For a detailed explanation see the ferret section. Additional suggestions for Doppler placement in the rodent include use of the tail artery. Blood pressure measurements may be limited due to the size of the tail and limited space for cuff placement. When masking a patient, complications may occur. Watch respirations and pulse oximetry closely to ensure adequate ventilation. If parameters are decreased, consider suctioning the oral cavity, and repositioning of the head and neck.
The recovery period is a very important time to continue monitoring. Assess vital signs during this time to ensure adequate recovery from anesthesia. See the ferret section for specifics on the recovery period because most objectives are similar for the rodent. It is important to monitor and evaluate for pain in the postoperative period. Indicators of pain include, but are not limited to, decreased appetite, self trauma, increased time to normal activity, and flinching, biting, or moving away from the gentle palpation of incision. To decrease the chances of a stress response, provide adequate analgesia. Because of pain this stress response leads to increased cardiac output, cardiac work, and oxygen consumption, which can be detrimental to the life of the patient. Using multimodal analgesia with an opioid and nonsteroidal antiinflammatory medication provides an overall better effect on relieving pain.
The rabbit’s anatomical structure is a round body made for agility and speed with powerful hindquarters, back legs, and large feet. They have a large abdominal cavity comprised mostly of stomach and cecum. Comparatively, the thoracic cavity is small with lungs and heart sizeproportional to the size of the body. Rabbits have a good sense of smell and are obligate nose breathers, using their diaphragm to ventilate. They have exceptionally long ears and good hearing for detecting predators. Being coprophagous, they get necessary vitamins, minerals, and proteins through their feces. Rabbits are also physiologically unable to vomit and possess sweat glands only on their lips. The water requirements of rabbits are much higher than that of other species, averaging 50–150 mL/kg/ day. Rabbits are territorial, nocturnal, and do not tend to hibernate. To sound alarm or show aggression rabbits sound a loud foot thump. Well-developed scent glands and urine marking are a primary means of communication among rabbits. Rabbits are exceptionally vulnerable to stress. They do not adapt well in stressful situations and thus can be very challenging patients in the hospital. The life span of the rabbit averages about 6–9 years (Quesenberry and Carpenter 1997).
Preparation for anesthesia
Every rabbit presenting for surgery and anesthesia should be given a thorough physical examination. Conduct a normal exam, starting at the nose and working backward. Pay special attention to the respiratory system, teeth, and hocks. Rabbits are prone to respiratory diseases and further workup or precautions are recommended in these patients. Weight loss or decreased appetite may be caused by malocclusion of the teeth. Improper housing may cause abscess or infection to form in the hock region (Thurmon et al. 1996). Assess overall health and condition of the rabbit to determine how critical its illness might be.
Rabbits have a high metabolic rate and inability to vomit; they normally require only 1 hour fasting time before anesthesia. This time allows stomach contents to empty somewhat and thus reduces the pressure placed on the diaphragm and lungs, facilitating better ventilation in the supine rabbit.
Proper restraint of the rabbit during physical examination or blood draw is imperative. The body structure and flight response combination makes rabbits subject to fractures of the back and legs when they attempt to flee their restrainer. It is important to limit stressful or harmful situations to decrease the chances of creating a dangerous situation for the rabbit. Extreme caution should be taken to handle rabbits appropriately. Do not use ears as a means to catch the rabbit. The scruff of the neck can be used if necessary; take care to support the hind end. For excitable rabbits or for IM or SC injections, snugly wrap the patient in a towel and support feet and spine. Ideally, these rabbits should be restrained on the floor to eliminate the possibility of a fall should the rabbit struggle during the injection. For SC injections, a preferred site is between the shoulder blades in the scruff of the neck. The lumbar muscle on either side of the spine just cranial to the pelvis is a preferred site for IM injections. When obtaining a blood sample for analysis, use the marginal ear, cephalic, or saphenous veins. Sedation may be required if there is a need to use the jugular vein.
Compile the information from the examination. Consider the ASA status of the patient presenting for anesthesia and surgery. Discuss with your attending veterinarian whether the patient needs any additional workup or stabilization before anesthesia and surgery. Additional diagnostic testing and stabilization may need to be performed before the patient undergoes anesthesia. Tests include, but are not limited to, radiographs, ultrasound, echocardiogram, and complete blood count and chemistry profile, and stabilization may include fluid and electrolyte therapy and institution of analgesic and antibiotic treatment.
