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  5. Scientific and welfare considerations when selecting anaesthetic agents for surgical or prolonged procedures
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  4. Scientific and welfare considerations when selecting anaesthetic agents for surgical or prolonged procedures

Scientific and welfare considerations when selecting anaesthetic agents for surgical or prolonged procedures

Selection of a particular anaesthetic agent or anaesthetic technique will depend upon a variety of factors. Some of these will relate directly to the anaesthetic agent and its potential interactions with the research protocol, and others to its ability to produce the required depth of anaesthesia. A further series of factors relate to the practicalities of cost, the availability of equipment and the expertise of personnel in the research unit. These various considerations are discussed in more detail below.

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Whichever method is chosen, it is important to keep in mind that two primary aims of anaesthesia are to prevent pain and provide humane restraint. The anaesthetic method itself should therefore be one which causes minimum distress to the animal. For example, the use of inhalational agents may involve exposure to irritant vapour (e.g. ether), and the restraint required for induction using a face mask may be stressful. Similarly, restraint for the administration of injectable agents can cause distress to the animal, as can the pain associated with injection of certain anaesthetics and the longer term consequences of myositis following intramuscular injection of irritant agents. 

Intravenous administration usually results in smooth and rapid induction of anaesthesia, provided that the animal is restrained effectively and that the injection is carried out with the required degree of skill. Consideration must be given to ways of minimising any fear or distress associated with handling or physical restraint and movement of the animal from its holding room to the operating theatre or laboratory.

Selecting a method of anaesthesia that is least likely to interfere with a particular research protocol is perhaps the most difficult task. The major pharmacological and physiological effects of the various anaesthetic agents should be reviewed, and this can at least minimize the interactions between the technique and the research protocol. It is important to appreciate that a superficial consideration of the compound’s effects may be insufficient. For example, if one concern is to maintain systemic blood pressure within the range found in conscious animals, then in the rat, pentobarbital might appear preferable to fentanyl/fluanisone/midazolam in some strains of the animal. However, the apparently normal blood pressure is maintained by peripheral vasoconstriction, and cardiac output is markedly depressed. Animals anaesthetized with fentanyl/fluanisone/midazolam have lower systemic blood pressure, but elevated cardiac output. Consequently, it is important to decide which is more important to a particular study – blood pressure or cardiac output. Other anaesthetics, such as urethane, may sustain blood pressure, but only because of their stimulatory effects on the sympathetic nervous system, so animals may have elevated plasma catecholamine concentrations. 

This information can only be gained by a careful search of the relevant literature. This then should allow evidence-based decisions to be made in relation to particular research projects. It is important not to assume that such an assessment has been carried out by other research workers whose publications include details of the anaesthetic technique used. 

Simply adopting the method of anaesthesia described in publications dealing with the particular animal model of interest will not necessarily ensure that an appropriate technique is used.

Having suggested that an assessment of anaesthetic–animal model interactions should be made, it is important to place such interactions in the context of the overall response to anaesthesia. There is little point in carefully selecting an anaesthetic, and then allowing the animal to become hypothermic, hypoxic and hypercapnic because of poor anaesthetic management. These common problems can have wide-ranging effects on the animal’s body systems, so attention to good anaesthetic management, described below, is of considerable importance. A second area to consider is the animal’s response to surgery. Surgical procedures produce a stress response whose magnitude is related to the severity of the operative procedure. In mammals, this response consists of a mobilisation of energy reserves, such as glucose, to enable the animal to survive injury. Although this response has clear evolutionary advantages, it is considered by many to be undesirable in humans and animals which are receiving a high level of intra-operative and post-operative care. It is also often undesirable because of the potential effects on particular research protocols.

Research workers are often reluctant to refine their anaesthetic methodology because it is thought that the anaesthetics used in the new technique may affect their animal model in the post-surgical period. In some instances, there will be a sound scientific basis for this opinion, based on a critical review of the relevant literature. In other circumstances, the effects of anaesthesia may be relatively unimportant when compared with the effects of surgical stress. Similar concerns are also expressed about the use of post-operative analgesics, and once again, the side-effects of any analgesics used should be considered alongside the other effects of surgery and anaesthesia. Clearly, it is logical to consider all of the factors that may interact with a particular study and to develop an anaesthetic and surgical procedure which is humane and provides minimum interference with the overall aims of the research project (Peterson et al, 2017).

Next Article : Practical issues when selecting anaesthetic agents for surgical or prolonged procedures

Updated on 12th May 2020

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