General Anesthesia: Loss of consciousness in addition to loss of sensation; ideally includes hypnosis, hyporeflexia, analgesia, and muscle relaxation; can be produced using a single drug (i.e. Isoflurane) or a combination of drugs (i.e. Ketamine and Xylazine).
Analgesia: Loss of sensitivity to pain.
Anesthesia does not necessarily equate with analgesia!
General anesthesia produces loss of consciousness, so the animal cannot perceive pain, but in unconscious animals, painful stimuli will still be transmitted and processed by the central nervous system. Although the animal does not perceive pain during the surgery, central hypersensitivity can still develop in the spinal cord and brain causing postoperative pain to be heightened. Some anesthetics do have analgesic properties such as the alpha-2 adrenoreceptor agonists (i.e. Xylazine, Medetomidine). In addition, analgesics can be used as part of the anesthetic regimen (i.e. opioids).
COMMONLY USED ANESTHETICS AND ANALGESICS IN RODENTS
The standard inhalant anesthetics for laboratory animal use are either isoflurane or halothane, delivered to effect in concentrations of 1-3% in oxygen (up to 5% for initial induction), using a precision vaporizer.
Advantages: Advantages of inhalant agents include rapid induction and recovery, with the ability to precisely titrate the level of anesthesia.
Disadvantages: Disadvantages include the cost and logistics of using precision vaporizers, human exposure concerns, the risk of fatal overdosage if an open system is used instead of a precision vaporizer, and depressed respiratory rate and decreased blood pressure. In addition, once animals awaken from gas anesthesia, there is no residual analgesic activity.
Concurrent use of ketamine combinations and/or opioid and/or non-steroidal anti-inflammatory analgesics is strongly encouraged if the procedure is likely to result in any residual pain.
Occupational safety is a serious concern. Inhalants must be directly vented out of the room, or, less reliably, adsorbed in a charcoal canister filter. Filters must be weighed and replaced before they reach target weight (usually an increase of 50 gm). Note that charcoal filtration is not accepted as a safe scavenging system by UBC Health, Safety and Environment.
Ketamine is a dissociative anesthetic used in a wide variety of species. In low doses, ketamine provides chemical restraint with minimal analgesia. In most instances, ketamine is used in combination with other injectable agents.
Advantages of ketamine: Advantages of ketamine are its wide margin of safety in most species and its NMDA receptor blocking action. In combination with other drugs, it can provide a surgical plane of anesthesia for about one half hour.
Disadvantages of ketamine: Disadvantages of ketamine include some irritancy due to low pH and insufficient anesthesia in some species and strains (especially mice) for some procedures. Ketamine is a Class III controlled substance and requires a license for use.
Ketamine may be combined with the alpha-2 agonists, Xylazine or Medetomidine, in the same syringe to produce a deep level of sedation. In some situations and in some species and strains, an adequate depth of anesthesia for surgery may be attained. In other cases, this sedation may require an inhalant agent to achieve surgical anesthesia. It is generally safer to titrate to effect with inhalant anesthetic from a precision vaporizer than with supplemental injections of ketamine.
Advantages: Advantages of ketamine/alpha2-agonist combinations are that they may be combined in one syringe, that they may produce short-term surgical anesthesia with good analgesia, and that recovery can be hastened by reversing the alpha2-agonist with Atipamezole or Yohimbine.
Disadvantages: Disadvantages of ketamine/alpha2-agonist combinations are that they will not reliably reach surgical plane of anesthesia in all cases, and that they can cause profound cardiac depression. The reversal of the alpha2-agonist results in the reversal of the analgesic component of the combination and should only be done when there is concern for the animal.
Caution for use: If a ketamine/alpha2-agonist combination is used for surgery longer than 20 minutes, animals will likely require additional anesthetic. Redosing with a lower dose of ketamine rather than the combination is usually safer, as the cardiovascular depression of alpha2-agonists is often longer-lasting than the sedation or analgesia produced. However, redosing repeated with ketamine alone will not produce a surgical plane of anesthesia. Preferably, a gas anesthetic should be used to continue the anesthetic for procedures longer than 30 minutes.
Adding acepromazine to the ketamine-alpha2-agonist combination may result in deeper and/or longer plane of anesthesia in small rodents, especially rats, and possibly some strains of mice as well. These combinations should be mixed immediately prior to use since the drugs are incompatible and their efficacy, once mixed, will decrease over time.
Ketamine may also be combined with the benzodiazepines, Midazolam or Diazepam, in the same syringe to produce a deep level of sedation. In most cases, this sedation will require an inhalant agent or other anesthetic to achieve surgical anesthesia. In most applications, Midazolam is preferred, as it can be injected intramuscularly; intramuscular injection of propylene glycol (the carrier in injectable diazepam) can cause painful, sterile abscesses and is discouraged.
