Anesthesia for Exotic Species

Lyon Lee DVM PhD DACVA

Introduction

Although there are wide variety of exotic species this lecture will primarily cover aspects related to anesthetizing birds, small mammals, reptiles and fish
In a survey carried out by the Association of Veterinary Anaesthetists (AVA), rodents and other small mammals had much higher clinical mortality associated with anesthesia (1 in 32) than in other domestic species (e.g. 1 in 1,000 for dogs). The cause of such high mortality probably results from unfamiliarity with the species and the generally less healthy state of the animals
There has been a growing popularity of exotic pet ownership in recent years, and as a result, more practitioners are faced with dealing with exotic species
Many exotic species become easily distressed to handling, increasing the risk of physical damage and of epinephrine release leading to development of complications under subsequent anesthesia
Proper preanesthetic examination of exotic species is often difficult and exact body weight and health status are difficult to pre-determine.
Previous status of nutrition, parasite load, pregnancy and other underlying disease are major anesthetic consideration but are also difficult to pre-determine
Many animals presented to the practitioners have respiratory disease so oxygen availability is important even if only injectable anesthetics are to be used
As a general rule, the principles of anesthesia as applied to domesticated animals should apply equally well in all these animals and the main differences arise from the need to protect the anesthetist and any assistants from injury by unanesthetized subjects.

Avian anesthesia

Physioanatomic peculiarities

Birds have air sacs (up to nine) acting as bellows to the lungs but these are not involved in gas exchange
In a tracheal obstruction, cannulation of caudal thoracic air sacs can still provide ventilation
Larger anatomic dead space for ventilation, but much higher surface to volume ratio along with thinner blood-air barrier, making the gas exchanges much more efficient
A greater percentage of oxygen is extracted from inspired air than mammals because of highly efficient crosscurrent flow of air and blood, and continuous gas exchange along the parabronchi
The trachea is composed of complete cartilaginous rings
Respiratory muscles are used both for inspiration and expiration
Vocal folds, arytenoids, thyroid cartilages, and epiglottis are absent
There is no diaphragm separating thoracic and abdominal cavities, and the thoracic space is at atmospheric pressure
Syrinx is found near the bifurcation of the trachea
Lungs are less elastic, and have smaller functional residual capacity
High basal metabolic rate results in rapid utilization of any tissue oxygen reserve
Renal portal circulation may reduce the efficacy of anesthetic drugs when injected in the leg muscles or veins. However, this is of little clinical significance as one can increase the anesthetic dose until the desired effect is achieved

Preparation for anesthesia

Thorough physical exam and history taking must be carried out: if non-approachable, behavioral characteristics are assessed in distance
They are very liable to develop handling stress so observation may be the only practical pre-anesthetic evaluation process. Any changes in body disposition, feather condition, grooming should be carefully assessed.
They can be brought into the pre-induction/procedure areas in advance to acclimatize so as to reduce the stress
The following laboratory data are generally required as minimum: PCV, TP, BUN, Glucose
Fasting requirement is contentious, but in general it is recommended in most birds as birds can regurgitate during anesthesia. However, in smaller birds with high basal metabolic rate fasting is better avoided to minimize the likelihood of hypoglycemia
Although not as commonly administered as in other domestic species, preanesthetic medication may be useful to minimize the stress and provide analgesia prior to anesthetic induction.
- In diving birds administration of midazolam with/out butorphanol can substantially reduce the diving reflex.
  - Midazolam can be administered at 0.2-1 mg/kg IM
    Butorphanol can be administer at 0.2-1 mg/kg IM

Figure 1. A sedated goose following midazolam and butorphanol premedication

Ratites or bigger birds may require deep sedation or chemical restraint prior to anesthetic induction: alpha 2 agonists with/out dissociatives are used for healthy animals

Inhalant anesthesia

Modern potent inhalants such as isoflurane, sevoflurane and desflurane are the preferred choice both for anesthetic induction and maintenance
In most birds that can be handled with little stress, anesthetic induction is achieved by using face mask induction technique, but chamber induction is better for birds that are difficult to control
Although face masks commercially available for small animals are appropriate for many birds, due to widely varying sizes in beaks, bills and ceres, air-tight face mask placement is not always easy, increasing anesthetic leakage
Intubating the birds once the anesthesia is induced will provide a secure airway as well as reducing the anesthetic leakage.
It is better to use non-cuffed ET tubes in birds, but if cuffed tubes are used, beware not to over-inflate the cuff to avoid damage to the trachea.

