Anaesthesia in the rabbit - Bibliovet The Encyclopaedia of Veterinary Medicine

Details: Category: Schede; Published: 02 May 2016; Hits: 10

Disciplina: Anestesiologia
Specie: Coniglio

ANAESTHESIA IN THE RABBIT: SAFETY AND RISK FACTORS

According to a study conducted in England (Broadbelt, 2008) overall anaesthesia-related deaths, regardless of the underlying conditions, are as follows:

Humans: 0,01-0,00167%
Dogs: 0,17%
Cats: 0,24%
Rabbits: 1,39%

It is evident from this study that in the rabbit, anaesthesia-related mortality is much higher than in the dog and cat. In addition, if mortality is considered based on the rabbit ASA class, mortality is as follows:

ASA I-II: 0,73%
ASA III-IV: 7,37%

It is therefore plausible that many rabbits initially diagnosed as clinically healthy have in fact an underlying disorder, and are actually in ASA class III.

The same study then assessed the moment of death in the perioperative period:

After premedication: 0%
At induction: 6%
During anaesthesia: 30%
After surgery: 64%

In most cases the cause of death is unknown, however in approximately 40% of cases it can be attributed to cardiorespiratory failure.

Table 1. ASA classification system

Category	Anaesthetic risk	Meaning
ASA I	Minimum	Normal healthy patient
ASA II	Low	Patient with mild systemic disease
ASA III	Medium	Patient with moderate to severe systemic disease
ASA IV	High	Patient with severe systemic disease that is a constant threat to life
ASA V	Very high	Moribund patient not expected to survive 24 hours
E	Emergency	The letter E is added to each class depending on the status of the patient.

Several anatomical and physiological factors contribute to making anaesthesia in the rabbit an objectively more delicate procedure than in the dog or cat:

Stress

The rabbiti s an animal of prey which is easily stressed when frightened or manipulated abruptly, with consequent release of catecholamines that adversely affect cardiac and renal circulation. Gentle handling, gentle ways and a peaceful and quiet environment, away from potential predators, helps to prevent such problems. After the trip to the clinic the animal should therefore be allowed to rest for a few hours; performing the anaesthesia just after arrival is not recommended. The rabbit should be left for a few hours - or for the night before the intervention - in a quiet and peaceful place, possibly in its cage or carrier.

Pain

Pain has profound negative effects on the physiology of the rabbit; it reduces hunger and intestinal motility, causing intestinal stasis, which may in turn be the cause of hepatic lipidosis.

Pain may induce the release of catecholamines and cause cardiac arrest; analgesia is therefore particularly important for surgical procedures. Rabbits appear particularly susceptible to pain secondary to abdominal surgery, while pain caused by limb fractures is apparently well tollerated. Analgesia should be started before the surgical procedure and continued for as long as necessary.

Anatomy of the glottis

In the rabbit the glottis is deep within the oral cavity and not easily visualized; it is easily traumatized following inept intubation attempts and is easily susceptible to laryngospasm. These factors make endotracheal intubation a delicate and not easy to learn procedure.

Nasal breathing

The rabbitis an obligate nasal breather. The presence of exudate in the nasal passages or inept manoeuvres that occlude the nostrils can result in rapid death of the animal; in this regard, it is important to remember that in the rabbit open mouth breathing is an ominous sign that often precedes death. Infections of the upper airways, which are a relative problem in the dog and cat, seriously hinder breathing in the rabbit. If possible, rabbits with such conditions should not be anaesthetized; in case of absolute need, intubation is necessary.

Apnoea induced by gas anaesthetics

The smello f gas anaesthetics, even for a light plane of anaesthesia, makes the rabbit hold its breath for up to three minutes, resulting in bradycardia and hypercapnia. Chamber or facial mask induction is therefore problematic if not impossible. Even in premedicated rabbits, mask induction should be done gradually, over several minutes.

Hypoxia

During anaesthesia the rabbit can easily enter into a state of hypoxia. The tidal volume of the rabbit is of only 4-6 ml/kg; the thoracic cavity is relatively small compared to the abdomen and breathing is predominantly diaphragmatic. During anaesthesia, the viscera weighing on the diaphragm may hinder breathing, a particularly serious problem in obese rabbits.Breathing may be helped by keeping the chest slightly lifted and particularly by ventilating the animal before problems may arise (intubation is necessary).

Occult disorders

As a prey animal, the rabbit tends to mask signs of illness. Pet rabbits may have a severe respiratory or heart disease while showing minimal symptoms and behaving normally. General anaesthesia can impair a state of precarious balance and cause failure of the cardiorespiratory system. For this reason, an accurate preanaesthetic assessment and collateral tests are necessary, especially hematochemical tests and radiographic studies.

Hypothermia

Pet rabbits are often small-sized and therefore predisposed to problems of intra- and postoperative hypothermia. Hypothermia jeopardizes the metabolism of anaesthetic agents, which may result in an excessive depth of the plane of anaesthesia, hypoxia, acidosis, cardiac arrhythmia and death.The use of warming strategies duringanaesthesia, such as using circulating warm water mats, clipping of only the minimum amout of hair possible, use of lukewarm liquid disinfectants (with no alcohol), monitoring of the body temperature and keeping the rabbit warm until complete recovery, is recommended.

Risk of anaesthesia overdose

The rabbit is more sensitive to the depressive effects of anaesthetic agents on the respiratory system, with a margin of safety which is relatively small compared to the dog and cat. Drug doses must therefore be calculated carefully, after weighing the rabbit with a precise scale.

Intolerance to fasting

Prolonged fasting may be the cause of gastrointestinal stasis and, already after 48 hours, of hepatic lipidosis. This, coupled with the fact that rabbits do not vomit, makes preoperative fasting contraindicated. Food should be suspended just one hour before the anaesthesia, in order to avoid that an excessively full stomach may hinder the excursion of the diaphragm and that any food residue may create problems with intubation. In any case, most of the ingesta are in the caecum, which requires days to empty.

Since pain causes anorexia, in all painful procedures analgesics should be continued in the days following anaesthesia. After recovery from the anaesthesia the rabbit should be encouraged to return to eating as soon as possible; should the rabbit not feed spontaneously, assisted feeding should be initiated together with parenteral rehydration. In the presence of an anorexic patient, prior to scheduling surgery assisted feeding should be started - this is contraindicated only in the case of gastric surgery or of intestinal obstruction – together with the administration of prokinetics (i.e. clebopride).

