Anesthetic Depth in Exotic Animals: Monitoring the Degree of Central Nervous System Depression

Key Points

  • Anesthetic depth is the degree to which the central nervous system is depressed by a general anesthetic agent.
  • Exercise caution when evaluating anesthetic depth in exotic animal patients, particularly when monitoring a novel species, as response to anesthetic agents can defy generalization in any given individual, particularly when anesthesia is achieved using a combination of drugs.
  • As a general rule of thumb, as anesthetic depth increases, muscle tone decreases and the respiratory pattern may become regular and even.
  • Respiratory rate and depth are important guides to anesthetic depth in birds and mammals, but respiration is a poor indicator in reptiles since apnea is often observed at a surgical plane of anesthesia.
  • As anesthetic depth increases, reflexes become attenuated. At a surgical plane of anesthesia, pedal withdrawal reflexes are often absent and the corneal reflex is present but sluggish in those patients possessing eyelids.
  • Although the palpebral reflex is often lost at a surgical plane of anesthesia in reptiles possessing eyelids and birds, this reflex may not be lost until dangerously deep levels of anesthesia in many mammals.
  • Excessive anesthetic depth is associated with severe, life-threatening cardiovascular depression manifested by bradycardia, hypotension, and hypercapnia. This stage can also be recognized by the loss of all reflexes, including corneal and palpebral. Death will ensue if the patient’s status is left unchecked.
  • The anesthetist must remain vigilant for any changes in patient status during the perianesthetic period. Changes in anesthetic depth can occur particularly quickly in birds.
  • This article is part of a RACE-approved Anesthetic Monitoring teaching module. Visit the articles on physiologic monitoring of vital signs, blood pressure, capnometry, pulse oximetry, and electrocardiography for additional information in exotic animal patients.
  • The content presented here is also available as an approximately 23-minute slideshow recording. More detailed information is available in the article version of this content, however it is not necessary to read the article to complete the teaching module quiz.

General principles

Anesthetic depth is the degree to which the central nervous system (CNS) is depressed by a general anesthetic agent, such as inhalant anesthetics, propofol, and ketamine.28 Anesthetic depth is usually determined by a variety of factors, including the dosage administered as well as the species and the physiological state of the individual animal.7,17

The anesthetist is fundamental to patient safety because she or he assures the patient is unconscious, immobile, and lacks a response to painful stimuli, all while maintaining stable anesthetic depth and safeguarding physiologic parameters (Fig 1). A dedicated anesthetist should be assigned to monitor every patient during the perianesthetic period. The anesthetist ideally remains with the animal continuously until the end of the anesthetic period.13 At minimum, a trained veterinary health professional should check the patient’s vital signs on a regular basis (at least every 5 minutes) during anesthesia and recovery.25

An anesthetist monitors this guinea pig premedicated with buprenorphine and midazolam, then mask induced and maintained on sevoflurane anesthesia

Figure 1. An anesthetist monitors this guinea pig (Cavia porcellus) premedicated with buprenorphine and midazolam, then mask induced and maintained on sevoflurane anesthesia. Photo credit: Katrina Lafferty, RLAT, VTS (Anesthesia/Analgesia). Click image to enlarge

Effective patient monitoring requires careful observation. The vigilant anesthetist monitors the presence or absence of reflexes and response to painful stimuli, changes in muscle tone, heart rate, and blood pressure, as well as changes in the pattern and character of respiration.18 Clear surgical drapes can facilitate visualization of tiny patients (Fig 2).13  The anesthetist should exercise caution when evaluating anesthetic depth in exotic animals, particularly when monitoring a novel species, as response to anesthetic agents can defy generalization in one individual animal, particularly when anesthesia is achieved using a combination of drugs.7

 clear surgical drape can facilitate visualization of small patients, like this toad.

