An introduction to anaesthesia for surgical or prolonged procedures

Overview

These articles provide guidance and information to individuals who, during their work with animals, anaesthetise them for surgical or other painful procedures, or will anaesthetise them for other procedures for longer than 15 minutes (the current guidance on “prolonged” anaesthesia).


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The techniques described in these articles are provided as examples of suitable anaesthetic methodology. The techniques shown aim to illustrate general principles of anaesthesia and highlight some key areas of good practice. It is not our intention to imply that these are the only appropriate techniques for the species concerned. A wide range of other methods is available, and some of these will be more suitable in particular circumstances and for particular research projects.

Some of the material in these notes has been adapted from Laboratory Animal Anaesthesia – 4th Edition. Paul Flecknell (2016), and this text provides more detailed information and extensive references to support the information provided.

Additional information is also available in a range of veterinary and medical anaesthesia texts.

Anaesthesia is a complex topic, and it is always advisable to discuss your proposed protocol with your veterinary advisor.

What is anaesthesia?

Anaesthesia means “loss of sensation”. Some anaesthetics (general anaesthetics) produce a loss of consciousness but the loss of sensation can also be restricted to a small area of the body, using local anaesthetics. Injecting local anaesthetics around nerve trunks can produce larger areas of loss of sensation, a technique termed regional anaesthesia. Although all of these techniques can be used in laboratory animals, general anaesthesia is most frequently used. This has the advantage of producing a loss of awareness, as well as a loss of sensation so that animals will not experience any distress associated with a procedure. General anaesthesia should also ensure the animal remains immobile since these agents usually produce muscle relaxation and suppress reflex activity. This can be an important advantage when carrying out complex surgical procedures, or procedures such as imaging when it is essential that the animal does not move during data acquisition.

General anaesthesia does not necessarily mean that the animal will be unresponsive to painful stimuli – the depth of anaesthesia can vary from very light, when an animal will simply be unconscious but responsive to even a minor stimulus such as a light ear pinch, through to a very deep plane of surgical anaesthesia, when it will be unresponsive to major surgical stimuli.

How do we anaesthetise animals?

General anaesthetics can be given by injection or inhalation, or by immersion of aquatic species in an anaesthetic containing solution. Although a single agent can be used, it is often better to give two or more agents in combination. This is because all anaesthetics have some undesirable side-effects. These side-effects are dose-dependent, so giving a high dose of one agent to produce all of the required features of general anaesthesia can severely depress vital body function. As well as combining anaesthetic agents, for example, an inhalational agent and an injectable anaesthetic, other agents such as analgesics or sedatives can be given, each making a contribution to the overall effect.

This approach often termed “balanced anaesthesia”, can produce all the required components of general anaesthesia: loss of consciousness, analgesia, suppression of reflex activity and muscle relaxation, without markedly depressing body systems. This has a further advantage when anaesthetising animals for research purposes. There may be potential interactions between the anaesthetic agents and the aims of each particular research study. Giving several drugs in combination, at relatively low dose rates, can result in fewer undesirable interactions and so result in better scientific data.

The combinations of agents used to produce general anaesthesia are often administered as a single injection in small rodents, since these species are relatively easy to restrain, and there may be little benefit to the animal of injecting a sedative, followed later by other agents. In larger species, sedatives and analgesics are usually given first, as pre-anaesthetic medication (or “pre-meds”), since this will calm the animal and make administration of subsequent agents either by injection or inhalation both easier for the operator and less stressful for the animal.

The initial onset of anaesthesia is termed “induction”, and its continuation is termed “maintenance”. Injectable and inhalational agents can also be combined, for example with anaesthesia being induced using an injectable agent, and then the period of anaesthesia prolonged or the depth of anaesthesia increased using inhalational agents.

Many anaesthetic agents have no specific analgesic effects, and additional analgesia can be provided either by adding an analgesic agent to the anaesthetic combination or by adding local anaesthesia.

Why do we anaesthetise laboratory animals?

Laboratory animals may be anaesthetised for a number of different reasons, for example:

  • To provide immobility so the animal does not displace catheters or move during imaging procedures.
  • To prevent pain during experimental procedures such as surgery.
  • To prevent distress during physical restraint

The degree or depth of anaesthesia produced can be varied, depending on the type of experimental procedures that are to be undertaken. If surgical procedures are to be carried out, then pain perception must be completely suppressed. If the surgery involves relatively little trauma to tissues, then a medium plane of surgical anaesthesia may be sufficient. For more major surgery, especially if this involves traction and tearing of tissues, a deep plane of surgical anaesthesia will be needed. In contrast, if anaesthesia is being induced simply to provide humane restraint while non-painful procedures are carried out, then only light anaesthesia, with little pain suppression, will be required. Different general anaesthetic agents may provide similar levels of hypnosis (sleep), but the degree of analgesia they produce can vary widely.

The duration of anaesthesia can also be varied depending upon the time needed to complete a procedure. Very brief periods of anaesthesia, for example for 30-60 seconds, may be sufficient to prevent pain or distress associated with a procedure such as blood sampling, and will also make the procedure easier for the operator by immobilising the animal. An inhalational anaesthetic agent such as isoflurane can be used for this purpose, and recovery is very rapid – within a few minutes of turning off the anaesthetic vaporiser. In contrast, animals may be anaesthetised for several days to allow prolonged studies to be completed with the minimum of distress to the animal. Prolonged procedures require special expertise in monitoring the animal to avoid major disturbances to its physiological processes. Since animals are often not required to recover from such prolonged studies, anaesthetic agents that would not be suitable for routine use may be selected.

Why is it important to optimise anaesthetic methods for laboratory animals?

Controlling unwanted variability is crucial to producing high-quality research data. Increased variation in your data caused by the use of an inappropriate or poorly conducted anaesthetic technique may lead to an increased number of animals being required for the study. Increased mortality from poor anaesthetic technique will also increase the numbers of animals required for the study. Carefully planned anaesthesia and pain management can help you to both refine your study and reduce the number of animals you use. These are two key principles of the 3Rs ethical framework, of Replacement, Reduction and Refinement.

Carefully planned anaesthesia is a crucial part of applying the principles of refinement and reduction to your research project. Protocols should outline the methods that will be used, and also summarise the measures that will be taken to minimize pain and distress throughout the preoperative, intraoperative and post-operative period.

Making anaesthesia safer for the animal

When we produce general anaesthesia, we depress activity in the brain and spinal cord, and this causes the animal to lose consciousness. At deeper levels of anaesthesia, the animal becomes insensible to painful stimuli.

As mentioned earlier, anaesthetic agents also affect other body systems, such as the heart and lungs. Often, cardiac output is reduced, blood pressure falls and respiration is depressed. If anaesthesia is too deep, the depression of these different body systems can be life-threatening.

To recognise when body systems are being depressed during anaesthesia, we can monitor the animal, and take measures to minimise these effects. For example, we can give oxygen, or infuse fluids to support the circulation. More details of anaesthetic monitoring procedures are given below.

When may anaesthesia affect the outcomes of your study?

It is important to consider whether anaesthesia may affect the outcome of your research work. This can be a result of specific interactions caused by the anaesthetic agents used – for example by their action at receptors or by interaction with other cell membrane components. It can also be as a result of the non-specific effects of anaesthesia, such as cardiovascular and respiratory depression and hypothermia.

We can minimise these effects by careful selection of the anaesthetic regimen, using methods such as balanced anaesthesia to reduce the dose of individual agents, and by providing support to the animal’s body systems and monitoring the degree of depression of these systems.

Next Article : Planning anaesthesia for surgical or prolonged procedures

Updated on 12th May 2020

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