Rabbits are a very common household pet in Europe as well as in the United States. With their increasing popularity, the complexity of diagnostic and surgical procedures performed in rabbits is growing. Nowadays, we have many types of anaesthetic regimes including the use of drugs that are injected intravenously or intramuscularly and inhalational anaesthetics. Patients are monitored throughout the surgical procedure with all necessary perioperative care. However, practitioner and nurse should be familiar with this species and its anatomy, physiology and other specific needs. Based on author experience, rabbits are at the same risk during anaesthesia as dogs and cats; the anaesthetic regime must be based on the health of a rabbit and the type of surgical procedure to be performed.

A thorough history of current and past medical problems can provide valuable information about rabbit health status. Physical examination may reveal pale mucosal surfaces, abnormalities of the heart or lungs, abdominal masses or other pathologies that may require further evaluation such as a haematological and plasma chemistry examination, urinalysis, radiography, electrocardiogram (ECG), echocardiography or any other diagnostic method prior to performing general anaesthesia. Thorough physical examination is therefore very important in establishing a further diagnostic and anaesthetic plan. Based on clinical examination, laboratory analyses and results of any imaging, the patient is placed in the appropriate American Society of Anesthesiologists (ASA) physical status scale (class I–V). Independent of the ASA group an additional examination should be performed and patient life functions must be stabilised before any anaesthesia or sedation. Even when a rabbit is placed in ASA I–II, the author recommends ensuring intravenous access.

Rabbits should be properly weighed and all drug dosages should be calculated carefully. When intramuscular injectable drugs are used with a potential pain effect or in larger volumes, the agent should be administered in more than one site to avoid any pain. Intraperitoneal anaesthetic administration is not recommended. The anaesthetist should monitor anaesthetic depth and all the vital signs to keep the rabbit safe and in an appropriate surgical plane. The objectives of surgical anaesthesia (stage 3, plane 2) are that the patient does not feel pain, is not moving, is not aware, has no memory of the procedure afterwards and all the vital signs are not dangerously depressed. Anaesthesia is achieved when there is a loss of sensation to either a part or all of the body. General anaesthesia also renders the patient unconscious. Sedation is similar to general anaesthesia but the patient remains semiconscious. Careful attention to intraoperative care is an important contributory factor to successful anaesthesia.

Respiration

Rabbits should be placed in a position which ensures the ability of the lungs to be fully expanded. Respiration is monitored by observing chest movements or by monitors detecting inspiration and expiration. A rise in respiratory rate during anaesthesia is mainly caused by a reduction of depth of anaesthesia. A fall in respiratory rate below 40% of the patient's normal rate (30–60 breaths / minute) indicates respiratory failure. If the animal is not intubated, it becomes necessary to intubate them if anaesthetic monitoring shows signs of cardiovascular or respiratory failure. The rabbit's airway should be inspected for obstruction, and oxygenation together with assisted ventilation should be started. Doxapram is also administered. The pressure of assisted ventilation should never be greater than 8 mmHg.

Capnography measures the amount of carbon dioxide in the air that is breathed in and out. The patients 'end-tidal' CO₂ refers to the amount of CO₂ measured at the end of expiration and it is used for the estimation of arterial carbon dioxide partial pressure. A sidestream capnograph is located between the breathing circuit and endotracheal tube to minimise the dead air space.

Supplemental oxygen, which maximises cardiac and pulmonary efficiency, should be provided in all cases of sedation or anaesthesia. In general a flow rate of 200 ml/kg/min will provide an inspired oxygen concentration of at least 40%. Doubling the flow rate could increase inspired oxygen concentration up to 80%. Humidifying and warming the oxygen is optimal.

Cardiovascular Function

Monitoring of cardiovascular function and circulation includes palpation of peripheral pulse to determine rate, rhythm and quality, and evaluation of mucous membrane colour and capillary refill time (CRT). Auscultation of heart beat, pulse oximetry and ECG are used to grossly assess peripheral perfusion and heart function. The indirect blood pressure measurement is most commonly used. The ultrasonic Doppler flow detector makes audible measurements of blood flow in an artery distal to the blood pressure cuff.

The mean arterial pressure (MAP) should be kept above 60 mmHg and systolic pressure above 90 mmHg to ensure adequate organ perfusion. A pneumatic cuff is usually placed on the shaved area above the elbow in a bed of ultrasonic gel and taped in place. Conjunctiva and gingiva should have a pink colour. CRT is best measured on the gingiva dorsal to the incisors and should be no more than 1–1.5 seconds.

