Special species

Anesthesia of Special Species

Presenter Information

  • Dr. André Escobar, DVM, Ms, PhD, DACVAA

  • Dr. Miguel Martinez, LV, Dip ECVAA

General Information on Special Species Anesthesia

  • Special species are not commonly encountered in general practice, leading to a unique interest and need for specialized knowledge.

  • Types of special species include:

    • Laboratory animals: mice, rats, guinea pigs, lagomorphs, ferrets, sugar gliders.

    • Ectotherms: reptiles (snakes, lizards, chelonians, crocodilians), amphibians, fish.

Anesthesia Considerations for Laboratory Animals

  • Millions of laboratory animals are anesthetized yearly, requiring awareness of their unique medical requirements.

  • Restraint can pose challenges due to:

    • Risk of bites

    • Agility and speed of animals

    • Easily stressed species

  • Special skills and environments (cages, holding areas) are essential due to the tiny size of these animals and their high metabolic rates.

  • Physiological parameters for a mouse include:

    • Heart rate: 300-800 bpm

    • Respiratory rate: 100-200 bpm

    • Cardiac index: 600-700 ml/kg/min

  • Feeding: Animals are typically fed ad libitum (freely available food).

Common Complications During Anesthesia

  • Potential complications include:

    • Drug over-dosage: Requires careful dilution.

    • Hypoglycemia: Fasting is not preferred; ensure food intake is maintained.

    • Hypothermia: Monitoring and management necessary.

    • Intravenous access difficulties: Especially an issue for respiratory systems where intubation is rarely performed due to anatomical challenges (obligate nasal breathers and narrow airways).

Routes of Anesthetic Administration

  • Common routes include:

    • Intra-peritoneal

    • Subcutaneous

    • Inhalation

    • Intravenous (especially via tail vein for mice/rats and auricular or cephalic vein for rabbits).

Commonly Used Anesthetic Drugs

  • Inhalation Anesthesia: Induction and maintenance typically with isoflurane or sevoflurane.

  • Injectable Anesthesia:

    • Ketamine (most common) used alone or with alpha-2 agonists (xylazine, dexmedetomidine) or benzodiazepines (midazolam).

    • Notably, some rabbits exhibit large amounts of atropinase, so glycopyrrolate is preferred.

Induction of Anesthesia

  • Induction methods include:

    • Inhalational: Via induction chamber or face mask.

    • Injectable: Combinations of ketamine with alpha-2 agonists or midazolam, alfaxalone, or propofol based on specific conditions.

Endotracheal Intubation

  • Intubation can be quite challenging in special species and is usually not performed due to anatomical constraints.

  • Techniques include:

    • Small endotracheal tubes

    • Blindly

    • Fiberoptic-guided

    • Otoscope-guided methods.

Monitoring During Anesthesia

  • Monitoring challenges arise due to the small size of the animals. Key physiological parameters to monitor include:

    • Depth of anesthesia via reflexes (withdraw, palpebral)

    • Heart rate and blood pressure (especially in rabbits)

    • Respiratory rate and measurements of EtCO2 and SpO2.

    • Thermoregulation is critical due to the risk of hypothermia.

Recovery from Anesthesia

  • Continuous monitoring is crucial during the recovery phase because of a high mortality rate.

  • Essential recovery protocols include:

    • Providing warmth

    • Administering fluids subcutaneously

    • Ensuring food availability to avoid hypoglycemia

    • Providing analgesia as needed to manage potential postoperative pain.

Anesthesia of Ectotherms

  • General characteristics of ectothermic anesthesia:

    • Snakes, lizards, crocodilians, chelonians, amphibians exhibit unique responses to anesthesia ('cold-blooded').

    • Body temperature regulation relies on external sources, affecting metabolic rates and anesthetic effects.

    • Ideal body temperature for anesthesia varies between types (20-25°C for aquatic/temperate species; 25-33°C for tropical).

  • Highlighted factors affecting anesthesia:

    • High anaerobic tolerance and ability to hold breath for extended periods.

    • Physiological considerations: slower heart rates and longer induction/recovery phases due to low metabolic rates.

Unique Features of Cardiovascular and Respiratory Systems in Ectotherms

  • Cardiovascular system highlights include:

    • A 3-chambered heart in lizards, chelonians, and snakes; a 4-chambered heart in crocodilians.

    • Lower mean arterial blood pressure compared to mammals, impacting anesthetic management.

  • Respiratory system characteristics:

    • Complete tracheal rings in some species, incomplete in others; risks associated with intubation techniques depending on anatomy.

    • Active inspiration/expiration is needed due to the absence of a muscular diaphragm; respiratory depression can be significant during anesthesia.

    • Gas exchange mechanics and breath-holding capabilities vary widely among ectothermic species.

Anesthesia in Fish

  • Fast for 12-24 hours to minimize regurgitation risks before anesthesia.

  • Induction typically occurs in tanks where anesthetic is added to water.

  • Maintenance involves the use of aerated water containing anesthetic; intubation of the buccal cavity can occur in some cases.

Unique Anesthetic Drugs and Techniques for Fish

  • Common drugs include:

    • MS-222: FDA approved with a withdrawal time of 21 days.

    • Other options: benzocaine, metomidate, isoeugenol.

  • Monitoring during fish anesthesia:

    • Parameters include jaw tone, righting reflex, heart rate/ECG, and gill movements.

  • Recovery involves transfer to an aerated recovery tank, possibly employing a 'reversal syringe' technique with pure water.