Anesthesia induction, care, and support
Before premedication ensure a complete setup. See setup for ferrets for a detailed list. The following are necessary tools and equipment for rabbit anesthesia:
- Endotracheal tubes, usually uncuffed, size 2–4 mm with a Murphy eye
The anesthesia protocol needs to accomplish several goals: provide good sedation, decrease stress, allow facilitation of IV catheter placement, and provide analgesia and a smooth transition to anesthesia. Oxymorphone, an opioid, provides good sedation and analgesia. Ketamine, a dissociative anesthetic, provides chemical restraint and analgesia. The alpha-2 agonist, medetomidine may also provide useful sedation, chemical restraint and analgesia. Consider omitting ketamine and medetomidine from the premed for the critically ill rabbit. In sick rabbits, midazolam, a benzodiazepine tranquilizer, can replace the ketamine or medetomidine to provide good muscle relaxation. The drug combination, along with a quiet stress-free environment will provide facilitation of IV catheter placement. If the rabbit responds adversely to restraint or a needle stick, consider masking it with sevoflurane inhalant anesthetic in oxygen until the rabbit is more tolerant. This technique is best for reducing stress for the rabbit and anesthetist. The rabbit should be monitored during this procedure. Usually, a pulse oximeter will work on the ear. For induction to anesthesia, the use of ketamine and midazolam, as was detailed for ferret anesthesia, is preferred. If tiny volumes are difficult to titrate, the induction mixture can be diluted in saline to make titration easier. There are several techniques that can be used for intubation of the rabbit. The rabbit’s long narrow oral cavity, inability to open the mouth wide, large tongue, and S-shaped neck all combine to make intubation challenging without sufficient training. Intubation of smaller pet rabbits is considered an advanced technique. Complications of intubation include damage to tongue and larynx (the latter can be fatal), hypoventilation, and hypoxemia. Pulse oximetry should be monitored closely. Time is an important factor when attempting intubation. In our hospital, we limit the total intubation attempt time to 10 minutes or less, after which time we proceed by mask maintenance unless an oral procedure is planned. The author’s preferred method is to use an otoscope with a high-intensity light source to visualize the larynx. An expensive but worthy purchase would be a Storz® Otoscope, which is equipped with its own light source. Blind intubation, orotracheal or nasotracheal, can also be successful, but also requires training and experience. Before intubation preoxygenate for 5 minutes. It may take extra time for intubation, so draw up extra induction agent, roughly twice the amount calculated. The following discussion describes intubation of the rabbit with a Storz Otoscope. The intravenous induction agent is given to effect until the rabbit does not swallow or respond to stimulation, such as a toe pinch, or attempts to open its mouth. Continue to have someone provide oxygenation by mask over the nose. Scoop the tongue out of the mouth with a cotton swab, and hold it gently to the side of the lower jaw. Place the rabbit in sternal recumbency with head and nose in a straight line pointed toward the ceiling. Place the otoscope in the oral cavity ventrally and dorsally and advance until the larynx is visualized.
To see the larynx, elevate the soft palate with the endotracheal tube or stylet. The larynx will be extremely ventral. Use 0.1–0.2 mL of 2% lidocaine on the larynx to prevent laryngospasm. Intubation is not possible if the rabbit is at a light plane of anesthesia. If at any time the rabbit swallows, gags, or moves, give more induction agent to effect. Place the endotracheal tube alongside the otoscope and advance until its tip is visualized.
Watch the tube move through the arytenoids into the trachea and remove the otoscope. The rabbit may cough or move at this time because intubation is very stimulating. Difficulty may arise when the proper angle or placement cannot be achieved. Manipulation of the head and neck to a different position may facilitate a better angle. Use the stylet through the endotracheal tube or otoscope. Watch as the stylet is passed through the arytenoids, and then pass the endotracheal tube over the stylet and advance into the trachea. Visualization must be maintained at all times to avoid esophageal intubation. Secure the endotracheal tube with Hy-Tape® or other available means. Place the endotracheal tube in the middle of the strip, crisscross over the tube and around the patient’s head, sticking it to itself in a modified figure eight. The tape is extremely sticky and works well to prevent the tube from sliding out of place. Use good judgment to decide what is best for the patient and realize that intubation may not be necessary or advisable without a properly trained anesthetist. Mask placement and delivery of inhalant anesthetic is an acceptable technique. During masking, ensure that the head and neck remain extended for the best possible ventilation. Suctioning of the oral cavity may be necessary due to mucus accumulation.
Connect to the maintenance inhalant anesthesia either isoflurane or sevoflurane via a precision vaporizer in 100% oxygen. Administer the inhalant through a non-rebreathing delivery system, such as a Bain modified Mapleson D that is connected and pressure-checked before anesthesia begins. The ratio of abdominal size to thoracic size in the rabbit predisposes to hypoventilation under anesthesia; thus, ventilation may be necessary to ensure oxygenation of tissues and vital organs. Maintain atraumatic technique when ventilating a small patient through a non-rebreathing system or mechanical ventilator.
There are many variables that affect the fluid therapy plan. The fluid requirements are higher than most small-animal exotic patients. Rabbits under anesthesia require fluid rates averaging 15 mL/kg/hr. See the ferret section for more information regarding fluids because products and administration are similar.