Figure 2. A clear surgical drape can facilitate visualization of small patients, like this toad. Photo credit: Katrina Lafferty, RLAT, VTS (Anesthesia/Analgesia). Click image to enlarge

 

Stages of anesthesia

In 1937, the anesthesiology pioneer, Arthur Ernest Guedel described a relationship between the level of anesthesia with diethyl ether and specific, physical descriptors.28 These classic stages and planes of anesthesia continue to be loosely applied to most inhalant anesthetics, like isoflurane and sevoflurane (Table 1).25 The widespread variation in response to anesthesia in exotic animals and the use of several drugs in combination makes the usefulness of such systems inconsistent in modern exotic animal practice.8

Table 1. Most general anesthetic agents produce progressive central nervous system depression that can be divided into stages
Stage IStage IIStage IIIStage IV
Respiratory rateN↑ to N to ↓Respiratory arrest
Heart rateN↑ to N to ↓↓Cardiac arrest
Reaction to surgical stimulation+++ to --
Muscle relaxation+/-+/-+ to +++++
Palpebral reflex+++ to --
↑: increase ↓: decrease +: present -: absent


Stage I is best described as altered consciousness. The patient is sedated or drowsy with decreased mobility, but still awake and responsive to stimulation.30 Eyelids often droop.20 Respirations may be deep or shallow, rapid and irregular depending on the degree of patient excitement.

The patient may next go through an excitatory phase (stage II) in which they are hypersensitive to sensory stimulation.6 The animal may react to stimuli like sound with exaggerated struggling. This delirium or excitement represents early loss of consciousness.25 As voluntary centers in the brain become depressed, the patient becomes unaware of its surroundings.25 Respirations are generally rapid and irregular.6,20,25 Breath holding may occur. Eyelids are open wide and pupils are dilated because of sympathetic nervous system stimulation.25 Reflex vomiting can occur if the patient has not been fasted, and defecation and urination can also occur.25

An excitatory phase of anesthesia can be particularly hazardous in exotic animal patients since increased sympathetic tone can promote arrhythmias, as well as struggling and physical injury. One important role of premedication is to reduce or eliminate this phase.25


Respirations become slow, deep, and more regular when the patient enters surgical anesthesia (Stage III), which is further divided into subjective planes of light, moderate, and deep (Table 2).25

Table 2. Surgical anesthesia is divided into planes6
Plane of anesthesia1 (Light )2 (Moderate)3 (Deep)
Respiratory rate+ to +++/—
Respiratory patternSmooth & regularNormal (slow & regular) or slightly decreased with an intercostal lagShallow with increased movement of abdomen or coelom*
Muscle relaxation+++++
Swallowing reflex
Withdrawal reflex+
* Increased abdominal or coelomic breathing is observed due to a gradual loss of movement by the intercostal muscles and/or diaphragm

— : absent; +/— : sometimes present; +: mildly present; ++: moderately present; +++: strongly present

A light surgical plane (plane I) is associated with minimal muscle relaxation. The jaw is relaxed, and the mouth is easily opened and withdrawal reflexes are present. In response to painful stimuli, such as a pin prick, the animal will move and physiological parameters like respiratory rate, heart rate, and blood pressure will rise. Swallowing reflexes are lost, so that intubation is now possible. A light surgical plane is indicated for restraint or minor, non-painful, non-invasive or minimally invasive procedures, like biopsy or laceration repair, when supplemental analgesia is provided.6,11,25

Plane II or moderate surgical anesthesia is characterized by an absence of consciousness, immobility, marked muscle relaxation, and lack of response to painful stimuli and is ideal for most invasive surgical procedures.25,30 The respiratory rate should be within normal range, or slightly decreased, and the heart rate is also within normal range. Heart rate, respiratory rate, and blood pressure can rise in response to pain.

Make every effort to distinguish between surgical pain and a light plane of anesthesia. A high degree of surgical pain will give the impression of a light surgical plane of anesthesia [and] the anesthetist will be inclined to increase the anesthetic delivered to the patient. As the plane deepens, less…response to…pain is seen, however when the surgical stimulus is removed, the patient will suddenly lapse into a dangerously deep anesthetic plane. Whenever possible, [provide] more analgesia not anesthesia [to these patients]”.—Joubert 2014 18

Deep surgical anesthesia (plane III) is associated with severe, life-threatening cardiopulmonary depression. Respirations often appear shallow and labored.

