Anesthesia and analgesia in the exotic patient


Chapter 6
Anesthesia and analgesia in the exotic patient


Amanda M. Shelby and Carolyn M. McKune


Exotic pet owners seek veterinary care when they observe an abnormal behavior in their pets. Often, these animals are not truly domesticated, which may take up to six generations. Therefore, handling for medical procedures often requires anesthesia. The infrequency and inexperience of the veterinary staff with exotic species may contribute to the increase in morbidity and mortality seen in these species [1]. Previous chapters have outlined the preanesthetic recommendations that apply to all species in regard to anesthesia. This chapter is designed to give information regarding differences and suggestions for performing anesthesia and providing analgesia. The reader is referred to additional sources (see end of chapter) for a more extensive review on specific species.


I. Common exotic mammals


Metabolic rate scales allometrically by species; that is, the smaller the species, the greater its metabolic rate. This will influence drug metabolism. Other parameters influenced by a change in metabolic rate include body temperature and oxygen consumption (Table 6.1).


A. Ferrets


1. Characteristics



  • Ferrets are often treated like cats in regard to drug dosing and intubation.
  • If not descented, ferrets may spray scent glands when stressed or threatened.
  • Ferrets have thick skin, requiring confidence when administering a SC injection.
  • Ferrets produce copious respiratory secretions, making inclusion of an anticholinergic a suitable choice for anesthesia when not contraindicated.

2. Anesthetic considerations



  • Handling and restraint: If required, squeeze cages might be helpful in restraining the unfriendly ferret. They have very sharp teeth and inflict severe bites. For the tame ferret, scruffing the nape of the neck and supporting the lower half of the body with the other hand works well for restraint (Figure 6.1).

    Table 6.1 Exotic small mammal vital normal ranges


    Sources: See Further reading.

















































































    Species Adult weights (kg) HR a(bpm) RR a(breaths/min) Temperature (°F) Fasting recommendation
    Ferret 0.7–3.0 200–275 30–35 100–104 6–8 h (unless the patient has a suspected insulinoma)
    Hedgehog 0.3–1.2 200–280 25–50 95–99 Not recommended
    Pot‐bellied pig 40–90 70–120 25–35 101–104 12–24 h
    Rabbits 1–6 130–325 30–60 99–103 2–4 h for food only
    Rats 0.25–0.5 250–450 70–130 99–100 Not recommended
    Mice 0.02–0.04 350–800 60–200 98–100.5 Not recommended
    Hamster 0.085–0.15 250–500 40–140 101–103 Not recommended
    Gerbil 0.05–0.10 250–500 90–150 99–102 Not recommended
    Guinea pig 0.7–1.2 230–380 50–100 101–103 2–4 h
    Chinchilla 0.4–0.6 200–350 45–80 102–103 Not recommended

    a Respiratory rates and heart rates may be lower during anesthesia.


    bpm, beats per minute; RR, respiratory rate.


  • Premedication: Agents that sedate and provide preemptive analgesia help to facilitate catheter placement and reduce induction and maintenance drug requirements; a variety of agents are suitable in the healthy ferret (Table 6.2). The hindlimb muscles are common locations for IM injections; sadly, for these little creatures, the patient is often sore on the limb afterwards and may limp where the injection was given, espe cially if large volumes are given. IV access includes the cephalic, jugular, and saphenous veins (Figure 6.2). It is helpful to nick the skin prior to attempting catheterization. Alfaxalone (5 mg/kg) has proven useful, in combination with butorphanol (0.2 mg/kg), with mild transient cardiorespiratory derangements [2].
  • Induction: If IV access is not achieved under premedication, mask or box inductions with isoflurane or sevoflurane is the most common approach. If IV access is available, IV induction agents are administered (see Table 6.2).
  • Intubation: Intubation is achieved with small endotracheal (ET) tubes similar to intubating a cat. The patient is placed in sternal recumbency with the head and neck hyperextended by an assistant. Some individuals suggest extending the head toward the person intubating while keeping the body straight. Lidocaine is used on the arytenoids to minimize laryngeal spasms. The tongue is pulled gently out of the mouth and a laryngeal scope is used for visualization. Small ET tubes ranging from 2.0 to 3.5 mm internal diameter (ID) (cuffed or noncuffed) are commonly used. Placement of the ET tube is verified by auscultation of lungs bilaterally for respiratory noises on inspiration or with the use of a capnograph.
  • Maintenance: All the modern inhalants are commonly used. Total intravenous anesthesia (TIVA) is also used if IV access is available.
  • Monitoring during anesthesia (see Figure 6.2): An ECG and Doppler are recommended. Blood pressure (BP) is obtained with a cuff and sphygmomanometer. Pulse oximetry is obtained by placing a probe over a foot. Anesthetic depth is monitored similarly to the average small animal patient. Jaw tone, eye position, and palpebral reflex are indicators of depth. Additionally, changes in respiratory rate (RR) and BP may indicate changes in anesthetic depth.
  • Recovery: Keep the patient warm in recovery and continue to monitor vital signs. Because ferrets have such a variety of endocrinologic disease requiring anesthesia and surgery, tailor each recovery to the specific patient. Additional analgesia is administered.
Photo depicts scruff restraint technique of a ferret.

Figure 6.1 Scruff restraint technique of a ferret.


Source: Courtesy of Jody Nugent‐Deal.


Table 6.2 Anesthetic protocol for ferrets.
























ASA 1 or 2 ASA 3, 4, or 5
Premedication

  1. Opioid (select most appropriate) [50]:


    1. Hydromorphone 0.1
      mg/kg IM or SC

    2. Butorphanol 0.4 mg/kg IM or SC
    3. Buprenorphine 0.02 mg/kg IM or SC


  2. +/− Glycopyrrolate 0.01 mg/kg IM or SC
  3. Sedative option (select most appropriate):


    1. Acepromazine 0.01–0.03 mg/kg IM or SC or
    2. Dexmedetomidine 0.003–0.01 mg/kg IM or SC


  1. Opioid (select most appropriate):


    1. Hydromorphone 0.1 mg/kg IM or SC
    2. Methadone 0.1–0.5 mg/kg IM
    3. Buprenorphine 0.02 mg/kg IM or SC


  2. Sedative option:


    1. Midazolam 1 mg/kg SC or IM
Induction Select one:

  1. Alfaxalone 3–5 mg/kg IM or if IV available, 1–3 mg/kg to effect following premed
  2. Propofol 4–6 mg/kg IV
  3. Ketamine 10 mg/kg + midazolam 1 mg/kg IM or IV
  4. Tiletamine‐zolazepam 1.5–3 mg/kg IV
Select one:

  1. Alfaxalone 1–3 mg/kg IV +/− midazolam 1 mg/kg IV
  2. Propofol 2–4 mg/ kg +/−midazolam 1 mg/kg IV
  3. Fentanyl 0.02 mg/ kg + diazepam 1–2 mg/kg IV
Maintenance, Intraoperative analgesia Gas inhalant with continued appropriate analgesia

  1. Local anesthetic block
  2. Additional opioid
Gas inhalant with:

  1. Local anesthetic block
  2. Fentanyl CRI 0.012– 0.042 mg/kg/h
Postoperative analgesic

  1. NSAID [51,52]:

    1. Carprofen 4 mg/kg SC SID or
    2. Meloxicam 0.2 mg/kg SC SID
    AND
  2. Opioid boluses


  1. Opioid boluses

IM, intramuscular; IV, intravenous; SC, subcutaneous; SID, semel in die (once a day).

