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Rabbit Anaesthesia – Understanding Your Patient.

Domestic rabbits are not native to the UK and were introduced by monks in the 5th Century.  However, the wild population didn’t become established until the Normans created warrens and introduced them in large numbers1.  

Broadbelt et al (2008) determined the perioperative mortality rate for rabbits was 7 times greater than for dogs, and even today, when rabbits are the third most commonly treated household pet they are still considered to be a challenge to anaesthetise2. Rabbits differ in anatomy, physiology, behaviour and responses to drugs when compared to dogs and cats, so it is necessary to become familiar with these traits in order to influence the assessment, monitoring and protocol selection for the patient, and to permit effective and informed communication with the client.

PREY SPECIES

Rabbits are sociable prey animals and are very aware of the sounds, sights and smells of potential predators.  They hide sickness and injury well as a wild lone or obviously sick rabbit will rapidly be predated.  Add to this the fact that some owners are unfamiliar with the care of the pet rabbit means that an apparently healthy animal presented to the clinic may actually be very sick.

GASTROINTESTINAL (GI) SYSTEM

Rabbits are hind-gut digesters, with digestible fibre undergoing bacterial fermentation in the thin walled caecum (comprising 40% of the volume of the GI tract) to produce volatile fatty acids (VFAs).  The VFAs are either absorbed through the caecal wall into the circulation or are excreted in caecotrophs that are then re-eaten for digestion in the small intestine. Approximately 40% of a rabbit’s maintenance energy requirements are obtained from caecally produced VFAs.  A stable caecal microflora contributes to a healthy GI tract and any disruption of the microfloral balance as a result of stress, disease, diet or medication can interfere with GI health3.

The bulk of the ideal diet for the pet rabbit should be grass and/or good quality hay.  This high fibre diet not only aids gut health but also reduces the chances of dental issues.  It is often necessary to educate owners on the correct diet for their pet rabbit as feeding high levels of pellet concentrate, often considered to be a “normal” diet by some clients, may result in obesity, poor body condition, dental problems and gastrointestinal issues.  Muesli-type diets allow selective feeding and should be avoided or restricted.  A rabbit patient with a healthy GI tract is much less likely to experience ileus in the perioperative period and will be more likely to respond positively to treatment if GI stasis should occur.  

The high metabolic rate of the rabbit requires a constant intake of food. Additionally, the continuous ingestion of fibrous material aids the maintenance of gut motility and energy balance.  Therefore, the rabbit patient should not be starved prior to anaesthesia and this should be carefully explained to the rabbit owner.  

RESPIRATORY SYSTEM

Rabbits are obligate nose breathers with a soft palate that apposes the epiglottis creating a tight pharyngeal seal1.  This tight seal means rabbits are unable to vomit, although they can still regurgitate.  The epiglottis of the rabbit is relatively small compared to a dog or cat and endotracheal intubation or use of supraglottic airway devices can take practice1,4.

At 4-6mg/kg the tidal volume of the rabbit is much smaller than a cat of the same weight and the diaphragm curves dorsally and caudally so any pressure from the large caecum, especially if the rabbit is positioned in lateral or dorsal recumbency, must be considered during treatment.  Ideally the rabbit should be maintained in sternal recumbency wherever possible and if it is necessary to place the animal laterally or in a supine position then a slight “head up” arrangement will help reduce gastrointestinal pressure on the diaphragm. Due to the small lung capacity of the rabbit care should be exercised during IPPV to reduce the possibility of barotrauma. 

Subclinical respiratory disease is relatively common in pet rabbits and can be difficult to detect but it can severely compromise respiratory capacity. Respiratory disease should therefore be considered in all rabbits, even those presenting as apparently healthy, as in reality they could actually be classified as ASA Physical Status II-IV5. An ASA score of >III has been shown to increase the risk of peri-anaesthetic mortality and may justify alterations to anaesthetic protocols2.

CARDIOVASCULAR SYSTEM

The heart of the rabbit is small (0.3% of bodyweight) and lies in the cranial part of the thoracic cavity. It has two (2) cusps in the right atrioventricular valve compared to three (3) in other commonly treated domestic species1. As heart rate is less variable than in the dog or cat it is a much less reliable indicator of anaesthetic depth or degree of pain. However, stress e.g. due to hospitalisation, will cause catecholamine release via stimulation of the sympathetic nervous system with a resultant persistently elevated heart and respiratory rates. Therefore, monitoring of these parameters should be approached with caution.