Anesthetic monitoring & maintenance

As in other domestic species heart rate, respiratory rate and body temperature are minimal for the physiologic monitoring
Anesthetic monitoring equipments can be useful and these include ECG and Doppler flow detector
Most veterinary pulse oximeters are calibrated with mammalian oxygen hemoglobin saturation dissociation curve, so accuracy is uncertain
For most small birds noninvasive blood pressure readings based on oscillometric principles (e.g. DINAMAP) are difficult to obtain, but for medium size to large size birds the pressure readings may be obtained on peripheral metatarsal arteries
Small birds may become very hypothermic and external heat sources must be supplied to prevent the animal becoming hypothermic
Fluids are administered to avoid dehydration and support hemodynamic stability

Recovery

Keep the patient warm (human infant incubator may be a good external thermal source as well as minimizing handling stress for small birds)
Provide analgesics
Attend to airway obstruction
Reverse any reversible drugs that may prolong the recovery

Anesthesia of small mammals

General considerations

Animals falling into this category include mice, rats, rabbits, hamsters, Guinea pigs and ferrets, most of which are extensively used for medical research
Small body size makes venous access, muscular injection and tracheal intubation more difficult
They have high metabolic rate and this characteristic must be taken into account for preanesthetic preparation and drug dosing

Preanesthetic preparation

Thorough physical exam and previous medical history check-up
Fasting is not necessary as vomiting during induction does not occur and also to prevent hyperglycemia due to their high basal metabolic rate
Water ad libitum until premedication
Stabilize any fluid deficit or electrolyte deficit
Most are easily distressed so ensure minimal handling to reduce the stress and excitement
Pre-existing respiratory disease is not uncommon, so availability to provide O2 is highly desirable
In ferrets preexisting adrenal tumors, cardiomyopathy, anemia and endocrinopathies including insulinoma are not uncommon, and thus this must be taken into account in the selection of premedicants

Anesthetic induction

Either injectables or inhalants are used, and the choice should be based on the animal’s temperament, health status, and availability of equipments and other supplies.
Administration of injectable premedicants or induction agents can be via intramuscular, intravenous, subcutaneous or intraperitoneal routes. Always aspirate before injection.
Generally, due to the small body size, anesthetic induction through IV route is less practical, and IP injections are more commonly carried out.
For IP injections, always restrain properly to avoid damaging important vital organs
Neuroleptanalgesic combination is useful for anesthetic induction
- Once Innovar-Vet® (Droperidol-Fentanyl) was used to be very popular for anesthetic induction and maintenance of small mammals, but this is no longer marketed in the US.
- Similar product, Hypnorm® (Fluanison-Fentanyl) is equally effective to provide deep sedation and light anesthesia, but is not available in the US.
- Other home made neuroleptanalgesic mixtures include
  - Fentanyl-midazolam
  - Fentanyl-acepromazine
  - Morphine-midazolam
  - Buprenorphine-midazolam etc.
Ketamine combined with neuroleptics are commonly used to provide anesthetic induction and maintenance and some examples are:
- Ketamine + medetomidine
- Ketamine + xylazine
- Ketamine + acepromazine
- Ketamine + midazolam
- Opioids may be added for synergistic effect to provide better degree of CNS depression, analgesia and muscle relaxation
Propofol may be used in animals following premedication with IV access: e.g. rabbits (ear vein) and ferrets (cephalic)
Other injectable anesthetics used for small laboratory mammals include pentobarbital, urethane, chloralose, chloral hydrate, inaction, and tribromoethanol
Face mask anesthetic induction can be carried out using isoflurane, desflurane or sevoflurane, but often times it is easier to place animals in anesthetic induction chamber (many commercial ones are available)

Endotracheal intubation

Rabbits and ferrets
- Intubating can be difficult in rabbits because of the long, narrow oropharynx, thick fleshy tongue and long incisors limiting access to the mouth.
- Although oropharyngeal passage is not as limited in ferrets, intubation is still not without any difficulty. Ferrets are treated similar to cats in dealing for endotracheal intubation and anesthetic management
- A straight infant size laryngeal, Miller, blade can facilitate the visualization of the laryngeal structure both in rabbits and ferrets\
- For blind intubation in rabbits, the head should be held on extension to provide straight line of the passage of the endotracheal tube
- Tubes of 2.5 to 4 mm ID are suitable for most rabbits and ferrets

Guinea pigs, hamsters, rats and mice
- 12 to 18 G plastic intravenous catheters can be used for tracheal intubation
- Intubation is not typically carried out due to their small tracheal size or narrow oropharynx, so use of face masks are more common for maintaining anesthesia
- In guinea pigs, maintenance of clear airway is not always easy since nasal and oropharyngeal secretions tend to become viscid during anesthesia and are liable to give rise to obstruction. To ensure a clear airway frequent aspiration of the secreted materials is necessary.