Photo depicts ferret under anesthesia, intubated. Endotracheal tube secured with suture. Monitoring devices in place with intravenous catheterization and arterial catheter in place.

Figure 6.2 Ferret under anesthesia, intubated. Endotracheal tube secured with suture. Monitoring devices in place with intravenous catheterization and arterial catheter in place.


Source: Courtesy of Jody Nugent‐Deal.


3. Common complications



  • Intraoperative hypothermia: Forced air warming units, circulating hot water blankets, and/or companion animal warming units are helpful to maintain body temperature.
  • Hypoglycemia: Hypoglycemia occurs due to high metabolic rates; 2.5% dextrose is added to the anesthetic maintenance fluids to correct this. The ferret with an insulinoma is a particular challenge in this regard, as it is a balance between providing enough dextrose without triggering release of an insulin surge.

B. Hedgehogs


1. Characteristics



  • Some species of hedgehog (European) hibernate when conditions are extreme (unlikely in the household pet).
  • Nocturnal (i.e., normally inactive during the day and active in the evening).
  • Spines cover the dorsal aspect of body.

2. Anesthetic considerations



  • Handling and restraint: Handling and restraint are difficult, especially if the patient rolls into a ball. Some will unroll if rump spines are stroked. Leather gloves are recommended. While these animals rarely bite, they are reclusive by nature and often resent handling, regardless of the degree of socialization.
  • Premedication: Often patients are masked or boxed down without the use of injectable premedications. However, SC injections are given with the assistance of forceps to grasp a fold of skin between the spines over the flank of the patient. IM injections are given in the thigh. Soreness results if large volumes are administered IM (Table 6.3).
  • IV access: Catheters are placed in the jugular or saphenous vein. Intraosseous (IO) catheters are placed in the proximal femur.
  • Induction: Attempts at intubation are made using a 1–1.5 mm ID ET tube; however, care is taken to avoid trauma to the larynx which causes swelling, bleeding, and airway obstruction. Supraglottic airway devices designed for rabbits have shown promise in reducing trauma to the airway while providing quick access and adequate ventilation as well as delivery of inhalant anesthetics to produce maintenance anesthesia (see Table 6.3).
  • Maintenance: Mask induction and maintenance with an inhalant anesthetic is common clinical practice.
  • Monitoring: A Doppler is vital for monitoring the heart rate (HR) in these animals. Pulse oximetry is useful if the probe is placed on a foot in the unrolled hedgehog. ECG provides information on HR, but most monitors are only capable of displaying up to 250 bpm. RR is monitored by watching chest excursions.
  • Recovery: Recover hedgehogs in a quiet, nonstimulating environment. Keep the patient warm in recovery and continue to monitor vital signs. Additional analgesia is administered as warranted.

Table 6.3 Anesthetic protocol for hedgehog.




















ASA 1 or 2 ASA 3, 4 or 5
Premedication

  1. Opioid (select most appropriate):


    1. Hydromorphone 0.1 mg/kg SC
    2. Butorphanol 0.4 mg/kg SC
    3. Buprenorphine [53] 0.01–0.05 mg/kg SC


  2. Sedative (select most appropriate):


    1. Midazolam 1 mg/kg
    2. Dexmedetomidine 0.03–0.05 mg/kg SC or IM


  3. Injectable anesthetic [54,55]:


    1. Ketamine 10–30 mg/kg SC or IM
    2. Alfaxalone 2–3 mg/kg SC or IM


  1. Opioid (select most appropriate):


    1. Hydromorphone 0.1 mg/kg SC
    2. Methadone 0.3–0.5 mg/kg SC
    3. Buprenorphine 0.02–0.05 mg/kg SC


  2. Sedative:


    1. Midazolam 1 mg/kg SC or IM


  3. Injectable anesthetic:


    1. Alfaxalone 2–3 mg/kg SC or IM
Induction, Maintenance, Intraoperative analgesia Mask/chamber with gas inhalant Mask/chamber with gas inhalant with:

  1. Fentanyl CRI 0.012– 0.042 mg/kg/h if IV or IO access available
Postoperative analgesic

  1. Meloxicam 0.2 mg/kg SC SID and/or
  2. Buprenorphine 0.02 mg/kg SC


  1. Buprenorphine 0.02 mg/kg SC

IM, intramuscular; IO, intraosseous; IV, intravenous; SC, subcutaneous; SID, semel in die (once a day).


3. Common complications


Complications are typically a result of the hedgehog’s high metabolic rate and the anesthetist’s underestimation of the patient’s requirements.



  • Hypoglycemia: Do not fast and administer 2.5% dextrose in fluids for extended procedures.
  • Hypothermia: Hypothermia due to small body size, use of nonrebreathing (NRB) circuits, and decreased metabolism is common. Esophageal temperature probes for continuous monitoring and forced air warming units are helpful.

C. Pot‐bellied pigs (PBPs)


1. Characteristics



  • PBPs have small tracheas compared to body weight; additionally, there is a sigmoid curvature to the airway and a ventral diverticulum anteroventral to the arytenoid fold preventing a straightforward intubation.
  • PBPs have a high fat to muscle ratio (anatomically present as a thick layer of subcutaneous fat), making the effect of drug injections less predictable and intra‐fat (rather than IM) injection more likely.
  • Pigs are cannibalistic.
  • Twelve‐ to 24‐hour fasting of adult pigs is recommended.
  • Difficulty accessing veins means most anesthetic plans are developed with minimal biochemical and hematologic information.

