Anesthesia and its monitoring in rabbits
Esther van Praag, Ph.D.
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Rabbits are often considered as difficult animals in relation to general anesthesia. This may relate to the fact that the doses needed to induce and maintain anesthesia and those producing toxic effects are close to each other, and to the variety of observed secondary effects related to stress and cardiac or respiratory reactions. Anesthesia in rabbits is, nowadays, considered as a safe procedure, when a minimum of safety measures is taken. This includes a complete check-up of the rabbit, the correct anesthetics agents, no malfunctioning equipment and doing the different steps on a calm rabbit.
Since rabbits are unable to vomit, there is no need to withhold the food and water longer than 2 to 4 hours before the planned surgical procedure. In fact, rabbits that do not eat over a longer period of time before the surgery show an increased tendency of becoming hypoglycemic during surgery or get post-surgical disturbances of the gastro-intestinal tract due to dysbiosis. Growth of pathogenic bacteria leads to the development of enterotoxaemia. The rate of recovery appears, furthermore, slowed down in rabbits whose food was taken away hours prior to surgery. It is thus advised to keep food available up to 2 to 4 hours (depending on the surgical procedure) before anesthetic preparations are started. This way the oral cavity does not contain food rests and that the stomach is not overloaded and distended. Food and water should be available immediately after the rabbit recovers from the anesthesia.
Pre-anesthetic steps are often useful in the preparation of surgical anesthesia. Click here for details.
Induction of general anesthesia
The choice of the anesthetic drug and its way of administration depend on the health condition of the rabbit, and the length of the surgery. Intravenous administration of anesthetic drugs is the least recommended way since the toxic doses of some anesthetic agents is close to that needed for surgical anesthesia.
When the injected subcutaneous or intramuscular way is chosen, higher doses of anesthetics must be used. As a consequence, waking-up time is slower and the risk of hypothermia is increased if no heating pad is used, or fluids are not administrated at body temperature. Some compounds also have known side effects or adverse effects, e.g. rabbits regularly anesthetized with the ketamine/xylazine mixture showed increased rates of heart disease and an increased death rate. It is necessary to take those adverse or side effects into consideration on an individual basis, according to the health state of the rabbit. Some rabbits also appeared to wake up more slowly and with more difficulty from gas anesthesia after need of regular teeth trimming.
A list of anesthetic drugs safe for use in rabbits, including their side effects, is available. Click here.
Gas anesthesia is commonly used in rabbits too. If this method is chosen, pre-anesthesia is necessary. Indeed, rabbits will often start to struggle and attempt to escape when they smell the agent, and may get hurt (e.g. spinal cord injury). Commonly used inhalant gas agents are isoflurane and sevoflurane.
Gas anesthesia can be induced by placing the rabbit in an induction chamber or by placing a mask tightly over its face and holding the rabbit firmly.
Rabbits often try to retain their breath independently of the method and the gas agent used. Rarely, bradycardia (unusually slow heartbeat) or respiratory distress is observed.
Preparing the rabbit for surgery and monitoring vital parameters during induction of anesthesia.
Video taken by Debbie Hanson.
When an induction chamber is used, it is advisable to remove the rabbit once it shown signs of relaxation and place him on a mask. If a nasal or face mask is used, it should be placed deep over its face in order to minimize dead-space.
Induction of anesthesia in a rabbit placed in an induction box.
Video taken by Elena Grisafi Favre.
The mask presents the advantage that it can quickly be removed, and be replaced as soon as the animal starts breathing normally again. In rare cases, when the rabbit refuses the breath the anesthetic gas, the induction can be started with propofol or with etomidate, before placing the mask, and continue the induction with the chosen inhalant anesthetic.
When the induction phase is finished, preparations for surgery can be started. This includes shaving, and disinfecting the skin. The depth of anesthesia must be verified before placing a surgical drape over the animal. Reliable methods in rabbits include:
• Pedal reflex, breath holding or hypoxia, and screaming: light depth;
• Palpebral (blink) reflex, ear pinch or left withdrawal reaction: medium depth;
• Corneal reflex: dangerously deep. When such depth is reached, cardiac arrest may occur. Emergency measures should immediately be implemented, by reducing or halting the administration of anesthetics. Exception: anesthesia with medetomidine.
