The ideal general anaesthetic agent has the following properties:

• provides humane restraint,
  
• is consistent and reliable, having the same effects each time it is used,
  
• has a wide safety margin, and is safe for both animal and user,
  
• does not interfere with the procedure or affect the scientific objectives,
  
• any alterations due to anaesthesia need to be minimal and consistent, to reduce any variability,
  
• keeps the animal physiologically stable,
  
• is cleared rapidly, so the animal recovers and returns to physiological normality quickly.

The production of analgesia is the single most important drug effect.

Balanced anaesthesia

No single agent alone will provide satisfactory narcosis, muscle relaxation and analgesia. This is therefore usually achieved by balanced anaesthesia, which is the administration of drugs in combinations, including premedicants, inhalation and injectable anaesthetics and analgesics, in order to produce stable anaesthesia and to reduce the potential side effects of any individual drug. Using several anaesthetics together can overcome many of the disadvantages encountered when using agents individually. Many anaesthetic drugs produce a synergistic (supra-additive) effect when combined, while decreasing the probability and seriousness of side effects.

Balanced anaesthetic regimes allow for minimal interference with the animal’s physiology, and a smooth, rapid and pain-free recovery. Administering sedatives prior to induction or inducing anaesthesia with an injectable agent avoids the stress otherwise caused by induction with inhaled agents. Some injectable agents produce little analgesia alone, and addition of a low concentration of an inhaled agent can provide this. Similarly, when using inhalation anaesthesia, the concentration required to produce surgical anaesthesia can be reduced by administering a potent analgesic such as fentanyl. Induction of anaesthetic can be carried out using injectable combinations, followed by inhaled agents for maintenance. In large animals, the administration of an injectable anaesthetic to induce anaesthesia is essential, as it is difficult to restrain these animals for induction by a facemask. Maintenance of anaesthesia can then be satisfactorily achieved by intubation and the administration of a volatile anaesthetic.

The combination of drugs selected to produce ideal anaesthesia should include a narcotic, a behaviour modifier, a muscle relaxant, an analgesic and a stress reducer.

Choice of anaesthetic

The agents chosen to achieve balanced anaesthesia for a particular procedure will depend on a number of factors, including:

• the species,
  
• the duration of the procedure,
  
• the depth of anaesthesia required,
  
• the purpose of the investigation,
  
• the equipment available,
  
• the experience of the staff.

Anaesthetic regimes of different durations and depths will be required for different procedures, and some anaesthetic drugs will be contraindicated in certain protocols. Therefore, advantage should be taken of the wide variety of anaesthetic drugs and combinations available to find one which is best suited to the experimental protocol.

Surgical techniques and anaesthesia have effects on the physiology of the animal, which can be prolonged. Surgery results in a significant stress response with corticosteroid release, and levels can remain high for several days. This is designed to assist the animal in overcoming injury, but may be undesirable in the experimental subject. The physiological effects of anaesthesia may be quite minor compared with those produced by the experimental surgery, but nonetheless an anaesthetic regime with minimal potential for adverse effects should be selected. Good anaesthetic practice is an important aspect of animal welfare, and is also relevant to the scientific validity of any study that makes use of the animals, since poor anaesthetic practice can produce unwanted variability.

The anaesthetic should be chosen when all factors potentially influencing the protocol have been considered. While the ideal drug does not exist at present, new preparations are in development continually and new anaesthetic protocols are constantly being evaluated. It is important to keep reviewing available literature to identify possible refinements.

Administration of anaesthetics

There are two common methods by which anaesthetics are administered: by injection or by inhalation. Each method has advantages and disadvantages (see Table 9.1). These can be used individually or combined. Injectable agents are often used for induction, with inhaled agents used for maintenance of anaesthesia.

Table 9.1 Advantages and disadvantages of injectable and inhalation anaesthesia.

Inhalation anaesthesia

Agents used for inhalation anaesthesia are volatile liquids, which are vaporized by passing a gas such as oxygen through or over them. The vapour/oxygen mixture is then inhaled. They are then absorbed from the lung alveoli into the bloodstream and are predominantly excreted the same way. This means that induction and recovery from anaesthesia are quicker with inhalation than injectable agents. This is important in regaining normal physiology, to control post-operative hypothermia and fluid or electrolyte imbalance. Recovery usually occurs within 1–10 min, although if animals have been maintained for an hour or more on inhalation anaesthesia, full recovery can take 20 min or longer.

The major advantage of inhalation anaesthesia is the ability to adjust the depth of anaesthesia. If the anaesthesia is too light, the concentration of vapour is simply increased, and within a few breaths the depth of anaesthesia will increase. Similarly, if the anaesthesia is too deep, the concentration can be reduced and the excess anaesthetic is breathed off until the depth of anaesthesia lightens. This makes inhalation anaesthesia relatively safe compared with injectable anaesthesia. Reversal of overdose of anaesthetic is quicker and usually more successful with inhalation agents than injectable ones, so reducing mortality. Recovery can be hastened if attention is paid to controlling the depth of anaesthesia during the procedure. As anaesthesia progresses the concentration of anaesthetic can usually be reduced slightly, and a further reduction in depth can be made during suturing of subcutaneous tissues and skin, so that recovery time following completion of surgery is reduced.

