Small mammal anesthesia – rabbits and rodents (Proceedings)
Rabbits and rodents are popular pets in urban society because they require little space, are relatively easy to care for, and are a species with which the client can develop a bond. Rodents are generally short lived with the exception of hystricomorphs (guinea pigs, chinchillas, etc.). They have evolved to be prolific to compensate for their short life span. A variety of medical conditions are best treated surgically. Before surgery can be performed, the patient must be anesthetized. The greatest challenges of anesthesia and surgery of pet rodents and rabbits stem from their small size. In recent years, drugs, equipment and techniques have been made available which allow the veterinarian to apply standard anesthetic techniques rabbits and rodents.
Careful and critical preanesthetic evaluation of the small rodent is essential for a successful outcome. A thorough history, physical examination and laboratory data base will provide the anesthesiologist with important information regarding the health status of the patient. Ideally, a preoperative CBC, chemistry profile and complete urinalysis are valuated and easily obtained in rabbits. In rodents, it may be difficult to get a blood sample due to size but many blood analysis machines can provide a lot of information with a very small sample of blood. Minimally, the PCV, total protein, BUN, blood glucose and urine specific gravity should be determined. Rodents urinate frequently, do not store large amounts in the bladder, and void when stressed making it difficult to get a sample by cystocentesis. Yet, renal insufficiency is common in older rodents, especially hamsters. A reasonable sample can be obtained by placing the patient in a clean glass or plastic container. Watch the patient closely and as soon as it urinates, remove the patient and collect the sample. Dilute urine with high protein is suggestive of renal insufficiency.An accurate body weight must be obtained prior to anesthesia to be able to calculate drug doses. The response to anesthetic agents is highly variable in small rodents and age, sex, weight and time of the day can affect the patient's response to anesthetic drugs. Before anesthesia, it is best to calculate doses and draw up emergency drugs. Epinephrine, atropine, and lidocaine are the most commonly used drugs during cardiopulmonary arrest and are inexpensive. Being prepared will avoid panic during an emergency and having to find and calculate emergency drug doses.
Prior to anesthesia, any underlying diseases, especially respiratory disease commonly seen in rabbits and rodents should be treated. Stress can induce respiratory disease in subclinical patients. Perioperative antibiotics can help keep subclinical respiratory infections from becoming a life threatening problem. If the animal is dehydrated, administration of fluids prior to anesthesia is a priority. Fluids are best administered IV or IOs in critically ill patients, but subcutaneous fluids are appropriate in many situations. Crystalloid fluids are administered during anesthesia at the standard rate of 10 ml/kg/hr. If the procedure is expected to take more than 20-30 min. the 5%dextrose in water should be used. Warming the fluids is helpful in minimizing hypothermia.
Since rabbits and rodents cannot vomit, pre-anesthetic fasting is not required. A short fast is beneficial in allowing the patient to swallow any food within the oral cavity minimizing the risk of aspiration or of food being carried into the trachea during intubation. A fast of 1-2 hrs is recommended prior to dental procedures.
Administration of preanesthetic medications is routine in rabbits and rodents. Parasympatholytics will decrease vagal tone as well as the amount of bronchial and salivary secretions. Atropine (0.05 mg/kg SQ; 0.08 mg/kg SQ in rabbits) or glycopyrrolate (0.01 to 0.02 mg/kg SQ or IM; 0.01-0.1 in rabbits) can be used. Atropine has a more rapid onset and is recommended for treating bradycardia and cardiac emergencies while glycopyrrolate is more slowly absorbed but has a longer duration. Up to 50% of rabbits have plasma atropinesterase and the dose is higher and it must be given more frequently. As a preanesthetic, glycopyrrolate is preferred in rabbits. These drugs can thicken respiratory secretions which can obstruct the airway.
Preemptive analgesia is recommended in patients undergoing surgery. Buprenorphine (0.01-0.05 mg/kg SQ) given 20-30 min. prior to induction will provide some sedation and preemptive analgesia. The dose of butorphanol varies widely across rodent species but is also a useful pre-anesthetic opioid (see table in Flecknell). A dose of 0.1-0.5 mg/kg IM or IV is recommended in rabbits. While nonsteroidal anti-inflammatory drugs are useful analgesics, they are best used postoperatively because they can have serious negative effects on renal function if the patient's blood pressure drops during anesthesia.
Preanesthetic administration of sedative tranquillizers is often beneficial in rabbits in rodents to help minimize stress and anxiety, and to minimize patient struggling. The benzodiazepines are known for their anxieolytic properties and midazolam is very useful in calming rabbits and rodents (0.5-5 in rabbits and 3-5 mg/kg in rodents IM or IV). It is also reversible with flumazenil (0.1 mg/kg IV – may precipitate seizure).
Ketamine HCl either alone or in combination with diazepam, midazolam or acepromazine may be given at the lower end of the dosage will allow faster induction of anesthesia with an inhalation agent and minimize stress. It also lowers the maintenance requirements of the inhalation agent.