Pre-anesthetic preparations of the rabbit
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. Indeed, rabbits are pre animals, which are easily distressed in a new environment (new persons, new odors, new noises, restrain, etc.). They may attempt to escape during the examination when frightened, when restrained too strongly, or poorly manipulated. This will affect various hormonal pathways and physiological processes, notably the release of catecholamines in the blood and a slowdown of the digestive system activity. The rise of catecholamine can have dramatic effects on the cardiac and respiratory activities, which lead to secondary adverse complications such as altered response to the surgical anesthetic drugs. Pain also triggers hormonal and physiological responses, it is thus necessary to give analgesic medication, when potentially painful procedures need to be done (e.g. blood sampling).
Anesthesia in rabbits can be considered as a safe procedure, when:
• the biology, physiology and anatomy of the rabbit are known;
• a minimum of safety measures are taken, including a complete check-up of the rabbit;
• the correct anesthetics agents, and dosages are used;
• there is no malfunctioning equipment;
• respiratory and cardiac monitors have no upper-limits set lower than use commonly observed in rabbits and other small animals.
Since rabbits are unable to vomit, it is not mandatory to withhold the food and water before a planned surgery. In fact, rabbits, whose accessibility to food is removed over a longer period of time, show an increased tendency of becoming hypoglycemic during surgery or became 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 is furthermore slowed down in rabbits whose food was taken away hours before surgery. It is nevertheless advised to remove food up to an hour before anesthetic preparations are started. Indeed, some rabbits tend to accumulate food and water in the oral cavity and the oropharynx; withdrawing food one hour prior to surgery will assure that the oral cavity does not contain food rests and that the stomach is not overloaded. Food and water should be available immediately after the rabbit recovers from the anesthesia.
It is important to do a full clinical examination and verify the general condition of the rabbit in a calm atmosphere. A complete examination, including accurate weight, a CBC, a blood biochemistry panel, X-rays, ultrasound or MRI, urinalysis, will help determine the health condition of the rabbit. Particular attention should be paid to respiratory and digestive troubles. It is best to refrain from anesthesia and postpone surgery if a rabbit presents any of the aforementioned problems. If the rabbit is found healthy, no major problems should be encountered.
The state of hydration is another important point. Indeed, a frequently observed problem is dehydration prior to surgery, due to a decreased intake of food and water and a travel to the veterinary clinic. This is particularly the case when dental or gastro-intestinal problems are involved. In this case, the surgery must be postponed in order to allow rehydration of the rabbit by means of administration of fluids. Various ways can be used:
• IV (intravenous) administration, directly in the marginal ear vein, lateral saphenous vein, or the cephalic vein, after placement of a catheter. Long-term catheterization of the jugular vein is not recommended; in rare cases, swelling of the head has been reported.
• SC (subcutaneous) administration is slow and not very effective in case of severe dehydration.
• IP (intraperitoneal) administration is slow and not very effective in case of severe dehydration.
• Intraosseus administration is useful when a prolonged administration of fluids is foreseen.
A light sedative or pre-anesthetic agent can be given during the pre-anesthetic phase in order to decrease stress in the rabbit, prior to anesthesia. They are furthermore indicated when:
• The concentration of volatile anesthetic, used to maintain anesthesia, must remain low. Preemptive analgesia by means of opioid agents, 30 to 45 min prior to surgery is indicated for this purpose (e.g. fentanyl/fluanisone (EU), fentanyl/droperidol (USA), or more commonly acepromazine/butorphanol).
• A rabbit is suffering from upper or lower respiratory problems, or when the irritable isoflurane gas is used for surgical anesthesia. The pre-anesthetic drug will help reduce the production of saliva or bronchial secretions. The use of atropine is not indicated in rabbits. Indeed, around 30 to 50% of all rabbits possess an endogenous atropinase enzyme. Glycopyrolate is the anti-cholinergic drug of choice in rabbits.
• Immediate reduction of post-surgical pain is needed. Opioid analgesic drugs like butorphanol or buprenorphine can be given during the pre-anesthetic preparations.
• A rabbit is stressed and anxious. The use of benzodiazepines (e.g. diazepam) provides good tranquilization and muscle relaxation. The can be particularly helpful, if intubation of the rabbit is needed during the surgical anesthesia stage.
(The regular use the ketamine/xylazine mixture has been linked to increased rates of heart disease and an increased death rate in some rabbits.)
The various pre-anesthetic agents that can be used in rabbits are listed in a table.
After the complete examination of the rabbit, it should be possible to have a good idea of the health status of the rabbit, and to assign him in one of the various ASA (American Society of Anesthesiologists) categories:
Thanks are furthermore due to Akira Yamanouchi (Japan) for the permission to use the pictures from VEIN (Veterinary Exotic Information Network, http://vein.ne.jp/).
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