Advantages: Advantages of ketamine/benzodiazepine combinations are that they may be combined in one syringe, are partially reversible and will produce deep sedation.
Disadvantages: Disadvantages of ketamine/benzodiazepine combinations are that they will not reliably reach surgical anesthesia. This combination, however, is preferred for imaging and other non-painful procedures as it is safer than the ketamine/alpha2-agonist combinations. Diazepam should be restricted to intravenous or intraperitoneal use. Pharmacologically, Telazol is a dissociate-benzodiazepine combination. These drugs are also controlled and require a license for use.
These combinations should be mixed immediately before use and protected from light. Benzodiazipines in mice may cause increased salivation and should be pre-treated with an anticholinergic.
The alpha2-agonists (Xylazine or Medetomidine) are hypnotic analgesics with significant analgesic properties. Used as sole agents, neither produce sufficient depth of anesthesia for even minor surgical procedures. Combined with ketamine, and possibly supplemented with inhalants or local or topical analgesics, they may be useful during surgery. In some species, medetomidine appears to lead to greater anesthetic depth than xylazine, and it is more reliably antagonized by atipamezole.
Advantages: Advantages of alpha2-agonists are that they produce profound analgesia of short duration, can be combined with ketamine +/- acepromazine to produce deeper anesthesia, they are not controlled substances, and they are reversible with IP or subcutaneous atipamezole (yohimbine is sometimes used for xylazine reversal). They are not irritating when injected via intramuscular or intraperitoneal routes.
Disadvantages: Disadvantages in most species include cardiovascular depression (decreased heart rate, decreased cardiac output, and hypotension). In addition, these drugs cause a significant “bulging” of the eyes, making eye protection and animal positioning crucial. They also act as diuretics making fluid replacement important. Oxygen supplementation is helpful to counteract the effects on the cardiovascular and respiratory systems. Alpha2-agonists cause a transient hyperglycemia which may have research implications. Rapid IV administration of reversal agent has produced seizures in some species.
Caution for use: If a ketamine/alpha2-agonist combination is used for surgery longer than 20 minutes, animals will likely require additional anesthetic. Redosing with ketamine initially rather than the combination is usually safer, as the cardiovascular depression of alpha2-agonists is often longer-lasting than the sedation or analgesia produced.
Though superseded in most applications by newer anesthetics, barbiturates still have their place in the research animal laboratory. They are most frequently used in terminal or acute studies, as recovery can be prolonged and unpleasant. Barbiturates are often the anesthetic of choice when neurophysiological recordings such as visual or auditory evoked responses are being conducted. Concurrent use of an analgesic (opioid or non-steroidal anti-inflammatory drug) is encouraged as it may improve pain relief with barbiturate use, and lower the required dose of barbiturate.
Sodium pentobarbital (Nembutal) and sodium thiopental (Pentothal) are currently the two most commonly used barbiturates. The duration of action of pentobarbital is considerably longer than that of thiopental.
Advantages: Barbiturates do not depress cortical evoked responses to the extent that other anesthetics might. Once stable anesthesia has been achieved, it may be longer lasting than with most other injectable agents. Barbiturates are the most common of the injected euthanasia solutions, as they reliably produce unconsciousness before respiratory depression and death.
Disadvantages: Disadvantages of barbiturates include a narrow margin of safety, primarily associated with respiratory depression. Pain sensation is only decreased at surgical planes of unconsciousness, and may even be heightened (hyperalgesia) at subanesthetic doses. Barbiturates for IP injection should be diluted to a strength of 6 mg/ml. Barbiturates are Class II controlled substances, except for some class III euthanasia solutions and require a license for use.
Avertin has been the standard anesthetic in much mouse transgenic work. It can produce short-term (15-20 minutes) surgical anesthesia with good muscle relaxation and moderate respiratory depression. It does not produce significant residual post-procedural analgesia. Because it can cause fatal peritonitis, it is not approved as a recovery anesthetic at UBC.
Opioid drugs are important components of many surgical anesthesia regimens, and are the most potent available post-procedural analgesics. Drugs in this group vary in their potency as well as their duration of action. Fentanyl, oxymorphone, buprenorphine and butorphanol are the most commonly used opioids in laboratory animal care, though others may be used on occasion. Fentanyl is the most potent of the three, but also the shortest acting. Buprenorphine is the longest acting and is good for most post-operative applications. Buprenorphine and butorphanol are mixed agonist/antagonists at different opioid receptors; they produce a less profound respiratory depression than full agonists, but also have a “ceiling effect” in the degree of analgesia produced with increasing doses.