The importance of ventilation

The main cause of anaesthesia-related death is poor ventilation (a concept that does not coincide with oxygenation). Ventilation corresponds to the air that enters and leaves the lungs; it is measured with a capnograph that monitors ETCO2 - the exhaled carbon dioxide - which in the anaesthetized patient is in turn related to PaCO2. In practice, it may be stated that the most common cause of death is respiratory failure.

Airway management is therefore essential to ensure a safe anaesthesia, as it allows oxygen administration and successful patient ventilation. Intubated patients may also undergo capnography, which allows real time monitoring of the efficacy of ventilation, in contrast to pulse oximetry which detects ventilation problems with a certain delay.

Table 2. Physiological parameters of the rabbit

Physiological parameters	Normal range
Body temperature	38,5 – 39,6 °C
Heart rate	130 – 325 beats/min
Blood volume	57 – 65 ml/kg
Respiratory rate	32 – 60 min
Tidal volume	4 – 6 ml/kg

PREANESTHETIC EVALUATION AND PREPARATION

Proper anaesthesia management starts long before the administration of the anaesthetic agent, it must start with the patient's physical examination. Ideally, when not dealing with an emergency, this should be done a few days before surgery. The rabbit to be anaesthetized should undergo a thorough physical examination, with the collection of a detailedclinical history, which should include:

the diet (dietary errors can have a negative impact on GI function);
appetite and faecal production (small and scarce droppings or the absence of faeces are indicative of stasis);
mental alertness (any decrease in alertness is indicative of the presence of problems);
variation in body weight;
current and past disorders, especially involving the cardiorespiratory system.

The physical examination must be thorough and must consider each organ system, the animal’s weight and the nutritional and hydration status. Of particular importance is the assessement of the cardiovascular system by means of auscultation, the examination of the nasal cavities (to exclude the presence of exudate or of abnormal respiratory movements) and of the colour of the mucous membranes.

Excessively lean rabbits may have health problems, such as dental disease or kidney failure. Obesity is also a major risk factor, as it hinders ventilation and facilitates the onset of hepatic lipidosis; in obese rabbits, elective surgeries are to be postponed until return to the target weight by correcting the diet and promoting physical activity.

The hydration status must always be verified; rabbits with dental or GI disorders may be dehydrated and demand adequate rehydration therapy, subcutaneously in mild cases or intravenously in more severe ones. In severe cases, if a vascular access cannot be established an intraosseous vascular access may be used.

It is important to verify the preservation of a normal appetite and faecal production, as anorexia is a significant risk factor for post-anaesthesia recovery.

Additional tests include CBC, clinical biochemistry and X-ray examination of the entire body. In specific cases, ultrasonography, CT or MRI may be useful.

Once the examination has been completed the ASA class can be assigned to the patient. Should the physical examination and the additional tests not reveal the presence of any disease and if a proper protocol is followed and if stress to the animal is avoided, problems during anaesthesia are unlikely

PRE-ANAESTHESIA

Before induction of anaesthesia the patient must be calm. It is recommended to wait a few hours after the arrival of the animal to the clinic, or to hospitalize the patient in its carrier in a quiet room overnight, in order to allow acclimatization to the new environment. If hospitalized, it is necessary to check that during the night the rabbit eats and defecates.

The rabbit should be handled gently and any self-induced injury in the attempt to break free must absolutely be avoided, as it can result in dislocation of the lumbar vertebrae. In order to facilitate the administration of intramuscular preanaesthetic drugs, if necessary the rabbit can be wrapped in a towel (Fig. 1); subcutaneous injections are instead well tolerated and usually require only a minimum restraint.

Fig. 1. Restraint method for intramuscular injections

Preanaesthetic drugs
In all cases of preexisting painful conditions or when surgery or painful maneuvres are expected, analgesics are to be used. A combination of NSAIDs (e.g. meloxicam) and opioids (butorphanol or buprenorphine) are normally used. In very agitated rabbits, benzodiazepines may be used, in view of their calming and muscle relaxant effect.

The drugs which are contraindicated in the rabbit, also in combination with other anaesthetic agents, are:

Acepromazine and chlorpromazine: they cause hypotension and hypothermia.
Xylazine: it causes cardiorespiratory depression.

Atropine is not listed among the preanesthetic drugs as a large percentage of rabbits possess the atropinase enzyme, which inactivates atropine; in its place, glycopyrrolate is preferable (not on sale in all countries).

Table 3. Preanaesthetic drugs

Drug	Dose	Route	Notes
Butorphanol	0,2-0,4 mg/kg	SC, IM	Opioid
Buprenorphine	0,01-0,05 mg/kg	SC, IM, IV	Opioid
Dexmedetomidine	0,05-0,125 mg/kg	SC, IM	See medetomidine
Diazepam	1 mg/kg	IM, IV	Moderate to deep sedation. Scarcely effective subcutaneously.
Fentanyl/droperidol	0,2 mg/kg	IM	Moderate to deep sedation. Good analgesia. The injection is painful. May induce bradycardia.
Ketamine	10 - 50 mg/kg	IM	Moderate to deep sedation. Moderate to good analgesia. High doses may induce depression and muscle rigidity.
Glycopyrrolate	5 – 11 mg/kg, up to 40 mg/kg	IV	Recommended in cases of respiratory diseases. Reduces salivation, bronchial secretions and bradycardia of vagal origin.
Glycopyrrolate	100 mg/kg	SC, IM
Medetomidine	0,1 – 0,5 mg/kg	SC, IM	Moderate to deep sedation. Good analgesic properties. Not recommendedin rabbitswith health problemsas it can causecardiorespiratory depression. Causesperipheral vasoconstriction and a mucosal “mauve” colour not to be mistaken for hypoxia.
Meloxicam	1 mg/kg	SC, IM	NSAID; initial dose
Meperidine	10 mg/kg	IM, IV	Sedation, analgesia. High doses cause respiratory depression.
Midazolam	2 mg/kg	IM, IV	Moderate to deep sedation. Better systemic absorption compared to diazepam. Minimal impact on the cardiovascular system.
Morphine	2 - 5 mg/kg	SC, IM	Sedation, analgesia. High doses cause respiratory depression. Not particularly recommended in the rabbit as it can induce severe ileus.

