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Preoperative considerations
Evaluation and Preparation
Delivery methods
Parenteral anesthetics
Inhalant anesthetics
Support and monitoring
Analgesics
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Health status- A variety of agents can adversely affect anesthesia.
In particular, Pasteurella spp. cause a variety of clinical syndromes, including pneumonia.
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Breed
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| Strain- Watanabe model of hypercholesterolemia and adriamycin-induced
cardiomyopathy
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Sex, cyclic
changes in does |
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Should be quarantined a minimum of 72 hours |
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Xylazine increases blood glucose and should be avoided in diabetic
animals.
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Evaluation and Preparation
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Rule out Pasteurella via chest
radiographs |
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Withholding food prior to
surgery is of little benefit; the stomach does not
completely empty even after 5 days of food withdrawal. A 12-hour fast may aid respiration due to degrees of emptying and
abdominal breathing. Blood glucose remains stable for 96 hours of fasting,
except in rabbits <3kg.
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Premedication
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Rabbits may produce atropinesterase
(AtrE). Is inherited and 50% have serum and/or tissue levels. |
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Glycopyrrolate is
a quaternary ammonium is effective in rabbits that make AtrE |
| Acepromazine causes vasodilatation
because it blocks adrenoceptors - good for phlebotomy. Chlorpromazine
causes muscle necrosis when given IM and should therefore only be given IV |
| Benzodiazepines
(diazepam and midazolam) have been used with ketamine and other agents for
short-term procedures. Diazepam (5mg/kg IM) with ketamine (30mg/kg IM)
provides insufficient analgesia for surgery.{4156} |
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a2-adrenergic
agonists
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Xylazine
produces excellent sedation, muscle relaxation and analgesia (high
doses), but depresses cardiopulmonary parameters at 5mg/kg |
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Medetomidine is
useful at 0.25mg/kg IM |
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Detomidine
needed in high doses, at which it is associated with myocardial
necrosis, therefore not recommended |
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Reversal:
yohimbine (0.2mg/kg IV) or atipamezole (1mg/kg IM) |
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Cholestyramine given post-operatively may reduce
antimicrobial-induced clostridial enterotoxemia caused by amoxicillin,
ampicillin, cephalothin, clindamycin and lincomycin.{4156} |
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| Avoid sciatic nerve |
| Chlorpromazine, ketamine/xylazine may cause site pathology |
| Injections should be limited to 1.5 ml in NZW and 1.0 ml in Dwarf rabbits |
| Lidocaine-prilocaine cream recommended on ear before venipuncture (EMLA
cream)
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| Some drugs have been given
intranasally, with mixed success:
| Telazol yields 40 minutes of anesthesia |
| Innovar causes bradycardia, apnea and 50% mortality |
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Tracheal Access
| Benzocaine (cetacaine) topical anesthetic associated with methemoglobinemia.
An AJVR article stated that this occurs in
dogs>mice>cynos>ferrets>cats>rabbits>pigs>Aotus>Rhesus>SD
rats>LE rats. {3703} |
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Respiratory monitor helpful in advancement of tracheal tube (Beck
Airway Airflow Monitor [BAAM])
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Breath-holding common in rabbits{4156}
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1. Barbiturates
| Pentobarbital anesthetic dose is very close to its apnea dose.
Apnea can be reversed by using the Hering-Breuer reflex (mild chest
compression??). Hallmark of pentobarbital is respiratory depression; also causes acidosis,
hypoxemia, hypercarbia and bradypnea.
Tachycardia is the result of baroreflex compensation. Pentobarbital causes less myocardial damage than halothane or
a-chloralose.
It may cause hypokalemia. Rabbits can become tolerant when anesthetized more
than once weekly.{4156} |
| Ethylmalonylthiourea (EMTU) is another barbiturate used in rabbits;
combined with xylazine it is said to provide safe and effective anesthesia.
