Small Mammal Anesthesia

A rabbit owner makes an appointment for her pet mini-lop because is not eating well and the owner is concerned about it being very listless, as well. When the rabbit is presented to you in the exam room, it is acting far from lethargic. Being a prey animal, it will do all that it can to hide its illness when in strange surroundings, which can make performing a thorough physical examination and procuring blood samples for lab testing very difficult and potentially dangerous for the patient. In many cases, it may be quicker and less stressful to anesthetize the rabbit for examination and venipuncture. Because rabbits possess hypsodontic teeth that grow throughout the life of the animal, it is important to evaluate the incisors and molars for overgrowth and possible mucosal ulcerations in the oropharynx. Examination of the teeth, other than the incisors, can be difficult without anesthesia and a proper speculum. There are many reasons why safe anesthesia protocols should be available to the small animal veterinarian who is treating ferrets, rabbits and rodents. Administering anesthesia to these small mammals requires different skills and protocols than those used for canine and feline patients, due to their unique anatomy and often very small size.

Rabbits are lagomorphs and are often high-strung and nervous due to their status as a prey animal. It is very important to learn proper restraint and handling of rabbits to prevent injury to the patient and also to staff involved in their care. Rabbits may kick out, resulting in luxation or fracture to lumbar vertebrae or limb fractures. A frightened rabbit may have very high levels of circulating catecholamine concentrations, which can seriously affect anesthesia. Several protocols have been developed using injectable anesthetics for rabbits, however, in most veterinary practices, inhalation anesthetics are used most commonly. Inhalation anesthetics are very safe for the rabbit patient when used correctly.

One potential problem with lagomorphs is their very small oropharyngeal opening. In addition to making examination of the oral cavity difficult, it also makes intubation of the trachea a very challenging procedure, due to their small mouth, large cheek teeth and tongue and deep oral cavity. Since rabbits cannot vomit, that eliminates the worries about aspiration of stomach contents and resultant aspiration pneumonia. For this reason, it is not necessary to fast rabbits prior to surgery. Some practitioners recommend fasting for approximately one to two hours, to allow the oral cavity to be cleared of food prior to the procedure. For most procedures, practitioners do not routinely intubate rabbits during anesthesia, as it is easy to traumatize the oral and respiratory structures during intubation. The only potential problem with not intubating rabbits is from excess saliva, and because rabbits produce atropine esterase, administration of atropine as a pre-anesthetic medication may require repeated injections as signs recur. As a pre-anesthetic, glycopyrrolate a better choice for controlling bradycardia, respiratory secretions and salivation, although excessive salivation is rarely a problem with isoflurane anesthesia.

Rabbits may also be administered antibiotics pre-operatively and pain medications may be administered during the procedure, to take effect as the patient wakes. Rabbits may become very depressed post-op due to pain and their strange environment, so appropriate medications must be administered as needed.

Prior to any surgical procedure, it is necessary to perform a complete physical examination, and appropriate lab testing, including a CBC, plasma chemistry panel, urinalysis and perhaps radiographs, ultrasound and ECG. Any abnormalities diagnosed should be treated and the patient should be stabilized prior to any surgical procedure.

All anesthetized small mammals should receive the same monitoring of vital signs as canine and feline patients. Pulse oximeters have been adapted for small animal patients, and these work well for small exotics. Doppler units can be utilized for monitoring blood pressure. It may be more difficult to monitor blood pressure in the smaller mammals. While esophageal stethoscopes are not used as often today, these may be a safe and effective alternative for monitoring heart and respiratory rates in very small patients. Electrocardiogram monitor may be safely used in small exotic mammals, but in the place of alligator clips, small needles are usually utilized instead and the unit must be able to monitor heart rates of 350-400 bpm.

Small mammals will quickly lose body heat while anesthetized, so it is vital that some method be employed to provide warmth during procedures requiring anesthesia. Thermal water blankets used for small animals will work well for small exotics. Heating pads can be used, and laminated rigid thermal beds are also used. In some cases, overhead heating is used. Warmed parenteral fluids may be administered during the procedure. Monitoring the core body temperature of small exotics is very necessary. A rectal thermometer is usually employed for monitoring the temperature.

Ferrets, which are mustelids that are common pet exotic mammals, often develop conditions requiring surgery. Insulinomas, adrenal tumors, tumors in the dermis and subcutis are but a few of the conditions that may require anesthesia. Some ferrets can be quite fractious and require general anesthesia for venipuncture. Proper restraint or general anesthesia is required for anterior vena cava venipuncture.

Anesthesia of ferrets is similar to that of cats. Since ferrets are prone to hypothermia during procedures, heat should always be provided to anesthetized ferrets. Despite their small size, ferrets are usually easily intubated, and do not seem prone to laryngospasm, as cats are. The endotracheal tube diameter used varies between 2.0mm and 4.0 mm. The GI transit time of the ferret is quite fast, so withholding food prior to anesthesia need only be three to four hours in most cases (especially in ferrets with suspected insulinoma) although many practitioners fast ferrets overnight, but they can be allowed access to water.

Atropine is usually used as a pre-anesthetic in ferrets. If antibiotics are warranted, they may also be administered prior to surgery. Many practitioners choose to use an induction chamber for ferrets and rabbits, to administer isoflurane. This chamber is also an excellent method of administering inhalation anesthesia to the smaller mammals, such as guinea pigs, chinchillas, hamsters, gerbils, rats, mice, short-tailed opossums and sugar gliders.

Injectable anesthetics such as ketamine/xylazine, ketamine/diazepam, propofol, TelazolTM and medetomidine/ketamine may also be used, and some of these anesthetics can be quickly and safely reversed.

For most of the very small rodents and marsupials, inhalation anesthetic is usually used via induction chamber. Isoflurane and sevoflurane work very well in these species. In most cases, endotracheal intubation is extremely difficult, if not impossible and is therefore not attempted. Anesthesia can also be induced via facemask. Chinchillas and guinea pigs may have copious and thick respiratory secretions, and unfortunately, glycopyrrolate and atropine are not usually helpful in controlling these secretions. Guinea pigs may also vomit or suffer from gastric reflux while under anesthesia, so for this species, and for all the small exotics, in general, it is best to maintain them with their head and chest elevated during anesthesia. This will reduce aspiration and will improve respiration during anesthesia.

Pain medications are usually warranted post-operatively. Meloxicam, dosed at 0.1-0.2 mg/kg, seems to be an effective and safe dosage for most exotics. Butorphanol and buprenorphine may be used, however, it must be remembered, that there is a higher incidence of ileus from these medications, which can be deleterious, especially in rabbits, and in patients that have undergone gastrointestinal procedures. Narcotics such as morphine, meperidine and pentazocine may be used when indicated, remembering that these medications may also cause ileus. Non-steroidal anti-inflammatories may also be used, however, some of these medications are not safe in patients with gastritis or enteritis and should not be used for more than five days in most patients.

When administered correctly and monitored properly, anesthesia is very safe in small exotic mammals. Very little specialized equipment is needed, however, some equipment must be modified or adapted to fit the small exotic mammal or marsupial patient.

Cadeusus
Copyright 2006 Margaret A. Wissman, D.V.M., D.A.B.V.P.
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