Chapter 6. Anaesthesia of Common Laboratory Species: Special Considerations
The earlier chapters in this book provide important general information needed to provide safe and effective anaesthesia. After having read these, and having considered the various aspects of the anaesthetic process, the final factor to consider is the species of animal involved. The small body size of rodents can make some procedures difficult or impracticable, the potential risk of physical injury to personnel may increase the need for chemical restraint in non-human primates and differences in anatomy and physiology influence the choice of anaesthetic agents for birds, reptiles, amphibia and fish.
This chapter provides suggested dose rates for a range of different anaesthetic agents for common laboratory species. It is important to recognize that individual responses to anaesthetic agents can vary considerably. In addition, different inbred strains also vary in their responses. Factors such as age, sex and environment can also influence responses. For this reason, dose rates and anaesthetic regimens often need adjusting to suit the particular animals being used in a research facility.
Careful consideration of these individual and species-specific factors, together with the general principles of anaesthesia and perioperative care, will help deliver anaesthetics that meet the scientific needs of specific projects, and ensure the maintenance of high standards of animal welfare.
Laboratory animals are anaesthetized either to provide humane restraint while relatively atraumatic procedures are carried out, or to eliminate the perception of pain during surgical operations. Two main factors influence the selection of a method of anaesthesia – concern for the welfare of the animal and the constraints imposed by specific types of research. An anaesthetic technique should therefore:
• Cause a minimum of distress
• Provide an appropriate degree of analgesia
• Result in an uneventful recovery, free from unpleasant side-effects.
Ideally, the technique should also be easy to implement, should have a high success rate and use anaesthetic agents that have a minimum influence on the experimental work that is being undertaken.
These factors have been considered carefully when selecting the methods recommended for each species in this chapter. The primary consideration has been the well-being of the animal, coupled with ease of use and safety of the drug or drug combination. For a more extensive discussion of selecting an anaesthetic agent, see Chapter 2.
Alternative anaesthetic regimes are included since some research protocols will preclude the use of the drugs recommended and, in addition, some drugs may not be readily available in certain laboratories. A comprehensive listing of anaesthetic drug dose rates for each species is also provided.
It is particularly important to read the general chapters on intra-operative care in conjunction with the notes below on anaesthetic techniques for different species.
The dose rates recommended in this chapter are those that have been found effective in the majority of individuals of the species concerned. The response to an anaesthetic drug can vary considerably and may be influenced by the strain of animal, sex, age and environmental conditions in which the animal is housed (Green, 1981; Lovell, 1986a, b, c; Mogil et al., 2005). Strain variations may also become apparent only during recovery from anaesthesia; for example, different inbred strains of mice have shown varying degree of respiratory depression immediately after isoflurane anaesthesia (Groeben et al., 2003). When using a drug or drug combination for the first time, or when anaesthetizing a different strain of animal, it is advisable to proceed cautiously. As experience is gained, a dose rate appropriate to the particular strain can be established.
In order to provide some guidance as to the predicted effect of the different anaesthetics and their duration of action, the dose rate tables for each species include an estimate of the likely depth of anaesthesia, its duration and the anticipated duration of loss of the animal’s righting reflex. It is important to note that considerable variation in response is to be expected. With many agents, a range of dose rates is given with a corresponding range of anticipated effects (e.g. light–deep anaesthesia). The terminology used is as follows:
Sedation (light, medium or heavy): The animal will have reduced activity and may become completely immobile, but is easily aroused, particularly by painful stimuli.
• Analgesia: Some pain-alleviating effect is present.
• Immobilization: The animal is immobilized but still responds to painful stimuli.
• Light anaesthesia: The animal is immobile and unconscious, but still responsive to even minor surgical procedures.
• Medium anaesthesia: Most surgical procedures (e.g. laparotomy) may be carried out without causing any response, but the animal may still respond to major surgical stimuli (e.g. orthopaedic surgery).
• Deep anaesthesia: The animal is unresponsive to all surgical stimuli.
These terms are used to provide a general guide, but in all instances the depth of anaesthesia produced in a particular animal should be assessed before commencing surgery (see Chapter 4).
Small Rodents
The problems that arise when anaesthetizing rodents are related primarily to the small body size of these species. Their high surface area to body weight ratio makes them particularly susceptible to the development of hypothermia; intravenous drug administration is limited by the size of the superficial veins, and the small and relatively inaccessible larynx makes endotracheal intubation difficult. A further consequence of the small size of these species is that the volumes of anaesthetic required may be very small. In many instances it might be convenient to mix together the required compounds and dilute them with saline or sterile water for injection. Suggested dilutions are given in Appendix 3. Given these practical constraints, it is often simplest to select an inhalational anaesthetic agent, as induction can be achieved smoothly and rapidly in an anaesthetic chamber, and anaesthesia maintained using a suitable breathing system.
Improvement in the health status of laboratory animals has greatly reduced the incidence of spontaneous disease; however, some rodent colonies still have endemic respiratory infections. The disease may not cause obvious clinical signs, but may cause respiratory failure during the period of anaesthesia or result in the development of severe clinical respiratory disease in the post-operative period.
It is unnecessary to withhold food and water before induction of anaesthesia since vomiting on induction or recovery does not occur in any of the small rodents. As mentioned earlier (Chapter 1), problems may be seen with some guinea pigs that retain food in their pharynx after being anaesthetized. If this occurs then a short period of pre-anaesthetic fasting (3–4hours) may be introduced.
Rats
Pre-anaesthetic Medication
Most rats can easily be restrained humanely to enable the intraperitoneal or intramuscular injection of an anaesthetic agent, provided they have become accustomed to being handled. One simple way to achieve this is to weigh the animal daily during its acclimatization period (see Chapter 1). This also provides useful information on its normal growth pattern and ensures that it has recovered from the stress associated with transportation. In general, intraperitoneal and subcutaneous injections are tolerated better than intramuscular injections as they cause less pain to the animal. Pre-anaesthetic medication to sedate the animal is not usually required as many injectable anaesthetics are given as a single mixture of two or more agents. If an intravenous induction agent is to be used, initial sedation with a tranquillizer or sedative/analgesic is recommended.
