Practical issues when selecting anaesthetic agents for surgical or prolonged procedures

Rodents and rabbits

The majority of injectable and inhalational agents available can be used in small mammals, but practical constraints such as the difficulties of intravenous induction may limit the use of some agents (e.g. propofol). Local anaesthesia is used less frequently in small mammals than in larger species because of the difficulties of physical restraint. However, even in these small animals it is practicable to use local anaesthetic techniques to provide additional analgesia in conjunction with low doses of injectable anaesthetics or low concentrations of inhalants.

The difficulty of administering anaesthetics by the intravenous route in smaller mammals has some important practical implications:

• When anaesthetics are given intravenously, the dose can generally be adjusted to provide the desired effect in a particular individual. Allowance can easily be made for individual and breed or strain variation, and over- or under-dosing is easy to avoid. 

• When administering anaesthetics by the intramuscular, intraperitoneal or subcutaneous routes, a calculated dose is given, and there is no opportunity to adjust it to the individual’s requirements. Large variations in response (up to 150% difference in sleep times) related to genetic background, age and sex have all been documented in small rodents.

• When giving injectable anaesthetics i/m, s/c or i/p, select an anaesthetic regimen with a wide safety margin, and if possible, one that is completely or partially reversible. 

A second consequence of administration by these routes is that relatively large doses of drug are given, and recovery times tend to be prolonged. In small mammals, this is undesirable because of:

• The risks of hypothermia during recovery
• The problems associated with prolonged respiratory depression
• The potential metabolic effects of several hours of unconsciousness or sedation. 

Once again, the use of a reversible anaesthetic regimen can be advantageous.

Although they have these potential disadvantages, injectable anaesthetic regimens remain popular in small rodents because administration using the subcutaneous, intramuscular or intraperitoneal route is relatively simple. The same general approach is adopted in all species – see Minimally Invasive Procedures without Anaesthesia. For intraperitoneal injection, it is easier if an assistant restrains the animal, and the anaesthetist can then extend one hind limb and inject into the middle of the right posterior quadrant of the abdomen. This minimises the risk of inadvertent puncture of the bladder, which lies in the midline just anterior to the pelvic brim. Injecting into the right side of the abdomen avoids the caecum, which is large and thin walled in all rodents, and so may reduce the risk of injecting into the gastrointestinal tract. Intramuscular injections can be made into the quadriceps, with the animal restrained by an assistant similarly. The anaesthetist immobilises the limb and the muscle, and injects into the middle of the muscle mass. Subcutaneous injections are made into the scruff (rat, mouse, hamster, gerbil, guinea-pig). 

Inhalational anaesthetics, however, may be considered even easier to administer, especially when an anaesthetic chamber is used. Although a calibrated vaporiser must be used for modern anaesthetic agents, the anaesthetic apparatus required need not be complex or expensive. 

Larger mammals

Use of intravenous induction agents can result in smooth, rapid, stress-free induction, provided the animal can be restrained effectively, and that the injection is carried out with the required degree of skill. A wide range of anaesthetic agents can be used successfully, and intravenous administration allows careful control of the depth of anaesthesia that is produced. If animals resent the physical restraint needed for intravenous injection, then a sedative can be administered (see Pre-anaesthetic medication). As mentioned earlier, in some species catheters can be placed in conscious animals without causing any discomfort by using EMLA cream. Depending upon the species, the cephalic vein in the foreleg (dog, cat, sheep, non-human primate), the ear vein (rabbit, pig, sheep) the saphenous vein (dog, non-human primate) and the tail vein (marmoset) are easily visible and placement of an “over-the-needle” catheter is relatively straightforward. It is advisable to do this even if anaesthesia with volatile agents is planned, as the catheter will enable fluid therapy to be administered, and also allow emergency drugs to be given rapidly if anaesthetic complications develop. 

Alternatively, after induction of anaesthesia with an injectable agent, anaesthesia can be maintained using an inhalational agent. Whichever technique is used, it is important to ensure the animal’s airway remains unobstructed, and it may be advisable to intubate the animal. 

When anaesthetising large and small animals, a significant advantage of using inhalational agents is that they are delivered in oxygen. This helps prevent hypoxia developing as a consequence of the respiratory depression caused by most anaesthetics. Administration of oxygen to animals anaesthetised with injectable anaesthetics is often overlooked, but is strongly recommended.