2. Anesthesia considerations



  • Handling and restraint: Pigs are difficult to restrain due to their shape and size. Often chemical restraint is used to avoid stress. Large pigs are controlled with a board or cage door front “squeezed” against a wall while an IM injection is given, or snared just behind the canine teeth. Small pigs are manually restrained (Figure 6.3).
  • Premedication: Behind the ear is the most common site for IM injections. In young PBPs, there is less fat in this area so the effectiveness of the drugs is more predictable. The epaxial muscles are avoided because of the large layer of back fat and the tradition of avoiding this muscle group in pigs for market. Injection in the semimembranosus and semitendinosus muscles with a 3.5 inch stylet from a catheter is a possible alternative. Intranasal midazolam 0.5 mg/kg may quiet the pig to a degree and facilitate limited handling. In general, pigs are less susceptible to the sedative effects of opioids, phenothiazines, and alpha‐2 agonists. Butyrophenones, such as azaperone (2.5 mg/kg IM), are often used for sedation in noncompanion pigs and are potentially useful in the PBP. Alfaxalone has proved useful in commercial swine, with or without the use of an alpha‐2 agonist, and may be useful in the PBP [3,4] (Table 6.4).
  • IV access: Veins are difficult to access. The easiest vessels for catheterization include the medial and lateral auricular veins (Figure 6.4) and lateral saphenous veins, which are typically catheterized following induction. Cephalic veins are occasionally attempted, but venipuncture in this area is performed blindly, as these vessels are difficult to palpate and visualize.
  • Induction: If IV access is obtained following premedication, injectable agents are given to facilitate intubation. Otherwise, mask induction with isoflurane or sevoflurane is most common until the patient is intubated (see Table 6.4).
  • Intubation: Intubation in pigs is a challenge for a number of reasons. The mouth does not open widely, and the larynx is at an angle to the trachea. Pigs have a pharyngeal diverticulum, as well as a tracheal bronchus; inaccurate placement of the tube in either of these areas results in trauma, swelling, or bleeding. Stylets or ET tubes inadvertently placed in the tracheal bronchus cause pneumomediastinum and inadequate ventilation. Cuffed ET tubes of various sizes (smaller than for an equivalent‐sized canine patient) are used; for an average 60 kg PBP, an 8–9 mm ID ET tube is used. Place the patient in sternal recumbency. Gauze or dog leashes facilitate visualization, when placed behind the canine teeth and used to open the mouth. A long blade laryngoscope is helpful in visualizing the larynx by placing the blade at the base of the tongue and pressing down to displace the epiglottis. Lidocaine is placed on the arytenoids to minimize laryngeal spasms, which readily occur. Introduce a long, thin stylet (crafted from small French urinary catheters connected together with tape or glue, approximately three times the length of the ET tube) into the trachea to assist intubation. If no resistance is encountered, an ET tube is passed over the stylet and gently rotated 180° once it passes beyond the larynx. Laryngeal mask airways are used with success in pigs and may reduce the potential for airway trauma. Auscultation and capnography confirm appropriate placement. If resistance is encountered, rotating the stylet gently while advancing it forward may help; however, significant resistance warrants backing out of the trachea and trying again. Intubation attempts are minimized to prevent trauma.
  • Maintenance: Inhalant anesthetics are most commonly used following intubation. IM injectable drugs, in combination with local blocks, facilitate adequate sedation and immobilization for minor procedures like castration, hoof trim, or tusk filing.
  • Monitoring anesthetic depth: Palpebral reflex and jaw tone will give an indication of the depth of anesthesia. PBPs have small eyes, making ocular signs of anesthetic depth difficult to obtain. Vital parameters may also indicate changes in depth. An ECG and Doppler are recommended for monitoring the HR and rhythm. Pigs have little excess skin, so using sticky pads to monitor ECG is preferable to ECG clips. The Doppler crystal, BP cuff, and a sphygmomanometer give an indication of trends. The Doppler is often placed distal to the accessory claw, with the cuff below the carpus on the forelimb, or on the hindlimb above the hock. Invasive blood pressure (IBP) is obtained from placing a catheter in the artery of the ear or dorsal pedal artery. It is also useful to monitor RR and EtCO2. Pulse oximetry is obtained by placing a pulse oximeter probe on the tongue. Temperature is continuously monitored during anesthesia and into recovery.
  • Recovery: Pigs are recovered without herd mates and only returned to the herd once completely recovered. They are placed in sternal recumbency for recovery, if possible. Pigs may obstruct post extubation due to laryngeal swelling. It is important to ensure they are capable of swallowing and adequate independent ventilation is present prior to extubation. Nonsteroidal antiinflammatory drugs (NSAIDs) are used, if not otherwise contraindicated, to minimize any airway swelling. Pigs experience postanesthetic sleep; although they appear conscious enough to extubate, once extubated, they sleep quite heavily, which contributes to the possibility of postextubation obstruction. Temperature is monitored during and following recovery. Additional analgesic drugs are administered as warranted.
Photo depicts PBP restraint and IM injection behind the ear.

Figure 6.3 PBP restraint and IM injection behind the ear.


Source:Courtesy of Amanda M. Shelby.


Table 6.4 Anesthetic and analgesic drugs for pot‐bellied pig.




























ASA 1 or 2 ASA 3, 4 or 5
Premedication Opioid (select most appropriate):

  1. Hydromorphone 0.1 mg/kg IM
  2. Methadone 0.1–0.5 mg/kg IM
  3. Buprenorphine 0.03 mg/kg IM
  4. Butorphanol 0.2–0.4 mg/kg IM

Sedative (select one):

  1. Dexmedetomidine 5–10 μg/kg IM
  2. Midazolam 0.2–0.5 mg/kg IM, intranasal

Injectable anesthetic (select one):

  1. Telazol 2–3 mg/kg IM
  2. Ketamine 7.5–10 mg/kg IM
  3. Alfaxalone 3–5 mg/kg IM
Opioid (select most appropriate):

  1. Hydromorphone 0.1 mg/kg IM
  2. Methadone 0.1–0.5 mg/kg IM
  3. Buprenorphine 0.03 mg/kg IM
  4. Butorphanol 0.2–0.4 mg/kg IM

Sedative (select one):

  1. Midazolam 0.2–0.5 mg/kg IM, intranasal

Injectable anesthetic:

  1. Alfaxalone 3–5 mg/kg IM
Induction Mask until intubation Alfaxalone 1–2 mg/kg IV to effect
Maintenance Gas inhalant Gas inhalant
Intraoperative analgesia

  1. Local blocks where possible
  2. Opioid bolus or CRI


  1. Local blocks where possible
  2. Opioid bolus or CRI
Postoperative analgesia Flunixin 0.5–1.0 mg/kg SC or IV
and
Opioid (select most appropriate):

  1. Buprenorphine 0.01–0.05 mg/kg IV or IM q 8 h
  2. Methadone 0.1–0.5 mg/kg IM q 4–6 h
Opioid (select most appropriate):

  1. Fentanyl patch 50–100 μg/h
  2. Buprenorphine 0.01–0.05 mg/kg IV or IM q 8 h
  3. Methadone 0.1–0.5 mg/kg IM q 4–6 h

CRI, constant‐rate infusion; IM, intramuscular; IV, intravenous.

Photo depicts IV access in the ear of a PBP.

Figure 6.4 IV access in the ear of a PBP.


Source:Courtesy of Patricia Queiroz‐Williams.


3. Common complications



  • Airway obstruction at extubation is possible. The anesthetist must be ready with additional induction agent (IV catheter remains in place until the patient is fully recovered) and a size smaller ET tube to intubate the patient if an obstruction occurs.
  • Hypoventilation, if not from excessive anesthetic depth, is often addressed with mechanical ventilation (MV).
  • Hypothermia (especially in in small pigs) occurs intraoperatively and postoperatively. Use of circulating warm water blankets and forced air warming units reduces the degree of hypothermia.
  • Dysphoric or rough recoveries are possible, especially in stimulating environments and with inadequate analgesia and sedation.

D. Rabbits


1. Characteristics



  • Rabbits are obligate nasal breathers (require patency of the nares and nasopharynx).
  • It is common for rabbits to have inconspicuous respiratory infections; as a prey species, they are remarkably good at hiding signs of illness and pain.
  • Rabbits do not regurgitate or vomit.
  • Some rabbits have circulating levels of atropinesterase [5], which makes the use of atropine less effective. Glycopyrrolate is a suitable alternative.
  • Rabbits have extremely powerful kick strength; their kick is strong enough to fracture their own spine if they are not properly restrained.
  • Because rabbits are a prey species, they are remarkably efficient at hiding signs of pain and distress. Preemptive and appropriate postoperative analgesia is administered to these animals without expecting a behavioral qualification of pain.