Clinical Tip: The irregular respiratory pattern typical of deep surgical anesthesia can unfortunately be mistaken for a much lighter plane of anesthesia.6,25

If this deep surgical plane is not corrected, the patient can progress to an anesthetic overdose (Stage IV). An anesthetic overdose is associated with respiratory arrest (hypoventilation and hypoxemia) followed by CNS depression and circulatory collapse, associated with reduced cardiac output, hypotension, and inadequate tissue perfusion. Mucous membrane color appears gray or even blue. All reflexes are absent, including the corneal reflex.20,25 If these changes are allowed to persist, death can occur within 1-5 minutes in mammals or potentially much more rapidly in birds.25 Some authors have described sudden piloerection and pupillary dilation with cardiac arrest.16

Clinical Tip: When there is evidence that the patient is too deep, halt inhalant anesthesia, disconnect the anesthetic line and flush the remaining inhalant from the system, then administer oxygen, gently stimulate the body, and begin (or continue) positive pressure ventilation.25

 

Monitoring tools

No single measure can be used to accurately pinpoint anesthetic depth.7 Depending on your species of interest, parameters monitored can include reflexes, eye position, the degree of muscle relaxation, response to surgical stimulation, and physiological parameters.7,25

Physiological parameters

Although physiologic parameters are subject to a variety of influences7, many anesthetic agents are cardiovascular and respiratory depressants. Therefore the respiratory rate tends to decrease as anesthetic depth increases.7 At a surgical plane of anesthesia, the respiratory rate should be within normal limits or slightly decreased and the pattern should be slow and regular, although this will depend on species. As a general rule, birds and reptiles do not tend to breathe well under general anesthesia, even when at an appropriate plane. Carefully monitor respiration in response to any painful stimulus. If the animal is too light and conscious of painful stimuli, the respiratory rate will increase or the respiratory pattern will become irregular.

Bradycardia also frequently occurs as the anesthetic plane deepens.7 Bradycardia can also be associated with specific anesthetic drugs, like opioids and alpha-2 agonists, even when the anesthetic plane is appropriate (Table 3). Progressive decreases in blood pressure and heart rate can indicate excessive anesthetic depth7, while a rapid increase in heart rate and blood pressure can be caused by painful surgical stimulation in a lightly anesthetized patient or in a patient lacking appropriate analgesia (Table 3).7

Table 3. Potential causes of alterations in heart rate 7,25
BradycardiaTachycardia

  • Increasing anesthetic depth

  • Opiods and alpha-2 agonists

  • Mesenteric traction

  • Intubation

  • Oculocardiac reflex*

  • Hypertension

  • Hypothermia

  • Hyperkalemia

  • Cardiac conduction disturbances


  • Inadequate plane of anesthesia

  • Painful surgical stimulation in a lightly anesthetized patient

  • Hypovolemia

  • Hypoxia

  • Hyperthermia

  • Hypercarbia

* The oculocardiac reflex, also known as the Aschner or trigeminovagal reflex, is a reduction in heart rate after direct pressure is placed on the eyeball.

Increases in heart rate, and systolic blood pressure of 20% or more over baseline can be due to painful surgical stimulation or inadequate anesthetic depth (Table 3).7,24 Lacrimation can also be observed at a light plane anesthesia.25

Muscle tone

The degree of muscle relaxation generally increases with the depth of anesthesia.18 Muscle tone can be evaluated using flexion and extension of the legs, as well as abdominal muscle tone or jaw muscle tone. Moderate resistance to fully opening the mouth is expected with moderate surgical anesthesia (plane 2) 25, however if an animal attempts to close its mouth when gentle traction is placed on the mandible, more anesthesia is generally needed. The ease of monitoring jaw tone will vary with the species due to differences in jaw size and muscle strength. For instance, jaw tone is relatively difficult to evaluate in rodents. Assessment of jaw tone is highly subjective and this parameter should be evaluated at regular intervals for comparison.

