2. Species-specific consideration
In general, smaller animals have higher metabolic rates and frequently require higher doses at more frequent intervals to achieve the desired effect. Species, strain and age differences often overshadow this general principle however. It is always best to start with a drug regimen developed in the species, age and strain with which the Principal Investigator is working, rather than extrapolate from one species to another. Also, when starting to use a new species or strain of animal, it is safer to administer the lower end of the dose range of parenteral anesthetics and analgesics. More can then be administered as needed. Safety and efficacy should thus be demonstrated in a pilot group of animals before a large-scale study is initiated.
Dilution of injected drugs allows more precise dosing, may make some drugs less irritating when injected, and may facilitate absorption, but also may shorten the shelf-life of the compound. Aseptic technique must be observed as mixtures (cocktails) are prepared; this includes wiping the cover of each vial or bottle with 70% ethanol or isopropanol, diluting with sterile water or sterile saline and not reusing needles used for dilution or administration. Only new, sterile needles must be used for withdrawing aliquots from the cocktail. Diluted drugs must be labeled with the contents and concentration of the dilution, the preparer’s name or initials, and dated, then discarded after 1 month, at the expiration date of any of the components, or as indicated by the manufacturer.
Isoflurane is encouraged as the first choice anesthetic in mice. It should be delivered as a known percentage (1-3% for maintenance; up to 5% for induction) in oxygen from a precision vaporizer or nose cone. See: Inhalant Anesthetics
Rodent Anesthesia Monitoring
Anesthetic monitoring of small rodents includes testing of rear foot reflexes before any incision is made, and continual observation of respiratory pattern, mucous membrane color and responsiveness to manipulations throughout the procedure. It is recommended that rectal temperature and heart rate are monitored electronically if possible during long or involved procedures. Monitor the rodent continually, documenting findings every 15 minutes and note the following:
Toe pinch method: The toe pinch method to evaluate depth of anesthesia is useful but not enough in itself. One must use two fingers and give the toe/foot a good squeeze. If there is no withdrawal reaction, the animal is judged deep enough to commence surgery. Remember that after this has been done the fingers are not sterile anymore. A sterile gauze pad may be used to protect the sterile gloves. Alternatively, a hemostat may be used to squeeze toe/foot. In this case, one must be careful not to squeeze too hard. Remember that after the hemostat has been used to squeeze toe, it is not sterile anymore and must not be used for surgery.
Respiratory pattern: Anesthesia will cause a distinct slowing of respiratory rate (RR). The surgeon must evaluate if RR becomes too slow and the anesthesia needs to be lightened and if the depth of respiration becomes too shallow. Increasing RR indicates the need for supplemental anesthesia.
Mucous membranes (MM): MM are evaluated by the color of the pinnae (ears) and toes. If these become bluish this is an emergency, indicating that the animal does not have enough oxygen. Pink is good and red MM usually indicates that the animal is too warm. This is not likely to occur during surgery but may occur during recovery from anesthesia, especially if a heat lamp is used to keep the animal warm. In such a case, the animal recovering from anesthesia must be protected and the lamp moved.
Reaction to surgical manipulation: If the animal makes any kind of move in response to incision or manipulation of organs, surgery must be temporarily stopped and anesthesia supplemented
Injectable anesthetics are typically administered by the intraperitoneal (IP) route. Injectable analgesics and reversal agents are often administered by the subcutaneous or the IP route. Intramuscular (IM) injections must generally be avoided because of the small muscle mass in this species. Diluting drugs in sterile saline solution will make it easier to accurately measure volume for injection.
Ketamine-xylazine and ketamine-medetomidine combinations produce short-duration surgical anesthesia in larger species, but may be insufficient for major surgical procedures in many strains of mice. An alternative approach is to use a ketamine combination, but then titrate to effect with isoflurane from a precision vaporizer. Partial reversal of the xylazine or medetomidine using yohimbine or atipamezole is possible, and will restore cardiovascular status more quickly. See: Dissociative Anesthetics
Mice are nocturnal animals, and are frequently housed in groups of nearly identical animals. These two factors make diagnosis of mild to moderate pain challenging. Weight loss is frequently monitored in animals at risk for ongoing pain. Pre-emptive treatment of pain before signs of pain are obvious is strongly recommended.