Some metabolic breakdown of inhaled agents does occur, although this is minimal with modern volatile agents such as isoflurane (<1%), and there is little or no induction of liver microsomal enzymes, which occurs with some injectable agents. This makes inhalation the anaesthetic method of choice where normal metabolism is required. Inhalation anaesthesia is particularly suited to short procedures, repeated anaesthesia and procedures where rapid return to normal metabolism is essential.

The disadvantages of inhalation anaesthesia are that a considerable amount of specialised equipment is required to deliver the anaesthetic vapour to the animal. Waste gases produced are a potential health and safety hazard to the user and have to be disposed of appropriately. Inhalation anaesthetics are greenhouse gases and globally contribute the equivalent of 1 million cars to global warming each year11.

Considerable skill is required to set up and operate an inhalation system. Despite these drawbacks, inhalation remains the anaesthetic method of choice for most procedures.

The volatile anaesthetic agents

Volatile agents are delivered to the animal from an anaesthesia machine using oxygen, or a mixture of oxygen and nitrous oxide, as the carrier. There are several volatile agents available. How these agents work remains incompletely understood.

The required concentration of volatile agent depends on the properties of the agent, not the weight of the animal. The dose an animal receives depends on the concentration of agent in the inhaled gas mixture, and its tidal volume. Therefore, heavier animals do not necessarily need a higher concentration of anaesthetic, but will receive a greater dose as they have a larger tidal volume. Each agent has a minimum alveolar concentration (MAC), which is the concentration at which anaesthesia is induced in 50% of animals (see Table 9.2).

Table 9.2 Induction and maintenance concentrations of volatile agents.

Isoflurane is a volatile liquid at room temperature which vapourises readily, with a boiling point of 48.5°C. It is completely nonflammable. Isoflurane produces very rapid induction and recovery from anaesthesia, allowing the depth of anaesthesia to be altered very readily. It reduces pain sensitivity and relaxes muscles, producing all three components of the anaesthetic triad to an acceptable degree. It can be used just with oxygen to produce anaesthesia suitable for most surgical procedures, and can be combined with opioid analgesics or nitrous oxide where additional analgesia is needed.

Isoflurane undergoes no metabolism and does not affect hepatic microsomal enzymes, so is suitable for studies requiring normal liver metabolism12. The maximum exposure limit for personnel is 50 ppm13.

Isoflurane causes some respiratory and cardiovascular depression, leading to a fall in blood pressure due to vasodilation. Although not irritant it has a pungent odour, which can cause some animals such as rabbits to breath-hold on induction. Although its use is declining in human medicine it is used commonly in veterinary anaesthesia.

Sevoflurane is a nonflammable liquid used for induction and maintenance of general anaesthesia. It has a boiling point of 58.6°C. Together with desflurane, it is replacing isoflurane in human anaesthesia. It is highly volatile, with a very fast induction and recovery. It is good where mask induction is needed as it is not irritant and is sweet-smelling. However, it produces profound respiratory depression in some species and there is a risk of accidental overdose due to the speed with which changes in inhaled concentration take effect. Sevoflurane is unstable in the presence of soda lime1 so should not be used in rebreathing anaesthetic circuits (see below). There is no workplace exposure limit defined for sevoflurane. The global warming potential of sevoflurane is lower than that of other inhalation agents.

Nitrous oxide is a colourless non-flammable gas with a slightly sweet odour. It is a weak general anaesthetic, produing euphoria on inhalation, but it is not potent enough for induction or maintenance of anaesthesia in most species. However, it has minimal cardiovascular and respiratory effects, and can be used together with a volatile agent to reduce the required concentration of that agent, reducing its side effects, while providing additional analgesia. It can be used 60/40 or 50/50 with oxygen to deliver the volatile agent for anaesthesia in larger species. It may also be beneficial in prolonged anaesthesia over 24 h, where administration of volatile agents in 100% oxygen can lead to oxygen toxicity. After prolonged anaesthesia, 100% oxygen should be given for 5–10 min, or oxygen can be displaced from the lungs by the nitrous oxide as it is breathed off, causing diffusion hypoxia, which can lead to suffocation. The occupational exposure limit for nitrous oxide is 100 ppm.

Anaesthetic machines

It is recommended that an anaesthetic machine with an appropriate vaporizer be used for inhalation anaesthesia. These allow precise control of the amount of volatile anaesthetic agent delivered to the animal, allowing precise control of the depth of anaesthesia. The components of an anaesthetic machine are:

• a source of oxygen (with or without nitrous oxide) as a carrier gas,
  
• a flowmeter to measure the oxygen flow,
  
• an anaesthetic vaporizer,
  
• a delivery system from the vaporizer to the animal,
  
• a mechanism to deal with waste gases.