Head withdrawal, or the degree of resistance encountered as the anesthetist attempts to withdraw the head from the shell, can be evaluated in turtles and tortoises. Wing tone can be assessed in birds. In some mammals, as well as snakes and monitor lizards, tongue withdrawal can also be evaluated. Tongue retraction often persists at a surgical plane of anesthesia and is lost when the patient is excessively deep. Finally, although anal or cloacal tone is not a particularly precise parameter, it can be useful when the head cannot be accessed.7 Anal or cloacal tone should be lax at a surgical plane of anesthesia. Keep in mind, anal or cloacal tone will also be lax if an epidural using a local anesthetic agent has been placed.

Ocular position

Ocular position is not a reliable indicator of anesthetic depth in exotic animal patients.27 Mammals typically rotate the ocular globe medially and centrally at a surgical plane of inhalant anesthesia, however globe position varies with the anesthetic agent used and changes are rarely observed in most exotic species. In birds and most reptiles, the large globe cannot move within the orbit. So again ocular position cannot be used to assess anesthetic depth. This is one reason exotic animal patients are at risk for corneal ulceration under anesthesia. Eyes should be lubricated well and often to prevent corneal dessication.13 Placement of damp gauze sponges over the eyes can help to maintain the moisture provided by eye lubrication.13

Nictitans

In species with eyelids, the third eyelid or nictitans often prolapses at a surgical plane of anesthesia.

Nystagmus

In some mammals, nystagmus is observed at a light plane of anesthesia.7,25 This finding is quite variable and cannot be relied upon as an indicator of anesthetic depth.8 Nystagmus is often seen with lacrimation, and it is most commonly observed when ketamine has been administered.25

Response to noxious stimulation

Monitor your anesthetized patient for chewing motions or voluntary movement involving the limbs or head, particularly in response to painful stimulation.7 A response to toe, ear, or tail pinch is usually lost at a surgical plane of anesthesia.18,25 One of the more noxious stimuli for birds is removal of feathers.15,16

Note: Purposeful movement in response to painful stimulation must be differentiated from spontaneous movement that can be seen with certain anesthetic agents, like ketamine or opioids.

Reflexes

Reflexes are involuntary and nearly instantaneous movements in response to a stimulus and are mediated via the reflex arc in the spinal cord (Fig 3). The presence of a reflex does not require the perception of pain.

A reflex arc is a neural pathway that controls a reflex. This reflex arc involves the spinal cord, but sensory neurons do not pass directly into the brain.

Figure 3. A reflex arc is a neural pathway that controls a reflex. This reflex arc involves the spinal cord, but sensory neurons do not pass directly into the brain. Source:  Wikimedia Commons via The Animals and Man, an Elementary Textbook of Zoology and Human Physiology. Click image to enlarge

Reflex activity is an important index for evaluating the degree of CNS depression, since increasing anesthetic depth produces characteristic changes in motor reflexes (Table 4).20,25 Some reflexes persist at a surgical plane of anesthesia while others are obliterated. The response of an individual animal can also vary widely and a single reflex response may not be adequate for assessing the plane of anesthesia or the level of analgesia.11

Table 4. How to perform and assess select reflexes 7,8,11,12,23
ReflexTechniqueAssessmentComments
Anal or cloacal pinchPinch the anus or cloaca to stimulate reflex contraction of the anal or cloacal sphincter.Retained at a light plane of anesthesia, dull to absent at a surgical plane of anesthesia 25The degree of response is species dependent7
Corneal Reflex
Gently touch the cornea with a moistened cotton-tipped applicator or apply a drop of sterile water to stimulate blinking or drawing the globe into the orbit.