The following drugs can be used to produce sedation and are listed in order of preference (see Table 6.1):
| Considerable variation in effect occurs between different strains. | ||
| Drug | Dose rate | Comments |
|---|---|---|
| Acepromazine | 2.5mg/kg im, ip | Light sedation |
| Atropine | 0.05mg/kg ip, sc | Anticholinergic |
| Diazepam | 2.5–5.0mg/kg ip, im | Light sedation |
| Fentanyl/dropiderol (Innovar-Vet) | 0.3–0.5ml/kg im | Immobilization/analgesia |
| Fentanyl/fluanisone (Hypnorm) | 0.2–0.5ml/kg im | Light/moderate sedation, moderate analgesia |
| 0.3–0.6ml/kg ip | ||
| Glycopyrrolate | 0.5mg/kg im | Anticholinergic |
| Ketamine | 50–100mg/kg im, ip | Deep sedation, immobilization, mild to moderate analgesia |
| Medetomidine | 30–100μg/kg sc, ip | Light to heavy sedation, mild to moderate analgesia |
| Midazolam | 5mg/kg ip | Light sedation |
| Xylazine | 1–5mg/kg im, ip | Light to heavy sedation, mild to moderate analgesia |
1. Hypnorm (fentanyl/fluanisone; Janssen) (0.2–0.5ml/kg im; 0.3–0.6ml/kg ip). At the lower dose rate, sedation and some analgesia is produced. The higher dose rate produces sufficient analgesia to enable procedures such as skin biopsy or cardiac puncture to be carried out (Green, 1975). Occasionally, marked respiratory depression is seen when the drug is administered at the higher dose rate. If this produces severe cyanosis, it can be reversed with nalbuphine, butorphanol or naloxone.
2. Medetomidine (30–100μg/kg sc) produces light to heavy sedation, and at the higher dose rate, many animals will lose their righting reflex. Some strains require significantly higher dose rates (300μg/kg) before becoming sedated and losing their righting reflex. The degree of analgesia produced is insufficient for anything other than very minor procedures, but is suitable for non-painful manipulations such as radiography (Virtanen, 1989). Medetomidine markedly potentiates the effects of other anaesthetic agents. For example, the concentration of volatile agent needed to produce surgical anaesthesia may be reduced by more than 60%.
3. Xylazine (1–3mg/kg im or ip) produces mild to moderate sedation. Although the drug provides little analgesia when used alone, it markedly potentiates the effects of other anaesthetic agents.
4. Ketamine (50–100mg/kg im or ip) produces deep sedation. The degree of muscle relaxation is poor, and the level of analgesia is insufficient for even superficial surgery (Green et al., 1981a).
6. Diazepam or midazolam (2.5–5mg/kg im or ip) produces light sedation, but neither drug has any analgesic action.
Atropine (0.05mg/kg ip or sc) or glycopyrrolate (0.5mg/kg im) (Olson et al., 1993) can be administered to reduce salivary and bronchial secretions and protect the heart from vagal inhibition.
Appropriate pre-anaesthetic medication will reduce the stress caused by induction of anaesthesia and also ease handling and restraint. In addition, it will reduce the amount of other anaesthetic agents required to produce general anaesthesia. The dose rates of anaesthetic drugs quoted in Table 6.2 apply to rats that have received no pre-anaesthetic medication unless otherwise stated. Generally, these dosages can be reduced by at least 30–50% if one of the drugs listed above has been administered.
| Duration of anaesthesia and sleep time (loss of righting reflex) are provided only as a general guide, since considerable between-animal variation occurs. For recommended techniques, see text. | ||||
| *Dose in millilitres per kilogram of a mixture of one part ‘Immobilon’, one part midazolam (5mg/ml initial concentration) and two parts water for injection. | ||||
| †Dose in millilitres per kilogram of a mixture of one part ‘Hypnorm’ plus two parts water for injection, and one part midazolam (5mg/ml initial concentration). | ||||
| Drug | Dose rate | Effect | Duration of anaesthesia (minutes) | Sleep time (minutes) |
|---|---|---|---|---|
| Alphaxalone/alphadolone | 10–12mg/kg iv | Surgical anaesthesia | 5 | 10 |
| Chloral hydrate | 400mg/kg ip | Light/surgical anaesthesia | 60–120 | 120–180 |
| Alpha-chloralose | 55–65mg/kg ip | Light anaesthesia | 480–600 | Non-recovery only |
| Etorphine/methotrimeprazine (Immobilon)+midazolam | 0.5ml/kg sc* | Surgical anaesthesia | 60–70 | 120–240 |
| Fentanyl/fluanisone+diazepam | 0.6ml/kg ip+2.5mg/kg ip | Surgical anaesthesia | 20–40 | 120–240 |
| Fentanyl/fluanisone/midazolam | 2.7ml/kg ip† | Surgical anaesthesia | 30–40 | 120–240 |
| Fentanyl/medetomidine | 300μg/kg+200μg/kg ip | Surgical anaesthesia | 60–70 | 240–360 |
| Inactin (thiobutobarbital) | 80mg/kg ip | Surgical anaesthesia | 60–240 | 120–300 |
| Ketamine/acepromazine | 75mg/kg+2.5mg/kg ip | Light anaesthesia | 20–30 | 120 |
| Ketamine/diazepam | 75mg/kg+5mg/kg ip | Light anaesthesia | 20–30 | 120 |
| Ketamine/medetomidine | 75mg/kg+0.5mg/kg ip | Surgical anaesthesia | 20–30 | 120–240 |
| Ketamine/midazolam | 75mg/kg+5mg/kg ip | Light anaesthesia | 20–30 | 120 |
| Ketamine/xylazine | 75–100mg/kg+10mg/kg ip | Surgical anaesthesia | 20–30 | 120–240 |
| Ketamine/xylazine/acepromazine | 40–50mg/kg+2.5mg/kg+0.75mg/kg im | |||
| Methohexital | 10–15mg/kg iv | Surgical anaesthesia | 5 | 10 |
| Pentobarbital | 40–50mg/kg ip | Light anaesthesia | 15–60 | 120–240 |
| Propofol | 10mg/kg iv | Surgical anaesthesia | 5 | 10 |
| Thiopental | 30mg/kg iv | Surgical anaesthesia | 10 | 15 |
| Tiletamine/zolezepam | 40mg/kg ip | Light anaesthesia | 15–25 | 60–120 |
| Urethane | 1000mg/kg ip | Surgical anaesthesia | 360–480 | Non-recovery only |
General Anaesthesia
Injectable Agents
The small body size of the rat makes intravenous injection difficult; hence, drugs are usually administered by the intraperitoneal or intramuscular route. If these routes are used, it is not possible to administer the drug gradually to effect and the anaesthetic must be given as a single, calculated dose. Because of the wide variation in drug response between different strains of rat, between male and female animals and between individuals, it is best to use a drug or drug combination that provides a wide margin of safety. Anaesthetic dose rates are summarized in Table 6.2.