Obesity is increasing in the pet rabbit population. In comparison to their leaner counterparts these overweight patients tend to have elevated resting heart rates and are at risk of developing hypertension and myocardial hypertrophy.  The increased abdominal pressure on the thorax reduces functional residual capacity with a resultant increased potential for hypoxaemia4

SKELETAL ANATOMY

The powerful hind leg and lumbar muscles of the rabbit allow rapid acceleration and high-speed evasion of predators, but this power is associated with a relatively lightweight and fragile spine. In fact, the skeleton of the rabbit accounts for only 7-8% of its total bodyweight compared to 12-14% for the cat and dog1,6.  Confident and knowledgeable handling is, therefore, essential to reduce the risk of spinal fractures (most commonly in the lumbar region) or other injuries and to protect the handler.  

ORAL CAVITY

The dental formula for the rabbit is:  I2/1; C0/0; P3/2; M3/3.

Rabbits have 6 incisors – 4 upper and 2 lower with the second upper incisors (peg teeth) located behind the primary incisors.  There is a diastema between the incisors and the  6 upper (3 each side) and 4 lower premolars.  The rabbit has 6 upper and 6 lower molars. The premolars and molars are commonly referred to as “cheek teeth”.  All rabbit teeth are elodont, or continuously growing (approximately 3mm/month1), requiring an abrasive diet to allow wear and maintenance of optimal length.  Grasses and hay, which are high in the abrasive silicates necessary for normal tooth wear, should form the bulk of the diet.

The rabbit has a small gape and long oral cavity together with a large and fleshy tongue.  When added to the large cheek folds at the diastema examination of the mouth and cheek teeth becomes challenging in the conscious rabbit, and visualisation for endotracheal intubation may be difficult although this does depend on the technique used.

PHARMACEUTICAL CONSIDERATIONS – DRUGS AND DRUG METABOLISM

The majority of pharmaceuticals are not licenced for use in the pet rabbit but there are exceptions in the UK: Alfaxan (Jurox UK), an intravenous anaesthetic induction agent; the neuroleptanalgesic Hypnorm (VetPharma).  This lack of products registered for use in rabbits means the Cascade for prescribing unauthorised medicines must be adhered to (for further clarification seek advice from the Veterinary Medicines Directorate) and the written, informed consent of the owner must be obtained prior to the administration of any drug not licenced for use in the target species.

The unique anatomy and physiology of the rabbit has resulted in distinctive responses to pharmaceuticals e.g. many rabbits have high circulating levels of atropinase resulting in rapid breakdown of atropine rendering this drug ineffective.  The expected response to drugs can also be unpredictable. Consequently, rabbits should not be treated as small cats or dogs and drug selection should be carefully researched prior to administration.  They should be treated with consideration and handled respectfully with attention paid to the requirements of a prey species.

Consult the most recent edition of formularies, manuals and texts for the current, most accurate, dose rates.  Avoid extrapolating dose ranges from cats and dogs and applying them to the rabbit as this may result in errors.  If in doubt, discuss protocols with a veterinary colleague experienced in rabbit anaesthesia.

For your downloadable summary of
Rabbit Anaesthesia - Understanding Your Patient

Please click here
Article by
Dr. Karen Heskin
BVSc CertSAO MRCVS

Veterinary Technical Manager, Jurox UK

Originally published: Wednesday, 17th April 2019

References

  1. Meredith A (2014). Chapter 1: Biology, anatomy and physiology. In: Meredith A and Lord B (eds). (2014). BSAVA Manual of Rabbit Medicine. 1-12.
  2. Broadbelt DC et al (2008). The risk of death: The Confidential Enquiry into Perioperative Small Animal Fatalities. Veterinary Anaesthesia & Analgesia. 35, 365-373
  3. Michelland et al (2010). Molecular analysis of the bacterial community in the digestive tract of the rabbit. Anaerobe 16(2), 61-65
  4. Grint, N. (2016). Chapter 1: Anaesthesia. In: Harcourt-Brown F and Chitty J (eds). BSAVA Manual of Rabbit Surgery, Dentistry and Imaging. 1-25.
  5. Eatwell K. (2014). Chapter 10: Analgesia, sedation and anaesthesia. In: Meredtith A and Lord B (eds). BSAVA Manual of Rabbit Medicine. 138-159
  6. Abdalla et al (1992). Comparative anatomical and biochemical studies on the main bones of the limbs in rabbit and cat as a medicolegal parameter. Veterinary Medical Journal 26, 142-153

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