Mammal Chemical Immobilization: Inhalant Anesthesia

Anesthesia is an integral part of exotic animal medicine, whether in the field, or in research or zoo settings. Even the most basic procedures typically require sedation or anesthesia, since most animals are either too shy, fast, or dangerous to handle without the aid of chemical immobilization. While the drugs and equipment used in the chemical immobilization of wild and exotic animals are inherently the same is in traditional veterinary and human medicine, there are often different and novel provisions that must be made in procedures and protocols. Often these have to do with the physiology or temperament of the species at hand, the lack of ability to assess individual animals beforehand or with conditions in the field.

The chemical immobilization of exotic mammals as a broad term encompasses an extremely wide range of variables, requirements, procedures and protocols owing to the sheer number and variation of exotic mammalian species. Elephants and mink are obviously both mammals, but the chemical immobilization procedures for these two species will be widely divergent.

General and Balanced Anesthesia

In most cases in which a mammal species is to be chemically immobilized, the attending veterinarian will be quite well-versed in what he or she has to do before, during and after a procedure. However, it is also the nature of exotic animal medicine that the veterinarian and support staff will encounter species with which they are relatively unfamiliar. Prior to attempting to anesthetize unfamiliar species in the field or otherwise, the veterinarian will evaluate as thoroughly as possible any anatomic or physiologic factors that might influence the anesthetic event.

Then, the level of anesthesia required for the planned procedure will be considered. While lighter levels of sedation or anesthesia may be sufficient for some procedures (e.g., the crating or loading of zoo animals, or simple clinical exams), more involved or invasive procedures will require heavier sedation or general anesthesia. More often than not, in domestic and exotic animal anesthesia, “general anesthesia” will necessarily mean balanced anesthesia, in which sedative and/or tranquilizing drugs are employed to immobilize (e.g., darting) or calm an animal prior to inhalant anesthesia being used. Smaller, more sedentary animals can often be handled and anesthetized without the use of sophisticated equipment, the majority of mammalian species will require specialized apparatus to allow safe and effective general (balanced) anesthesia.

Inhalant Anesthesia for Mammals

While the precise mechanisms how inhalant anesthetics produce their general anesthetic effects are not precisely known, it is believed that they interfere with functioning of nerve cells in the brain by acting at the lipid matrix of cell membranes.³ Once restrained, most smaller mammals can be induced using isoflurane or sevoflurane in oxygen, or a mix of oxygen and nitrous oxide delivered by facemask or chamber.²

Isoflurane provides rapid and smooth induction and emergence from anesthesia, and is an excellent anesthetic for exotic animals. When compared with older anesthetics (e.g., halothane or methoxyflurane), isoflurane has less myocardial depressant and catecholamine sensitizing effects and it can be used safely in patients with hepatic or renal disease.²

In the case of mask inductions, sevoflurane is often preferred over isoflurane due to the faster onset of action; similarly, sevoflurane is often preferred over desflurane since it is better accepted and brings about smoother inductions.¹Sevoflurane is also thought to be better for debilitated or geriatric patients, since it is more easily dosed. For larger mammals, halothane was once the inhalant anesthetic of choice, but is no longer available.

If an animal is relatively calm, pre-oxygenation prior to a gradual increase in vaporizer setting is often used.² The animal can then be maintained on a mask or intubated depending on its size, the duration of the procedure and necessary depth of anesthesia. Once the procedure is completed the animal is allowed to breathe oxygen until it has recovered.

In the field, minimal monitoring equipment typically includes electronic thermometers to monitor for hypothermia or hyperthermia. A photodetector clip is attached to the tongue or ear; this provides information on the respiratory function of the animal.¹ An oxygen supply should be available in the event that the animal’s SpO₂ level falls below 90%. Unfortunately, oxygen is often unavailable under field conditions and it is not uncommon for anesthetized animals to have SpO₂ values that fall below 90%.¹ Thus, venous access should always be maintained in order to provide rapid IV medication or application of an antagonist in the event of a respiratory crisis.