INDUCTION AND MAINTENANCE OF ANAESTHESIA

In view of the availability of many anaesthetic protocols the final choice will depend on the conditions of the rabbit as well as on the type and duration of surgery. Each rabbit should be weighed accurately prior to calculating the anaesthetic dose; but apart from the animal’s weight, it is necessary to consider the general overall conditions of the patient, bearing also in mind that in any case the individual response may be variable.

Routes of administration of induction drugs

In the rabbit, chamber induction with a gas anaesthetic is not indicated; the smell of the anaesthetic agent stimulates a prolonged apnoea, with resulting hypercapnia and acidosis, the release of catecholamines and a high risk of mortality. During the excitatory phase of induction the rabbit can become agitated and injure itself. Moreover, the only parameter that can be monitored during chamber induction is breathing.

Mask induction is not usually used, unless sevoflurane is available, as also in this case the odour of the anaesthetic agent may be the cause of prolonged apnoea, hypercapnia and death. Should mask induction be used, the animal must be adequately premedicated in order to reduce stress, although this does not prevent apnoea. The gas anaesthetic should initially be deliverd with a mininum flow, and then increased in a stepwise manner; should the animal stop breathing, the anaesthetic flow is to be again decreased.

The best induction technique consists of a balanced mix of injectable anaesthetics administered subcutaneously, intramuscularly or intravenously. During maintenance intravenous medications are instead easy to overdose. The intramuscular route allows for a more rapid and regular absorption than with the subcutaneous approach, even if it is more annoying and necessitates good animal restraint. The disadvantages of the injectable approach consist in the impossibility of controlling the plane of anaesthesia when it is too deep and the prolonged awakening time if reversible agents are not used.

To facilitate intramuscular injections the rabbit may be completely wrapped in a towel, leaving only the rear part of the back free, so as to allow the injection in the epaxial muscles (on the side of the spine). The rabbit is then placed in a pet carrier until the anaesthetic agent takes effect. As soon as the upright posture is lost therabbit is tranferred to the operating table, while administering oxygen via a face mask. Subsequently, if indicated, the rabbit is intubated.

For the intravenous route a catheter is inserted into the cephalic vein of the forearm (Fig. 2),into the lateral saphenous vein or, in large breed rabbits, into the auricular vein. The auricular vein is not used for the administration of diazepam as this agent may cause tissue necrosis.

The insertion of an intravenous catheter in a healthy rabbit requires sedation; the intradermal inoculation of small amounts of local anaesthetic in the site of cather insertion, a few minutes prior to the procedure, may be helpful (see “Local anaesthesia”).

Fig. 2. Insertion of the intravenous catheter

Table 4. Examples of anaesthesia protocols

*Premedication*	*Induction (10–15 minutes after premedication)*	*Maintenance*
Butorphanol 0,2–0,4 mg/kg SC, IM Buprenorphine 0,01–0,05 mg/kg SC, IM, IV Meloxicam 1 mg/kg (in addition to one of the previous agents)	Ketamine 10 - 15 mg/kg + medetomidine* 0,15 - 0,25 mg/kg, SC, IM Ketamine 5–10 mg/kg + midazolam 0,25–0,50 mg/kg, IV Medetomidine 0,25 mg/kg + fentanyl 0,02 mg/kg + midazolam 1 mg/kg, IM Medetomidine 0,1 mg/kg + ketamine 3 mg/kg + butorphanol 0,3 mg/kg, IV Induction with sevoflurane at 8%	Isoflurane 1,5–2,5% Sevoflurane 2–4%

*Dexmedetomidine may be used instead of medetomidine, at half the dose (same volume)

The combination tiletamine-zolazepam is not recommended in the rabbit as it may induce renal necrosis.

Positioning of the animal

The patient must be positioned so as to minimize as much as possible breathing constraints. Dorsal recumbency is the position that most hinders diaphragmatic excursion; keeping the chest slightly raised above the abdomen is helpful. The head and neck must be physiologically extended. If the limbs are tied, they should not be unduly extended. Instead of being tied in the usual position, the front legs may be positioned to the side of the chest.

Oxygen administration

Anaesthetized rabbits should always receive oxygen (regardless of the type of anaesthesia), as spontaneous ventilation with atmospheric air is insufficient to maintain adequate oxygenation. In the case of manual or mechanical assisted ventilation, the respiratory rate must be of 25-50 breaths per minute. Oxygen can be administered using various techniques: face masks, endotracheal or intranasal tubes, supraglottic airway devices.

Face masks

Face masks have the advantage of being simple to use and they do not irritate the airways as there is no contact with the larynx. Should intubation not be possible, face masks represent the only method to administer oxygen. In case of minor procedures on healthy subjects, face masks may be preferable to endotracheal intubation as they prevent the risk of iatrogenic damage to the airways. Oxygen is to be administered at a flow rate of three times the respiratory minute volume.

The disadvantages of face masks are: inability of guaranteeing an effective assisted ventilation, increased dead space, dispersion of the anaesthetic gas and inhalation of atmospheric air if the mask is not perfectly tight, apnoea if isoflurane is also administered, inability to protect the airways from the entry of foreign material.

Precautions for face mask use are: the nostrils should not be obstructed and the eyes should not be harmed with the rubber mask edge; masks with less dead space and with a better seal are recommended. Since the rabbit is a nasal breather, it is sufficient for the mask to cover just the nose; if necessary, this solution leaves space to work within the oral cavity (Fig. 3).

Face masks are not recommended for direct induction as they may be the cause of prolonged voluntary apnoea, with the consequent marked increase of carboxaemia, agitation and with all the previously mentioned adverse events.

Fig. 3. Being the rabbit an obligate nasal breather it is sufficient for the face mask to cover only the nostrils.

Table 5. Advantages and disadvantages of face masks

Advantages

Disadvantages

· Easy to use

· No contact with the larynx

· Alternative method when intubation is not possible

· Preferable to endotracheal intubation in minor or short procedures

· Increased dead space

· Inhalation of ambient air and dispersion of gases

· Apnoea

· IPPV difficult or impossible

· No protectionof theupper or lowerairwaysfrom obstructionor aspiration

Nasal cannula

When orotracheal intubation is impossible in view of the small size of the animal or because the head or the oral cavity must remain accessible, a nasal cannula for the administration of oxygen and of gas anesthetics may be used. Intranasal intubation exploits the fact that in the rabbit the epiglottis is trapped against the dorsal surface of the soft palate, so from the nasopharynx air can directly reach the larynx and trachea. A tube inserted nasally through this aperture provides oxygen at tracheal level and is effective even if the mouth remains open.

