Anaesthesia and Surgery - Small Mammals

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Introduction

Because small mammals are prey species, they do not take to intensive conditions of surgery and hospitalisation very readily. Those that are handled very frequently by their owners are more bonded with humans and better surgical subjects as a result.

Rodents and rabbits are particularly susceptible to the surgical complications of dehydration (blood and fluid loss), core temperature depression, hypovolaemic shock, ileus and renal and respiratory depression.

Surgical Principles

Pre-operative Preparation

A thorough pre-operative examination is required, and may bring to light problems which means surgery has to be delayed.

Pre-operative haematology, biochemistry and clotting times may be indicated if organ failure is suspected.

Pre-operative antibiosis can be considered in debilitated animals. Many small mammals have subclinical respiratory infections which can lead to problems during surgery and post-operatively.

Hydration and nutritional shortfalls should be addressed, especially in cases which have had a prolonged history of anorexia due to dental disease for example.

Starving is not usually necessary in small mammals due to their high metabolic rate. Rabbits cannot vomit and so food does not need to be withheld. There is often food in the oropharynx of these animals, and the mouth can be flushed out before anaesthetic induction to ensure a patent airway.

Ferrets should be starved for 1-4 hours prior to surgery.

If possible, intravenous access should be obtained. In rabbits, commonly used veins include the cephalic and marginal ear veins.

Preparation for surgery:

Small mammal fur (especially rabbits, chinchillas and ferrets) is usually quite difficult to clip, and can clog clippers, so special clippers should be used. The skin is very delicate and prone to tearing or developing clipper rash. The amount of fur clipped should be kept to a minimum.

Clear self-adhesive plastic drapes are useful as they are lightweight and enable monitoring of the patient.

Antiseptic fluids used for aseptic preparation should be warmed before use, and alcohol-based antiseptics should be avoided as they lead to excess heat loss.

The patient can be insulated using blankets, bubblewrap, and foil blankets.

Care must be taken to avoid hyperthermia as well. Electric heat pads are prone to overheating, and surgeons tend to prefer hot water bottles or incandescent lights. Operating in a warm room with additional heat from the operating light is usually enough.

Rabbits are prone to developing respiratory distress at ambient temperatures above 25°c.

Surgical Technique

Haemostasis is very important in small mammals, as the loss of small volumes of blood is much more significant for them than in cats and dogs.

Haemostasis can be achieved by ligation, or using vascular clips. Diathermy units are also popular.

Choice of suture material and method of closure:

Rabbits readily develop caseous suppurative responses to foreign bodies and suture material like catgut. Polyglycolic acid sutures cause the least reaction. The size of the suture material is also important, and fine instruments and material should be used.

Rabbits and rodents are very likely to groom skin sutures out if left above the surface of the skin. Subcuticular suture patterns and a buried knot, such as the Aberdeen knot, are recommended.

Tissue glue can be used in some cases, such as rodent castration.

Anaesthesia Principles

Analgesia

Analgesia is essential for any anaesthetic procedure, as small mammals do not show signs of pain very well and it may go unnoticed. It can be given pre-operatively to pre-emp the painful surgery.

Drugs available include:

Opiates: pethidine, buprenorphine
NSAIDs: meloxicam, carprofen
Local anaesthetics: lidocaine, bupivicaine (low safety margin), EMLA cream for venepuncture.

Local anaesthetic blocks can be used in ferrets. Nerves commonly blocked include: infraorbital, zygomatic, mandibular, maxillary and mental. This is usually helpful for dental procedures.

Epidural blocks can also be performed in ferrets at the lumbosacral joint.

Intubation

This can be difficult in small mammals, but it provides access to the airways and is preferable in all cases.

In rabbits, intubation is difficult due to: large tongue and cheek teeth, small airway, large epiglottis, acute angle between mouth and larynx, prone to laryngospasm.

It can be done blind, which requires practice, or visually using an otoscope.

Most smaller rodents are not intubated, but techniques have been described.

Ferret intubation is usually straightforward after spraying the epiglottis with lignocaine.

Considerations during surgery

Position is important, as most small mammals have a significant weight in their abdominal viscera which may press against the diaphragm and cause breathing difficulties. They should be positioned with the thorax slightly above the abdomen to avoid this problem.

Ferrets have a strong vagal reflex, making them more susceptible to development of arrhythmias and so agents should be selected to try and minimise this development.