Medical-grade oxygen and nitrous oxide can be provided from cylinders of compressed gas attached to the anaesthetic trolley, or piped into the facility. Where cylinders are used a reducing valve attaches to the cylinder to ensure that a constant low pressure of gas is delivered to the system. From the cylinder, the carrier gases pass through flowmeters, which allow volume of gas passing to the animal to be monitored and adjusted. The flowmeter consists of a graduated glass tube with a bobbin, which floats at a level determined by the amount of gas passing through the tube. The tube is calibrated such that the top of the bobbin is in line with the marking indicating the flow rate of the gas. The bobbin should rotate in the tube. Each flowmeter is calibrated specifically for a particular gas.

The carrier gases are then passed through a vaporizer, over the volatile agent. The vaporizer delivers a known concentration of the anaesthetic vapour over a given range of flow rates and temperatures. Each vaporizer is calibrated for a particular agent and should not be used with other agents.

All anaesthetic equipment must be serviced regularly to ensure the correct quantities of vapour are delivered. The machine must be checked before each anaesthetic is given to ensure the gas cylinders and vaporizer are full, there are spare cylinders, a key to turn them on is available and the flowmeters are working. The anaesthetic circuit, endotracheal tubes and scavenging system must be checked to make sure they are intact, correctly attached and functional. If any monitoring equipment is to be used it must be set up properly and in working order. Only once all these have been checked should the animal be anaesthetised.

Induction of anaesthesia

Inhalation anaesthesia can be induced in larger animals, such as pigs, using a face mask. Animals may need sedation for this, as many resent the handling required. In small animals they can be placed in an induction chamber, and oxygen containing anaesthetic vapour piped into the chamber. The chamber is a simple box with transparent sides so that the animal can be observed during the induction, and must be easy to clean. Absorbent paper can be placed on the floor of the chamber. The inlet pipe is at low level and the extract at high level, since the volatile agents are heavier than air. Anaesthesia is induced by piping in the induction concentration of the agent (see Table 9.2). As the animal breathes the anaesthetic vapour, it is absorbed in the lungs and begins to circulate. The concentration in the CNS starts low and gradually builds, until enough has been absorbed to induce anaesthesia. This can take several minutes and there is a time lag between inhaling the vapour and the onset of anaesthesia. The speed of onset depends on the concentration of vapour in the inspired gas and the particular agent used. To increase the speed of induction, the concentration of vapour in the inspired gas may be increased to a level which, if it were maintained, would cause excessive depression of the CNS and death. This is known as the induction concentration. Once anaesthesia is induced – the animal has lost its righting reflex, is immobile in the chamber and breathing regularly – the animal can be removed from the chamber. Anaesthesia will persist for a brief period (about 1 min), which may be sufficient for short procedures (such as ear punch). For longer procedures, anaesthesia can be maintained via a face mask or endotracheal or intranasal tube for as long as is required. The concentration of anaesthetic vapour must be reduced to a suitable maintenance concentration (Table 9.2). The concentration needed will vary with the individual animal, the nature of the procedure, and whether other drugs have been given to provide additional analgesia. The major advantage of inhalation anaesthesia is that the concentration can be adjusted to meet the exact needs of the individual animal.

After the procedure the chamber should be tipped to empty out any residual anaesthetic gas, and be cleaned between animals; otherwise pheromones left by previous occupants can cause stress to animals newly placed in the chamber.

Maintenance of anaesthesia: anaesthetic circuits

To maintain anaesthesia, the mixture of gases and volatile agent is delivered to the animal through an anaesthetic circuit. There are essentially two types of circuit: those where carbon dioxide is absorbed (rebreathing circuits) and those where it is not (non-rebreathing circuits). The particular one chosen depends on the size of the animal, the site of the operative field, whether intermittent positive pressure ventilation (IPPV) is to be used, and personal preference. The quantity of oxygen piped into the circuit depends on the needs of the animal (which depends on its body weight), and the type of circuit.

For non-rebreathing circuits, the expired air is not re-inhaled, and fresh gas from the anaesthetic machine is delivered continually to the animal. The exhaled gases are swept out of the open end of the circuit by the fresh gases flowing in during the expiratory phase. If the fresh gas flow rate is too low, rebreathing may occur, and carbon dioxide can build up to dangerous levels as there is no method of removing it. It is essential, therefore, to have a high-enough flow rate to remove the alveolar gas when it is expired so the animal does not rebreathe exhaled gases.

Most laboratory animals are small, and therefore have small tidal volumes. They also have limited capacity to continue breathing if there is resistance in the anaesthetic circuit. For these animals it is important that an anaesthetic circuit is used with low resistance to breathing and a low dead space.

T-piece circuits have minimal resistance and little dead space and are the circuits of choice for animals up to 3 kg in body weight (Figure 9.2). The open tube acts as a reservoir of fresh gas. Exhaled gases are pushed out of the open end of the reservoir tube by fresh gases flowing in from the anaesthetic machine during the expiratory phase. T-piece circuits require a high gas flow rate, at least twice the animal’s minute volume, and are therefore expensive to use for larger animals.

FIGURE 9.2 T-piece circuits.