Small diameter endotracheal cannulas (2-2.5) may be used, as well as nasal or soft catheters, according to the size of the patient. The rabbit must be properly anaesthetized and placed in dorsal recumbency. Before insertion, the length from the nostrils to the glottis is measured and the tip of the cannula is lubricated with water-soluble gel. The rabbit is placed in dorsal recumbency, with the head extended; the tip is inserted into one nostril and advanced in ventro-medial direction until reaching the glottis (Fig. 4). If the tube is too large or if the manoeuvre is not properly performed, a mild epistaxis may result.

Once the catheter has been insertedit is connected to the anaesthetic circuit; the gas anaesthesia flow used is similar to the one administered via a face mask (three times the respiratory minute volume).

Fig. 4.I nsertion of the ntranasal tube

Endotracheal tubes

For prolonged anaesthesias, endotracheal intubation is essential. Before attempting intubation the rabbit must be oxygenated for about three minutes. No more than three insertion attempts should be tried in order not to damage the delicate structures of the upper airways that can lead to bleeding, swelling and spasms, with potentially fatal consequences. Intubation should be performed only if the rabbit is under an adequate plane of anaesthesia; in practice, no retraction reflex or increased respiratory rate should be present when compressing the digit at the base of the claw.

In small-size patients intubation is not only more difficult but also results in a significant increase in air flow resistance.

In view ofthe anatomical peculiarities of the rabbit, endotracheal intubation presents some objective difficulties:

Long and narrow oropharynx
Large andl ong tongue
The soft palate covers the larynx
Big incisors
The opening of the oral cavity is limited by the temporomandibular joint
Easily induced laryngospasm
Possibility of damaging the larynx

Usually, size 2,5-3 endotracheal tubes are used in rabbits under 3 kg; size 3.5-4 in rabbits of 3-5 kg.

In the rabbit, three techniques for endotracheal intubation are available. In all cases the animal is placed in sternal recumbency, with the head in hyper-extension - in straight line with the body - in order to align the airways. Since rabbits are prone to laryngospasm, prior to intubation a local anaesthetic must be administered at the level of the larynx and the tube must be lubricated with sterile gel

Blind intubation

Blind intubation requires some experience and cannot be used in apnoic patients. Since the airways are not visualized, a slight risk of introducing foreign material is present. While the tube is being advanced the anaesthesiologist must bring her/his ear to the external opening of the endotracheal tube and listen to respiratory sounds; when the noise reaches its maximum intensity it signifies that the end of the tube is next to the glottis. The tube is advanced into the trachea during inhalation.

Intubation with an otoscope or laryngoscope. This technique is easier to learn and allows the visualization of the glottis and of the possible presence of foreign material. It cannot be used in patients under 1 kg of body weight. With regard to the otoscope, a dedicated sterile cone must be used. The tube may be advanced also through the cone of the otoscope. The tongue is moved to one side and the cone or the laringoscope is inserted until visualization of the soft palate (Fig. 5), which is moved with the tip of the instrument; the larynx is right behind; the tube is advanced into the trachea and the scope is then extracted. The technique must be performed rapidly in order to avoid hypoxia. As a variant, a guide can be inserted first (i.e. a dog urinary catheter) and the tube is then slid over it.

Fig. 5. Observation of the glottis through an otoscope

Intubation with the endoscope. With this technique the visualization of the glottis is further facilitated. The tube can be advanced next to the endoscope, or the endoscope may be inserted into the tube and is then removed once the tube has entered into the trachea.

Table 6. Advantages and disadvantages of endotracheal intubation

Advantages

Disadvantages

· Relatively simplewith practice

· Relativeprotectionfrom aspiration

· Theobstructionof the airways is unlikely

· Particularly usefulfor head procedures

· Possibility of IPPV

· Absenceof gas-induced apnoea

· Possibility of monitoring ventilation

· Significantrisk ofcausing trauma

· Potential infiltration of fluids

· The cuff can damage the tracheal mucosa

· Risk of bronchialoroesophagealintubation

· Risk of inhalation of foreign materialif doneblindly

· Riskof infectionor irritation caused by the disinfectants used to sterilize the tube

Tracheal complications resulting from the use of tracheal tubes:

Onset of wheezing, cyanosis and death 48 hours post anaesthesia
Fibrosis
Stenosis
Venous dilatation and congestion
Oedema
Haemorrhage

To minimize the risk of complications the following precautions are necessary:

The tubes should not be disinfected with chlorhexidine
Apply lidocaine on the glottis
Provide an appropriate plane of anaesthesia to prevent spasm of the glottis
Do not repeat more than three intubation attempts and limit the overall intervention time

Supraglottic airway device

The supraglottic device consists of a tube that ends with a non-inflatable cuff made of soft material, similar to a gel, which fits atraumatically above the glottis (Fig. 6). It is designed specifically for the rabbit and cannot be used in other species. Different sizes are available for patients of all sizes.

Fig. 6. Supraglottic device

The device presents the following characteristics:

The cuff of the device adjusts to the anatomy of the pharynx and of the surrounding structures: when inserted, it displaces the soft palate and it frees the epiglottis, which fits into the opening of the cuff. The larynx and trachea are not touched (Fig 7).
The tip of the cuff engages with the oesophagus, sealing it and preventing the insufflation of the stomach in case of forced ventilation.
The side projections automatically lock the device in the correct position, allowing a precise blind insertion and preventing its rotation.
It is made of very soft, medical-grade silicone, which is atraumatic for airway tissues.
The shape of the device allows a very easy and blind insertion in a matter of seconds, without requiring the visualization of the airways. However, it is always better to first check the oral cavity and pharynx in order to rule out the presence of foreign material.
Thanks to the large internal diameter and the opening of the cuff the device does not cause any narrowing of the airways and eliminates dead space.
The connection to the capnograph allows an effective monitoring of ventilation; this reduces problems and mortality during anaesthesia (Fig. 8).
The cuff seals the airways, allowing IPPV in case of need and avoiding the dispersion of anaesthetic gases.
The speed of insertion makes it the first choice device in emergencies.
The device on the market can be autoclaved after use (up to 40 times), eliminating the risk of disinfectant residues, which are potentially damaging to the tissues.