The anaesthetic combination of choice for rats is fentanyl/fluanisone (Hypnorm, Janssen) together with diazepam or midazolam (0.6ml/kg ip ‘Hypnorm’, and diazepam 2.5mg/kg ip). When using midazolam the components are mixed together with water for injection (see Appendix 3). These combinations provide good surgical anaesthesia with excellent muscle relaxation lasting about 20–40minutes (Green, 1975; Flecknell and Mitchell, 1984). Longer periods of anaesthesia can be achieved by the administration of additional doses of Hypnorm (about 0.1ml/kg im every 30–40minutes). Following the completion of surgery, the anaesthesia can be reversed using nalbuphine (0.1mg/kg iv, 1.0mg/kg ip or sc) or butorphanol (0.1mg/kg iv, 2mg/kg ip or sc).
A second effective alternative is to administer medetomidine (0.5mg/kg ip) or xylazine (10mg/kg ip) in combination with ketamine (75mg/kg ip). The two compounds can be mixed in the same syringe to provide good surgical anaesthesia, although the depth of anaesthesia may be insufficient for major surgery in some animals (Van-Pelt, 1977; Green et al., 1981a; Hsu et al., 1986; Wixson et al., 1987; Nevalainen et al., 1989). This combination provides about 30minutes of surgical anaesthesia. The combination can be partially reversed using atipamezole (1mg/kg sc or ip), but early reversal (10–20minutes after induction) may be associated with undesirable behavioural disturbances due to the effects of ketamine (Morris, personal communication). Ketamine/xylazine has been reported to cause an increased incidence of post-anaesthetic corneal ulceration, although the incidence can be reduced by reversal of xylazine (Turner and Albassam, 2005). Ketamine and xylazine can also be administered in combination with acepromazine, enabling the use of lower doses of the individual components (Welberg et al., 2006).
A third safe and effective anaesthetic regimen is fentanyl (300μg/kg ip) and medetomidine (300μg/kg ip). The two agents can be mixed and administered as a single injection. Fentanyl/medetomidine provides about 60minutes of surgical anaesthesia (Hu et al., 1992). Sufentanyl (40μg/kg) and medetomidine (150μg/kg), mixed and administered subcutaneously as a single injection, may also be used (Hedenqvist et al., 2000). Both combinations produce respiratory depression and this can be severe, so administration of oxygen is strongly recommended. However, recovery is very rapid provided anaesthesia is reversed by administration of atipamezole (1mg/kg sc or ip) (to reverse medetomidine) and either nalbuphine (0.1mg/kg iv, 1.0mg/kg ip or sc), butorphanol (0.1mg/kg iv, 2mg/kg ip or sc) or another mixed agonist/antagonist opioid analgesic (see Table 5.5–5.8 and 5.8 and Table 6.3). Experience has shown that the quality of induction and recovery with this method of anaesthesia is greatly improved by allowing the rats to acclimatize for 1–2hours after movement into the room in which the procedure is undertaken (Drage, personal communication).
| Compound | Anaesthetic regimen | Dose rate | Comments |
|---|---|---|---|
| Atipamezole | Any regimen using xylazine or medetomidine | 0.1–1mg/kg im, ip, sc or iv | Highly specific alpha2 adrenoreceptor antagonist; dose required varies depending on dose of xylazine or medetomidine administered |
| Buprenorphine | Any regimen using μ opioids (e.g. fentanyl) | See Table 14c | Slower onset than butorphanol and nalbuphine, but longer-acting analgesia |
| Doxapram | All anaesthetics | 5–10mg/kg im, iv or ip | General respiratory stimulant |
| Flumaznil | Benzodiazepine (e.g. midazolam) | 0.1–10mg/kg | Dose varies depending upon dose of benzodiazepine; resedation may occur |
| Butorphanol | Any regimen using μ opioids (e.g. fentanyl) | See Table 14c | Almost as rapid-acting as naloxone, maintains post-operative analgesia |
| Naloxone | Any regimen using μ opioids (e.g. fentanyl) | 0.01–0.1mg/kg iv, im or ip | Reverses analgesia as well as respiratory depression |
| Yohimbine | Any regimen using xylazine or medetomidine | 0.2mg/kg iv 0.5mg/kg im | Relatively non-specific antagonist; not recommended |
If intravenous administration of drugs is feasible, then propofol (10mg/kg iv) (Glen, 1980; Brammer et al., 1992; Cockshott et al., 1992) or alphaxalone/alphadolone (6–9mg/kg iv) (Green et al., 1978) produces surgical anaesthesia, and both compounds are especially useful for administering by continuous infusion to provide stable, long-lasting anaesthesia. The maintenance dose of propofol can be markedly reduced if buprenorphine is administered as pre-anaesthetic medication (Penderis and Franklin, 2005). When administered by the intraperitoneal route the effects are less predictable and these drugs can only be relied upon to produce heavy sedation. It is likely that alphaxalone will have similar properties.