See: Mouse Formulary
Dilution of injected drugs allows more precise dosing, but may shorten the shelf-life of the compound. Aseptic technique must be observed as mixtures (cocktails) are prepared; this includes wiping the cover of each vial or bottle with 70% ethanol or isopropanol, diluting with sterile water or sterile saline and not reusing needles used for dilution or administration. Only new, sterile needles must be used for withdrawing aliquots from the cocktail. Diluted drugs must be labeled and dated, then discarded after 1 month, at the expiration date of any of the components, or as indicated by the manufacturer.
Rat anesthesia and analgesia considerations are similar to mouse anesthesia considerations, though some doses vary. In rats, ketamine combinations are more likely to provide adequate surgical anesthesia than in mice, and so may not require supplemental isoflurane. See: Dissociative Anesthetics, Inhalant Anesthetics, and,
Hamster anesthesia is similar to rat and mouse anesthesia, though some anesthetic doses differ. Peripheral veins are extremely difficult to access in hamsters, limiting some of the anesthetic options.
2.4 Guinea Pigs
Guinea pigs can be difficult to anesthetize, especially on a survival basis. Intravenous injection is difficult. Intramuscular injection is acceptable, though animals may self-mutilate at injection sites when they have recovered from anesthesia. Intraperitoneal (IP) administration works well, if the large cecum can be avoided. Guinea pigs may be anesthetized by face mask with volatile anesthetics; endotracheal intubation requires specialized training.
Long procedures are best performed using inhalant anesthesia with an endotracheal tube in place. LASC, LACF and IACUC staff are available to train researchers in this technique. See: Rabbit Formulary
Cats are readily anesthetized using a variety of injectable or inhalant methods.
Initial restraint of a fractious or frightened cat can be a challenge for the researcher’s safety and for the animal’s welfare; choice of technique will depend on the skill level of the researchers as well as the individual cat’s temperament. It is recommended that a tranquilizer such as acepromazine together with ketamine be administered IM or even SC initially to provide chemical restraint. After this has taken effect an IV catheter may be placed and general anesthesia may be induced. Alternatively, chamber induction with isoflurane may be considered. This can be stressful to the cat, and may pose an occupational exposure risk to the workers. Intramuscular or subcutaneous injection of sedatives requires a moderate level of skill, and carries some risk of cat bites and scratches. Training is available, and veterinarians and veterinary technicians can provide direct assistance when necessary.
Non-steroidal anti-inflammatory drugs may be useful, but must be used with extreme caution in cats. Do not exceed recommended doses or frequencies of administration. Acetaminophen is never used in cats. See: Cat Formulary
Dogs are easily anesthetized with a variety of techniques. Intramuscular injection of ketamine or ketamine combinations are to be avoided, because of the incidence of behavioral disturbances.
2.8 Nonhuman Primates
Nonhuman primates require specialized handling and restraint to deliver anesthetics without compromising human safety. Ketamine alone or ketamine-diazepam are typically used by intramuscular injection for initial sedation. Once sedated, primates are easily anesthetized with a variety of techniques. Use of palatable oral medications decreases the need for restraint for medication. See: Nonhuman Primates Formulary
Swine are easily anesthetized with a variety of techniques. However, sedation to achieve endotracheal intubation can be a challenge. Ketamine-xylazine is a common intramuscular sedative, but requires a large volume of injection. Use of Telazol® or Telazol® combinations can significantly reduce the volume of injection for larger animals. See: Swine Formulary
2.10 Frogs and amphibians
Immersion anesthetic (tricaine methanesulfonate, or MS-222 ) is common, especially for fully aquatic species like Xenopus. Because MS-222 is an acidic solution that can cause stress, discomfort, and prolonged anesthetic induction time when concentrations are above 0.5 g/L it must be titrated to a normal pH (~7) by adding sodium bicarbonate. Once a surgical plane of anesthesia has been reached, anesthesia may be supplemented, but not eliminated, by maintaining the animal at 4o C. Post-operative pain management can include local infiltration of bupivacaine. See: 3.13 Local anesthetics
Immersion anesthetic (Tricaine methanesulfonate, or MS-222) is the most common anesthetic in use with fish.
Small birds may be anesthetized by inhalation anesthetics (such as isoflurane) or injectables. Fasting is not generally required in advance. It is vital to maintain adequate warmth during the anesthetic period. Birds, because of their flexible neck and trachea, must be placed with neck stretched and linear so that the airway remains open while the animal is under anesthesia.