2. Anesthesia considerations



  • Handling and restraint (Figure 6.5): Rabbits are very easily stressed; it is wise to handle them in a quiet, dim environment to minimize stress until they are anesthetized. One method of restraint is to cradle the rabbit in one arm with the head tucked into the body of the handler. Alternatively, the handler grasps the patient by the scruff and supports the patient’s hind end. Do not pick up by the ears!
  • Premedication: SC injections are an effective technique for premedication, and preferred, if possible, to IM injections. IM injections are administered in the muscular bodies on either side of the spine or hindlimb. Squeeze boxes work nicely for IM injections in feral rabbits. Intranasal administration for soluble drugs, such as midazolam, is an alternative to a needle. While a variety of injectable agents are useful in rabbits, alfaxalone alone or in combination with an opioid and/or an alpha‐2 agonist such as dexmedetomidine is useful to achieve moderate (alfaxalone alone) to deep sedation (combination) in rabbits [68].
  • Induction: Because rabbits become stressed and excited, it is recommended that they are sedated prior to boxing down with a gas anesthetic when IV induction cannot be performed.
  • Intubation: Intubation is a precarious maneuver in the rabbit (Figure 6.6). For an average size rabbit, a 2–2.5 mm uncuffed ET tube or supraglottic airway device (SGAD) such as a v‐gel® is used to secure an airway [9]. A SGAD may result in a quicker and less traumatic intubation [10]; if rabbits are a significant part of one’s practice, investing in these tubes is appropriate. Traditional intubation is only attempted a maximum of two or three times. Traumatic or repetitive intubation attempts cause laryngeal swelling and airway obstruction after extubation. See Table 6.5 for intubation techniques.
  • Vascular access: Catheterization is performed when patient allows, often after induction. When clipping a site for catheterization, use gentle technique as the rabbit’s thin skin is easily traumatized. The cephalic vein is suitable for venous access, and may reduce the possibility of damage to the ear. The lateral saphenous vein is accessible as well. IO catheters are placed in the proximal femur, tibia, or humerus. In an emergency, the auricular artery is used to provide arterial access; however, this may result in sloughing of the ear due to reduced perfusion. Therefore, routine catheterization of the auricular artery is not advised.
  • Maintenance: The patient is typically maintained on gas inhalants, but a TIVA technique is possible if surgeon access to the airway is required and a catheter is in place (Table 6.6). Certain drug combinations, such as alfaxalone with midazolam and dexmedetomidine for premedication, may result in hypoxemia and hypoventilation; capnography and ventilatory support are advised [11].
  • Monitoring: An ECG will display the patient’s cardiac rhythm and HR; however, most monitors only read up to 250 bpm. A Doppler over the radial artery or directly over the heart works well to confirm mechanical (not just electrical) function of the heart. The forelimb is also a useful site for BP monitoring with a sphygmomanometer and Doppler crystal. Fluctuation in the volume of the Doppler may indicate changes in BP. Obtain RR by watching the chest rise and fall; if the patient is intubated, ETCO2 confirms cardiac output and gives information about ventilation. Esophageal temperature probes are recommended for continuous monitoring of core body temperature.
  • Monitoring anesthetic depth: Most reflexes are absent at a surgical plane of anesthesia but the palpebral reflex is maintained even at a deep anesthetic plane. A light plane of anesthesia is identified by response to a toe pinch, ear twitch, or surgical incision. A rabbit that is at a light plane of anesthesia may shake its head in response to painful stimuli. A deep plane of anesthesia is identified by fixed dilated pupils unresponsive to light with no corneal reflex, depressed ventilation, and minimal response to surgical stimulus.
  • Recovery: Rabbits are recovered in a quiet, dim environment to prevent startling and kicking at arousal. Additional analgesia is administered as warranted.
Photo depicts (a–c) Restraint techniques for rabbits.

Figure 6.5 (a–c) Restraint techniques for rabbits.


Source:Courtesy of Jody Nugent‐Deal.

Photo depicts blind technique for intubation in a rabbit.

Figure 6.6 Blind technique for intubation in a rabbit.


Source:Courtesy of Jody Nugent‐Deal.


Table 6.5 Blind intubation technique in rabbits.



















Materials:Appropriately sized endotracheal tube (2–4 mm ID)
Blind technique:


  1. The patient is anesthetized adequately with a regular respiratory pattern.


  1. Maintain the patient in sternal recumbency with the head and neck hyperextended. It is best if the patient is grasped from behind the head with the holder’s thumb and forefinger by the ramus of the mandible. Nose is essentially directed toward the ceiling.


  1. Place a lidocaine drop on the arytenoids to decrease spasms.


  1. Advance the ET tube over the base of the tongue.


  1. The anesthetist places their ear near the end of the ET tube and advances toward the larynx until respiratory noises are the loudest. Advance the ET tube into the trachea during the next breath; it is helpful to rotate the tube as you advance to pass between the arytenoids. Alternatively, the capnograph adaptor is placed on the end of the ET tube. When EtCO2 is seen on the capnograph, advance the ET tube into the trachea during the next breath.


  1. If no respiratory noise is ausculted with administration of a breath, the ET tube is likely in the esophagus and should be withdrawn.

ET, endotracheal; ID, internal diameter.


Table 6.6 Anesthetic protocol for rabbits.




























ASA 1 or 2 ASA 3, 4 or 5
Premedication

  1. Opioid (select most appropriate):


    1. Hydromorphone 0.2 mg/kg IM
    2. Butorphanol 0.4 mg/kg IM
    3. Buprenorphine 0.03 mg/kg IM


  2. Sedative (select most appropriate):


    1. Midazolam 1 mg/kg IM, SC, or intranasal
    2. Acepromazine 0.025–0.05 mg/kg IM or SC
    3. Dexmedetomidine 0.1–0.2 mg/kg IM or SC


  3. Injectable anesthetic (select most appropriate):


    1. Alfaxalone 2.5–5 mg/kg IM
    2. Ketamine 5–10 mg/kg IM


  1. Opioid (select most appropriate):


    1. Hydromorphone 0.2 mg/kg IM
    2. Butorphanol 0.4 mg/kg IM
    3. Buprenorphine 0.03 mg/kg IM


  2. Sedative:


    1. Midazolam 0.5–1 mg/kg IM or SC or intranasal


  3. Injectable anesthetic (consider if desired):


    1. Alfaxalone 2.5–5 mg/kg IM
Induction

  1. Injectable induction (choose one):


    1. Alfaxalone 1–3 mg/kg IV to effect
    2. Propofol 2–6 mg/kg IV to effect

  2. Mask/chamber with gas inhalant (author strongly opposes inhalant inductions when avoidable)
Injectable induction (choose one):

  1. Alfaxalone 1–3 mg/kg IV to effect
  2. Propofol 2–6 mg/kg IV to effect
Maintenance Gas inhalant Gas inhalant
Intraoperative analgesia

  1. Local blocks where applicable
  2. Opioid bolus or CRI


  1. Local blocks where applicable
  2. Fentanyl CRI 0.010–0.06 mg/kg/h
Postoperative analgesia

  1. NSAID (choose one):


    1. Flunixin 0.3–1.0 mg/kg SC or IV SID
    2. Meloxicam 1.0 mg/kg PO SID


  2. Opioid (choose most appropriate):


    1. Buprenorphine 0.05–0.3 mg/kg SC q 8 h
    2. Hydromorphone 0.2 mg/kg IM q 4 h
    3. Methadone 0.1–0.3 mg/kg IM q 4 h
Opioid (choose most appropriate):

  1. Buprenorphine 0.05–0.3 mg/kg SC q 8 h
  2. Hydromorphone 0.2 mg/kg
    IM q 4 h
  3. Methadone 0.1–0.3 mg/kg IM q 4 h

CRI, constant‐rate infusion; IM, intramuscular; I, intravenous; PO, per os (by mouth); SC, subcutaneous; SID, semel in die (once a day).