Tiletamine/zolezepam generally only produces light anaesthesia, but surgical planes of anaesthesia can be produced in some strains of rat. In these animals the degree of cardiovascular system depression seemed less than with other anaesthetic agents (Saha et al., 2007).
Pentobarbital should be diluted to provide a 30mg/ml solution and up to 40–50mg/kg administered intraperitoneally. Severe respiratory depression invariably accompanies the onset of surgical anaesthesia and this agent has a narrow safety margin. Until an appropriate dose rate is established, both inadequate and excessively deep anaesthesia may result, so this drug is best avoided in rats. Intraperitoneal administration of pentobarbital may also cause pain, as a result of the low pH of the solution (Svendsen et al., 2007).
Inhalational Agents
The most convenient method of inducing anaesthesia in the rat is to use an anaesthetic chamber. This should be constructed from perspex, so that the animal can be observed during induction. Anaesthetic vapour should be supplied from an anaesthetic machine, and the chamber should be designed so that excess anaesthetic gas can be ducted to a gas-scavenging device or removed from the room via the ventilation system. Following induction of anaesthesia, the rat should be removed from the chamber and anaesthesia maintained using a small face mask connected to the anaesthetic machine (see Chapter 3).
Endotracheal Intubation
The major disadvantage of using a face mask for connection of the animal to the anaesthetic gas supply is that it is difficult to assist ventilation should this prove necessary. Endotracheal intubation, together with use of an appropriate anaesthetic circuit, allows easy control of ventilation. Intubation is not a difficult procedure to master, especially if specialized apparatus is purchased. Further details of the technique are given in Chapter 3.
Mechanical Ventilation
Several different ventilators are available for use in rats, with a variety of mechanisms of action. All aim to achieve controlled ventilation of the lungs by means of the application of intermittent positive pressure to the patient’s airway. The ventilators manufactured by Harvard Apparatus Ltd. (Appendix 4) are the most widely used in the UK. A volume-cycled piston model and a pressure-cycled ventilator are available. Neither of these ventilators has facilities for humidification of anaesthetic gases, but this can be achieved quite readily by bubbling the gases through a water bath. An economic approach is to purchase a human infant feeding bottle warmer, fill it with water and pipe anaesthetic gases through a glass aerator placed in the unit.
Anaesthetic management is particularly important to prevent the development of hypothermia in rats. Ideally, the rat should be placed on a thermostatically controlled heating blanket (Harvard Apparatus Ltd.) or alternatively, heating lamps can be used. Both measures can be combined with the use of insulating material such as cotton wool, aluminium foil or bubble packing (Chapter 4). These measures must be continued in the post-operative recovery period.
Mice
Pre-anaesthetic Medication
Mice are easily restrained humanely and it will rarely be necessary to produce sedation before induction of anaesthesia. If sedation is required, the following drugs can be used (Table 6.4):
| Considerable variation in effects occurs between different strains. | ||
| Drug | Dose rate | Comments |
|---|---|---|
| Acepromazine | 2–5mg/kg ip, sc | Light sedation |
| Atropine | 0.04mg/kg sc | Anticholinergic |
| Diazepam | 5mg/kg im, ip | Light sedation |
| Fentanyl/dropiderol (Innovar-Vet) | 0.5ml/kg im | Immobilization, analgesia |
| Fentanyl/fluanisone (Hypnorm) | 0.1–0.3ml/kg ip | Light sedation, moderate analgesia |
| Ketamine | 100–200mg/kg im | Deep sedation, mild to moderate analgesia |
| Medetomidine | 30–100μg/kg sc | Light to deep sedation, mild to moderate |
| Midazolam | 5mg/kg im, ip | Light to moderate sedation |
| Xylazine | 5–10mg/kg ip | Light sedation, mild to moderate analgesia |
1. Hypnorm (Janssen; 0.1–0.3ml/kg ip) provides sedation and sufficient analgesia for superficial procedures such as ear punching (Green, 1975). The drug is most conveniently administered as a 1:10 dilution of the commercial preparation. The effects of this mixture can be partially reversed using nalbuphine (4mg/kg sc or ip) or butorphanol (2mg/kg ip or sc).
2. Medetomidine (30–100μg/kg ip) produces light to deep sedation. As with the rat, considerable strain variation may occur. Sedation can be completely reversed using atipamezole (1mg/kg ip).
3. Xylazine (5–10mg/kg ip) produces sedation but appears to have little analgesic action when used alone in mice.
4. Acepromazine (2–5mg/kg ip) produces sedation but has no analgesic action.
5. Diazepam (5mg/kg ip) or midazolam (5mg/kg ip) produces sedation but no analgesia.
General Anaesthesia
Injectable Agents
As with rats, drugs are most conveniently administered by the intraperitoneal route. Dose rates of anaesthetic agents are summarized in Table 6.5.