Fig. 7. The cuff of the supraglottic airway device accurately reflects the anatomy of the perilaryngeal region of the rabbit

The device is available in 6 sizes, adjusting to rabbits ranging from 0.6 kg to more than 5 kg; the size is chosen based on the patient's ideal weight.

The device can be used for intraoraldental procedures, such as the filing of molariform teeth, in medium- and large-size subjects, but not in small ones as it does not allow enough space to manoeuvre inside the mouth.

The insertion of the device is very simple and takes only a few seconds. The rabbit must be on a surgical plane of anaesthesia. An assistant maintains the rabbit in sternal recumbency with the head extended, as for endotracheal intubation. An otoscope or a laryngoscope is used to verify that no foreign material is present in the airways. Lidocaine is instilled on the glottis, waiting for one minute for full effect. In the meantime, the device is prepared and the cuff is lubricated with sterile, water-soluble gel, making sure that the opening is not occluded by an excessive amount of lubricant.The mouth of the rabbit is opened with one handand the tongue is gently moved to one side. The device is inserted through the diastema and is advanced into the oral cavity until feeling a certain resistance; this signifies that it is in place and that it is not necessary to advance it further. The head of the animal is lowered, the device is connected to the capnograph and to the anaesthetic circuit and the device is tied behind the head by tightening the knot ventrally. The correct positioning is confirmed by the movement of the pilot balloon and by the wave of the capnograph.

At the end of the procedure the device is simply extracted.

Fig. 8. Supraglottic device inserted and connected to the capnograph

Vascular access

Brief interventions excluded, a vascular access must always be established: gas anaesthetics have a dose-dependent hypotensive effect; the intravenous administration of fluids is therefore necessary to offset hypotension and to stabilize haemodynamic conditions. Moreover, an established vascular access is required for the administration of drugs in case of emergencies.

The vascular access is usually through the cephalic vein of the forearm, with a technique totally similar to the one employed in the cat. Alternatively, the lateral saphenous vein may be used (except in the case of gastric dilatation), or, in large size rabbits, an auricular vein. Ringer's, electrolytic or saline physiological solutions can be infused, warmed to body temperature. The perioperative dose is of 5-15 ml/kg/h. In small size patients the use of an infusion pump may be useful for a more precise calculation of the dose.

MAINTENANCE OF ANAESTHESIA

Anaesthesia is normally maintained with gas anaesthetics (usually isoflurane) via endotracheal tube, face mask or supraglottic airway device. The face mask is adequate for short procedures in healthy subjects; for longer procedures, ventilation of the patient is preferable, as problems of apnoea may easily arise. A non-rebreathing circuit is used with a gas flow of 250 ml/kg/min.

In order to ensure an adequate plane of anaesthesia, monitoring must be constant. An excessively deep plane of anesthesia can be the cause of cardiac arrest, but even an excessively light one may entail substantial risks. The rabbit, in fact, responds to painful stimuli with apnoea, with a long expiratory phase (often accompanied by a scream in non-intubated subjects). It is therefore necessary to deepen the plane of anaesthesia by increasing the percentage of gas and by using forced ventilation; this would be difficult if the rabbit were not already intubated as the smell of the gas anaesthetic may cause respiratory arrest and forced ventilation would therefore not be possible.

Deepening the plane of anaesthesia with intravenous medications must be done with extreme caution, in order to avoid a possible overdose. Deepening is typically done using one-tenth of the dose used for induction, monitoring its impact.

Minimizing the duration of the anaesthesia

The longer the duration of anaesthesia, the greater the possible incidence of complications and of respiratory depression. At the time of induction, everything must be ready in the surgical theatre: the anaesthetic circuit, surgical instruments, clippers, warm disinfection material, fluid therapy equipment, anaesthetic drugs and drugs for emergencies, staff, etc. The intervention must be carefully planned in advance and every staff member must be fully aware of what they have to do.

Maintenance of normothermia

For the anaesthesia to be successful, hypothermia must be avoided; during the preparatory stages and in the surgical theatre patients must be isolated from the cold table by means of circulating hot water mattresses or convective warming devices. The use of hot water bottles or other similar solutions is not recommended: initially they can cause burns and with time they cool down.

The hair coat is to be clipped as little as possible and the surgical field should be cleansed with warm antiseptic solutions, avoiding the use of alcohol-based disinfectants. During the intervention the body temperature must be monitored, i.e. with a rectal digital thermometer that is turned on periodically. The body temperature can vary rapidly, going both up or down.

Other precautions

The rabbit is an obligate nasal breather: the nostrils must always be patent, unless the rabbit is intubated.
When blocking/tying the limbs for surgery they should not be extended excessively, so as not to obstruct breathing. During the surgical procedure, do not rest your arms on the chest of the rabbit.
The corneas must be moistened frequently with artificial tears in order to prevent corneal drying.
Special attention must be paid to preoperative hair clipping. Rabbit skin is very delicate and easily torn; precision hair clippers with a small cutting head are particularly helpful. While clipping, the skin must be kept distended, with the head of the clipper parallel to the skin. The clipper must be fully functional, in order to cut to a minimum surgical-field preparation times.

EMERGENCIES

Respiratory arrest

In the case of respiratory arrest, positive pressure ventilation (IPPV) must be started immediately, which is only possible if the rabbit is intubated. Intubation during respiratory arrest is very difficult and may require more time than the time available, while the insertion of a supraglottid airway device is simple and immediate, also in case of apnoea. In extreme cases, the use of an emergency tracheal 14-16 G intravenous catheter is possible. A tracheotomy - with the insertion of an endotracheal tube - is also possible, but although effective, for this procedure to be carried out quickly previous practice is required.

In the case of overdose of an anaesthetic agent, naloxone can be used as an opioid antagonist, flumazenil as a benzodiazepine antagonist and atipamezole as an antagonist of dexmedetomidine and medetomidine.

Cardiac arrest

In the rabbit, in case of cardiac arrest the resuscitation success rate is in fact quite low; also in this case, to increase the probability of success the rabbit must already be either intubated or have a supraglottic airway device inserted. The overall approach is similar to what already done in other species:

Start immediately with IPPV and administer any necessary medication, i.e. for respiratory arrest
Begin to perform a cardiac massage by compressing the chest laterally at the rate of 60-120 compressions per minute
Administer adrenaline intratracheally or intravenously
Administer crystalloids at the dose of 60 ml/kg/h
Eventually administer colloids as a bolus, total dose of 5-10 ml

The dosage of drugs for emergencies is calculated based on the patient’s weight before the induction of anaesthesia; in order to allow a prompt intervention in case of need, the drugs must be readily available.

