Anaesthesia and Surgery - Small Mammals
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.
Starvation 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.
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.
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
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.
Eye lubricants should be used to protect the eyes as they are often bulging.
Monitoring
Analgesia
This is essential for any anaesthetic procedure, as small mammals do not show signs of pain very well and it may go unnoticed.
Analgesia can be given pre-operatively to pre-emp the painful surgery.
Drug available include:
Opiates: pethidine, buprenorphine
NSAIDs: meloxicam, carprofen
Local anaesthetics: lidocaine, bupivicaine, EMLA cream for venepuncture.
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.