| Duration of anaesthesia and sleep time (loss of righting reflex) are provided only as a general guide, since considerable between-animal variation occurs. For recommended techniques, see text. | ||||
| *Dose in millilitres per kilogram of a mixture of one part ‘Hypnorm’ plus two parts water for injection, and one part midazolam (5mg/ml initial concentration). | ||||
| Drug | Dose rate | Effect | Duration of anaesthesia (minutes) | Sleep time (minutes) |
|---|---|---|---|---|
| Alphachoralose | 100–120mg/kg ip | Light anaesthesia | 300–420 | Non-recovery only |
| Alphaxalone/alphadolone | 10–15mg/kg iv | Surgical anaesthesia | 5 | 10 |
| Chloral hydrate | 400mg/kg ip | Light anaesthesia | 30 | 60–90 |
| Fentanyl/fluanisone (Hypnorm)+diazepam | 0.4ml/kg ip+5mg/kg ip | Surgical anaesthesia | 30–40 | 120–240 |
| Fentanyl/fluanisone (Hypnorm)/midazolam | 10.0ml/kg ip* | Surgical anaesthesia | 30–40 | 120–240 |
| Ketamine/acepromazine | 100mg/kg+5mg/kg ip | Immobilization/anaesthesia | 20–30 | 40–120 |
| Ketamine/diazepam | 100mg/kg+5mg/kg ip | Immobilization/anaesthesia | 20–30 | 60–120 |
| Ketamine/medetomidine | 75mg/kg+1.0mg/kg ip | Surgical anaesthesia | 20–30 | 60–120 |
| Ketamine/midazolam | 100mg/kg+5mg/kg ip | Immobilization/anaesthesia | 20–30 | 60–120 |
| Ketamine/xylazine | 80–100mg/kg+10mg/kg ip | Surgical anaesthesia | 20–30 | 60–120 |
| Ketamine/xylazine/acepromazine | 80–100mg/kg+10mg/kg ip+3mg/kg ip | Surgical anaesthesia | 30–40 | 60–120 |
| Methohexital | 10mg/kg iv | Surgical anaesthesia | 5 | 10 |
| Metomidate/fentanyl | 60mg/kg+0.06mg/kg sc | Surgical anaesthesia | 40–60 | 90–120 |
| Pentobarbital | 40–50mg/kg ip | Immobilization/anaesthesia | 20–40 | 120–180 |
| Propofol | 26mg/kg iv | Surgical anaesthesia | 5–10 | 10–15 |
| Thiopental | 30–40mg/kg iv | Surgical anaesthesia | 5–10 | 10–15 |
| Tiletamine/zolezepam | 80mg/kg ip | Immobilization | 60–120 | |
| Tribromoethanol | 240mg/kg ip | Surgical anaesthesia | 15–45 | 60–120 |
The anaesthetic combination of choice is fentanyl/fluanisone (Hypnorm, Janssen) together with midazolam or diazepam (0.4ml/kg ip ‘Hypnorm’, and diazepam 5mg/kg ip). When using midazolam the components are mixed together with water for injection (see Appendix 3). These combinations provide good surgical anaesthesia lasting about 20–40minutes (Green, 1975; Flecknell and Mitchell, 1984). Anaesthesia can be prolonged by the administration of additional doses of Hypnorm (0.3 every 30–40minutes). Following the completion of surgery, anaesthesia can be partially reversed by the administration of nalbuphine (4mg/kg ip or sc) or butorphanol (2.0mg/kg sc or ip).
A combination of ketamine (75mg/kg ip) and medetomidine (1.0mg/kg ip) produces moderate surgical anaesthesia in most strains of mouse. In some strains, however, the degree of analgesia is insufficient for major surgery (e.g. laparotomy) (Voipio et al., 1990; Cruz et al., 1998). In contrast, this dose of medetomidine caused increased mortality; better results were obtained with a dose of 0.25mg medetomidine and 100mg/kg ketamine (Kilic et al., 2001). This illustrates the importance of assessing the effects of injectable anaesthetics in the particular strain, age and sex of mouse that is to be used.
Ketamine and medetomidine can be mixed in the same syringe and given as a single injection. Anaesthesia can be partially reversed by administration of atipamezole (1mg/kg sc).
An alternative combination is that of ketamine (80–100mg/kg ip) and xylazine (10mg/kg ip). This combination can also be pre-mixed in the correct proportions and administered as a single intraperitoneal injection. It provides 20–30minutes of anaesthesia, but the depth of anaesthesia is often insufficient to enable major surgery to be carried out humanely (Mulder and Mulder, 1979; Green et al., 1981a; Erhardt et al., 1984). The depth and duration of anaesthesia can be increased by the addition of acepromazine (ketamine 100mg/kg, xylazine 10mg/kg, acepromazine 3mg/kg, all administered ip) (Arras et al., 2001; Buitrago et al., 2008). Anaesthesia can be partially reversed by administration of atipamezole (1mg/kg sc).
The combination of metomidate (60mg/kg) and fentanyl (0.06mg/kg) produces stable surgical anaesthesia in mice (Green et al., 1981b). The two drugs are combined and given as a single subcutaneous injection.
If the technique of intravenous injection can be mastered, then either propofol (26mg/kg iv) (Glen, 1980) or alphaxalone/alphadolone (Child et al., 1971; Green et al., 1978) can be used to provide short periods (5–10minutes) of anaesthesia. An advantage of these compounds is that repeated administration to prolong anaesthesia is not associated with prolongation of the recovery period (see section on Long-Term Anaesthesia in Chapter 5). It is likely that alphaxalone will have properties similar to those of alpaxalone/alphadolone. Intraperitoneal administration of propofol, either alone or together with opioid analgesics, has unpredictable effects and is not recommended in this species (Alves et al., 2007).
If pentobarbital is to be used, it should be diluted to provide a 6mg/ml solution and administered at a dosage of 40–50mg/kg ip. The variation of effect in different strains of mice is very considerable, sleep times ranging from 10 to 300minutes (Lovell, 1986b) with identical doses of anaesthetic (45mg/kg), so that over- or under-dosage with this drug frequently occurs.
