Surgical considerations (Proceedings)
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General Considerations and Pre-operative PreparationSince rabbits are a highly sensitive prey species, a very conscientious effort needs to be made to reduce stress, pain and any other factors that could be a detriment to their health in the peri-operative period. The surgical principles are the same for rabbits as in other species, but there are some unique anatomical, physiological, postural, and behavioral aspects that need to be considered when dealing with the rabbit as a surgical patient.
One of the most common responses of a rabbit body to stress is ileus. Because rabbits have a very tight oesophageal sphincter, they cannot vomit. Therefore, they do not need to be fasted prior to an anaesthetic episode. In fact, fasting a rabbit can predispose them to ileus.
Rabbits have very thin skin and fine hairs. Great care needs to be taken not to nick the rabbit's skin as the surgery site is prepared. Setting aside one very sharp blade that is not to be used for shaving heavily soiled or rough coated animals and using it on rabbits only is useful. Shaving the hair off is best accomplished by gently stretching the skin over the area to be shaved, and clipping slowly.
For general peri-operative pain management, buprenorphine can be given 30 minutes prior to anaesthesia induction at a dose of 0.05-0.10mg/kg SC (Bradley, 2001). Good pre-anaesthetic sedation can be obtained with either ketamine at 10-15mg/kg IM or medetomidine at 100-250mcg/kg IM. Once the sedative has taken effect, anaesthesia can be induced via mask with isoflurane. Once anaesthesia is fully induced, tracheal intubation can be accomplished. If an endoscope is available, it can be used to assist in tracheal intubation. If not, the following technique can be used: The rabbit is positioned on a table between the restrainer and the intubator. Gauze strips or a mouth gag are used to hold the mouth open. The restrainer extends the head forward with the maxilla slightly extended more forward than the mandible. The tongue is gently pulled out by the intubator, and a Miller -0- laryngoscope blade is inserted into the mouth on it's side with the side that has the light bulb on it towards the roof of the mouth and the other side along the tongue. The blade is gently inserted to the back of the mouth then pushed down on the back of the tongue. With the neck extended forward the pharynx can be visualized. It is common to see the epiglottis above the slightly translucent soft palate. The soft palate can be gently pushed with the end of the endotracheal tube to flip the epiglottis down onto the back of the tongue. At this point, the glottis can be well visualized and 0.05mL of injectible lidocaine can be sprayed on it to prevent spasm when intubation is attempted. Most rabbits require a 2.5-3.0mm uncuffed tracheal tube. It is useful to use a stylet for the actual intubation process. Since the opening into the rabbit's airway is located very caudally and curves downward at an angle, it is useful to bend the distal 7-8mm of the tip of the stylet at an approximately 30 degree angle. Care needs to be taken to be sure the end of the stylet does not extend past the tip of the tracheal tube. Once intubated, the tube is secured with gauze strips and fastened behind the neck. Since the tracheal tubes are so small, the gauze should be secured around the adaptor rather than the tube itself. The intubated rabbit can usually be maintained at 2-2.5% isoflurane with an oxygen flow rate of 2 litres/min. If tracheal intubation is unsuccessful, the rabbit can be maintained on a mask and usually requires a rate of 3-3.5% isoflurane with an oxygen flow rate of 2 litres/min. Rabbits can also be maintained with a tube that is positioned in the nasal passageway. To use this method, a topical anaesthetic is applied to the nasal cavity and a 1.0-1.5mm tube is passed medially and ventrally. This technique is useful for very small rabbits. They can be maintained with the tube in the nasal passageways or the tube can be pushed in further to enter the trachea through the nasal passages (Harcourt-Brown, 2002).
Due to its small size, a rabbit has a relatively high surface area to body mass ratio. Several methods can be used to supply supplemental heat to keep the patient warm during the procedure. These include the convective heaters (Orcutt, 2000), a circulating warm water pad, or 1 litre bags filled with either warm water or sand.