Ultrashort-acting barbiturates have also been used to effect. |
2. Dissociatives
| Ketamine is the most widely used,
most often with xylazine. When given IM, will result in respiratory
depression, hypoxemia and increased CO2
with minimal metabolic alkalosis, decreased heart rate. |
| Ketamine/xylazine combinations
with either acepromazine or butorphanol extend the period of anesthesia, but
may increase hypothermia and hypotension. |
| Telazol® is nephrotoxic
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3. Neuroleptanalgesia/anesthesia
| Fentanyl-droperidol (Innovar-Vet®) - droperidol acts longer causing prolonged
sedation after neuroleptanalgesia has passed. Bradycardia, hypotension,
bradypnea and hypoxia result. Use glycopyrrolate or atropine to avoid bradycardia. Neither
diazepam nor detomidine improved the consistency or reliability of Innovar. |
| Fentanyl-fluanisone (Hypnorm®) -
by itself causes respiratory depression, bradycardia and poor muscle
relaxation. Diazepam or midazolam produce surgical anesthesia. Respiratory depression of this drug can be reversed by
naloxone, doxapram and various opioids. |
| The use of mixed agonist/antagonists following neuroleptanalgesia has
been termed anesthésie analgesique sequentielle. Buprenorphine provides the
best reversal of Hypnorm-associated respiratory depression, normalizes
hypoxemia and CO2 levels, while preserving
analgesia. |
4. Miscellaneous agents
| Alphaxalone-alphadolone (Saffan® or
Althesin®) produces light to medium
anesthesia for 8-10 minutes. At low doses
given by infusion, peripheral vasoconstrictor reflexes
are preserved; this is useful in pharmacologic studies
requiring these intact reflexes. |
| Propofol only good for induction and diagnostic
procedures, as it produces light anesthesia with little reflex depression.
Blood gas values and blood pressure are unchanged. When using the new
oil-in-water emulsion instead of Cremaphor, induction and recovery are
smooth and rapid, but there is hypoxemia, hypertension and maintained
reflexes when used for long-term infusion. |
| Urethane (ethyl carbamate)
produces long sleep times (5-6 hours) and excellent muscle relaxation, but
it is carcinogenic and causes hemolysis and must be monitored by safety
personnel |
| Chloralose-urethane preserves or enhances baroreceptor reflexes
and has been used for long-duration non-survival studies. Urethane has also
been combined with acepromazine for over 12 hours of anesthesia. |
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| Bain and Rees modified T-piece circuits most
appropriate |
| Isoflurane vs. halothane: both
decrease arterial pressure and increase heart rate. Halothane also decreases
PaO2, cardiac output, and respiratory rate and increases PaCO2,
while isoflurane has no effect on these. Isoflurane advantages include
cardiac safety, rapidity, minimal hepatic transformation and reduced
viscerotoxicity. Disadvantages are cost, breath holding, reduced respiratory
anesthetic index and hypotension. One reference claims high death losses in
rabbits anesthetized for >2 hours on halothane, potentially due to
ventricular arrhythmias (although they weren't doing ECGs){4582} |
| Nitrous oxide increases cerebral blood flow to varying degrees
depending on background anesthesia
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Regional anesthesia
| Preoperative incision infiltration with lidocaine or bupivacaine may
decrease need for post op analgesics; specific local nerve
blocks have not been described |
Hypnosis
| Not reliable enough to consider for replacement of
analgesics or anesthetics. |
Support and monitoring
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Ear (pinna) reflex
(ear movement in response to compression) best detection of depth
followed by pedal (rear)
withdrawal, corneal and palpebral reflexes, in that order |
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End-tidal
CO2 measured at the tip of the ET tube differs from that measured
at 12cm from the tip by only 1mm Hg, and is a good indicator of PaCO2 |
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IV fluids should be given at a rate of
10ml/kg/hr during surgery. Daily fluid needs are 100ml/kg/day. |
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Anorexic rabbits develop hypoglycemia
rapidly due to their high metabolic rates. Give 5% dextrose subcutaneously,
50% dextrose orally, supplements, and/or Nutrical to avoid hepatic
lipidosis. |
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Pain assessment
| Decreased food and water consumption, failure to groom, reduced
activity to immobilization, guarding behavior. |
Analgesics
| There is a great need for
research on postoperative analgesia in rabbits. |
| Standard analgesiometric tests (quantitative pain tests) are useful in
assessing opioids, but not NSAIDS, in the rabbit. Tests include
thermal stimulation of the muzzle, electrical stimulation of the ears or
tooth pulp, and pressure on the hindlimb. Inflamed paw pressure tests are
used for NSAIDS, but have not been developed in rabbits. |
| Buprenorphine is the authors’ choice
(0.01-0.05mg/kg SC or IV provides 10 hours of analgesia). Butorphanol,
pentazocine and nalbuphine have also been tried. Fentanyl and its congeners
cause too much respiratory depression. Morphine produces hypertension and
hyperglycemia. Oral opioids have low bioavailability and are unpalatable. |
| Piroxicam and flunixin have been shown to reduce inflammation (not
assessed for pain). Aspirin has been used clinically, as have ketoprofen and
acetaminophen. |
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