3. Common complications



  • Rabbits are very susceptible to corneal ulcers. The eyes cannot be lubricated enough; constant lubrication is recommended. Alternatively, some anesthetists tape the eyes shut.
  • Fracture of the spine during improper restraint or boxing down occurs. Avoid boxing down the nonsedated rabbit.
  • Hyperthermia is common during restraint and handling due to stress. Try to minimize stress during the preanesthetic and recovery phase.
  • Hypothermia is common during anesthesia. The use of plastic surgical drapes, forced air warming units, and circulating warm water blankets is helpful in maintaining body temperature.
  • The thoracic cavity of the rabbit is quite small compared with the large abdominal cavity, thus increasing the chance of hypoventilation. Intermittent hand ventilation is recommended.
  • Gastrointestinal complications noticed by decreased stool production and stasis commonly occur in rabbits post anesthetic procedures [12].

E. Rodents (chinchillas, gerbils, guinea pigs, hamsters, mice, rats)


1. Characteristics



  • Many rodents are obligate nasal breathers.
  • Undetected respiratory disease is common in rodents.
  • Some species, such as hamsters, are nocturnal.
  • Hamsters and guinea pigs have cheek pouches, for storage of food particles, which may result in aspiration pneumonia in the sedated or anesthetized animal.
  • Rodents are prone to self‐mutilation. Avoid giving IM injections especially with irritating solutions or in large volumes.
  • Several species of rodent are capable of shedding their tails if picked up.
  • Guinea pigs have a palatal ostium, which makes traditional intubation of this species virtually impossible [13].
  • Some species, such as the rat, mouse, and gerbil, do not vomit.

2. Anesthetic considerations



  • Handling and restraint: Grasping the tail of rodents is avoided. If the rodent is tame, simply cupping it in the palm of a hand is ideal. Grasping the nape or scruff of the neck is the most acceptable means of restraint, especially when giving injections.
  • Premedication: A variety of premedicants are suitable for rodents; in addition to the SC and IM routes, some medications are administered intraperitoneally (IP). Administration of drugs IP takes practice, so it is advisable for only experienced personnel to use this route (Table 6.7). The muscle bodies for IM administration are small, and certain drugs may irritate tissues, so SC injection is often preferable. Generally speaking, the larger rodents (i.e., guinea pig) will require the lower dose range of agents listed in Table 6.8, while small rodents (i.e., mouse) will require the higher end of the dosage range.
  • Induction: Usually, rodents are masked or boxed down in a small enclosure with inhalants (Figure 6.7). The patient is removed from the box when the “righting” reflex is absent and a facemask is placed until the patient is relaxed. Alternative options include injectable anesthetic such as alfaxalone and ketamine administered by a variety of routes and in combination with opioids and sedatives to produce a balanced, effective anesthetic plan [14,15].
  • Intubation: Larger rodents are intubated, but care is taken during the intubation process to avoid trauma, which may lead to swelling and subsequent airway obstruction. Minimal attempts are made, especially in small rodents. Rats are intubated with the assistance of a modified #2 otoscope ear speculum to allow visualization of the larynx. Lidocaine on the arytenoids will help minimize laryngeal spasms. Catheters (16–20 G) work well in place of ET tubes.
  • IV access: Catheterization in large rodents is accomplished in the cephalic, the tail (in rats), or saphenous veins. IO catheters, if used, are commonly placed in the proximal femur, tibia, or humerus.
  • Maintenance: Typically, gas inhalants are used to maintain anesthesia in nonlaboratory settings.
  • Monitoring during anesthesia.


  • – Vitals: HR is monitored with a Doppler over an artery or directly over the heart. ECG may be of limited use, as most monitors only read up to 250 bpm. In small rodents, a wire suture is placed in the skin of the animal to clip ECG leads to. Capnography is often inaccurate in small rodents but may assist with confirmation for placement of the ET tube. Instead, monitor chest wall excursions visually to obtain RR.
  • – Anesthetic depth: Trends in HR and RR assist with assessment of depth of anesthesia. Responses to toe pinch, tail pinch, or skin incision all suggest too light a plane of anesthesia for surgery. A guinea pig may shake its head in response to surgical stimulus.
  • – Recovery: Continued thermal support is important in the recovery phase, as is oxygen supplementation and additional analgesic drugs as warranted.

Table 6.7 Intraperitoneal injection in rodents.





Materials: Second person to restrain, 25 G needle, drug for administration
Technique:

  1. It is helpful for an additional team member to restrain the animal. This individual inverts the rodent with the head down, to displace abdominal viscera.
  2. Enter the caudal quadrant of the abdomen at a 20° angle. The right caudal quadrant is used for most small rodents. The exception is the rat, in which the left caudal quadrant is used.
  3. Aspirate prior to drug administration.
  4. Inject drug; maximum volume is 1–3 mL.

Table 6.8 Anesthetic protocols for rodents.a
























ASA 1 or 2 ASA 3, 4 or 5
Premedication

  1. Opioid (select most appropriate):


    1. Buprenorphine 0.05–0.5 mg/kg SC
    2. Butorphanol 0.5–5 mg/kg SC


  2. Sedative (select one):


    1. Midazolam 0.5–2 mg/kg SC
    2. Acepromazine 0.5–5.0 mg/kg SC


  3. Anticholinergic (select one):


    1. Atropine 0.05 mg/kg SC
    2. Glycopyrrolate 0.01–0.02 mg/kg SC


  4. Injectable anesthetic (select one):


    1. Ketamine 5–10 mg/kg SC
    2. Alfaxalone 2–20 mg/kg SC, IM


  1. Opioid (select most appropriate): Buprenorphine 0.05–0.5 mg/kg SC
  2. Morphine 2–5 mg/kg SC
  3. Sedative option:


    1. Midazolam 0.5–1 mg/kg SC


  4. Anticholinergic (select one):


    1. Atropine 0.05 mg/kg SC
    2. Glycopyrrolate 0.01–0.02 mg/kg SC
Induction Mask/chamber with gas inhalants Mask/chamber with gas inhalants
Maintenance Gas inhalants Gas inhalants
Postoperative analgesia

  1. Carprofen 4–10 mg/kg SC SID
  2. Buprenorphine 0.05 mg/kg SC
    q 6–8 h


  1. Opioid (select most appropriate):


    1. Buprenorphine 0.05 mg/kg
      SC q 6–8 h

    2. Morphine 2–5 mg/kg SC q 4 h

a great variation in dosing exists between species of rodents.


IM, intramuscular; SC, subcutaneous; SID, semel in die (once a day).

Photo depicts masking a rodent.

Figure 6.7 Masking a rodent.


Source: Courtesy of Anderson da Cunha.


3. Common complications



  • Hypoxemia may result because an airway is difficult to secure in several rodent species. Oxygenation by mask is a reasonable part of the plan.
  • Hypoglycemia occurs in small rodent species that are used to free choice availability of food. Unfortunately, lack of venous access for administration of 2.5–5% dextrose complicates this. Therefore, preemptive steps, such as access to food before and immediately after anesthesia, are crucial to preventing hypoglycemia.
  • Hypothermia occurs due to large surface area to body mass ratio, use of NRB circuits, and depressed metabolism. External heat support is necessary.
  • Respiratory obstruction results from traumatic intubation; in some cases, it is preferable to use a mask as opposed to attempting intubation. Copious respiratory secretions may also obstruct the ET tube; look for increased respiratory effort, which may indicate the ET tube is obstructed.