Typically brisk in conscious animals 7, sluggish at a surgical plane of anesthesia,
and completely lost at an excessively deep plane of anesthesia.25,27
However loss of corneal reflex varies significantly among individual animals and anesthetic agents. Therefore loss of this reflex in a patient that previously had it indicates anesthesia should be lightened.25
CAUTION: It is possible to traumatize the cornea so take care and only evaluate this reflex when it is absolutely necessary.7,23
Ear pinch/
ear flick/
auricular reflex

Pinch the pinna of the ear or gently stroke or tickle the inside of the ear with a cotton-tipped applicator to stimulate head shaking, ear twitching, whisker movements, or even vocalization in some guinea pigs.The ear pinch is usually not lost until a deep plane of surgical anesthesia.
Palpebral reflex (lateral and medial)
Lightly touch the medial or lateral canthus of the eye or stroke the eyelashes to stimulate blinking.7Often lost at a surgical plane of anesthesia in birds and reptiles with eyelids.

A sluggish palpebral reflex is often retained in rabbits and may not be entirely lost until the patient reaches excessively deep levels of anesthesia. 8,13
Lost early with barbiturates and most inhalation agents; often retained with dissociative drugs, like ketamine 6,12

Some patients never display a palpebral reflex or this response can become fatigued with repeated testing.12
Pedal withdrawal reflex
See withdrawal reflex below
Pupillary light responseShine a bright light into the eye to stimulate constriction of the pupil.As anesthetic depth increases, pupil diameter increases. A fixed, dilated pupil that is unresponsive to light is associated with a dangerously deep plane of anesthesia.18
Righting reflex
Place the patient on its back or side and then observe for attempts to return to a normal, upright positionLost at a surgical plane of anesthesiaLoss varies with the species and anesthetic agent used 7
Withdrawal reflex (e.g. pedal withdrawal reflex)Pedal withdrawal reflex: Extend one leg and pinch the web of skin between the toes. Apply pressure for up to 1 second per site. Depending on patient size, use your fingers, fingernails, or forceps to ensure adequate pressure. A positive reflex is indicated by flexion (or withdrawal) of the extended hindlimb.
Present at a light plane of anesthesia; lost at a moderate surgical plane

If the animal makes general body movements or cries out this is not a positive reflex but instead indicates that the animal has felt the painful stimulus.
When evaluating your patient’s response to pain, always try at least two toes as well as the ears so that you are sure a response to pain is indeed absent.

In many species, loss of hindlimb withdrawal is lost at lighter planes of anesthesia than forelimb withdrawal but this is usually sufficient to allow surgery to proceed.8

 

Exotic companion mammals

Rabbits, ferrets, and larger rodents can be anesthetized as with cats or dogs using premedication, intravenous catheter placement, followed by injectable induction agents. Smaller mammals, such as hedgehogs and sugar gliders, may need to be induced with a face mask or chamber, however these patients frequently breath hold and then take deep, rapid breaths that can quickly result in dangerously high levels of inhalant anesthesia.25 To minimize risk, the use of pre-anesthetic agents and/or low induction settings should be considered.25

Physiological parameters

It is important to be vigilant when monitoring anesthesia in exotic companion mammals. Rabbits, ferrets, and rodents can easily become stressed, leading to marked catecholamine release and irregular physiologic responses, including potentially fatal arrhythmia.11

Muscle tone

Ferrets that undergo isoflurane mask induction, without premedication on board for quick procedures like blood collection or survey radiographs, will quickly reach a deep plane of anesthesia, usually within 1-2 minutes. Nevertheless, jaw tone in these ferrets will remain very strong for several minutes until a surgical plane of anesthesia is reached.

Reflexes

Traditional reflexes used to monitor the rabbit include the righting reflex and corneal reflex.5,7,11,25 The righting reflex is lost with surgical anesthesia, while a slight corneal reflex is often retained in mammals at a moderate plane of surgical plane anesthesia (Table 4).7

The palpebral reflex can be difficult to assess in small rodents (Fig 5).8 The palpebral reflex has also been described as unreliable in rabbits 21, although this reflex can become fatigued with repeated testing in any patient. In most mammals, a sluggish palpebral reflex is retained at a moderate surgical plane of anesthesia (Table 4) and is not completely lost until the patient reaches dangerously deep levels of anesthesia.7,8

Response to noxious stimuli

The ear pinch or ear flick is considered the most sensitive indicator of anesthetic depth in the rabbit.5,11,25 At a light plane of anesthesia, an ear pinch can stimulate head shaking in rabbits or rodents, or even vocalization in guinea pigs.8 A response to ear pinch is usually not lost until a deep plane of surgical anesthesia (Table 5).

