Exotic mammal anesthesia (Proceedings)

The biggest concern with all exotic animals under anesthesia is hypothermia. Other main concerns include airway access, intravenous access, pain management, and hemorrhage.

Their body mass to surface area ratio is so small that they become much colder than the average small animal patient. Warming devices include heating pads, warming disks (SnuggleSafes), warm bags of fluids, warm blankets, heating lamps, and forced air warmers (Bair Huggers).

Some exotic species are very difficult to intubate, and instead of spending time attempting to intubate, a mask is used to maintain inhalant anesthesia. The specific benefits of intubation are a reduction of waste gas exposure for employees, the ability to administer positive pressure ventilation, and less bulky equipment around the patient's face. Each patient should have the options considered and a decision made about attempting intubation or not.

Many exotic animals are so small that intravenous access is very difficult to accomplish and may not be possible in some cases. Depending on the procedure and the species, it may be beneficial or even necessary. IV access during a procedure is helpful for administration of fluids, antibiotics, analgesics, and CPR drugs. Keeping IV catheters in place as long as possible after a procedure is recommended for continued administration of fluids, or to reverse anesthetic drugs if necessary. If intravenous catheterization is not possible, intraosseous catheterization may be considered. Drugs and fluids can be administered intraosseously. Care should be given when administering large volumes through this route. Subcutaneous fluid administration is another option for animals in which we are unable to get an IV catheter.

Pain management is another consideration in these species. We are limited with how much knowledge we have of certain species, and sometimes it is very difficult to assess pain status because certain species are especially good at masking their pain. Consideration should be given to drugs that are able to be reversed.

Hemorrhage is much more significant with exotic animals due to their small blood volume. For more invasive procedures, blood volumes should be calculated for each patient. Blood loss should be replaced with crystalloids (lactated Ringer's solution) at approximately 2 – 3 times the volume lost or with colloids (hetastarch or Oxyglobin) one to one with the volume lost. In some cases the clinician may want to consider having another animal of the same species ready to donate blood if needed.

Sevoflurane is our inhalant anesthetic of choice for exotic animals because it allows for rapid changes in anesthetic depth. Isoflurane is also a safe choice for inhalant anesthetic in exotic animals and less expensive than sevoflurane. Use of sevoflurane is only possible with sevoflurane compatible vaporizers.

Monitoring of exotic animals can be more difficult than in most small animals. Due to their small size, many monitors are unable to give accurate measurements and readings. We generally try to monitor HR, RR, and temperature in all animals. Electrocardiogram (ECG) can be measured by attaching clips to the skin. However, some exotic animals have very friable skin so we use gauze to pad the ECG clips or even needles through the skin with ECG clips attached to the needles. Pulse oximeters are very helpful, however often they will not read, because the pressure of the probe compresses the blood flow through the tissue at that point. Small probes are available but can sometimes be hard to obtain. Blood pressure monitoring is definitely recommended when possible. Most exotic animals are too small to make this possible. Applying a Doppler to a foot or even attaching it near the patient's heart can serve as a good heart rate monitor. Temperature should be monitored even if just occasionally throughout a procedure to facilitate appropriate warming.

The recovery period for exotic animals is especially critical. Temperature, respiratory rate, and heart rate should be monitored throughout recovery until after extubation. If recovery is prolonged, a blood glucose level should be checked in order to supplement if necessary. All exotic animals should be offered food as soon as possible once awake (procedure allowing). Fluid administration may be required during this period. Reversal of certain anesthetic drugs may allow for a smoother and quicker recovery in some cases.

Rodents

The different species that we most commonly see are rats, mice, and guinea pigs. Common procedures performed on these species are ovariohysterectomies, mass removals, and cystotomies. Premedications can be given into the lumbar muscle of bigger rodents or the quadriceps muscle. Intravenous catheterization is possible in rodents via a cephalic, saphenous, or lateral tail vein in rats and mice. Intraperitoneal is another option for fluid administration when IV access is not possible.

Induction of anesthesia is possible with injection (IV or IM) or via mask using inhalant anesthesia. Intubation is not always possible in rodents. Some larger rodents can be intubated using a small IV catheter as the endotracheal tube. Often, inhalation anesthesia with a mask is the easiest method of maintaining general anesthesia. Monitoring of rodents under anesthesia is tricky due to the small size of the patients and should include HR, RR, temp. Two Dopplers are frequently applied to our smallest rodents since we typically lose one or two monitors while moving patients into the OR.

The recovery period is typically very brief in the healthy rodent. Ideally they are up and moving around and eating shortly after recovering. Re-warming is usually necessary during recovery due to their small size. Sometimes during the recovery period fluid administration is required, in addition to analgesics, or glucose supplementation.