II. Avians


1. Characteristics



  • Birds have a highly efficient respiratory system compared to mammals. Birds have higher VT, lower RR, and larger minute ventilation. Inspiration and expiration are both active phases in the bird; that is, respiratory movements are accomplished by cervical, thoracic, and abdominal muscle contractions (there is no diaphragm to contribute to the active phase). Two complete respiratory cycles exchange inhaled gases via cross‐current gas exchange.
  • The trachea is longer and has an increased diameter compared to mammals, resulting in more dead space. Unlike mammals, the tracheal rings are complete. For this reason, the anesthetist utilizes uncuffed ET tubes. It is wise to review the specific species with regard to variations in respiratory anatomy; for example, some penguins have a double trachea and emus have a tracheal diverticulum.
  • The epiglottis is absent.
  • Birds have high metabolic rates which require higher substrate levels (blood glucose [BG] is greater than 200 mg/dL) and higher fluid rates (10–30 mL/kg/h).
  • Cardiovascular differences include larger stroke volume (SV), increased cardiac output (CO), and higher mean arterial pressure (MAP). HR varies considerably depending on the size of the patient.
  • Circulation influences the effect of various drugs. Birds possess a renal portal system. This network of vessels around the kidney, which selectively channels blood through or past it, results in a potential first‐pass effect of drugs administered in the caudal half of the body, altering the drug’s intended effect. When a drug is given in the lower half of the body, it is metabolized by the renal portal system before reaching the central nervous system, the target site for effect.
  • Fasting is highly variable depending on the size of the bird; in general, fasting is usually less than 3 hours, but may not be performed at all for very small birds.

2. Anesthetic considerations



  • Handling and restraint: It is important to minimize stress while restraining birds. Ideally, a bird is allowed to acclimatize to the hospital environment before handling. In an emergency situation, acclimatization is not possible. Covering the eyes of the patient with a hood or towel helps to reduce stress. For smaller birds, utilize the hand technique: place the head gently between the thumb and forefinger with the body supported in the palm.
  • Preanesthetic assessment: The physical examination (PE) begins before the bird is removed from its housing. Visually assess RR and effort prior to handling. Look at the bird’s posture and whether the feathers are “fluffed.” When the bird is removed from its surroundings, a hands‐on examination of the bird commences. This includes ausculting air sacs ventrally, lungs dorsally, and over the trachea for abnormal airway sounds. The heart is ausculted as well. Obtaining an accurate body weight is key to drug dosages and fluid rates; palpation of the keel gives an indication of body condition of the bird to know whether this weight is appropriate for the patient. Color of the urate is helpful in suggesting liver dysfunction when preoperative bloodwork (BW) is unavailable. Hydration status is assessed by moistness of the cloaca, ocular mucous membranes, and elasticity of skin. Sunken eyes and cold extremities suggest dehydration and/or shock.
  • Premedication: Anticholinergics are controversial for use in the bird; while an increase in HR is desirable, increased thickness of respiratory secretions increases the chance of ET tube occlusion. Additionally, some herbivorous birds may possess atropinesterases. Injectable premedication with sedatives, analgesics, and anesthetics will minimize stress during induction (Table 6.9). The pectoral muscles are utilized for IM injections (Figure 6.8), although consideration is given to a captive bird that may be released, as soreness may result in limited flight activity. However, when administered into the lower half of the body, the drugs pass through the portal venous system, which may result in altered drug metabolism.
  • Induction: Preoxygenate if possible. The most common means of induction is masking the patient down with an inhalant (Figure 6.9). In the small avian, the mask is made with a syringe casing so the whole head of the patient fits in the “mask.” An examination glove with a hole cut in the end is used as a diaphragm to create a sealed fit. The patient’s HR and RR are monitored closely during induction because there is a tendency to achieve an excessive depth resulting in hypoventilation and bradycardia. Injectable agents are used if an IV catheter is available. Vessels suitable for catheterization include the cutaneous ulnar or basillic vein, medial metatarsal, or jugular vein (Figure 6.10). Preference is given to the upper half of the body due to the circulatory pattern and its influence on drug metabolism. IO catheters are placed in the proximal ulna or cranial tibiotarsus; other bones are pneumatic.

    Alfaxalone and its favorable IM route is gaining attention for avian anesthesia. Several species, including parrots, lovebirds and budgerigars, exhibit sedation after an IM injection with alfaxalone [1619] and, indeed, alfaxalone may prove a suitable alternative to other sedatives or isoflurane for minimally noxious procedures [20,21]. If only alfaxalone is used for induction, muscle tremors and hyperexcitation may occur during induction, Premedication with midazolam may help alleviate that [18]. Dosage of alfaxalone for effect varies considerably by the breed of bird, so the reader is referred to species‐specific dosing when possible.


  • Intubation: The glottis is readily visible for intubation (Figure 6.11). Uncuffed ET tubes are used.
  • Maintenance: Inhalants in oxygen or TIVA are the most common techniques.
  • Analgesia: Local techniques are used when possible. Toxic doses vary among species and between local anesthetics [22]. Eutectic mixture of local anesthetic (EMLA) cream is used but is limited to small areas. In some species of birds, there is a prevalence of kappa receptors, making kappa agonists such as butorphanol or nalbuphine suitable analgesics in birds. However, there is conflicting information about pure mu agonists, such as hydromorphone or tramadol, altering a bird’s response to nociception [2326]. The reader will note that a variety of opioids are listed in Table 6.9. Some newer data suggest that some raptorial species of birds may have more mu receptors compared to kappa receptors [27] and thus a partial or full mu opioid may be more appropriate for these species [28]. Most studies in parrots and chickens show that full mu opioids are likely not as effective for analgesia [26,29], but may work for sedation [30] or maintenance agent reduction [3134]. With a huge number of species to evaluate, it is not possible to create a blanket statement on which opioid is most appropriate for all avian species, and the reader is encouraged to look for studies that pertain to the species of bird they most often encounter.
  • Monitoring suggestions: A Doppler is considered essential monitoring for assessing HR; while an ECG is useful, many ECG units only count up to 250 bpm. Additionally, the thin skin of the bird makes ECG clips unsuitable for use, so a small wire suture or needle is placed in the skin as an alternative site for clips. An esophageal stethoscope is a suitable means of HR monitoring as well. Use of a Doppler and sphygmomanometer, with a BP cuff, allows the anesthetist to obtain BP trends. Additionally, in large birds, arterial access is possible. The brachial or carotid arteries are accessible; the use of EMLA cream may facilitate arterial line placement. Respiration is monitored. Ideally, there are minimal respiratory noises in the intubated patient, although the intubated bird, unlike mammals, has the ability to vocalize due to the caudal location of the syrinx. Apnea for a period greater than 10–20 seconds warrants a manual breath; most birds benefit from MV under anesthesia. Capnography is helpful in assessing quality of ventilation and confirming that perfusion of the lungs is occurring; however, birds are one of the few animals where the EtCO2 is possibly greater than PaCO2. The benefits are considered in light of the possible increase in dead space or oversampling of a capnograph adapter. Pulse oximeters are not accurate in birds, due to the difference in avian hemoglobin. A cloacal or esophageal temperature is monitored to guide thermoregulatory management.
  • Monitoring anesthetic depth: Reaction to toe pinch, cere pinch, cloacal pinch, or feather plucking, and increased jaw tone are all indications of a light plane of anesthesia. Muscle tone, such as neck or wing tone, may give an indication of depth as well. Slight palpebral and corneal reflexes are typical in the surgical plane of anesthesia. Decreases in HR, BP, RR, and apnea are indications of a deep plane of anesthesia.
  • Recovery: Indications of recovery include wing and leg movement. Jaw tone returns as anesthetic depth decreases and the patient is extubated when this occurs. Continued oxygen supplementation is administered after extubation. Additional analgesics are administered as warranted.

