Anaesthesia and Analgesia of Exotic Animals: Rodents and Rabbits
Ignacio Alvarez Gómez de Segura
Hospital Universitario La Paz
Anaesthesia and analgesia of less common companion animals like rodents and rabbits has evolved over the last years with the use of relatively newer drugs and techniques previously employed in more common species like the dog or the cat. However, rodents need to be managed according to species. Physical restraint is easy in most rodents and rabbits and use of sedatives may not be considered necessary before anaesthetic induction. However, when indicated, common sedatives and tranquillisers include those employed in larger species such as ketamine, alpha-2 agonists (medetomidine or xylazine), benzodiazepines, opioids like fentanyl, or phenothiazines like acepromazine.
Anaesthesia of rodents
Common difficulties encountered when anaesthetising rodents are related with their reduced body size. Body weight and size limit the availability of easily accessible administration routes like peripheral veins, so the route of choice in rodents is the intraperitoneal. Heat-loss is a common adverse effect during anaesthesia and it is fairly important in rodents since their relatively high surface-to-body weight ratio favours heat dissipation. Electric or water heating blankets are relatively cheap and adequate to prevent heat loss. Alternatively, animals can be covered with isolation material to reduce heat loss.
Drugs or a combination of drugs with a wide safety margin are preferred, due to the high variability in response to anaesthetics by the different species and strains of rodents. Known injectable anaesthetic drugs include ketamine and potent opioids combined with tranquillisers. Fentanyl provides excellent analgesia and may be combined with medetomidine. In some European countries fentanyl is commercially available combined with fluanisone (Hypnorm®), which may be combined with benzodiazepines (diazepam, midazolam) to provide surgical anaesthesia. Ketamine is a versatile drug that may be combined with most tranquillisers to provide anaesthesia. However, its combination with alpha-2 agonists probably provides a better degree of analgesia compared to other combinations. Other common anaesthetics like propofol may also be given to rodents but this requires an intravenous route. The market availability of antagonists to opioids or alpha-2 agonists (atipamezole) facilitates their use, and allows animals a fast recovery from anaesthesia, known as reversible anaesthesia. However, care should be taken to avoid the sudden appearance of pain produced by the antagonism of drugs with analgesic properties. When potent opioids are given, antagonism is better achieved by giving the partial agonist buprenorphine instead of naloxone, and postoperative analgesia can be prolonged for several hours.
Inhalatory anaesthesia with either of the common drugs halothane or isoflurane is the technique of choice for most procedures due to the easiness of its administration and relative safety. Anaesthetic induction is fast with little or no distress and maintenance of anaesthesia is easily performed by applying a simple facemask. The anaesthetic potency of isoflurane and halothane is similar to that in larger species and therefore the values of inspired concentrations of these halogenated agents are similar. Low doses of these agents allow minor procedures whilst higher doses are adequate for major surgery. However, side effects of these drugs derived from the use of high doses are also observable in rodents and careful monitoring of vital signs is required. Opioids can be given to provide intraoperative analgesia and reduce the amount of inhalatory anaesthetics needed to provide surgical anaesthesia. Recovery from inhalatory anaesthesia is fast.
Ventilation requires endotracheal intubation and this is not easy given the reduced size of the oral cavity, larynx and trachea. However such procedure can be performed using an otoscope, although it is rarely necessary in the clinical setting.
Most analgesics used in larger species are also adequate in rodents and rabbits, including opioids (e.g., buprenorphine) or non-steroidal anti-inflammatory drugs (NSAIDs, e.g., carprofen, meloxicam). Nevertheless, pain assessment is still difficult in rodents although initial research focused on behaviour based pain assessment has been developed. The goal in pain assessment is to easily identify signs of pain and individually provide adequate analgesia.
Monitoring of small rodents is a challenging situation since most vital sign monitors have been developed to reflect human standards. Common heart rates are usually out of the specified range for most ECG or pulse oximetry monitors. Furthermore, reduced body size also limits measurement capabilities. Fortunately, in the last ten years, a wide range of veterinary monitors has overcome these limitations and much multiparametric equipment has incorporated monitoring of parameters like ECG, pulse oximetry and temperature in a single device that is suitable for use in small mammals, as well as in larger species.
Rabbits are normally docile, relatively easy to restrain, animals but easily stressed and prolonged restraint is probably best achieved with tranquillisers. Their fur covering and larger body size compared to most rodents reduces heat loss, but, nowadays, manoeuvres to prevent heat loss are also highly recommended. Ear veins are readily accessible for catheterisation and allow the administration of parenteral drugs through this route. The central ear artery should not be inadvertently employed to administer anaesthetics or other drugs to avoid harmful reactions.
Injectable anaesthetics are usually given by the intravenous route although intramuscular administration is also acceptable for low volumes. Ketamine is the most common drug used to induce and maintain anaesthesia lasting 20-45 min depending on the dose and combined tranquilliser used. As in rodent anaesthesia, alpha-2 agonists are the preferred combinations. Fentanyl combinations are also adequate combined with medetomidine, or fluanisone and benzodiazepines. Propofol can be given by continuous infusion but it provides light anaesthesia with poor analgesic properties.
Anaesthetic induction with inhalatory agents is not a recommended procedure since it is stressful to the rabbit and can produce dangerous hypoxia due to the appearance of apnoea. A better approach is to induce anaesthesia with, for example, intravenous propofol and then maintain anaesthesia with halogenated drugs. The anaesthetic potency of common inhalatory drugs (isoflurane, halothane) is lower in the rabbit compared to the dog, cat or rodents and therefore higher concentrations of these drugs must be given to obtain a similar degree of anaesthetic depth (10-20% increase). Endotracheal intubation in the rabbit is also difficult due to the small opening of the oral cavity and protrusion of the tongue, which impedes visualisation of the larynx. However, blind intubation is a relatively easy procedure and, also, an otoscope may be used to visualise the larynx. Postoperative analgesia can be obtained with common analgesics used in other species. Monitors suitable for cats or small dogs are also adequate in rabbits.
Ignacio Alvarez Gomez de Segura