Anesthetic monitoring and maintenance

Anesthetic depth monitoring using physiologic reflexes such as palpebral, muscle tone, withdrawal reflex, and gross purposeful movement
Basic physiologic monitoring includes heart rate, respiratory rate, and body temperature
Because of the very high heart rates, the ECG is almost exclusively required to obtain correct heart rate
Noninvasive blood pressure monitoring is best achieved in rats and mice using Doppler flow detector and sphygmomanometer
In ferrets and rabbits, noninvasive blood pressure monitoring can be carried out using oscillometric device (e.g. DINAMAP) and blood pressure cuff
Pulse oximetry and capnogrpahy provide useful adjunctive monitoring to assess ventilatory efficiency
Fluids need to be administered to avoid hypovolemia and maintain stable hemodynamics

Recovery

Similar precautions practiced in other species should just as well apply to these species, with particular emphasis on maintaining optimal body temperature, securing a clear airway and providing adequate pain relief

Anesthesia for Reptiles

Physioanatomic peculiarities

Reptiles are poikilothermic or ectothermic (cold-blooded) and their body temperature and metabolic rates are governed by the ambient temperature
In snakes the tracheal ring is incomplete but turtles and crocodilians have complete tracheal rings and it is important not to over inflate the ET tube cuff in these species
Except in crocodilians where the pulmonary morphology is similar to that of the mammals, most reptilians have more primitive lung structures and possess air sacs which do not involve in gas exchanges
Crocodilians have a similar heart structure to mammals but most reptiles have a three chamber heart with two atria and one ventricle. The ventricle is functionally subdivided into cavum arteriosum, cavum venosum and cavum pulmonale
Reptiles have an extensive pulmonary shunting. They also undergo extensive anaerobic metabolism which is particularly well developed in aquatic reptiles such as sea turtles. These evolutionary adaptation enable them to sustain hypoxic insult much better than mammalians in a low oxygen environment.
Renal portal circulation may reduce the efficacy of anesthetic drugs when injected in the leg muscles or veins. However, this is of little clinical significance as one can increase the anesthetic dose until the desired effect is achieved
Reptiles do not posses a true diaphragm but negative pressure pumping system is still used to ventilate
Respiratory muscles are used both for inspiration and expiration
In apneic Chelonians ventilation can be supported by moving the legs in and out by changing the volume of coelomic cavity
The reptile glottis is slit like opening between the arytenoid cartilages and located at the base of the tongue on the floor of the oral cavity
Low basal metabolic rate

Preparation for anesthesia

Thorough physical exam and history taking must be carried out. However, due to the danger involved in handling some vicious and venomous species observation of behavioral characteristics in distance may be the only practical pre-anesthetic evaluation process. Any changes in body disposition, the skin condition, discharges from the nostrils and eyes should be carefully assessed.
The following laboratory data are generally required as minimum: PCV, TP, BUN, Glucose. Blood glucose level is generally lower than mammals (30 – 100 mg/dl)
Any abnormalities (dehydration, anemia, acid-base imbalance, hypoglycemia) must be corrected prior to anesthetic induction
Although regurgitation and aspiration is unlikely, fasting is recommended because of impaired digestion.
Injectable premedicants can provide sedation and facilitate the anesthetic induction using inhalants, and the following agents are used for this purpose
- Dissociatives (ketamine) and alpha 2 agonist (medetomidine, xylazine) combinations
- Dissociatives (ketamine, tiletamine) and benzodiazepine (diazepam, zolazepam, midazolam) combinations
- Muscle relaxants (succinylcholine, gallamine, atracurium)
- Opioids (etorphine, carfentanyl)
- Barbiturates (pentobarbital)
- Sodium channel blockers (tricaine methanesulfonate)
- Each agent can be further added to different classes for balanced anesthetic approach

Anesthetic induction

Inhalants are commonly used using clear plastic chamber
IV injection of anesthetics is not generally practical in these species but induction via IM injection can be carried out using pole syringes or blow pipes

Propofol IV induction may be carried out in well sedated animals following premedication or in well restrained animals by an expert handler

Figure 2. A Gecko being induced with sevoflurane via a face mask

Use of inhalant anesthesia

Modern potent inhalants such as isoflurane, desflurane and sevoflurane are the preferred choice both for anesthetic induction and maintenance
Their ability to withhold breath and extensive pulmonary shunting can significantly delay inhalation anesthetic induction
Due to the danger involved an anesthetic chamber is best utilized for induction
For many reptiles face masks commercially available for small animals are appropriate
Intubating the animals once the anesthesia is induced will provide a secure airway as well as reducing the anesthetic leakage.