Inhalational Agents
Induction of anaesthesia using an anaesthetic chamber is simple and convenient. Maintenance using a face mask is straightforward, but may require a suitably sized mask to be constructed, for example from the end of a disposable syringe barrel.
Endotracheal Intubation
Endotracheal intubation is technically difficult to carry out in mice and requires the use of a purpose-built laryngoscope (Costa et al., 1986) or purchase of specialist apparatus (see Chapter 3).
Mechanical Ventilation
Mechanical ventilation can be carried out using one of a number of purpose-designed ventilators (Schwarte et al., 2000) (Appendix 7, Chapter 5).
Anaesthetic Management
Mice are even more prone than rats to develop hypothermia, and it is essential to take measures to maintain body temperature (see Chapter 4).
Hamsters
Pre-anaesthetic Medication
Hamsters are easily restrained humanely and pre-anaesthetic sedation is rarely necessary. If restraint is a problem, an anaesthetic chamber should be used for induction of anaesthesia. If sedation is required, the following drugs can be used (see also Table 6.6):
| Considerable variation in effects occurs between different strains. | ||
| Drug | Dose rate | Comments |
|---|---|---|
| Acepromazine | 2.5mg/kg ip | Light sedation |
| Atropine | 0.04mg/kg sc | Anticholinergic |
| Diazepam | 5mg/kg im, ip | Light to moderate sedation |
| Fentanyl/dropiderol (Innovar-Vet) | 0.9ml/kg im | Analgesia; unpredictable degree of sedation |
| Fentanyl/fluanisone (Hypnorm) | 0.2–0.5ml/kg im 0.3–0.6ml/kg ip | Light/moderate sedation, moderate analgesia |
| Glycopyrrolate | 0.5mg/kg im | Anticholinergic |
| Ketamine | 50–100mg/kg ip | Deep sedation, immobilization, mild to moderate analgesia |
| Medetomidine | 30–100μg/kg sc, ip | Light to heavy sedation, mild to moderate analgesia |
| Midazolam | 5mg/kg ip | Light sedation |
| Xylazine | 1–5mg/kg im, ip | Light to heavy sedation, mild to moderate analgesia |
1. Hypnorm (0.5ml/kg ip) provides sufficient analgesia for superficial procedures.
2. Medetomidine (100μg/kg sc) produces moderate sedation in hamsters, but animals do not lose their righting reflex even at high dose rates (Morris, 1991).
3. Diazepam (5mg/kg ip) or midazolam (5mg/kg ip) produces sedation but no analgesia.
Atropine (0.04mg/kg sc, im or ip) can be administered to reduce salivary and bronchial secretions. Dose rates for general anaesthesia given below should be reduced by 30–50% if one of the sedative drugs listed above has been administered.
General Anaesthesia
Injectable Agents
As with other small rodents, drugs are most conveniently administered by intraperitoneal injection (see also Table 6.7).
| Duration of anaesthesia and sleep time (loss of righting reflex) are provided only as a general guide, since considerable between-animal variation occurs. For recommended techniques, see text. | ||||
| *Dose in millilitres per kilogram of a mixture of one part ‘Hypnorm’ plus two parts water for injection, and one part midazolam (5mg/ml initial concentration). | ||||
| Drug | Dose rate | Effect | Duration of anaesthesia (minutes) | Sleep time (minutes) |
|---|---|---|---|---|
| Alpha-chloralose | 80–100mg/kg ip | Immobilization | 180–240 | |
| Alphaxalone/alphadolone | 150mg/kg ip | Immobilization/anaesthesia | 20–60 | 120–150 |
| Fentanyl/fluanisone (Hypnorm)+diazepam | 1ml/kg im or ip+5mg/kg ip | Surgical anaesthesia | 20–40 | 60–90 |
| Fentanyl/fluanisone (Hypnorm)/midazolam | 4.0ml/kg ip* | Surgical anaesthesia | 20–40 | 60–90 |
| Ketamine/acepromazine | 150mg/kg+5mg/kg ip | Immobilization/anaesthesia | 45–120 | 75–180 |
| Ketamine/diazepam | 70mg/kg+2mg/kg ip | Immobilization/anaesthesia | 30–45 | 90–120 |
| Ketamine/medetomidine | 100mg/kg+250μg/kg ip | Surgical anaesthesia | 30–60 | 60–120 |
| Ketamine/xylazine | 200mg/kg+10mg/kg ip | Surgical anaesthesia | 30–60 | 90–150 |
| Pentobarbitone | 50–90mg/kg ip | Immobilization/anaesthesia | 30–60 | 120–180 |
| Tiletamine/zolezepam | 50–80mg/kg ip | Immobilization/anaesthesia | 20–30 | 30–60 |
| Tiletamine/zolezepam/xylazine | 30mg/kg+10mg/kg ip | Surgical anaesthesia | 30 | 40–60 |
| Urethane | 1000–2000mg/kg ip | Surgical anaesthesia | 360–480 | Non-recovery only |
The anaesthetic combination of choice for Syrian hamsters is fentanyl/fluanisone with midazolam or diazepam (1.0ml/kg ip ‘Hypnorm’, and diazepam 5mg/kg ip). When using midazolam the components are mixed together with water for injection (see Appendix 3). These combinations provide good surgical anaesthesia lasting about 20–40minutes (Flecknell and Mitchell, 1984) and can be partially reversed with nalbuphine (2mg/kg sc) or butorphanol (2.0mg/kg sc).
An alternative, equally satisfactory combination in the Syrian hamster is ketamine (100–200mg/kg ip) and xylazine (10mg/kg ip), which in this species appears to reliably produce surgical anaesthesia (Curl and Peters, 1983). Ketamine (100mg/kg) mixed with medetomidine (0.25mg/kg) also appears to produce effective surgical anaesthesia (Erhardt et al., 2001). Anaesthesia can be partially reversed using atipamezole (1mg/kg).