The level of surgical anesthesia is reached when reflexes to ear pinch and jaw tone are lost.
During the anesthesia, the color of the mucous membranes (eye, lips tongue), the respiratory rate, the heart beat and the rectal temperature should be monitored.
Monitoring vital parameters
Monitoring of the respiratory rate, depth and rhythm
The rate of respiration depends on the used anesthetic drug. The general tendency is a decrease of the number of breaths per minute, to about 30 to 60. When the rate is lower than 30 breaths per minute, or less that 50% of the normal rate, there should be concern. Once the anesthesia and vital parameters are stable, the respiration rate should remain regular and slow, though sudden changes are not uncommon in rabbits
Respiratory rate can be monitored by observing the movement of the chest, when no surgical drape is used. If this is not possible, respiration should be monitored. It should be mentioned that some electric monitors for respiration are unable to measure the rapid respiratory rate of rabbits and other small animals, and thus become inefficient.
The physiological status of the rabbits can also be followed by monitoring the exhaled CO2 (capnography). In this case, it is important to note the general trend, and not individual numbers. If capnography is used, it is important to take into account the dead-space of the equipment and mask into the anesthetic equipment. Mainstream capnography is thus not advisable. Side-stream capnography can be used, though the volume of the sampled gas may be very large in comparison to the tidal volume.
Monitoring of the heart and heart rhythm
It is important to monitor the cardiac activity in anesthetized rabbits. Indeed, pre-anesthetic examination and handling can cause a dangerous raise the level of catecholamine. This can have disastrous consequences such as a lack of correct oxygenation of the blood (hypoxemia) or an excess of CO2 in the blood (hypercapnia).
Further problems seen in rabbits include hypotension or an unusually slow heart rhythm (bradycardia).
The heart beat rate of rabbits is rapid and may exceed 250 beats per minute. ECG may thus be difficult, since the upper limits of the monitoring device are reached (usually set at 250 or 300 beats per minute), except when medetomidine or ketamine/medetomidine is used (decrease to 120-160 beats/min).
Pulse oximeters can be used to follow the heart rate and the level of oxygenation. The values typically should remain higher than 90%; lower values typically indicate that the level of oxygen in the blood is insufficient (hypoxemia).
Reliable signals are obtained when the probe is fixed to the tail, the ear, the tongue, the hand or a digit of a limb in larger rabbits.
Cardiac arrest rarely occurs during anesthesia. This is an emergency situation that leaves little time for intervention; in rabbits cardiac arrest is quickly followed by respiratory arrest and death.
Monitoring of the body temperature
Control of body temperature of the rabbit during anesthesia is essential due to:
• The large surface area to body mass ratio; leading to rapid loss of temperature;
• Convective and radiant heat loss;
• The effect of anesthetic agents on the body temperature mechanisms;
• Pre-surgical and surgery itself. A good balance should be found between the minimum area to shave at the surgery site, the minimum shaved area ensuring asepsis, and the use of minimal quantities of disinfecting solutions.
Hypothermia tends to increase dangerously the anesthetic depth during the surgical procedure. This may be accompanied by hypoxia, acidosis, cardiac arrhythmia and a disturbance of the metabolism of blood platelets.
The body temperature of an anesthetized rabbit can be monitored with an electronic thermometer, or a thermo-sensor inserted deep in the rectum.
The use of heating pads, hot water pads or bottle, or convective heat sources help maintain body temperature during the surgical procedure, during the waking-up and recovery phases. When fluid therapy is necessary, the fluids must be heated at body temperature by warming them in a microwave or passing the tube through a bowl of lukewarm water.
Many thanks to Amir Maurer, DVM (Companion and Exotic Animal Veterinary Center, Holon, Israel), Elena Grisafi Favre (la Colline aux Lapins, Switzerland), and to Akira Yamanouchi (Veterinary Exotic Information Network, Japan) for the permission to use their pictures to illustrate this page. Thank you also to the veterinarians Thomas et Caroline Pilloud (Cabinet vétérinaire du Brevil, Boudevilliers, Suisse) for their cooperation.
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