Co-axial circuits are a modification of the T-piece where the fresh gas inflow pipe runs inside the reservoir limb. During expiration the dead-space gases pass down the outer corrugated tubing to the scavenger. Some co-axial circuits, for example the Bain circuit (Figure 9.3), have a valve on the expiration arm. As the system fills, the pressure increases to a level at which the expiratory valve opens and then expired alveolar gas escapes through the valve. Since the valve increases resistance to breathing these circuits cannot be used with small rodents. They are suitable for animals of 3–10 kg, and a minimum flow rate of 100–300 ml/kg/min is needed to prevent rebreathing.

FIGURE 9.3 Bain co-axial circuit.

The Magill circuit (Figure 9.4) is suitable for animals between 10 and 40 kg. A fresh gas flow of 80–90% of minute volume (respiratory rate×tidal volume) is needed, making this economic to use in these larger animals. This circuit is only suitable for animals of 10 kg and over because the valve causes high resistance and it has a relatively high dead space.

FIGURE 9.4 Magill circuit.

In rebreathing circuits, carbon dioxide in the exhaled gases is absorbed by soda lime so that the anaesthetic vapour can be re-circulated with the addition of fresh oxygen. The required gas flow rate is low, and re-use of expired anaesthetic vapour makes them very economical. Nitrous oxide should not be used with closed systems, since it is impossible to monitor the concentration in the recycled gas. One disadvantage is that the packed soda lime increases the resistance to respiration and dead space. This makes these systems unsuitable for animals under 15 kg in weight. The conservation of heat in the system may give rise to heat stroke in sheep. Sevoflurane should not be used with rebreathing systems.

In the To and fro system (Figure 9.5), the canister containing soda lime is placed between the animal and the reservoir bag to absorb the carbon dioxide. The dust from the soda lime may be inhaled and cause inflammation in the respiratory tract.

FIGURE 9.5 To and fro system.

In Circle systems (Figure 9.6) there is a circular flow of gas controlled by two unidirectional valves. They are designed for large animals, as there is high resistance in the system.

FIGURE 9.6 Circle system.

Maintenance of anaesthesia: face masks and endotracheal tubes

The anaesthetic vapour and carrier gas are delivered from the circuit to the animal via a face mask or endotracheal tube. Face masks must fit snugly over the nose and mouth but not rub the eyes. Gas invariably leaks from around the mask, contributing to pollution of the atmosphere, so these should either be used in a flow hood, or a face mask incorporating a gas-scavenging system used. Face masks require a flow rate of three times the animal’s minute volume.

Endotracheal tubes are rubber or plastic tubes inserted into the trachea. These are available in different sizes for different species, with or without an inflatable cuff, which is used to make an airtight seal in the trachea. The tube provides a patent airway so that artificial respiration may be given if required. Endotracheal intubation requires skill, and in some species the use of a laryngoscope. After induction of anaesthesia, the lubricated tube is passed over the tongue and epiglottis, and through the larynx. Once in situ, the cuff, if present, is inflated to seal the trachea and prevent the animal from breathing around the tube. Inflatable cuffs should not be used for tubes smaller than 5 mm diameter.

The size of tube used depends on the size of the animal, but as a rough guide adult beagles need a 9 or 10 mm tube with a cuff, cats need a 3–5 mm tube without a cuff and rats need a 16–12 G arterial cannula or modified urinary catheter. Dogs, cats and primates may be intubated while lying in left lateral recumbency (right-handed operator). An assistant holds the head and neck extended and lifts the upper jaw. The operator pulls the tongue forward and down, and usually the larynx can then be visualised. The tube is then passed gently through the glottis into the trachea. In cats, the larynx should be sprayed with local anaesthetic prior to passage of the tube to prevent laryngeal spasm.

Pigs have a particularly long larynx, and a laryngoscope is required to illuminate the larynx and hold down the tongue and epiglottis.

Small animals can be intubated via surgical implantation of a tube in the trachea in non-recovery procedures, or by passing a tube carefully into the mouth. For rats the animal is placed on its back and a laryngoscope or otoscope inserted into the mouth to visualise the larynx14. An alternative to a laryngoscope can be fashioned from a syringe barrel15. The tube can then be inserted through the glottis. For rabbits, the animal is placed in ventral recumbency and the head and neck extended by an assistant so the neck is straight. The tube is inserted in the mouth to one side of the incisors then straightened and advanced as far as the glottis. The tube is then advanced into the larynx during the inspiratory phase of respiration. Alternatively, an assistant stands in front of the prone rabbit holding its head up and pulling the tongue forward. The operator stands behind the rabbit, and places the tube as above16,17,18.

Endotracheal intubation in mice, gerbils and hamsters is difficult and requires skill and purpose-made apparatus1. This often has to be done following induction with injectable anaesthetics, because the animal may wake from inhalation anaesthesia in the time taken to place the tube. A method has been described in which intubation is facilitated, allowing for intubation after inhalation induction19.

An alternative to endotracheal intubation is the use of a nasal catheter. This is effective in many rodents as they are nasal breathers. Small nasogastric tubes, urinary catheters or intravenous catheters can be used. After induction of anaesthesia the catheter is inserted through the external nares and into the nasal passage, by applying gentle pressure and directing the catheter medially and ventrally. The typical gas flow rate required is three times the minute volume. Animals may sneeze if too lightly anaesthetised, and there may be mild epistaxis, which can be stopped by applying pressure to the side of the nose20.

