Preoperative considerations
Methods of anesthetic delivery
Inhalants
Parenteral general anesthetics
Recovery from anesthesia
Rodent analgesics and methods of delivery
Pregnant rodents
Neonates
Take the quiz!
I.
Preoperative Patient Evaluation and
Care
A.
Water should not be restricted.
B.
CT
37:3; An Alternative to
Overnight Withholding of Food for Rats.
1. Withholding food overnight causes undesirable side effects such as
loss
of body and hepatic weight and decreases in blood glucose concentration. Other potential problems include ingestion of bedding.
2.
Providing rats with sucrose cubes
overnight ameliorated these undesirable effects while still reducing the size of
the GIT. Removing
or reducing access to bedding provided additional reduction in weight of the
GIT.
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A.
Intraperitoneal drug injection
-
Lower
left abdominal quadrant
-
Fasting
the animal 4-8 hours prior minimizes errors
-
Use
a 20-22 gauge needle instead of a
smaller (25-26 gauge) which may not penetrate the subcutaneous fat
and abdominal wall.
B.
Intravenous:
Rarely performed in the guinea
pig or hamster
C.
Intramuscular:
Use is discouraged in small
rodents
D.
Inhalation:
Rats are difficult to intubate due to their large tongue, large molars,
pendulous soft palate, small larynx, and small trachea.
A variety of techniques and home-made equipment is available to make the task
easier.{4553}
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III.
Inhalant Anesthetics
A.
Ether
1.
The Guide recommends replacing ether with less hazardous inhalants
[Note: the Guide doesn't actually say this. It says on page 17: "If ether
is used, personnel safety should be ensured by proper use of signs and by using
equipment and practices to minimize risks associated with its
explosiveness"].{3948}
2.
Advantages
a.)
low cost
b.)
suitable for minor procedures
3.
Disadvantages
a.)
Induction is unpleasant and existing respiratory disease will be
exacerbated.
b.) Carcasses must be well
ventilated
c.)
Storage of carcasses must be in explosion proof refrigerators or freezers
4.
Species/strain differences (lethal effects at various concentrations)
a.)
C57BL/6 most resistant,
followed by ICR
b.)
DBA/2,
BALB/c, and
C3H/He
were most sensitive
c.) Use in guinea pigs is unsafe
since they tend to hold their breath and salivate profusely
d.) Hamsters require very careful monitoring, and an AMBU bag should be nearby
e.) Ether is the least satisfactory inhalant for gerbils due to rapid induction
B.
Carbon dioxide:
1.
Low (50%) CO2 concentrations produce an excessive time of
induction, with frequent severe and possibly fatal adverse effects with moderate
distress and discomfort for the animals.
2.
High (100%) concentrations produce rapid anesthesia with fewer adverse
effects but raise serious ethical and moral concerns.
3.
Suggest 70% + oxygen as the optimal concentration based on practicality
and humane acceptability.
4.
LAS
47:4; Humane and Practical
Implications of using Carbon Dioxide Mixed with Oxygen for Anesthesia or
Euthanasia of Rats {4212}
a.)
Human volunteers judged the humaneness of breathing 50% to 100% CO2.
Increasing concentrations were rated as highly unpleasant (50%) to
painful at 100%.
b.)
Practical implications were measured in SD rats.
c.)
Lower concentrations were least likely to cause pain and distress but had
the longest times to anesthesia and death.
It also had the highest incidence of unwanted side effects (seizures,
etc.) and the most severe histologic changes in the lungs (pulmonary edema and
hemorrhage).
d.)
Higher concentrations induce more rapid anesthesia and death but are more
likely to cause appreciable pain and distress.
e.)
Recommendations:
Use a non-precharged chamber and a low gas flow rate so that conscious
animals are never exposed to CO2 concentrations greater than 70%.
5.
LAS
46:4; Effect of In Vivo
Administration of Anesthetics on GABA Receptor Function
a.) Most studies involving GABA receptors use either cervical dislocation or
decapitation as euthanasia without the use of anesthesia.
b.) Known modulators of GABA receptor
system
1.)
Benzodiazepines
2.)
Barbiturates
3.)
Ethanol
4.)
Stress related steroids
D.
Methoxyflurane (Metofane, Penthrane)
1.
May be administered by an open drop or induction chamber
2.
Highly soluble in blood and tissues, making for prolonged recovery
3.
Diabetes
insipidus like syndrome has been observed in F344 rats
from fluoride ion-induced renal damage.
E. Halothane
1.
Nonflammable, non-explosive and nonirritating
2.
Requires careful monitoring to prevent overdose and requires the use of a
calibrated vaporizer.
3.
Induces microsomal enzymes in rats. Other immune system
effects include decreased rat lymphocyte response to mitogens, decreased
chemotaxis, decreased phagocytosis.{4177}
Direct effect on myocardial and vascular smooth muscle via interference with
calcium-dependent processes{4177}.
4.
Hepatotoxicity occurs in some
rodents.
The guinea pig is an animal model for
acute halothane-associated hepatotoxicity.
5.
Halothane is specifically a very useful anesthetic in guinea pigs, a
species in which reliable anesthesia is very difficult to obtain. [Note the
inconsistency in the above two statements!]