Figure 3. A small snapping turtle is intubated using an 18 G catheter and is being monitored using a Doppler flow detector

Anesthetic monitoring & maintenance

As in other domestic species heart rate, respiratory rate and body temperature are minimal for the physiologic monitoring
An esophageal stethoscope can be useful to monitor both cardiac rate, rhythm, intensity and respiratory rate and rhythm.
Anesthetic monitoring utilizing combination of an ECG and a Doppler flow detector (typically placed in a site near the heart) will provide useful monitoring of electrical and mechanical activities of the heart
Due to their thick skin (scales) pulse oximetry and noninvasive blood pressure readings are difficult to obtain.
Most veterinary pulse oximeters are calibrated with mammalian oxygen hemoglobin saturation dissociation curve, so its accuracy is uncertain
Small reptiles may become very hypothermic and external heat source (heating pad, forced warm air blanket etc.) must be supplied to prevent the animal becoming hypothermic

Recovery

Ensure to maintain optimal temperature of the particular species for faster drug metabolism (and recovery)
Provide a secure and clear airway
Provide adequate analgesia
Reverse any reversible drugs that may prolong the recovery

Fish anesthesia

Preanesthetic preparation

Many species of fish exist with widely varying shape and sizes, and these significantly influence the choice of pre/anesthetics and anesthetic management
A thorough pre-anesthetic physical exam of the cardiorespiratory system is not readily achievable.
Although fish do not have lungs and can not aspirate, fasting is recommended as aspirated materials can dislodge within the gills and degrade the water quality

Anesthetic induction

Anesthetic induction can be as simple as adding anesthetic into the fish tank, but strict planning and attention to the aquatic environment are still of paramount importance for smooth operation and decreased morbidity
Several anesthetics available as mixed in water include tricaine methanesulfonate, clove oil (eugenol as active ingredient) and isoflurane. In larger fish such as shark, ketamine-medetomidine combination has been successfully used for anesthetic induction and short maintenance of anesthesia

Anesthetic maintenance and monitoring

For procedures lasting longer than 10 minutes an anesthetic system that allows water to flow over the gills is used.
A submersible recirculating water pump within the fish tank pumps the anesthetic diluent water into the mouth, past the gills and out through the operculum while the fish is placed in a presoaked foam bed (see figures below)
The skin that is not in contact with moistened foambed should be kept wet by spraying water periodically to prevent desiccation
Anesthetic depth monitoring can be carried out by monitoring changes in cardiorespiratory system. An ECG or Doppler flow detector can be used to monitor the rate and rhythm, and respiratory rate can be counted by observing the movement of the operculum.
If respiration ceases the anesthetic concentration delivered to fish must be reduced or stopped until the fish resumes the respiration
The fish’s heart operates differently to mammalians’. The myocardial cells can utilize local glycogen stores for energy instead of relying on blood glucose supply, so the fish’s heart can still contract in a clinically dead animal. It is therefore important to evaluate the animal’s heart rate in conjunction with other variables when determining anesthetic depth.

Figure 4. A basic set-up for inducing and maintaining small fish using tricaine methanesulfonate (MS 222) diluents via a submersible recirculating water pump and hoses

Figure 5. A gold fish is placed on a presoaked foambed and ready for a quick surgical removal of the head tumor. The diluent anesthetic is constantly being circulated through the open mouth, gills and out of the operculum using the water pump and hoses

Recovery

Recovery is achieved by placing the fish in an anesthetic-free water
The water is aerated and the animal’s mouth is oriented to the water flow
If the animal is not breathing on its own, it is assisted with water to flow into the mouth and over the gills.
As the fish recovers, respiration increases, fin starts to move and then the fish swims with progressively better coordination.
Most recover within 5 minutes after being placed in clear water, and animals taking longer than 10 minutes are suspected to have been overdosed or medically compromised

Anesthesia for Exotic Species

Introduction

Avian anesthesia

Physioanatomic peculiarities

Preparation for anesthesia

Inhalant anesthesia

Anesthetic monitoring & maintenance

Recovery

Anesthesia of small mammals

General considerations

Preanesthetic preparation

Anesthetic induction

Endotracheal intubation

Anesthetic monitoring and maintenance

Recovery

Anesthesia for Reptiles

Physioanatomic peculiarities

Preparation for anesthesia

Anesthetic induction

Use of inhalant anesthesia

Anesthetic monitoring & maintenance

Recovery

Fish anesthesia

Preanesthetic preparation

Anesthetic induction

Anesthetic maintenance and monitoring

Recovery

Further readings