Health and safety

Volatile anaesthetic agents pose some risk to human health. There may be alterations in cardiorespiratory function, hepatotoxicity occurs infrequently but has been reported and all modern agents can trigger malignant hyperthermia21. Prolonged exposure to some agents can cause abortion. Each volatile agent has a defined occupational exposure limit (OEL), which should not be exceeded, and so either an active or passive scavenger system must be used to extract the waste gases and dispose of them safely. A length of tubing attached to the expiratory port of the circuit can be used to duct waste gases to the outside, or into activated charcoal. Alternatively, the apparatus may be used in an extraction cabinet. Activated charcoal canisters must be weighed regularly and changed when they reach a specified weight. Activated charcoal will not remove nitrous oxide.

Injectable anaesthesia

Injectable agents are simple to administer with a minimum amount of equipment. This is useful when operating in the field or when equipment for administration of volatile anaesthetic agents is not available. With intravenous injection, anaesthesia is induced immediately, minimising stress. They are particularly useful for the induction of anaesthesia, which can then be maintained by inhalation, or for short-duration anaesthesia.

Modern injectable agents are available that are short-acting, and which can be given by intravenous infusion, allowing for improved control of the depth of anaesthesia and overcoming some of the disadvantages of using injectable agents.

When injectable agents are used it is still sometimes helpful to administer oxygen, to prevent development of hypoxia, for example during long-term anaesthesia lasting for more than 1 h, in which respiratory depression can lead to hypercapnia and acidosis. The ability to administer oxygen and control ventilation helps control many potential difficulties encountered in injectable anaesthesia.

Induction

Injectable agents are given either intravenously directly into the bloodstream, or via another parenteral route from which they are absorbed into the blood. Intravenous injections usually act very rapidly, producing loss of consciousness almost immediately. The calculated dose can be administered slowly, allowing time for the drug to circulate before deciding whether more is required. The drug can, therefore, be administered to effect and titrated to the exact requirements of the animal. Intravenous administration can be facilitated by applying local anaesthetic to the site of venipuncture prior to injection.

The small size or problems of restraint in many laboratory animals means that injectable agents are most frequently given via the intramuscular or intraperitoneal routes as a single dose. With this route, a large total dose is needed, and absorption can be slow and unpredictable, leading to variable induction. Giving the drug as a single bolus can result in overdosage, which is difficult to reverse, and the animal must be weighed first to ensure accurate dosing. Residual drug effects persist for long periods and full recovery can be prolonged. There is also the possibility of accidental injection into a blood vessel or major abdominal organ, which could have serious consequences. Ideally injectable anaesthetics should have a wide safety margin, and be able to be given in a small volume through a small needle.

A considerable amount of skill is required to perform intravenous injection. For all other routes less skill is required, but it is not possible to dose to effect, since the response to injection is slow. For injectable agents, more than inhaled agents, the level of anaesthesia achieved depends on the level of stimulation of the animal at the time. If an animal is stressed, for example due to poor handling or restraint in the pre-anaesthetic period, it will require a higher dose of anaesthetic agent, leading to a greater risk of overdosage and adverse effects. An animal that is given an injection and left undisturbed may be apparently deeply anaesthetised, but if stimulated it may respond by reflex movements of the limbs and an increase in the depth and rate of breathing. This does not necessarily mean that a further injection of anaesthetic is required: if more is given, and stimulation then stops, there is a risk that the level of anaesthesia will become dangerously deep. The depth and duration of anaesthesia produced is therefore variable.

Maintenance

Injectable anaesthetics produce variable periods of anaesthesia depending on the agent used. If maintenance is required this can be achieved by giving intermittent top-up injections, by continuous intravenous infusion, or by using an inhaled agent for maintenance. The response to injectable anaesthetic agents varies with the individual animal, being affected by sex, strain and age. This is true for all anaesthetics, but adjusting the depth of injectable anaesthesia to account for these differences is more difficult than for inhalation anaesthesia.

Intravenous injections allow dosing to be adjusted according to the individual animal’s response, so there is less likelihood of over- or under-dosing. Many drugs and combinations can be given by constant intravenous infusion for prolonged anaesthesia.

Giving intermittent injections for maintenance inevitably results in variable depth of anaesthesia during the procedure. Monitoring the depth of anaesthesia is therefore particularly important, so that supplemental injections can be given in good time to prevent the animal from feeling pain, as there will be a time lag between the top-up injection and deepening of anaesthesia. It is not acceptable to give a further dose by injection as the animal starts to wake up. It is important to be familiar with the techniques required for the administration of injectable agents, and the type of anaesthesia obtained with particular anaesthetics in the type of animal to be used.

Recovery

Most injectable anaesthetics are excreted by the kidneys after metabolism in the liver. This process can be prolonged, leading to slow recovery from anaesthesia, but may also affect hepatic enzymes, interfering with some studies.

The action of some injectable anaesthetic agents can be reversed using specific antagonist drugs. Several antagonists are available commercially, and these can partially reverse some anaesthetic combinations. They have several major benefits.