Table 7. Emergency drugs in the rabbit

Drug	Concentration	Dose	Route	Note
Atipamezole	5 mg/ml	0,5 mg/kg	IM, SC	Antagonist of medetomidine and dexmedetomidine
Atipamezole	5 mg/ml	0,25 mg/kg	IV	Antagonist of medetomidine and dexmedetomidine
Epinephrine (1:1000)	1 mg/ml	0,2 mg/kg	IV, IO, IT	In case of cardiac arrest
Flumazenil	0,1 mg/ml	0,01-0,1 mg/kg	IM, IV	Benzodiazepine antagonist
Furosemide	50 mg/ml	2 mg/kg	IV, IO	Diuretic
Glycopyrrolate	0,2 mg/ml	0,02 mg/kg	IV, IO, IT	To counteract bradycardia
Naloxone	0,4 mg/ml	0,01 mg/kg	IV, IO	Opiate antagonist

MONITORING DURING ANAESTHESIA

Table 8. Monitoring systems

Tool	Advantages	Disadvantages
Vigilant anaesthesiologist	Only the anaesthesiologist can assess changes in the respiratory pattern, the best parameter to assess the adequacy of the plane of anaesthesia	Experience is required and the person needs to be focused exclusively on this monitoring function
Stethoscope	Reliable and cheap	Requires the use of both hands. Findings altered by movement. Less effective in case of hypovolaemia
Oesophageal stethoscope	Reliable and cheap and can be used with both hands free
Doppler	Extremely reliable, also in very small patients. It allows the monitoring of the heart rate while leaving both hands free The intensity of the signal correlates with the blood pressure value	Movements may move the probe. Fixation of the probe to the patient may be difficult
Indirect monitoring of arterial pressure	It allows the monitoring of pressure variations	Limited correlation with central blood pressure The procedure is performed manually and is rather labour intensive
Pulse oxymetry	It allows the monitoring of oxygen saturation It does not require a dedicated operator	In smaller patients the probe may be difficult to attach It does not immediately identify decreases in oxygenation
Capnography	It allows the monitoring of exhaled CO₂ - saome capnographs are equipped with a monitor It identifies the onset of hypercapnia in real time	It requires intubation of the patient The smaller sidestream monitors may be too big for smaller sized patients For it to be reliable the dead space must be minimal
Elettrocardiography	It allows the monitoring of heart activity and rate	The device must be capable of identifying very high frequencies Data interpretation requires a certain experience

Modified from: Pre- and Perisurgical Preparations: Optimizing the Little Stuff (1510) AAV 2011 - Angela M. Lennox, Larry Nemetz

Anaesthesia monitoring allows the identification and correction of potentially fatal imbalances (such as bradycardia, hypotension, hypoxaemia, hypercapnia) before they may become dangerous.

Monitoring is based on the observation of reflexes, cardiopulmonary parameters and body temperature.

The most reliable reflexes are, respectively: the auricular reflex (movement of the pinna in response to compression), foot retraction, corneal and palpebral reflexes. Also useful is the monitoring of the muscle and mandibular tone. The presence of vocalizations or of voluntary muscle movements are clearly indicative of an insufficient plane of anaesthesia. The reflex response may vary depending on the type of anaesthetic protocol used. Extremely reliable indications result from the monitoring of the breathing pattern.

Table 9. Reflexes in the course of an adequate plane of anaesthesia

Reflex	Notes
Palpebral reflex	Not reliable
Corneal reflex	Present
Eye position	Not reliable
Foot withdrawal reflex	Should be absent
Mandibular tone	Loss of tone is indicative of a surgical plane of anaesthesia
Auricular reflex	Loss of reflex is indicative of a surgical plane of anaesthesia
Respiratory reflex	A uniform respiration in terms of rate, depth and type is a reliable indication of a surgical plane of anaesthesia

Modified from: Vella D. Rabbit General Anaesthesia. AAVAC-UEP 2009.

Body temperature can be measured continuously or intermittently by means of a rectal thermometer or probe. Body temperature measurement is all the more important the longer the duration of surgery and in the case of visceral exposure.

Indirect measurement of arterial pressure

Arterial pressure is used to determine whether the patient is normotensive, hypotensive or hypertensive, to monitor treatment in patients udergoing shock therapy and for intraoperative monitoring. In smaller sized rabbits the measurement of arterial pressure is more difficult.

The animal is positioned in sternal or lateral recumbency. The cuff may be placed in different body regions, more commonly above the elbow. The width of the cuff should ideally be 40% of the circumference of the region to which it is applied. Wider cuffs underestimate the real pressure reading. The hair is clipped on the medial side of the central area of the radio-ulnar region; the probe is positioned covered with a generous amount of ultrasound gel and is then held in place.

The Doppler unit is turned on with the volume initially set to zero, and is then slowly turned up. The Doppler sound should be heard clearly. If nothing is heard, or if the signal is too weak, the probe is moved until the vessel is localized. When a good signal is established, the probe is fixed on site with some tape. The cuff is inflated until the sound disappears; the pressure is then slowly released from the air valve (about 3 mm Hg per second) until the sound is again herd. As soon as the sound returns the pressure value is read on the sphygmomanometer, value which corresponds to the systolic pressure.

Maintenance of the Doppler unit

At the end of the procedure the probe is cleaned with cotton or gauze, moistened with water or chlorhexidine, and the protective cap is then applied. Alcohol or ECG gel should not be used on the probe to avoid damaging it. The cable must be left attached to the unit, coiling it so as not to create sharp bends or twists, to prevent damage. When not in use, the Doppler unit should always be left connected to the power line in order to maximize battery life.

Capnography

Capnography is the measurement of the concentration of carbon dioxide (CO2) in exhaled air - ETCO2 (end tidal CO2, end-tidal carbon dioxide) - which reflects the value of arterial CO2 and allows to assess whether the patient's ventilation status is appropriate or not. In addition, it also allows the assessment of CO2 variations in real time (capnogram). Capnography thus provides information on the need for assisted ventilation, as anaesthesia often results in a more or less severe respiratory depression. It also allows the assessment of the correct positioning of the endotracheal tube or of the supraglottic device and of the proper functioning of the anaesthetic circuit.