Table 6.9 Common anesthetic and analgesic protocols in birds.








































ASA 1 or 2 ASA 3, 4 or 5
Premedication

  1. Butorphanol 1.0–2.5 mg/kg IM [56,57] (chickens, parrots)
    OR

  2. Methadone 6 mg/kg IM [58] (chickens)
    OR
  3. Concentrated buprenorphine 0.3 mg/kg, SC (raptors) [59]


  1. Butorphanol 1.0–2.0 mg/kg IM [57] (chickens, parrots)
    OR
  2. Fentanyl 20–30 μg/kg [60] (raptors)
+/−
Midazolam 1–2 mg/kg IM
+/−
3. Midazolam 1 mg/kg IM
+/−
Ketamine 10–20 mg/kg IM
+/−
4. Alfaxalone 10 mg/kg IM
OR
Alfaxalone 10–20 mg/kg IM
Induction Mask with gas inhalant Mask with gas inhalant
OR
If IV access present, alfaxalone 2 mg/kg IV to effect
Maintenance Gas inhalant Gas inhalant
Alfaxalone for short procedures
Intraoperative analgesia

  1. Local blocks where appropriate
    AND

  2. Butorphanol boluses (chickens, parrots)
    OR
  3. Methadone boluses 6 mg/kg IM [58] (chickens)


  1. Local blocks where appropriate
    AND

  2. Butorphanol CRI 0.1–0.4 mg/kg/h following loading dose premedication (chickens, parrots)
    OR

  3. Fentanyl CRI 30 μg/kg/h following loading dose [61,62] (raptors)
    OR
  4. Methadone 6 mg/kg IM [58] (chickens)
Postoperative analgesia

  1. Butorphanol 1.0 mg/kg IV or IM (chickens, parrots)
    OR

  2. Concentrated buprenorphine 0.3 mg/kg, SC (raptors) [59]
    AND

  3. Carprofen 1 mg/kg IM SID
    OR
  4. Meloxicam 1.0 mg/kg PO SID


  1. Butorphanol 1.0 mg/kg IV or IM (chickens, parrots)
    OR
  2. Concentrated buprenorphine 0.3 mg/kg, SC (raptors) [59]

CRI, constant‐rate infusion; IM, intramuscular; IV, intravenous; SC, subcutaneous; SID, semel in die (once a day).

Photo depicts IM injection in the pectoral muscle of a bird.

Figure 6.8 IM injection in the pectoral muscle of a bird.


Source: Courtesy of Anderson da Cunha.

Photo depicts masking a parrot.

Figure 6.9 Masking a parrot.


Source: Courtesy of Anderson da Cunha.

Photo depicts brachial vein for IV catheterization in a bird.

Figure 6.10 Cutaneous ulnar or basillic vein for IV catheterization in a bird.


Source: Courtesy of Anderson da Cunha.

Photo depicts avian larynx.

Figure 6.11 Avian larynx.


Source:Courtesy of Anderson da Cunha.


3. Common complications



  • Hypothermia occurs due to large surface area to body mass ratio, use of NRB circuits, and depressed metabolism. External heat support is provided.
  • ET tube occlusions secondary to mucus plugs are extraordinarily common in birds; any resistance to administering a breath or difficulty ventilating a patient warrants an exchange of ET tube due to the possibility of occlusion.
  • Hypoventilation which is prevalent in this species often necessitates MV.

III. Reptiles


1. Characteristics



  • The heart has three chambers (two atria, one ventricle). This allows for mixing of oxygenated and deoxygenated blood with changes in vascular resistance under anesthesia. This “shunting” significantly worsens perfusion of vital tissues.
  • Reptiles are ectothermic; therefore, body temperature influences drug metabolism and clearance, as well as physiologic parameters.
  • Prior to anesthetizing a reptile, the reader is advised to review the individual species variations in regard to respiratory anatomy and physiology, as this is beyond the scope of this chapter. For example, many reptiles do not possess a diaphragm. Instead, abdominal, pectoral, and pelvic muscles control ventilation. However, crocodilians have a nonmuscular tissue which functions as a rudimentary diaphragm. Most snakes possess a single elongated right lung, although boas possess the left lung lobe in addition to the right. Most reptiles have simplified sacs with folds allowing for respiratory gas exchange, but there is a whole spectrum of complexity. The trachea in a turtle is short and bifurcates early compared with most reptiles. Some reptiles, such as turtles, possess complete tracheal rings.
  • A renal portal system is present in reptiles as it is in birds. This network of vessels around the kidney, which selectively channels blood through or past it, results in a potential first‐pass effect of drugs administered in the caudal half of the body, altering the drug’s intended effects.
  • Although fasting is often not required, timing of last large meal (especially for snakes) is important to know. A recent large meal affects the patient’s ability to adequately ventilate.