Eye lubricants should be used to protect the eyes as they are often bulging.

Monitoring

The best monitoring equipment is a nurse's close observation. Written records allow trends to be seen and provide a reference if problems occur.

Eye reflexes are difficult to monitor as the pupillary response is lost when ketamine is used.

The depth and pattern of breathing gives information on: the depth of anaesthesia, the level of analgesia, respiratory problems.

But it can be hard to monitor due to a high respiratory rate in small mammals.

The pedal withdrawal response is lost at medium and deep planes of anaesthesia.

Cardiac monitoring is important, and a sudden tachycardia may indicate an inadequate plane of anaesthesia.

Temperature monitoring is essential: all anaesthetics depress thermoregulation, and some cause peripheral vasodilation. Opening body cavities and removing fur removes insulation, and water loss from lungs and viscera removes heat.

Other monitoring equipment that can be used includes: ECG, oesophageal stethoscope, capnography, doppler.

Anaesthetic protocols

In all cases the animal should be weighed to gain an accurate figure and to avoid over or under-dosing.

Rabbits

Rabbits breath-hold if a face mask or an induction chamber is used, and so these methods are not preferred.

Pre-medicants such as diazepam or acepromazine can be used in particularly nervous or fractious animals.

Medetomidine and ketamine provide good surgical anaesthesia and will enable intubation.

The triple combination of: medetomidine, ketamine and butorphanol is useful for short procedures and prior to anaesthetic maintenance.

Fentanyl and fluanisone ('Hypnorm') can be combined with diazepam.

Inhalation agents include isoflurane and sevoflurane. Sevoflurane is the agent of choice if chamber induction is essential, as it is less likely to irritate the airways.

Guinea-pigs

Xylazine can be combined with ketamine and given i/m or s/c, but this can lead to a prolonged recovery.

Guinea pigs can also be anaesthetised in an induction chamber with isoflurane, after several minutes of pre-oxygenation.

Chinchillas

Ketamine and acepromazine i/m or s/c works well but can also lead to a prolonged recovery.

Inducing with isoflurane in an induction chamber is also possible.

Other Rodents

Isoflurane induction and maintenance is the method of choice.

Various injectable combinations can be injected intraperitoneally, but the response is variable and there is little control over the depth of anaesthesia.

Ferrets

Medetomidine and ketamine can be given s/c. A small amount of isoflurane may be necessary to allow intubation.

Maintenance is with isoflurane, and antipamezole can be given to reverse the medetomidine, given at the same volume of medetomidine that was injected.

Alternatively, acepromazine can be given as a premedicant, and the ferret can be masked down with isoflurane.

Propofol can be used if an intra-venous line is available.

Post-Operative Considerations

Rabbits should have a warm environment in which to recover to help with any hypothermia issues that may have been encountered. Artificial fleece bedding should be used then moved onto a straw/hay based bedding and not sawdust. Water should be provided and the patient should be encouraged to eat as soon as possible to help prevent post operative gastrointestinal issues. Analgesia should also be given to help prevent development of post operative ileus.

Rodents are often placed in incubators after the procedure is completed upon recovery to help maintain and increase the body temperature. This can also be aided by the use of subcutaneous or intra-peritoneal warmed fluids. Bedding should be provided such as artifical fleece or similar, but no sawdust should be used. Patients should be encouraged to eat as soon as they have fully recovered. Prokinetics may be given to guinea pigs and chinchillas to help prevent postoperative ileus. Non steroidal anti inflammatories are useful in rats also for preventing ileus. Patients should be kept separate from dogs and cats to minimise stress upon recovery.

Ferrets should be maintained on oxygen until breathing spontaneously and their circulation is stable. The ET tube should be removed when they start to move or gag. It is common for them to curl up into the sleep position during recovery and heat and blankets should be provided for burrowing. It is not uncommon postoperatively for blood pressure to drop after it appears they have recovered, particularly in upper gastrointestinal surgeries or those that have significant blood loss.


Anaesthesia and Surgery - Small Mammals Learning Resources
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References

Malley, D. (2010) Anaesthesia and analgesia of rabbits RVC student notes

Sayers, I. (2010) Rabbit Surgery RVC Elective student notes

Lewis, W. (2010) Small Mammals RVC Elective student notes




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