In awake subjects, the normal CO2 value is of 35-45 mm Hg. Values over 45 mm Hg signify that ventilation is inadequate and that the potential causes must be identified and corrected:

an excessively deep plane of anaesthesia, with consequent depression of the respiratory centre;
inability to adequately expand the chest due to the animal’s positioning.

In order to correct such problems, ventilation of the patient is started at a pressure of 15-20 cm H2O - corresponding to about 15 ml/kg of air - while in the meantime the plane of anaesthesia is lightened or the positioning of the animal is changed.

Should CO2 decrease to values close to zero, the possible causes are:

Disconnection of the anaesthetic circuit from the airways
Dislodgement of the endotracheal tube or intubation of the oesophagus
Complete obstruction of the endotracheal tube
Malfunctioning of the ventilator

Pulse oximetry

Pulse oximetry is used to evaluate the oxygen saturation of haemoglobin in arterial blood (SpO2) - to determine if the blood is properly oxygenated - and the frequency and rhythm of cardiac activity; it is typically used in anaesthetized animals for anaesthesia monitoring.

Oxyhaemoglobin absorbs light at a wavelength of 940 nm, while deoxyhemoglobin at 660 nm. The LED in the pulse oximeter probe emits wavelengths of 940 and 660 nm; the device measures the amount of absorbed and reflected light for each wavelength and calculates the oxygen saturation level.

In normal subjects breathing atmospheric air the oxygen satuturation reference values are 95-97%. In subjects breathing oxygen the normal values are 99-100%; values below 90% are indicative of severe hypoxaemia.

Pulse oximeter limitations:

It is not very reliable when used on poorly-perfused tissues and in cases of vasoconstriction, anaemia, hypovolemia, low cardiac output or hypothermia.
In the presence of methemoglobin or carboxyhemoglobin the values are distorted.
Darkly pigmented skin hinders the reading of the probe.
At saturation values below 70% the measurement is unreliable.
Movements alter the reading of the probe.

The best site on which to apply the pulse oximeter clip is the tongue; it can also be placed on any body part hosting an arterial vessel: the pinna, limb extremities, the tail, as long as the pigmentation is not excessive. Hair-covered areas are to be previously clipped. To reduce motion artifacts the probe should preferably be applied to the areas least subject to possible movement during surgery.

ASSISTED VENTILATION (IPPV)

In case of inadequate or absent spontaneous ventilation in a rabbit under anaesthesia, assisted ventilation is necessary. Assisted ventilation is used when ETCO2 (end-tidal CO2) exceeds the value of 45 mmHg, or when Sp02 (blood oxygen saturation level) drops below 95%. In order to perform assisted ventilation the animal must clearly be either intubated or have a supraglottic airway device inserted.

ETCO2 should be maintained within the normal range of 35-45 mm Hg.
Monitoring of chest expansion is recommended: it must be perceptible but not excessive.
During IPPV the heart and the major thoracic vessels are compressed during the inspiratory phase, contrary to what happens during spontaneous breathing, as a positive pressure is generated. This impairs cardiac function in view of the loss of the thoracic pump and may result in a pressure drop. To overcome this problem the inspiratory phase must be as short as possible, but not so much as to cause an excessive pressure increase in the airways; the ratio between inspiration time and expiration time should therefore be maintained at 1:3 or 1:4.
Maximum inspiratory pressure must be of 15 cm H2O.
In healthy patients, at the end of the intervention the return to normal breathing typically occurs smoothly once manual ventilation is interrupted. A few moments are often necessary prior to recovery as a rise of blood CO2 is first necessary, with consequent stimulation of the respiratory centre.

Assisted ventilation may be manual or mechanical.

Manual ventilation

Manual ventilation requires a dedicated person for the entire duration of the procedure. Respiratory gases are delivered into the breathing circuit by compression of the breathing bag by the operator, after closing of the pop-off valve. The efficacy and safety of the technique are dependent on the operator’s experience, whom, based on practice, direct patient observation and the resistance encountered while compressing the bag, must calculate how much bag compression is necessary (the tidal volume), at which speed and at which rate. If SpO2 falls to values close to 90%, ventilation is insufficient.

Oxygen volume on the anaesthesia unit and the opening of the pop-off valve must be set in such a way that the bag fills regularly without having to continuously adjust the opening of the valve.

Mechanical ventilation

Mechanical ventilation requires the use of a ventilator, which delivers fresh gasses into the patient’s respiratory tract. Two modalities of mechanical ventilation are possible: pressure-controlled and volume-controlled.

Values in the rabbit

Tidal volume: 7-10 ml/kg
Respiratory rate: 25-50 breaths/min

The baseline parameters (respiratory rate, inspiratory time, inspiratory-to-expiratory time ratio and tidal volume) are the physiological ones for the species.

The respiratory minute volume is given by the respiratory rate per minute x the tidal volume.

The gas flow volume is set at 3 times the minute volume.

Possible complications of assisted ventilation in the case of excessive insufflation pressures or tidal volumes are lung barotrauma (pressure injury) or volutrauma (volume injury). When using an automatic ventilator, the unit must be controlled and pre-set before induction of the anaesthesia.

LOCAL ANAESTHESIA

Also in the rabbit local anaesthesia is useful in order to reduce the dose of general anaesthetic used and to contrast postoperative pain. Lidocaine and bupivacaine are used for the purpose; lidocaine is of rapid onset of action but of short duration of action, while bupivacaine has a slower onset of action but a longer duration of action. The two agents may thus be used in combination, exploiting the advantages of both drugs in order to have a rapid and persistent action.

The addition of buprenorphine to the mixture, at a dose of 0.003 mg/kg, extends the duration of the anaesthetic effect by several hours.

The use of local anaesthesia is contraindicated in infected and inflamed tissues, as the action of anaesthetic agents is inhibited by acidic environments.

Table 10. Drug dose for local blocks

Drug	Dose*	Onset of action	Duration of action
Lidocaine 2% (20 mg/ml)	2-4 mg/kg SC	5-10 minutes	0,5-1 hour
Bupivacaine 0,5% (5 mg/ml)	1-2 mg/kg SC	5-30 minutes	4-8 hours

* If used in combination, the dose of each drug must be reduced by 1 mg/kg

To avoid even severe adverse events (seizures, death), intramuscular and intravascular injections must be strictly avoided - as they may result in an excessively rapid systemic absorption - and the dose must be accurately calculated. If necessary, local anaesthetics may be diluted with sterile water.