2. Anesthesia considerations



  • Restraint and handling: This varies greatly by species restrained. All reptiles are capable of biting; many have claws and tails that are used in defense. Leather gloves are useful in preventing trauma to the handler for large reptile species. Nonpoisonous snakes are grasped at the base of the head with one hand while the other hand supports the body. Lizards are similarly handled, although the body is often tucked under the arm of the handler or restrained in the other hand (Figure 6.12). Turtles and tortoises are held at the shell; the length of the neck determines how far caudal on the carapace the animal is restrained. Some species of turtles have extremely long necks and can deliver a powerful bite to an unsuspecting restrainer.
  • Preanesthetic assessment: In nonpoisonous and tame species, perform a PE prior to anesthesia to obtain a baseline HR, RR, body temperature, and accurate body weight. Physical parameters such as HR are greatly influenced by body temperature, environmental stress, and presence of noxious stimuli. Due to the large variation in reptilian species, normal physical parameters are based on species‐specific reference material and the patient’s examination.
  • Premedication: Benefits of a premedication in animals include reduced dosages of induction drugs, analgesia, and sedation. However, clearance is prolonged in reptiles compared to mammals. Therefore, reversible drugs are chosen when possible. Some reptiles have mu opioid receptors, making mu opioids such as morphine an effective analgesic option, although respiratory depression may result [35,36]. Other opioids may be effective at providing analgesia but differences between species are notable [37,38]. Other alternative analgesic drugs, including alpha‐2 agonists such as dexmedetomidine and medetomidine, may provide analgesia in some retiles [3941]. Any injectable premedication is administered in the cranial portion of the animal if possible [42]; more effective sedation with injectable drugs such as alfaxalone was evident when the drug was administered cranially [43]. However, safety of the handler is prioritized over concern regarding the renal portal system. There is also evidence that intracelomic administration of alfaxalone is a viable route in snakes [44,45]. Indeed, with its favorable routes of administration, alfaxalone is a versatile drug for use in many reptile species [46,47].
  • Induction: When premedication is insufficient, reptiles can be induced with inhalants via mask or chamber (Figure 6.13). Inhalants are administered to the patient until it is relaxed and intubation is accomplished (Table 6.10). Turtles and tortoises may be induced with IV induction agents by IV access or injection through the subcarapacial sinus under their shells (Figure 6.14). Intubation of snakes and lizards is straightforward, as the position of the glottis is rostral (Figure 6.15). Noncuffed ET tubes, Cole tubes, or 16–19 G catheters are used for smaller reptiles. Typically, the best method of securing the ET tube is taping it to the patient’s lower jaw. In turtles (Figure 6.16), the glottis is located at the base of the tongue. They have short tracheas so care is taken to ensure ET tubes are not advanced too far, causing an endobronchial intubation. In reptiles, IV access is infrequently achieved. In turtles and tortoises, catheterized vessels include the jugular or coccygeal veins; alternatively, IO catheters are commonly placed (Figure 6.17).
  • Maintenance: General anesthesia is maintained primarily with inhalant anesthetics. Alfaxalone has potential for a surgical plane of anesthesia in some lizard species [48]. Because these animals are ectothermic, external heat support is necessary throughout the anesthetic period to keep them in their optimal thermoregulatory range.
  • Monitoring vitals: A Doppler is useful for monitoring HR (Figure 6.18); a pencil probe is ideal for the anesthetist to obtain HR via the ocular artery by placing this probe (well lubed) onto the eye. An esophageal stethoscope is also useful for obtaining HR. RR is visually observed. Assisted ventilation of 1–4 breaths per minute is recommended due to the long periods of apnea; MV is indicated for most reptiles. Capnography is not an adequate representation of ventilation due to intrapulmonary shunting (i.e., EtCO2 is increased compared to true PaCO2), but does provide some evidence for the presence of CO2. Pulse oximetry is not accurate in reptiles due to their alternative form of hemoglobin and difficulty placing the probe.
  • Monitoring anesthetic depth: Muscle tone will decrease in the reptile as it becomes anesthetized. In the snake, relaxation occurs initially at the head, then through the torso, and proceeds caudally, lastly affecting the tail; recovery occurs in the opposite order. In snakes, the tongue retraction reflex will only subside in a deep plane of anesthesia. Palpebral reflexes may give an indication of anesthetic depth. Corneal reflexes are also lost in a deep plane. Toe or tail pinch is absent during the surgical plane of anesthesia. Jaw tone is also monitored in species where this is safe to assess.
  • Recovery: Snakes typically take longer than lizards to recover; turtles, especially those that are capable of holding their breath, take an extraordinarily long time to recover. There is evidence that administration of epinephrine may expedite recovery from inhalant in sea turtles [49]. Maintaining optimal body temperature is critical to recovery. All reptiles are placed on room air to recover due to the downregulation of a reptile’s respiratory system secondary to increased FiO2. Continuing assisted ventilation is often necessary on room air (Figure 6.19). Extubation occurs once the patient is capable of spontaneous ventilation and pharyngeal reflexes have returned. Additional analgesia is administered as warranted.
Photo depicts restraint of a bearded dragon.

Figure 6.12 Restraint of a bearded dragon.


Source: Courtesy of Amanda M. Shelby.

Photo depicts masking a snake.

Figure 6.13 Masking a snake.


Source: Courtesy of Anderson da Cunha.


Table 6.10 Anesthetic protocol for reptiles.


































Snake Lizard Turtle/tortoise
Premedication (Note: No premedication in aggressive or poisonous species) Opioid premedication:

  • Butorphanol 20 mg/kg IM

Sedative agent (select one):

  1. Midazolam 0.5–1.0 mg/kg IM
  2. Dexmedetomidine 0.05–0.1 mg/kg IM/SC

Injectable anesthetic (select one):

  1. Ketamine 5–10 mg/kg IM
  2. Alfaxalone 5–20 mg/kg IM or 10–30 mg/kg intracolemic
  3. Propofol 15 mg/kg IV


  1. Morphine 0.4–1.5 mg/kg
  2. Midazolam 1.0–2.0 mg/kg IM
  3. Dexmedetomidine 0.1–0.2 mg/kg IM
  4. Injectable anesthetic (select one):


    1. Ketamine 5–10 mg/kg IM
    2. Alfaxalone 5–15 mg/kg IM or 10–20 mg/kg SC [63]


  1. Opioid (select one):


    1. Hydromorphone 0.5 mg/kg IM
    2. Morphine 0.4–1.5 mg/kg IM


  2. Sedative (select one):


    1. Midazolam 1.0–2.0 mg/kg IM
    2. Medetomidine 0.05 mg/kg IM


  3. Injectable anesthetic (select one):


    1. Ketamine 5–10 mg/kg IM
    2. Alfaxalone 10 mg/kg IM
Induction

  1. Proper premedication dosing can facilitate intubation
  2. Mask/chamber with gas inhalants; due to breath holding, this can take several minutes and exposes staff to waste anesthetic gas


  1. Proper premedication dosing can facilitate intubation
  2. Mask/chamber with gas inhalants


  1. Mask/chamber with gas inhalants
    OR

  2. Propofol via frontal sinus or IV 2–4 mg/kg to effect
    OR
  3. Alfaxalone via frontal sinus or IV 3–10 mg/kg to effect (Note: assisted ventilation is necessary if the high end of the dose range is used) [64]
Maintenance Gas inhalants Gas inhalants
OR
Alfaxalone in some lizard species [65]
Gas inhalants
Intraoperative analgesia Local blocks where applicable Local blocks where applicable (Note: Neuroaxial anesthesia with lidocaine or bupivacaine is feasible in the bearded dragon [66,67]) Local blocks where applicable
Postoperative analgesia

  1. Butorphanol 20 mg/kg IM
  2. Carprofen 1–4 mg/kg IM SID


  1. Morphine 0.4–1.5 mg/kg IM
  2. NSAID (select one):


    1. Carprofen 1–4 mg/kg IM SID
    2. Meloxicam 0.2 mg/kg PO or IV


  1. Tramadol 5–10 mg/kg PO
  2. Carprofen 1–4 mg/kg IM SID

IM, intramuscular; IV, intravenous; NSAID, nonsteroidal antiinflammatory drug; PO, per os (by mouth); SC, subcutaneous; SID, semel in die (once a day).

Photo depicts subcarapacial sinus injection in a tortoise.

Figure 6.14 Subcarapacial sinus injection in a tortoise.


Source:Courtesy of Anderson da Cunha.

Photo depicts intubation of a snake.

Figure 6.15 Intubation of a snake.


Source: Courtesy of Anderson da Cunha.

Photo depicts larynx of a tortoise.

Figure 6.16 Larynx of a tortoise.


Source: Courtesy of Anderson da Cunha.

Photo depicts (a) IO catheter in a tortoise.

Figure 6.17 (a) IO catheter in a tortoise.


Source:Courtesy of Anderson da Cunha. (b) Jugular catheter in a tortoise. Source: Courtesy of Jody Nugent‐Deal.

Photo depicts doppler and blood pressure monitoring of a reptile.

Figure 6.18 Doppler and blood pressure monitoring of a reptile.


Source: Courtesy of Jody Nugent‐Deal.

Photo depicts snake placed on Ambu-bag on room air for recovery.

Figure 6.19 Snake placed on Ambu‐bag on room air for recovery.


Source: Courtesy of Jody Nugent‐Deal.


3. Common complications



  • Hypothermia: Hypothermia is reduced by using circulating hot water blankets and forced air warming units.
  • Hypoventilation: Hypoventilation secondary to apnea and respiratory depression is common in most reptiles, requiring assisted ventilation.
May 13, 2023 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Anesthesia and analgesia in the exotic patient

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