The pedal withdrawal reflex usually disappears in mammals at a surgical plane of anesthesia (Table 5).18,33 In guinea pigs, the pedal reflex is not always a reliable measure of anesthetic depth because cavies sometimes make involuntary leg movements, even at a surgical plane of anesthesia.7,11

In many exotic companion mammals, test at least two toes as well as the ears to ensure the patient does not respond to noxious stimuli before beginning a potentially painful procedure. In small rodents, it can be difficult to assess toe pinch or stimulation of interdigital tissue. Alternatives include pinching abdominal skin with mosquito forceps11, rectal pinch, or tail pinch.8 In fact, lack of response following tail pinch is the most reliable indicator of surgical anesthetic depth in rats (Table 5).11

Ocular position

Some sources describe the globe rotating ventromedially in the rabbit at a moderate surgical plane of anesthesia 7,30, however this is rarely observed in clinical patients.

Table 5. Evaluation of anesthetic depth in the exotic companion mammal
Stage of anesthesiaAltered Consciousness + Excitement
(Stage 1 to Stage 2)
Light Surgical Plane
(Stage 3)
Moderate Surgical Plane
(Stage 3)
Deep Surgical Plane
(Stage 4)
Muscle relaxation-+/-++++++
Corneal reflex+++-
Ear pinch+++/--
Palpebral reflex*+++/--
Pedal/tail withdrawal++/---
- : absent; +/-: sometimes present; +: mildly present; +++: strongly present

 

Birds

As a group, birds are incredible athletes. The same remarkably efficient and sensitive respiratory system that allows birds to fly at heights where humans require supplemental oxygen, also allows relatively minor changes in inhalant anesthesia levels to cause rapid and dramatic changes in anesthetic depth. The anesthetist must be prepared to identify and address these changes just as promptly.17

During Stage 1 anesthesia (altered consciousness), the bird is initially lethargic with droopy eyelids. Over time, the head lowers, plumage becomes fluffed and ruffled, and increased third eyelid movement is observed. The bird is still rousable but it typically does not resist handling. The excitatory phase (Stage 2) is more likely to be observed in larger avian species.20

Physiological parameters

Respirations are classically rapid, regular, and deep at a light surgical plane.20 At a moderate surgical plane of anesthesia respirations may become slow, deep, and regular.17,20 At an excessively deep plane of anesthesia, respirations are slow and shallow or intermittent.20 Unfortunately, avian patients will often defy expectations and display a highly irregular respiratory pattern or even become entirely apneic regardless of anesthetic depth so careful observation is required.

Muscle tone

Successful induction of stage 3 or surgical anesthesia has occurred when the legs and wings are sufficiently relaxed so that they can be extended without being withdrawn (Videos 1 and 2).17,20 In many patients, lax wing tone signals that the patient is ready for endotracheal intubation.17 Lax cloacal tone and some jaw tone is expected at a light surgical plane (Table 6).17,20

Video 1. Presence of wing tone in a budgerigar (Eublepharis macularius) and absence of normal wing tone in a cockatoo (Cacatua alba). Video produced by Katie Lennox-Phillibeck; technique demonstrated by Dr. Angela Lennox (no audio).

Video 2. Evaluating wing tone in an anesthetized hyacinth macaw (Anodorhynchus hyacinthinus). Source: Ryan O’Shea, CVT.