The use of pentobarbital (50–90mg/kg ip) in hamsters is particularly hazardous and a high mortality often occurs. If pentobarbital is to be used in any small rodent, it is best to administer a dose sufficient to produce light anaesthesia (50mg/kg ip) and then administer a volatile anaesthetic to produce full surgical anaesthesia.
Inhalational Agents
Induction of anaesthesia using an anaesthetic chamber is simple and convenient, and halothane, isoflurane and methoxyflurane provide effective and safe anaesthesia. A suitably sized mask may need to be constructed for anaesthetic maintenance, or a commercially available system purchased (e.g. IMS, Appendix 7).
Endotracheal Intubation
Endotracheal intubation is difficult to carry out in hamsters and requires the use of a purpose-built laryngoscope (Costa et al., 1986) (see Chapter 3).
Mechanical Ventilation
Mechanical ventilation can be carried out using a purpose-designed rodent (Appendix 7, Chapter 5).
Gerbils
Pre-anaesthetic Medication
Initial restraint of gerbils for intraperitoneal administration of anaesthetics is reasonably simple, but young or particularly active individuals may be better anaesthetized using an anaesthetic chamber.
Information on the effects of sedative agents is limited in gerbils, but the following agents appear reasonably effective in this species (see also Table 6.8).
| Considerable variation in effect occurs between different strains. | ||
| Drug | Dose rate | Comments |
|---|---|---|
| Acepromazine | 3mg/kg im | Light sedation |
| Atropine | 0.04mg/kg sc | Anticholinergic |
| Diazepam | 5mg/kg im, ip | Light sedation |
| Fentanyl/fluanisone (Hypnorm) | 0.5–1.0ml/kg im, ip | Moderate sedation, moderate analgesia |
| Ketamine | 100–200mg/kg im | Heavy sedation, mild to moderate analgesia |
| Medetomidine | 100–200μg/kg ip | Light to heavy sedation, mild to moderate analgesia |
| Midazolam | 5mg/kg im, ip | Light/moderate sedation |
| Xylazine | 2mg/kg im | Light sedation, mild to moderate analgesia |
Hypnorm (0.5–1.0ml/kg ip) provides sufficient analgesia for superficial procedures. Partial reversal is possible using nalbuphine (4mg/kg ip or sc) or butorphanol (2mg/kg ip or sc).
Diazepam (5mg/kg ip) or midazolam (5mg/kg ip) produces sedation but no analgesia.
Atropine (0.04mg/kg sc, im or ip) can be administered to reduce salivary and bronchial secretions.
Dose rates for general anaesthesia given below should be reduced by 30–50% if one of the sedative drugs listed above has been administered.
General Anaesthesia
Injectable Agents
Drugs are most conveniently administered by intraperitoneal injection in gerbils (see also Table 6.9).
| Duration of anaesthesia and sleep time (loss of righting reflex) are provided only as a general guide, since considerable between-animal variation occurs. For recommended techniques, see text. | ||||
| *Dose in millilitres per kilogram of a mixture of one part ‘Hypnorm’ plus two parts water for injection, and one part midazolam (5mg/ml initial concentration). | ||||
| Drug | Dose rate | Effect | Duration of anaesthesia (minutes) | Sleep time (minutes) |
|---|---|---|---|---|
| Alphaxalone/alphadolone | 80–120mg/kg ip | Immobilization | – | 60–90 |
| Fentanyl/fluanisone (Hypnorm)+diazepam | 0.3ml/kg im or ip+5mg/kg ip | Surgical anaesthesia | 20 | 60–90 |
| Fentanyl/fluanisone (Hypnorm)/midazolam | 8.0ml/kg ip* | Surgical anaesthesia | 20 | 60–90 |
| Ketamine/acepromazine | 75mg/kg+3mg/kg ip | Immobilization | – | 60–90 |
| Ketamine/diazepam | 50mg/kg+5mg/kg ip | Immobilization | – | 30–60 |
| Ketamine/medetomidine | 75mg/kg+0.5mg/kg ip | Medium anaesthesia | 20–30 | 90–120 |
| Ketamine/xylazine | 50mg/kg+2mg/kg ip | Immobilization | – | 20–60 |
| Metomidate/fentanyl | 50mg/kg+0.05mg/kg sc | Surgical anaesthesia | 45–90 | 180–240 |
| Pentobarbital | 60–80mg/kg ip | Immobilization/anaesthesia | 20 | 60–90 |
| Tribromoethanol | 250–300mg/kg ip | Surgical anaesthesia | 15–30 | 30–90 |
The combination of fentanyl (0.05mg/kg sc) and metomidate (50mg/kg sc) appears most reliable in producing general anaesthesia in gerbils (Flecknell, 1983).
Fentanyl/fluanisone with midazolam or diazepam (0.3ml/kg ip ‘Hypnorm’, and diazepam 5mg/kg ip) is less satisfactory in gerbils than in other rodents, and only light anaesthesia may be produced. When using midazolam the components are mixed together with water for injection (see Appendix 3).
Ketamine (75mg/kg) and medetomidine (0.5mg/kg), mixed together and administered by intraperitoneal injection, produce medium planes of anaesthesia in gerbils (Perez-Garcia et al., 2003). The medetomidine may be reversed using atipamezole (1mg/kg sc or ip).
As in hamsters, the use of pentobarbital in gerbils is particularly hazardous and a high mortality often occurs if this drug is used to produce surgical anaesthesia (80mg/kg ip). Lower dose rates (60mg/kg ip) produce light anaesthesia which can be deepened using low concentrations of volatile anaesthetics (e.g. 0.5% halothane).
Inhalational Agents
Sevoflurane (Henke et al., 2004), halothane, isoflurane and methoxyflurane can be used to provide effective and safe anaesthesia. Induction using an anaesthetic chamber is simple and convenient, followed by maintenance if required using a face mask, as with other small rodents.