• By reversing the sedation and reducing sleep time, animals become active more rapidly, and are less at risk of hypothermia.
  
• Many anaesthetic drugs depress respiration, and this depression can be promptly and effectively reversed with antagonist drugs.
  
• An additional advantage of sequential analgesia can be obtained when reversing pure agonist opioids, such as fentanyl, with partial agonists such as buprenorphine. The side effects produced by fentanyl are reversed, but the analgesia persists because of the action of the bupenorphine1.

Injectable agents

Many injectable agents are available. The quality of anaesthesia produced by different agents varies considerably. Very few anaesthetic agents can produce general anaesthesia when used alone, and most injectable agents are given in combinations. Even with agents that can induce general anaesthesia alone, reduction of anxiety with suitable premedicants, additional analgesia or use of an inhaled agent for maintenance may be required. Table 9.3 gives doses of anaesthetic drugs for small laboratory animals.

Table 9.3 (a) Anaesthetic and other drug dose rates for in rodents and rabbits1, 20, 25,26. (b) Analgesic doses for small animals1, 20,25. (c) Drug doses for use in dogs and ferrets 27,28. (d) Anaesthetic and analgesic dose rates for chickens. (e) Doses for sedatives, anaesthetics and analgesics in ruminants. (f) Sedative, anaesthetic and analgesic drug dose rates for pigs. (g) Dose rates for anaesthetics and analgesics in primates.

Propofol

This should be given by slow intravenous injection, as rapid administration may induce apnoea. It acts rapidly, inducing anaesthesia smoothly without excitatory side effects. It is ultra-short acting, and recovery is rapid and smooth. It can be used by continuous infusion for prolonged anaesthesia. It can be used safely in cats, dogs, monkeys, pigs and rabbits, and can be combined with a wide range of premedicants, analgesics and inhaled agents. Utility in small rodents is limited, however, due to the need for intravenous administration.

Alphaxalone

This is non-cumulative and produces good muscle relaxation with excellent cardiovascular stability and minimal respiratory depression except at high doses22. Intravenous administration rapidly results in anaesthesia, but the calculated dose should be given slowly, over 60 s, to avoid apnoea. It can be used for induction of anaesthesia prior to maintenance with inhalation agents, for short-duration anaesthesia or for long-term anaesthesia by intermittent injection or continuous infusion. Analgesia produced is limited, and additional analgesia may be required.

Recovery following anaesthesia is rapid, but it is preferable that animals are left undisturbed in the recovery period, otherwise paddling, minor muscle twitching or more violent movements may be seen.

At the time of writing, there has been limited use of alphaxalone in laboratory animals. However, it has been shown to produce dose-dependent pre-emptive analgesia in rats23, and has been used for anaesthesia in rats in studies requiring avoidance of significant respiratory and haemodynamic depression while facilitating mechanical ventilation24. It is likely that it will have a place in laboratory animal anaesthesia.

Pentobarbitone

This is a barbiturate that depresses the CNS and produces marked cardiovascular and respiratory depression. It has poor analgesic properties and very large doses are needed to reduce pain perception. It has a low therapeutic index: the lethal dose is only slightly above the clinical dose. Even after intravenous injection onset of anaesthesia is slow. Injections must be given very slowly to allow anaesthesia to deepen to its full extent before giving extra doses. If it is given intraperitoneally as a single bolus it has a poor safety margin, and mortality can be high. Recovery from anaesthesia is slow, and may be associated with convulsions and paddling. Its use is not recommended for recovery anaesthesia. In concentrated form, pentobarbitone is used for euthanasia.

Combination anaesthetics

Ketamine

This is an N-methyl-D-aspartate (NDMA)-receptor antagonist, which can be given by intramuscular or intravenous injection. It circulates rapidly after administration and produces analgesia but little muscle relaxation, and animals may exhibit tremors and remain responsive to external stimuli, particularly sound. Animals seem not to be aware of their surroundings, and analgesia varies with the species. It is the agent of choice for sedation of Old World monkeys. The corneal reflex is lost very quickly, and it is essential to protect the eyes with a bland ophthalmic ointment. The laryngeal and pharyngeal reflexes are maintained, but there is an increase in salivary secretions, which can cause airway obstruction, so ketamine is sometimes used with a drying agent such as atropine. There is good maintenance of blood pressure, but in most species the dose required to produce surgical anaesthesia causes severe respiratory depression and it is not used alone. It can be combined with xylazine, medetomidine, diazepam, midazolam or alphaxalone to produce general anaesthesia in many species (sheep, primates, pigs, rabbits and rodents). Using ketamine in combination with other agents also avoids side effects, such as muscle tremors.

Narcotic analgesics (opioid analgesics)

Opioids such as fentanyl, alfentanil and sufentanil are useful as premedicants, components of balanced anaesthetic regimes and in neuroleptanalgesia (see below). They are potent analgesics but have significant side effects such as respiratory depression. Fentanyl is a potent analgesic that lasts for 15–20 min. In rats, dogs and primates it has sedative effects, but in mice, cats and horses it causes excitement. Alfentanil is less potent than fentanyl but the onset of activity is more rapid. Sufentanil is 10 times more potent than fentanyl, but has not been used extensively in laboratory animals.