Lidocaine-bicarbonate mixture

To reduce theburning sensation caused by the inoculation of local anaesthetics and the consequent reaction of the animal, sodium bicarbonate may be added.

A1:10 bicarbonate-lidocainesolution is used:

- 0.9ml oflidocaine2%

- 0.1mlof sodiumbicarbonate8.4%

Fora higher dose precision, the mixture can be diluted with sterile water for injection.

The mixturemust be prepared just before use. A higher amount of bicarbonate cannot be used as the solution would precipitate (Fig. 9). Again to prevent precipitation phenomena, lidocaine must be aspirated into to syringe first, and then bicarbonate. If a combination of lidocaine and bupivacaine is to be used in a conscious patient, lidocaine with the addition of bicarbonate is injected first, and then, after a few minutes, bupivacaine.

Fig. 9. Above: with an excessive amount of bicarbonate the solution precipitates. Below: the correct lidocaine-bicarbonate mixture remains clear.

Local anaesthesia along the incision lineThis procedure is helpful in controlling intraoperative and postoperative pain. A mixture of lidocaine and bupivacaine is used, eventually with the addition of buprenorphine. The maximum injectable dose must be calculated with great careand is then loaded into a syringe with a 25 G needle, or thinner; if necessary, the mixture can be diluted with sterile water for injection in order to obtain a greater volume of fluid.

After clipping of the injection site,the needle is inserted full length into the subcutaneous space along the expected incision line. Aspiration prior to injection is recommended to check for blood return; the anaesthetic is injected while retracting the needle until its final extraction. If the incision line is longer than the needle, the needle is reinserted and the operation is repeated. Once completed this operation the surgical site is then disinfected, giving time for the anaesthetic to take action before starting with the surgical incision.

The dripping of local anaesthetic directly into the skin incision is less effective.

The procedure can be used routinely in laparotomy interventions (e.g., ovariectomy and ovariohysterectomy) and inorchiectomy.

Local anaesthesia for venipuncture and catheterization

To facilitate venipuncture and catheterization the skin may be anaesthetized at the entry point of the needle or catheter. The skin is moved to the side and a small amount of lidocaine - with the addition of sodium bicarbonate - is inoculated (Fig. 10); a few minutes are then necessary before it takes effect.

Fig. 10. Local anaesthesia for venipuncture: the skin is moved to the side before inoculating the local anaesthetic

AWAKENING AND POSTOPERATIVE PERIOD

The postoperative period is particularly critical as most of the anaesthesia-related deaths take place during such time. Careful monitoring, especially during the three hours following the end of the procedure, may help to reduce risks and decrease mortality. The main complications of the postoperative period are circulatory shock, pulmonary oedema, hypoglycemia, hyperthermia, oliguria and seizures.

Various measures may be taken in order to optimize the recovery phase, decrease risks and increase patient comfort:

Upon completion of the intervention the rabbit, if in supine position, must beplaced in lateral or sternal recumbency, in order to facilitate breathing.
Monitoring must be continued until awakening.
Following removal of theendotracheal tube or of the v-gel device, oxygen administration must be continued with the mask.
Awakening must take place in a comfortable, dimly lit, warm, clean and quiet environment; the animal can be wrapped in absorbent mats and fleece blankets, or kept it in a thermostat unit; the animal must be kept under observation, without bothering it excessively; the temperature must be kept under control: both hypothermia and hyperthermia must be avoided.
Once the rabbit has entirely recovered the upright position and coordination it shouldbe transferred to its cage with bedding, food, water and litter box.
Should surgical wounds be present, especially on the animal’s ventral areas, beddings consisting of particulate substrates - that may adhere to the wound - should be avoided.
Following awakening, the return to feeding must be encouraged as soon as possible; should the animal not resume eating within a few hours, the administration of prokinetics, assisted feeding and the continuation of parenteral rehydration are necessary.
Faecal production is another important parameter to be monitored; should the rabbit not defecate, prokinetics are to be administered.
The eventual surgical wound must be checked regularly; rabbits do not generally pay particular attention to the suture, unless pain or discomfort are present; measures to minimize the risk of suture material removal by the animal are delicate handling of tissues, infiltration of local anaesthetics along the incision line and the execution of an intradermal suture.
The use of patches to protect the surgical wound is contraindicated as it may result in excessive skin irritation as the skin of rabbits is particularly delicate.
Preferably, no substance should be applied on the rabbit’s wound, as it may promote licking, with consequent drug ingestion.
Rabbits do not generally tolerate Elizabethan collars well, as they can induce anorexia; they should be used only when strictly necessary and under continuous monitoring.
The administration of analgesics is of fundamental importance to avoid pain related complications; the treatment is typically continued for 2-3 days after surgery.

PAIN RECOGNITION AND TREATMENT

Signs indicative of pain must be promptly identified as pain induces significant physiological adverse events: tachycardia, hypertension, immunosuppression, bowel stasis, delayed healing. In order to recognize such signs one must be familiar with the normal and abnormal behaviour of rabbits, taking into account also the temperament of the individual subject. The rabbit may at times tend to mask signs of pain, while in other circumstances the presence of pain is evident.

Signs indicative of pain in the rabbit are:

Reluctance to move or immobility
Abnormal posture, sunken abdomen
Abnormal breathing
Bruxism
Dysorexia/anorexia
Increased thirst
Attempts to hide
Aggressiveness
Unkempt hair coat
Closed eyes
Indifference to the surrounding environment

Table 11. Analgesic drugs

Drug	Dose
Carprofen	1–2 mg/kg PO, SC q12–24h
Ketoprofen	1-3 mg/kg IM q12-24h
Meloxicam	1 mg/kg initial dose, then 0,5 mg/kg SC, IM, PO q12-24h
Buprenorphine	0,01–0,05 mg/kg SC, IM, IV, q6–12h
Butorphanol	0,02-0,05 mg/kg IM, IV, SC q12h
Tramadol	10 mg/kg PO q12-24h

The non-pharmacological treatment calls for the use of soft, clean and dry bedding, easy access to food and water, a quiet and peaceful setting, warmth and gentle handling.