Endotracheal Intubation
Endotracheal intubation requires the use of a purpose-built laryngoscope (Costa et al., 1986).
Mechanical Ventilation
Mechanical ventilation can be carried out using a purpose-made ventilator (Appendix 7, Chapter 5).
Anaesthetic Management
Like other small rodents, gerbils are especially prone to develop hypothermia, and heating pads or lamps should be used to prevent this (see Chapter 4).
Guinea Pigs
Guinea pigs are among the most difficult rodents to achieve safe and effective anaesthesia. Their response to many injectable anaesthetics is highly variable, and post-anaesthetic complications such as respiratory infections, digestive disturbances and generalized depression and inappetence are frequently seen. Many of these problems can be avoided by careful selection of anaesthetic agents and a high standard of intra- and post-operative nursing care.
Pre-anaesthetic Medication
Guinea pigs are non-aggressive animals that are generally easy to handle and restrain. When frightened they run around their cage at high speed, making safe handling difficult. It is important to approach guinea pigs quietly and handle them gently but firmly. They should be picked up around the shoulders and thorax and the hindquarters supported as they are lifted clear of their cage. Intramuscular or intraperitoneal injection of anaesthetic agents can then be carried out. Pre-anaesthetic medication is therefore not usually required but, if an anaesthetic is to be administered by intravenous injection into an ear vein, cephalic vein or medial saphenous vein, initial sedation is advantageous.
The following drugs can be used to produce sedation and restraint (see also Table 6.10):
| Considerable variation in effects occurs between different strains. | ||
| Drug | Dose rate | Comments |
|---|---|---|
| Acepromazine | 0.5–1.0mg/kg im | Light to moderate sedation |
| Alphaxalone/alphadolone | 40mg/kg im, ip | Heavy sedation, mild analgesia |
| Atropine | 0.05mg/kg sc | Anticholinergic |
| Diazepam | 2.5mg/kg ip, im | Heavy sedation |
| Fentanyl/dropiderol (Innovar-Vet) | 0.44–0.8ml/kg im | Sedation, analgesia |
| Fentanyl/fluanisone (Hypnorm) | 1.0ml/kg im, ip | Moderate sedation, moderate analgesia |
| Ketamine | 100mg/kg im, ip | Heavy sedation, light analgesia |
| Midazolam | 5mg/kg im, ip | Heavy sedation |
1. Fentanyl/fluanisone (Hypnorm, Janssen) (1.0ml/kg im or ip) will produce restraint, sedation and sufficient analgesia for minor procedures such as skin biopsy.
2. Diazepam (5mg/kg ip) or midazolam (5mg/kg ip or im) produces heavy sedation and immobility, but no analgesia. The animal is easily roused by painful stimuli or other disturbances such as noise. This agent can be useful in providing sufficient sedation to allow local anaesthetic techniques to be used humanely.
3. Ketamine (100mg/kg im) immobilizes guinea pigs but does not produce good analgesia.
4. Medetomidine and xylazine administered alone have very little sedative effect in guinea pigs, but do potentiate the effects of other anaesthetic agents (see below).
Atropine (0.05mg/kg sc) should be administered to minimize the volume of bronchial and salivary secretions. It is particularly useful in guinea pigs because of their relatively narrow airways, which are prone to obstruction.
The dose rates of drugs listed below apply to guinea pigs which have received no pre-anaesthetic medication unless otherwise stated. The dosages of general anaesthetics should be reduced by 30–50% if one of the drugs listed above has been administered.
General Anaesthesia
Injectable Agents
Intravenous administration of anaesthetics is difficult to achieve in guinea pigs, and drugs are usually administered by the intraperitoneal, subcutaneous or intramuscular route. The animals should be carefully weighed and dose rates calculated accurately. Anaesthetic dose rates are summarized in Table 6.11.
| Duration of anaesthesia and sleep time (loss of righting reflex) are provided only as a general guide, since considerable between-animal variation occurs. For recommended techniques, see text. | ||||
| *Dose in millilitres per kilogram of a mixture of one part ‘Hypnorm’ plus two parts water for injection, and one part midazolam (5mg/ml initial concentration). | ||||
| Drug | Dose rate | Effect | Duration of anaesthesia (minutes) | Sleep time (minutes) |
|---|---|---|---|---|
| Alphaxalone/alphadolone | 40mg/kg ip | Immobilization | – | 90–120 |
| Alpha-chloralose | 70mg/kg ip | Light to medium anaesthesia | 180–600 | Non-recovery only |
| Fentanyl/fluanisone (Hypnorm)+diazepam | 1.0ml/kg im or ip+2.5 mg/kg ip | Surgical anaesthesia | 45–60 | 120–180 |
| Fentanyl/fluanisone (Hypnorm)/midazolam | 8.0ml kg ip* | Surgical anaesthesia | 45–60 | 120–180 |
| Ketamine/acepromazine | 100mg/kg+5mg/kg im | Immobilization/anaesthesia | 45–120 | 90–180 |
| Ketamine/diazepam | 100mg/kg+5mg/kg im | Immobilization/anaesthesia | 30–45 | 90–120 |
| Ketamine/medetomidine | 40mg/kg+0.5mg/kg ip | Moderate anaesthesia | 30–40 | 90–120 |
| Ketamine/xylazine | 40mg/kg+5mg/kg ip | Surgical anaesthesia | 30 | 90–120 |
| Methohexital | 31mg/kg ip | Immobilization | – | 20 |
| Pentobarbital | 37mg/kg ip | Surgical anaesthesia | 60–90 | 240–300 |
| Tiletamine/zolezepam | 40–60mg/kg im | Immobilization | – | 70–160 |
| Urethane | 1500mg/kg iv, ip | Surgical anaesthesia | 300–480 | Non-recovery only |
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