Neuroleptanalgesia

Combinations of opioid analgesics and sedatives produce neuroleptanalgesia, which is similar to a light plane of anaesthesia. The animal no longer responds to stimuli, but is not totally unconscious. Neuroleptanalgesics produce some respiratory depression and poor muscle relaxation, but when combined with benzodiazepines these problems are markedly reduced and a state of neuroleptanaesthesia may be produced, under which surgical procedures may be carried out. Combinations of fentanyl/fluanisone with either midazolam or diazepam can provide good anaesthesia for rodents and rabbits. Midazolam is water-soluble and can be administered in the same syringe as the fentanyl/fluanisone. It can also be diluted with sterile water for accurate dosing in small rodents.

To speed recovery and counteract side effects, the opioid component of neuroleptanalgesics can be reversed at the end of surgical procedures by either narcotic antagonists (such as naloxone) or partial agonists (such as buprenorphine or butorphanol), which also provide post-operative analgesia. Naloxone reverses analgesia as well as any side effects, so it must not be used unless surgery has been completed and adequate analgesia provided by some other means. Reversal is not always required, as most neuroleptanalgesics will wear off by themselves, but recovery without the use of antagonists can be prolonged.

Medetomidine and atipamezole

Medetomidine can be given by intravenous, intramuscular or subcutaneous injection. Animals should be left undisturbed for 10–15 min after administration for the drug to reach maximum effect, preferably in a quiet environment, as they may still react to noise. Medetomidine does not induce general anaesthesia when used alone, but is excellent when used in combinations. If used as a premedicant it reduces the requirement for inhaled anaesthetics markedly, and can be used with fentanyl/fluanisone to produce good anaesthesia and analgesia. It may also be used successfully with ketamine in many species. The biggest advantage of medetomidine is that it can be reversed very rapidly with the specific antagonist atipamezole. See also the section on premedication, above.

Long-term and non-recovery anaesthesia

Long-term anaesthesia may be required for some procedures. This can be achieved with intermittent top-up injections by the intramuscular or intraperitoneal routes, although the depth of anaesthesia can vary markedly. If the top up is not administered in time the animal may experience pain, so good anaesthetic monitoring is essential. Alternatively an anaesthetic can be given by continuous intravenous infusion (e.g. alphaxalone, propofol or fentanyl/midazolam). Short-acting agents enable the plasma concentration and depth of anaesthesia to be adjusted quickly, and lead to a rapid recovery. Many different types of infusion pump are available to facilitate administration. Long-term anaesthesia can also be achieved by the use of inhaled agents for maintenance, after induction with inhaled or injectable agents.

Many experimental protocols require terminal anaesthesia. There is little difference between recovery and non-recovery anaesthesia in terms of animal care and management, although some more invasive surgical techniques such as surgical exposure of a blood vessel for catheterisation and tracheostomy creation are more often used if the animal is not to be allowed to recover from anaesthesia.

Some anaesthetic agents are only suitable for non-recovery experiments, e.g. alpha-chloralose results in a very prolonged recovery period associated with considerable involuntary excitement, and urethane can produce severe peritoneal irritation and is carcinogenic. Chloral hydrate is a very poor analgesic in many species, and can cause paralytic ileus in the rat.

Recovery and post-anaesthetic care

At the conclusion of a procedure it is important that the animal wakes up as soon as possible, so that its physiology returns to normal. It is better to have a rapid recovery and provide adequate post-operative analgesia than to have prolonged anaesthesia. With inhaled agents the vaporizer can simply be switched off and the animal allowed to breathe oxygen for a few minutes until the anaesthetic gas has been exhaled, resulting in recovery within a few minutes. Injectable agents usually have to be metabolised by the liver and excreted by the kidneys. This process can be prolonged, and recovery from injectable anaesthetics usually takes longer than for inhaled agents. This increases the risk of hypothermia or dehydration. Some injectable drugs have specific reversal agents, the administration of which will lead to a return to consciousness within minutes. Reversal agents can also be used to reverse any respiratory depression, which develops in the post-operative period. Some injectable agents are ‘ultra-short-acting’, and are metabolised very quickly. Use of these newer drugs has overcome some of the problems previously associated with injectable agents.

The quality of care following recovery from an anesthetic can have a dramatic effect on the speed of recovery. If not cared for properly, semi-conscious animals may lie in their urine or faeces and develop skin lesions, or get bedding material in their eyes and noses. Their cage mates may attack them, and they may remain inappetant for long periods. Many of the monitoring procedures described below should be continued in the immediate recovery period. This helps to ensure that optimal attention is given to the animal, and that all the measures needed to ensure its comfort and well-being are undertaken promptly and effectively.

Since all animals will require some degree of special attention in the recovery period, it is preferable to provide a separate recovery area. This not only enables more appropriate environmental conditions to be maintained but also encourages individual attention and special nursing. Individual housing will often be indicated anyway to prevent problems such as injury of semi-conscious animals by their cage mates.