Ferrets

Ferrets are often treated similarly to small cats. Typical procedures for ferrets include foreign body removals, mass removals, dentals, and adrenalectomies. Intramuscular premedications can be given into the lumbar muscle, or into the quadriceps muscle. Intravenous catheter placement can be achieved via a cephalic vein, saphenous vein, femoral vein, or even jugular vein. Ferret arterial catheters are difficult, but possible in larger ferrets in the dorsal pedal artery.

They can (and should) be intubated with a 3 – 3.5 mm cuffed endotracheal tube. Similar to cats, it helps if lidocaine is applied to the arytenoids before attempting intubation. Monitoring should include HR, RR, temp, and BP. ECG, and pulse ox should work well and be used in addition to a Doppler for BP. The tail can be used for blood pressure if there is not an available limb. Temperature should be monitored when possible as the patient may need use of many different warming devices.

During the recovery period ferrets may need fluids, analgesics, glucose supplementation, or even reversal of some of the anesthetic drugs. Heat should be provided until the ferret is moving around and able to maintain its own body temperature. Ideally the ferret would be offered food straight away.

Rabbits

Rabbits are an easily stressed species and are at an increased risk of anesthetic-related death, and therefore handling should be done very carefully. Rabbit restraint should not be taken lightly as rabbits can kick out and fracture their own backs. Depending upon the temperament of the rabbit, they should be cradled in order to carry them, preferably covering their eyes so that they are less likely to jump. We give IM injections into the back muscles, and prefer to restrain rabbits on the floor so that they can't jump off a table. Most commonly, rabbits present for an ovariohysterectomy, or a dental procedure. Occasionally, we see rabbits for other more invasive, less common procedures such as liver biopsies or enucleations.

Intravenous access is desirable when possible. The cephalic, saphenous, or ear veins can be used to place a catheter depending upon the size of the rabbit. Ear catheters should be wrapped well to help keep them in place after the surgery. Arterial catheterization is possible through a relatively large artery running along the middle of the ear. Depending on the procedure, this may be helpful as additional monitoring.

Intubation is possible, and the ease of intubation depends largely upon the size of the rabbit. While maintaining inhalation anesthesia with a mask is one method of maintaining general anesthesia in rabbits, it is not the safest way. To minimize risk of hospital personnel exposure to waste gas, allow for positive pressure ventilation (when patient light or during CPR), and to perform procedures in the mouth or around the face, a sealed airway is necessary. Esophageal intubation and laryngeal damage (possibly leading to tracheal obstruction) are potential risks of intubation in rabbits. The difficulty with rabbit intubation is that the arytenoids are at the back of the mouth and rabbit jaws do not open very wide. This makes visualizing the larynx very difficult. Some people will therefore intubate blindly. This is a risky procedure because it can often be very time consuming, and there is an increased chance of intubating the esophagus. Cyanosis occurs almost instantly after the esophagus has been intubated in rabbits. Our preferred method for intubating rabbits is using a rigid scope with a camera attached, which then projects to a screen. Images from the scope in the mouth are displayed on the screen and a stylet is advanced between the arytenoids when visualized (just like a video game). Once the stylet is between the arytenoids, the scoping instrument is removed from the rabbit's mouth leaving the stylet in place. Then the endotracheal tube is passed over the stylet and between the arytenoids. The stylet is removed, and the endotracheal tube is secured in place. We use tape rather than a tube tie because we find the tube ties slip which could cause extubation of the rabbit. A size 3 – 3.5 mm uncuffed endotracheal tube should be used. There are several methods to double check proper intubation. One is visualizing the arytenoids around the endotracheal tube; this is difficult due to the lack of space in the rabbit's mouth. Watching for condensation (in the endotracheal tube, on the end of the laryngoscope, or on a glass slide) is a second method. Other methods are palpating the neck area to feel the trachea alone or a second tube next to the trachea (esophageal intubation), and watching for evidence of a breath through the end of the tube by holding a small amount of the rabbit's fur at the end of the tube. Possibly the easiest method (if equipment is available) is attaching a capnograph to the end of the endotracheal tube. Values of <5 mmHg could suggest esophageal intubation (if patient ventilating well), values of >30 mmHg would confirm proper intubation.

Monitoring under anesthesia should include HR and RR. A pulse oximeter is an excellent monitor for these patients. ECG is also recommended if available. Blood pressure can be monitored on bigger rabbits using a Doppler or certain oscillometric machines. Also, direct measurement can be performed using an arterial catheter. Temperature should be monitored when possible as the patient may need many different warming devices.

The recovery period is just as critical as the anesthetic period. During this time, the temperature should be regulated gradually. A quiet, dimly lit room is ideal for rabbits to reduce stress while they are still recovering from anesthesia. Blood glucose levels should be checked in rabbits that appear to have a delayed recovery in case supplementation is necessary. Fluid administration may be required during this period. Sometimes, with slower recoveries, reversal of certain anesthetic drugs is helpful.