Opioids are most often administered by injection. Oral use can be effective, but requires much higher doses because of “first-pass” liver metabolism when absorbed from the gut.
Pre-emptive analgesic involves using appropriate analgesics before they are needed (i.e. prophylactically) and is strongly recommended as it will prevent pain “windup”, decreasing the requirement in the post-operative period -- buprenorphine may be administered when the general anesthetic is administered, or at any time during surgery. Respiratory depression is minimal, though sleep time may be lengthened. Pre-emptive use enhances pain management during the immediate post-surgical period. Though it increases animal handling (a stressor), administration of the analgesic 30 minutes prior to the initial surgical incision maximizes the analgesic efficacy in most situations.
Advantages: Opioids are potent analgesics. Concurrent use with inhalant or injectable general anesthesia will lower the required dose of the anesthetic to maintain a surgical plane of anesthesia.
Disadvantages: Opioids can suppress respiration (more marked effect with the use of fentanyl than with buprenorphine). Opioids may increase locomotor activity, and may cause pica (abnormal ingestion of non-food items such as bedding) in rats. Alternatively, they may sometimes cause sleepiness and slower recovery from general anesthesia. Opioids are controlled substances and require a license for use.
Cautions for use: Buprenorphine has found favor as the longest-acting opioid analgesic. However, this duration of action is closer to 6 hours in most situations than it is to 12 hours. Twelve hours is the absolute maximum dosing interval for use of buprenorphine for post-procedural pain in rodents. To prepare a dilution for smaller rodents, one 1 ml vial of Buprenex (0.3 mg/ml) is drawn into a sterile syringe and added to 19 ml of sterile 0.9% NaCl and dispensed into a sterile vial (e.g. a sterile serum tube). Label the tube Buprenorphine HCl 0.015 mg/ml solution and the date prepared. Expiration date is either that from the original package or 1 month from the date prepared. The solution must be protected from light
The advent of newer, more potent, more specific anti-inflammatory agents has increased their usefulness in laboratory animal use. Most reduce fever, reduce inflammation, and provide varying degrees of analgesia (acetaminophen does not significantly reduce inflammation).
Advantages: Carprofen, ketoprofen, and meloxicam may have duration of analgesic action up to 24 hours. They may be used concurrently with anesthetics, with opioid analgesics, and with local anesthetic/analgesics. Injectable NSAIDs are useful for accurate dosage and administration to small rodents. They are not controlled substances (some are by veterinary prescription only) and must be obtained through the Animal Care Centre.
Disadvantages: NSAIDs may decrease clotting ability, of possible concern following surgery. Gastric upset and even ulceration may occur, especially with prolonged use. Prolonged use carries the risk of kidney or liver disease. Undesired side effects are more likely with increasing length of usage -- for most situations, limit use of NSAIDs to 3-4 days per animal, except under veterinary supervision. Do not use in dehydrated animals or in animals with kidney or liver dysfunction.
Local anesthetic/analgesic drugs (lidocaine and bupivicaine) may be useful both during surgery, and post-operatively. They block nerve conduction when applied locally at sufficient concentration. In larger animals, Lidocaine has a fast onset of action and provides a couple of hours of analgesia. Bupivicaine has a slower onset of action (up to 30 minutes) but provides up to 12 hours of residual analgesia in some animals. Both of these drugs are thought to provide less duration of analgesia in rodents. Both can be infiltrated subcutaneously at the surgical site. Lidocaine cream (EMLA) is used topically on shaved, intact skin prior to venipuncture, though it requires 30-60 minutes or more of contact with skin to reach full effect. It is not a substitute for injectable local anesthetics for surgical procedures as it only penetrates the skin and is often removed by the animal before it can have an effect.
Even with general anesthesia, the topical or subcutaneous (at surgical incision site) administration of a local anesthetic is recommended in order to provide additional post-surgical analgesia. Local anesthetics should not be used alone to provide post-surgical/post-procedural analgesia.
Advantages: Intra-operative use can augment the pain relief of general anesthetics and reduce the need for frequent redosing of opioids and/or NSAIDs. They are not controlled substances. At appropriate doses, they have minimal cardiovascular effect.
Disadvantages: Intramuscular and intravenous injection should both be avoided. Systemic toxicity (including seizures and death) can result from overdosage (more likely to occur with smaller animals) and with accidental intravenous injection. Lidocaine may sting when first injected. It can be mixed with Sodium Bicarbonate, in a 9:1 Lidocaine to Sodium Bicarbonate ratio to decrease its local irritation.
Urethane, choral hydrate, and alpha-chloralose have some specialized use in laboratory animal anesthesia. Their use should be discussed with a veterinarian as they are considered human health risks and can lead to fatal peritonitis and/or ileus in rodents.