Reflexes

At a light surgical plane, the palpebral reflex is usually absent and the pedal withdrawal reflex is present but sluggish.16,20 The corneal reflex is also present but there is no response to postural changes (negative righting reflex).20 The corneal reflex is still present but sluggish at a moderate surgical plane of anesthesia (Table 6). 17,20

Response to noxious stimuli

At a light surgical plane, there is pain on feather pluck but there is a lack of voluntary patient movement and no response to sound. At a moderate surgical plane of anesthesia there is no response to painful stimuli, like pinpricks of the cere, toe pinch, cloacal pinch, or feather plucking (Table 6).15,17,25 In fact, birds are often more likely to respond to feather plucking than skin incision.16 This may be due to the cluster of pain and pressure receptors at the base of each feather follicle.16 However in red-tailed hawks (Buteo jamaicensis) and great horned owls (Bubo virginianus) anesthetized with propofol via constant rate infusion, toe pinch was considered a better guide to a surgical plane of anesthesia than plucking contour feathers.14

Nictitans

Most birds retain a slow third eyelid response at a surgical plane.25

Table 6. Evaluation of anesthetic depth in the avian patient 17,20
Stage of Anesthesia Altered Consciousness + Excitement
(Stage 1 to 2)
Surgical
Anesthesia
(Stage 3)
Excessive Depth
(Stage 4)
PlaneLightMediumDeep
Muscle relaxation-+/-+++++
Wing tone+++---
Cloacal toneMay void++/---
Pedal withdrawal++++/---
Cere reflex++/----
Movement with feather plucking+++++/---
Corneal reflex++++++/---
Palpebral reflex+++---
- : absent; +/-: sometimes present; +: mildly present; ++: moderately present; +++: strongly present

 

Reptiles

It can be challenging to accurately assess anesthetic depth in reptiles.2,19,31,32 There are many aspects of reptile physiology that remain poorly understood, and the tremendous diversity of species seen in clinical practice means that it can be difficult to generalize. Nevertheless, the clinical signs associated with induction of general anesthesia are relatively consistent.2

Induction time, recovery time, and all physiologic values are temperature dependent in reptiles.2 It is crucial to maintain these ectothermic patients within the preferred optimum temperature zone for their species.2

Physiologic parameters

Observation of respiration is a poor indicator of anesthetic depth in reptiles, since most members of this taxonomic group rely upon intercostal musculature to breathe.32 Breath holding or hypoventilation can be observed with light anesthesia 22, particularly in chelonians.32 Apnea is often observed at a surgical plane of anesthesia and most reptile patients will require ventilatory support.2

Although heart rate should be carefully monitored in reptiles 25, there are multiple factors that can influence this parameter, including temperature and oxygenation, which can make it unreliable for the assessment of anesthetic depth.32 Of course a significant decrease in heart rate can indicate excessive anesthetic depth.23 Muir (2013) 25 recommends that anesthesia be lightened when heart rate falls to less than 80% of the stabilized or maintenance rate.

Muscle tone 

A surgical plane of anesthesia in the reptile is characterized by immobility and profound muscle relaxation. The limbs will hang limply from the chelonian shell.1 In snakes, muscle relaxation has been described as beginning cranially and moving caudally, then reversing during recovery.2,22,31 Muscle relaxation in lizards ideally begins mid-body and moves forward and then backwards so tail tone is lost last (Fig 4). 2,10,31 In varanids (monitor lizards) induced with gas anesthesia, muscle relaxation has been described as beginning first in the forelimbs. The hind limbs and neck then lose muscle tone almost simultaneously, followed by loss of the righting reflex. Finally there is loss of tail tone.2 This same pattern has been demonstrated in other lizards administered halothane and sevoflurane and in turtles anesthetized with ether. 1,3,4,24

Evaluation of head withdrawal and neck tone can also prove useful in turtles and tortoises and tongue withdrawal can be evaluated in snakes and varanids.2,22,31 Tongue retraction often persists until the patient enters a deep surgical plane of anesthesia.2,22

The normal or light chelonian will attempt to withdraw its head into the shell. As muscle relaxation increases, the response to head withdrawal decreases until head withdrawal is absent in chelonians at a surgical plane of anesthesia.32

Reflexes

As muscle tone decreases and anesthetic depth increases, reflexes are lost.29 The righting reflex is one of the first reflexes lost in snakes and lizards at a surgical plane of anesthesia (Video 3). 2,26,29,31 The righting reflex is not as useful in turtles and tortoises.
29,31

The palpebral reflex is lost in chelonians and most lizards at a moderate surgical plane of anesthesia and response to vent stimulation is also sluggish.2 The Bauchstreich reflex, in which stroking of ventral scales or scutes stimulates movement of the body wall, is also much reduced at a surgical plane of anesthesia.9

Due to the lack of eyelids and the presence of a spectacle, corneal and palpebral reflexes cannot be evaluated in snakes and most geckos, with one exception being the leopard gecko (Eublepharis macularius).