F.
Isoflurane
1.
Anesthetic of choice for procedures requiring low risk and reliable rapid
recovery
G. Sevoflurane
1.
Blood solubility one-half to one-third of isoflurane, approaching that of
nitrous oxide.
2.
Rapid induction which is not accompanied by struggling
3.
Degraded by soda lime or Baralyme in a temperature dependent manner.
Breakdown product (an olefin
called compound A) is lethal in rodents at high concentrations.
It may not approach lethal levels in clinical use.
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A.
Barbiturates- the thiobarbiturates (thiopental, thiamylal and ethyl
(1-methylpropyl) malonylthiourea or EMTU) are irritating when administered
peri-vascularly, and are infrequently used in rodents due
to difficulty obtaining vascular access.
1.
EMTU (Inactin) at 80-100mg/kg causes marked hypercapnea and acidosis in
rats, but produced surgical anesthesia for 3-4 hours. It has been
used in rat renal studies for its long, stable anesthetic plane, but it also
causes lowered blood pressure, renal blood flow, and GFR.{4161}
2. Methohexital (Brevital)
is known for its rapid induction and brief duration, but unfortunately does not
produce surgical anesthesia in the rat, only profound restraint (40mg/kg IP).
Mixing it with pentobarbital produced profound life-threatening respiratory
depression and poor analgesia. In hamsters, methohexital mixed with diazepam
could be tweaked to produce surgical anesthesia, although respiratory depression
was always a risk (3 parts 1% methohexital mixed with 1 part 0.5% diazepam,
given at 4-5 ml/kg).{4154}
2. Pentobarbital
(Nembutal)
a. poor
analgesic
b. Male mice are more sensitive than
females
B.
Dissociatives
1.
Ketamine- highest recommended doses produce deep sedation but incomplete
analgesia. Muscle
rigidity when used alone.
2.
Ketamine/xylazine
a.) judged to be superior to ketamine/diazepam, pentobarbital, and
Innovar-Vet
b.)
transient hyperglycemia (xylazine)
c.)
increased intraocular fluid glucose levels
d.)
Polyuria and excessive salivation
e.)
Bradycardia
f.)
Reversal with yohimbine and tolazoline (as well as the CNS stimulant
4-aminopyridine or 4AP)
g.)
LAS 46:4; Effects of Two
Injectable Anesthetic Agents on Coagulation Assays in the Rat
1.)
Examined the effects of urethane or ketamine/xylazine anesthesia on the
aPTT (Activated partial thromboplastin time), TT (thrombin time) and the PT
(Prothrombin time)
2.)
Urethane increased the aPTT
compared to the controls
3.)
Ketamine/xylazine
increased TT.
4.)
No changes were seen in the PT for either agent
h.)
LAS 47:5; Prolonged (12 hours)
Intravenous Anesthesia in the Rat
1.)
Ketamine and xylazine in a ratio of 30:1 were infused IV by use of an
adjustable syringe pump
2.)
Rectal temperatures were maintained at 35-36C rather than 37-38C which
improved the survival of the animals
i.)
CT
35:2; Comparison of Five
Anesthetic Agents Administered Intraperitoneally in the Laboratory Rat.
Ketamine/xylazine was superior at
inducing analgesia while provoking only minimal abdominal inflammation. Although tiletamine/zolazepam of all the agents incited the least amount
of abdominal inflammatory response, this combination provided inadequate
analgesic anesthesia. Pentobarbital,
chloral hydrate and TBE given IP caused appreciable abdominal inflammation and
induced inconsistent analgesic anesthesia.
j.) CT 37:4; Evaluation of
Injectable Anesthetics for Major Surgical Procedures in Guinea Pigs.
Surgery could not be performed on any of the animals given ketamine or
tiletamine-zolazepam/detomidine. There was a high rate of adverse
effects in guinea pigs receiving detomidine. Four of six in the tiletamine-zolazepam/detomidine group died.
Those animals receiving tiletamine-zolazepam/xylazine and
tiletamine-zolazepam/medetomidine underwent successful surgery. A combination of
tiletamine-zolazepam and xylazine or medetomidine was effective for inducing
anesthesia and providing sufficient analgesia to perform a surgical procedure. This is in contrast to what the blue
book says on page 176, but is more recent information and is therefore probably
the correct answer on the boards.
C.
Neuroleptanalgesics
1.
Fentanyl-droperidol (Innovar Vet)
a.
Human and veterinary drugs vary in strength. Human drug has approx.
one-tenth the potency per ml of the veterinary formulation.
b.
Usefulness is compromised by its
irritant nature, which may cause tissue necrosis and self trauma to the digits
following IM dosing.
2.
Fentanyl-fluanisone (Hypnorm)
a.
Judged to be superior
neuroleptanalgesic in rats
b.
Addition of midazolam creates a
reliable and longer lasting surgical anesthetic with good muscle relaxation and
analgesia.
Dose-dependent respiratory depression can be reversed, if needed, with
naloxone.