Pulse oximetry is a non-invasive continuous measurement of oxygen saturation in tissues. A SpO₂ greater than 91% usually indicates adequate oxygenation; however, pulse oximetry may be inaccurate by as much as 5%, so the SpO₂ level should be kept above 95%. Pulse oximetry fails to detect hypoventilation, hyperventilation or other respiratory problems; capnography is necessary to detect these changes. Pigmented skin could make the reading variable. A good quality signal may be unobtainable in patients that are hypothermic and/or hypotensive.

Hypotension and Fluid Therapy

Inhalational agents are naturally hypotensive, and untoward effects are dose dependent. The use of inhalational agents as sole anaesthetics necessitates higher doses and increases the risk of potential adverse effects. Therefore, the use of analgesia and injectable premedication is recommended. It was reported that use of medetomidine often causes a profound negative impact on the cardiovascular and respiratory system and should be used with caution. If hypotension occurs during surgery, the inhalational anaesthesia is reduced first, while the continual rate infusion is increased.

An intravenous catheter is placed in a lateral ear vein, cephalic or saphenous vein. Before catheter placement a layer of EMLA is applied to provide local anaesthesia. Administering perioperative fluids subcutaneously or intraperitoneally is unreliable due to peripheral and visceral vasoconstriction. Intraosseous administration is an alternative to intravenous access.

Fluid therapy is an important component of haemodynamic stabilisation minimising drug-exacerbated hypo-tension and risks related to anaesthesia. Perioperative fluids are given via precise syringe pumps. A dosage rate for perioperative intravenous fluid therapy is 10–15 ml/kg/ hour. In cases of gastric dilatation, fluid administration into the saphenous vein should be avoided.

Recommended first-line therapy for hypotension consists of intravenous boluses of polyionic fluids of 5–15 ml/ kg. Overhydration should be avoided. If a rabbit is refractory to isotonic crystalloid therapy, the use of synthetic colloids should be considered. Hetastarch is administered at 5 ml/kg i.v. over 5–10 minutes. Another possibility is to use 7.5% hypertonic saline in 1–3 ml boluses over 10 minutes until normal heart rate and blood pressure are obtained. Dopamine or norepinephrine can be used to treat refractory hypotension. Checking glycaemia, packed cell volume (PCV), total protein and blood gas analysis intraoperatively is recommended.

Blood Loss

Blood losses comprising less than 10% of blood volume in a patient with normal PCV could be corrected with isotonic fluids at a rate of three times the estimated blood loss amount. In case of chronic (PCV < 20–25%) or acute blood loss, transfusion of whole blood is recommended. Synthetic colloids should also be added to the crystalloid therapy in rabbits with hypoproteinaemia. Rabbit clinical examination and laboratory analyses are important to determine whether an animal is suitable as a donor and to calculate the proper amount of blood needed for a recipient. Rabbits have four blood groups; however, no problems in the first transfusion were noted by the author. The major cross-match is optimal. The estimation of the amount of whole blood required by the recipient can be calculated as:

Whole blood is administered by syringe pump or in boluses into the intravenous catheter or intraosseously. The haematocrit and total plasma protein concentration should be evaluated 1, 2 and 24 hours post transfusion. Blood transfusions should be administered within 4–6 hours to prevent bacterial overgrowth.

Temperature

The most common method of monitoring temperature is with the use of a rectal thermometer. Heat loss occurs by convection, radiation, conduction and evaporation. Rabbits lose heat rapidly because of their high surface area relative to their bodyweight. Use of heating blankets/pads set at specific temperature (35–37°C) is strongly recommended. Also fluid and oxygen should be prewarmed. Using heating infusion devices is recommended.

References

1.  Lichtenberger M, Ko J. Critical care monitoring. Veterinary Clinics of North America: Exotic Animal Practice 2007;10:317–344.

2.  Meredith A, Flecknell P, eds. BSAVA Manual of Rabbit Medicine and Surgery. 2nd ed. Gloucester: British Small Animal Veterinary Association, 2006.

3.  Thomas JA, Lerche P, eds. Anesthesia and Analgesia for Veterinary Technicians. 4th ed. St. Louis: Mosby, 2011.