The recovery area should be stocked with a range of emergency drugs and equipment. Additional apparatus such as monitoring equipment may be needed after major surgery and/or prolonged anaesthesia.

The prevention and management of pain following a surgical procedure is a particularly important aspect of care in the recovery period. Pain can produce physiological changes that may alter the rate of recovery from surgical procedures, and that these changes may also affect the experimental results. Acute pain can be alleviated by the use of centrally or peripherally acting analgesic drugs administered systemically, by local anaesthetics or by the application of supporting bandages or other means of protecting and immobilising damaged tissues. The selection of a particular treatment will vary, depending upon the nature of the pain, its cause and its estimated severity and duration. For mild to moderate pain, non-steroidal anti-inflammatory drugs (NSAIDs) such as meloxicam, carprofen or flunixin may be sufficient. For moderate to severe pain, opioid analgesics such as buprenorphine may be needed.

The degree of personal attention given to animals will depend upon the species. Some animals may respond well to personal contact, whereas this may be stressful for others. Recovering animals should be left in a warm environment with subdued lighting, and be disturbed as little as possible. The animal should be on comfortable bedding and kept clean and dry. Animals may need additional fluid, given by injection or orally, and it may be appropriate to offer palatable food. Urine and faecal output should be monitored. All clinical observations and drugs administered must be recorded.

Local anaesthetics

Local anaesthetics act directly on nerves to block the conduction of impulses, thus eliminating pain and other sensations. Both local and regional anaesthesia are widely used in the management of pain in humans, and there are also well-established techniques in clinical veterinary practice. They are still relatively under-used in the smaller species of laboratory animals, principally because although pain sensation may be abolished there still remains the problem of restraint in these species.

Local anaesthetics may be applied topically, by injection, locally into the skin and/or underlying tissues in a specific area, or as a nerve block, for example epidural or paravertebral block, which results in desensitisation of a large area, known as regional anaesthesia. Local anaesthetics have an important role in providing anaesthesia and pain relief for large animal species. When combined with a sedative or hypnotic to provide humane restraint they can be used for major surgery and remove any of the risks associated with general anaesthesia.

Topical application

Some agents are available for direct administration to tissues. Drops can be used to desensitise the cornea and conjunctiva. An aerosol may be used for the larynx prior to intubation to prevent laryngospasm. For the skin, cream (e.g. EMLA, AstraZeneca) can be used to facilitate venipuncture.

Infiltration of the skin and underlying tissues

This method is useful for minor techniques, such as skin biopsies or suturing minor wounds. Injectable local anaesthetic is administered using a very fine needle. Agents such as lignocaine or bupivacaine can be infiltrated around the surgical site to provide analgesia during surgery or post-operatively. This method can be very useful for provision of post-operative analgesia, since there are no systemic side effects. Lignocaine preparations typically last 1–2 h, and bupivacaine can provide analgesia for up to 8 h20. Some preparations of lignocaine contain adrenaline to prolong efficacy: care should be taken not to overdose with these as this may delay wound healing.

Regional anaesthesia

Injectable local anaesthetic can be injected around a major nerve to desensitise a particular area. The use of nerve blocks for regional anaesthesia requires a detailed knowledge of anatomy and specific training in the techniques. The technique can be used for specific nerve blocks, such as the pudendal nerve for work in the pudendal region, and the cornual nerve block for dehorning or foot blocks using specific nerves to regions of the feet in horses. Epidural or spinal blocks involve injection of local anaesthetic into the epidural space or directly into the spinal cord to produce desensitisation of the region posterior to the site. This is a useful technique to provide anaesthesia of the pelvic region in cattle and sheep, and may be used post-operatively to provide continuing analgesia. A paravertebral block involves injecting local anesthetic around several lumbar spinal nerves as they emerge from the vertebral column to produce desensitisation of the flank. This method is useful in ruminants.

Although local anaesthetics may be very successfully used in the post-operative period as an adjunct to other methods of pain control, to provide adequate restraint and to reduce stress in most laboratory animals it is usually preferable to use general anaesthetics.

For doses of anaesthetic agents for each species, see Table 9.3.

Anaesthetic Management

All anaesthetic drugs will, if given in sufficient quantity, cause death. On the other hand, if an insufficient amount is given, the animal will feel pain. To maintain the animal between these two extremes, care must be taken in the maintenance of physiological stability, particularly airway patency, respiratory function, circulatory function and body temperature, and in monitoring the depth of anaesthesia, to ensure that the parameters of unconsciousness, analgesia and muscle relaxation are being met. Poor animal care during anaesthesia can result in prolonged recovery times, or in extreme cases, even death. The animal should be monitored frequently, at least every 5 min, or more often if stability has not been established, to ensure the animal is physiologically stable, and to determine the depth of anaesthesia. Records should be kept of the measurements to enable adverse trends to be spotted easily and corrective action taken. It also allows a series of anaesthetics to be compared. Anaesthetic monitoring depends on watching the animal closely and recording the measurements. An example of an anaesthetic record chart is shown at Figure 9.7.

FIGURE 9.7 Record of general anaesthetic.

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