Top
D. Miscellaneous Anesthetics
1. Alpha chloralose
a. 9% of rats develop
seizures
b. Very poor analgesic; do not use as sole anesthetic
2. Alphaxalone-alphadolone (Saffan,
Althesin)
a. Steroidal anesthetic
b.
In rats, anesthesia can be maintained for hours with IV infusion or boluses. Repetitive boluses are contraindicated in the guinea pig because it
causes respiratory distress and pulmonary edema.
3. Chloral hydrate
a.
Classically associated with concentration-dependent gastric mucosal injury, adynamic
ileus and peritonitis with IP administration. Effective dose in
rats approaches LD50.
4. Propofol
(Diprivan, Rapinovet)
a.
Poor analgesic, but good for short-term procedures such as
tattooing or blood sampling. Can induce rats with Hypnorm and
maintain on propofol infusion for things like long-term vascular
catheterizations; stable blood pressure, heart rate and respiration.
b. Requires IV access, so not good for hamsters, guinea pigs or gerbils
5. Tribromoethanol (Avertin)
a. Used in transgenic research to
facilitate such procedures as embryo transfer, vasectomy or distal tail
amputation for Southern blot analysis.
b. Major disadvantages
1.) Cardiovascular and respiratory depression at high doses
2.) Toxic by-products
if exposed to light or improperly stored. Decomposes into dibromoacetaldehyde and hydrobromic acid which are potent
gastric irritants leading to fibrinous peritonitis, ileus and fatalities. Particularly prominent in mice on second exposure of
TBE.
3.) Proper storage is in the dark at
4C.
4.)
LAS 48:2; Doppler
Echocardiographic Analysis of Effects of TBE Anesthesia on Cardiac Function in
the Mouse Embryo: A Comparison with Pentobarbital
TBE can induce transient changes
in cardiac function parameters between day 11-14 of gestation. All arrhythmias were associated with the atrium.
There were no arrhythmias in the pentobarbital groups.
6. Urethane (Ethyl carbamate){4733}
a. At anesthetic doses (1-1.2g/kg=1-1.2mg/gm) in rats, it has a wide margin of safety and causes minimal changes in blood pressure, aortic blood flow and blood gas values. Rats
can be anesthetized for up to 24 hours, but they develop acidosis, hypocapnia,
hyperoxia, hypotension and bradycardia. Transient narcosis occurs even with skin exposure in mice. It is also well absorbed when given SQ or orally.
b. In combination with alpha chloralose, surgical anesthesia can be obtained. The urethane component suppresses muscle activity and reflexes and the CNS, while maintaining stable respiratory patterns. But when given IP to rats, peritoneal fluid accumulates, the kidneys stop responding to NaCl and water load, and the body becomes hyperosmolar. Renin and aldosterone increase and the pressor response to aldosterone is reduced. Following IP injection, urethane distributes evenly to all tissues. It is metabolized to ammonia, CO2 (which is expired) and ethyl alcohol. Given IP it can cause irritation, peritoneal fluid accumulation, hyperosmolarity, increased plasma renin and aldosterone.
However, if combined with alpha-chloralose, there is less effect on cardiac and
respiratory systems.
c. Major disadvantages: proven status as a
carcinogen
and mutagen in rodents. Mouse strains that normally develop pulmonary tumors will do so at higher incidence and earlier in life after a single anesthetic dose.
d. Human health and safety precautions: Very soluble in water, alcohol and lipids; it volatilizes at room temperature. It is highly mutagenic to fruit flies, and at the usual anesthetic dose , it's cytotoxic to dividing cells. Its antineoplastic properties are likely due to this arrest of chromosomal division at metaphase. Although there are no studies of the carcinogenic potential in humans, it is assumed, as is the potential for immunosuppression. For safety, urethane should be mixed only in a fume hood, and open containers are never permitted.
e. Should
be limited to non-survival procedures.
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V. Recovery
from Anesthesia
The time to "recovery" of normal function after even a simple
surgical procedure (such as a catheter implant) is determined in part by what
parameter one uses to define "recovery." The type of anesthetic may
not be as important as the parameters that are monitored. Halothane,
pentobarbital, and ketamine+ace+xylazine all had similar effects in this study.
Parameters used included body weight, food/water consumption, MAP, activity and
heart rate.{4527}
Rats lose weight following surgery (about 30 grams in a 225-250 gram male),
and they don't gain it back until about 4 days postop. Food and water
consumption, however, return to baseline earlier, at around 1-2 days. There are
some differences in MAP, heart rate and activity levels depending on whether the
light or dark phases are examined. HR and MAP are elevated for 24 hours postop
op when measured during the light phase. Previous work has documented that body
weight and autonomic nervous system activity (HR and MAP) are related. Rats are
less active during the dark phase for up to 3 days. The authors concluded that
(1) if body weight is used as the "recovery" parameter, 4 days should
be allowed; (2) for studies of exercise and circadian activity, 2 days should be
allowed; and (3) for studies of autonomic control of the heart, 2 days should be
allowed. It was noted that the ketamine cocktail prolonged return to normal
activity more than either halothane or pentobarbital.{4527}
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VI. Methods of Analgesic Drug Delivery to
Rodents
A. Oral delivery
1. Disadvantages