Video 3. Presence of a brisk, normal righting reflex in a juvenile bearded dragon (Pogona vitticeps). Sluggish to absent righting reflex in first a boa constrictor (Boa constrictor) and then an adult bearded dragon. Video produced by Katie Lennox-Phillibeck; technique demonstrated by Dr. Angela Lennox (no audio).

Response to noxious stimuli

Reptiles can retain the ability to move slightly in response to painful stimuli even after the loss of the righting reflex and tail tone.2,29 Therefore evaluate the patient’s response to pain, using a toe, tail, and/or vent/cloacal pinch, before initiating any invasive procedure.2,19,25,26,29

Nictitans

The third eyelid typically prolapses at a surgical plane of anesthesia.

Table 7. Evaluation of anesthetic depth in the reptile 2,6,9,23
Stage of anesthesiaLight
(Stage 1)
Light
(Stage 2)
Surgical
(Stage 3)
Excessively Deep
(Stage 4)
Limb movementsSlowFew--
Muscle relaxation-±++
Pedal withdrawal reflex,
tail pincha
+±+-
Righting reflex+±--
Corneal reflex*+++-
Head withdrawal**++--
Tongue withdrawal***+Much reduced
+/--
+-
Palpebral reflex*++/---
Vent stimulation reflex++/-Much reduced
+/--
-
a Voluntary response to painful stimuli
* Cannot be evaluated in snakes and some lizards due to the presence of a spectacle.
** Evaluated in chelonians
*** Evaluated in snakes and monitor lizards
- : absent; +/-: sometimes present; +: present

 

Recovery

During recovery the patient should be kept warm.22 There is typically a progressive return of the palpebral reflex, jaw tone, and the swallowing reflex.7 Uncoordinated attempts to move will be observed, and nystagmus may also be seen in mammals. There is generally an increase in both spontaneous respirations and heart rate.7,18,19 Recovery may also be associated with a period of excitement or dysphoria (Video 4).

Video 4. Recovery of a hyacinth macaw (Anodorhynchus hyacinthinus). Note the patient is manually restrained upright. Source: Ryan O’Shea, CVT.

The patient can be extubated once spontaneous breathing is observed and jaw tone and tongue movement have returned.22 Birds will often vocalize as they begin to recover. Due to the anatomic position of the bird’s “voice box” or syrinx, birds can even vocalize while intubated.

Recovery times in reptiles are often prolonged when compared to birds and mammals.2 It is important to maintain the patient at their preferred optimum temperature zone because body temperature affects both the speed and quality of recovery. 2 Manually ventilate the patient every 1-5 minutes with room air using a manual resuscitator (i.e. Ambu-bag) to allow a buildup of carbon dioxide.2 Maintain the patient intubated until consistent, spontaneous ventilation and voluntary movements are observed.32

 

Conclusion

Anesthetic depth in exotic animal patients can be assessed by a variety of factors, including the presence or absence of reflexes, response to noxious stimulation, changes in vital signs, evaluation of muscle tone, eye position (in mammals), and pupil size. Assessment of anesthetic depth is challenging in all animals. Unlike physiologic measures that can be objectively measured, the parameters used to evaluate anesthetic depth are observed on a continuum. There are also specific considerations for evaluation of each taxonomic group. Therefore, this important monitoring skill is frequently considered more of an art than a science that can only grow with time, experience, and careful attention to detail.18

 

Acknowledgement

Thank you to Dr. Graham Zoller, who provided invaluable feedback and advice during the early phases of manuscript development.

 

Appendix 1

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Appendix 2

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References