Ovariohysterectomy is indicated in the pet rabbit to prevent unwanted pregnancies and pseudopregnancies, to modify territorial behaviour, and to prevent or remove neoplasias of the reproductive tract. An incidence of uterine neoplasias as high as 80% has been reported in rabbits greater than 4 years of age (Paul-Murphy, 1997). The rabbit has a bicornate uterus, two cervices and a very large mesometrium that is used to store large amounts of fat. Upon opening the abdomen of a mature female rabbit, the uterus can be found located caudally and ventrally. In an immature rabbit, the uterus is found in a more dorsal location. It is not uncommon to find a small to moderate amount of free fluid in the abdomen. The suspensory ligaments are fairly lax compared to other species, and the ovaries are relatively easy to exteriorise. The fallopian tubes often are very long and extend quite far cranially. Care needs to be taken to exteriorise them and remove them in their entirety. They can be quite friable. The ovarian arteries are ligated in a routine manner with an absorbable suture material. The lateral uterine ligaments (round ligaments) are then broken down to allow the entire uterus to be exteriorised. The vaginal body is relatively large and flaccid. It is possible for urine to reflux up into the vaginal vestibule. If this happens, the urine can be "milked" down and a routine cerclage ligature can be placed approximately 1-2cm distal to the cervices. The body wall is then closed in a routine manner. Even in over weight rabbits, there is very little subcutaneous tissue. A separate subcutaneous closing layer is not routinely necessary. A subcuticular skin closure holds more than adequately and is well tolerate by most rabbits.
Rabbits have a high tendency for forming adhesions. They are so good at it that they are used as models for adhesion formation and prevention in human medicine. A protocol of 200mcg of verapamil SC q8h for a total of nine doses was shown to reduce adhesion formation in rabbits which had one uterine horn traumatized by ligation followed by burns induced with thermocautery. The suggested mechanisms of action by which calcium channel blockade mediates this process is that the drug (verapamil) may be 1- intervening at sequential loci of the adhesion formation cascade, 2-reducing vasoactive/inflammatory mediator production during the acute inflammatory response, 3-inhibit irreversible platelet aggregation, 4-protect against acute granulocyte-mediated tissue injury, 5-reduce microvascular permeability which results in decreased exudation of fibrin-rich plasma as substrate for clot formation and 6-inhibit fibroblast penetration into clot matrices (Steinleitner, 1990).
It is not uncommon for female rabbits to become inappetent after being spayed. This can result in a dangerous situation as the inappetance can lead to ileus and hepatic lipidosis. Administration of cyproheptadine at a dose of 1 mg PO BID is a useful appetite stimulant in rabbits. A post-operative dose of metoclopromide at 0.5mg/kg is helpful for encouraging intestinal motility post surgically. Another dose of buprenorphine can be given 6-8 hours post operatively as needed for additional pain management. Some individuals seem to be more sensitive to the procedure than others. It is often beneficial to send the owner home with 1-2 doses of meloxicam (0.2mg/kg) to be given orally q24h for the first 24 to 48 hours post op.
Indications for castration are population control as well as behaviour modification. Intact male rabbits exhibit distinct urine marking behaviour. Although neoplasia of the testicles does occur, it is relatively uncommon and tends to manifest only in older rabbits (>8 years). Rabbits have relatively large inguinal rings that remain patent throughout their lives. Many practitioners castrate rabbits via a scrotal approach in a manner similar to feline castration and report rarely having problems with abdominal organs herniating through the canals. Since closing them is not difficult or time consuming it is best to take the precaution of closing the inguinal canals at the time of surgery. One method of castration is as follows: a 2cm pre-scrotal incision is made through the skin. The testicles easily slip back up into the abdominal cavity when pressure is applied to the scrotum in order to push them up to the incision. It is helpful to occlude the inguinal canal by applying pressure to the pubic bone with an index finger while pushing the testicle up to the incision with the thumb. Once exteriorised, the procedure can be completed with either an open or closed castration technique. The vaginal tunic covering the testicle is torn away from the scrotum. The spermatic cord is then be ligated in a routine manner with absorbable suture material. The inguinal rings can be somewhat vague; this is especially true in older or obese rabbits. They are visualized most easily by grasping the cranial ventral border of the ring with a thumb forceps and pulling that border up towards the surgeon (as the rabbit is positioned in dorsal recumbency). In a smaller rabbit (<2-2.5kg) it is possible to ligate the rings with one cruciate suture that encompasses the lateral aspect of the right ring, the medial aspect of both rings and the lateral aspect of the left ring. In a larger rabbit, it is often necessary to close each ring separately.
Rabbits that have become obese commonly develop a large flap of skin that folds over the genital area. This flap can be so extensive that large amounts of faeces and debris can get caught up in there and cause severe infections. Even after the diet is corrected and the rabbit loses weight, the flap often remains and needs to be addressed surgically. The area is prepared surgically as well as possible. The skin of these flaps is often very irritated from the alkaline urine and is very friable. A semicircular incision is made around both sides of the flap and a half moon shaped piece of skin is removed. The remaining wound edges are sutured together with an absorbable 4-0-suture material.