essential not to occlude the nostrils accidentally. This obligate nasal respiration is, however, useful when it is necessary to work on the animal’s oral cavity, because the guinea pig can be oxygenated in any case by applying the mask over the nostrils leaving the mouth free.
Given the disproportion between the volume of the abdomen and that of the thorax, it is useful to hold the thorax slightly lifted in order to facilitate respiration; care should be taken not to extend the neck excessively, but to maintain it at a physiological inclination.
When the level of anaesthesia is adequate, the reflexes are lost and breathing is regular and shallow. The reflexes to evaluate are:
- Foot reflex: pinching a toe stimulates withdrawal of the limb;
- Auricular reflex: the guinea pig tries to retract if the ear is pinched;
- Mandibular tone: resistance to an attempt to open the mouth.
Some subjects do not lose all their reflexes, even when adequately anaesthetised. The adequacy of the anaesthesia is also evaluated by observing the rhythm and depth of respiration and the colour of the mucosae.
The anaesthesia can be monitored using a pulse oximeter applied to the extremity of a limb and a Doppler probe on the thorax. A digital thermometer, introduced into the rectum, is useful for measuring body temperature.
INJECTABLE ANAESTHESIA
Various anaesthetic protocols, using different combinations of injected drugs, have been published in the literature. The level of anaesthesia can be deepened, if necessary, by using inhaled anaesthesia (with isoflurane). Indeed, injectable anaesthesia alone may not be sufficient for major operations.
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Protocol 1 (intramuscular)
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Light anaesthesia |
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Protocol 2 (intramuscular)
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Light anaesthesia, prolonged emergence |
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Protocol 3 (intramuscular)
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Diazepam can cause muscle irritation; it is better to use midazolam. |
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Protocol 4 (intraperitoneal and intramuscular)
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Anaesthesia lasts 20-30 minutes. Deepening of the anaesthetic level with isoflurane may be necessary. The level of anaesthesia can also be deepened by injecting butorphanol (0.1 mg/kg intramuscularly or subcutaneously) at the time of induction. At the end of the operation, the anaesthesia can be reversed by the administration of atipamezole. |
INHALED ANAESTHESIA
Isoflurane is the drug of choice for inhaled anaesthesia. Induction can be performed directly with this anaesthetic gas or with an injectable anaesthetic agent. Given the difficulty of intubation, maintenance anaesthesia can be achieved using a face mask (Fig. 1). The volumes of isoflurane necessary are 2-5% for induction and 0.25-4% for maintenance. The level of anaesthesia is adequate when the straightening reflex is lost.
POST-OPERATIVE PERIOD
The emergence from anaesthesia should take place in a calm, comfortable environment. The guinea pig’s respiration, mucosal colour and temperature should be monitored until the animal has recovered consciousness. The assumption of food and water should be encouraged as soon as possible. In the case of painful operations, analgesics and stimulators of gastric motility (e.g. clebopride) are administered. If hydration is necessary, this should be performed with warmed fluids administered subcutaneously, preferably when the guinea pig is still under the effect of the anaesthetics in order to minimise the distress. Once the guinea pig has recovered consciousness, the presence of a companion can help considerably to reduce stress. The animal should be encourage to eat as soon as possible; if it does not start feeding spontaneously, assisted alimentation must be used.
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Analgesics |
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