anestesia

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

o Dissociatives (ketamine) and alpha 2 agonist (medetomidine, xylazine) combinations

o Dissociatives (ketamine, tiletamine) and benzodiazepine (diazepam, zolazepam, midazolam) combinations

o Muscle relaxants (succinylcholine, gallamine, atracurium)

o Opioids (etorphine, carfentanyl)

o Barbiturates (pentobarbital)

o Sodium channel blockers (tricaine methanesulfonate)

o 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

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.

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