Control
of Animal Pain and Distress
The
Nature of pain -
Pain is the sensory experience evoked
by noxious stimuli. Noxious stimuli
are those that damage or threaten
to damage tissue such as excessive
mechanical, thermal, chemical or
electrical energies being applied
to the body. Such stimuli cause
the release of substances (histamine,
serotonin, bradykinin, ATP, Prostagland
ins, H +, and K +) that stimulate
nociceptors. Recognition of pain
depends upon intact nerve pathways
from nociceptors to the thalamus
and cerebral cortex, as well as
a functional cerebral cortex and
subcortical structures. Acute or
short lived pain produces signs
that include nociceptive reflexes
(withdrawal) and sympathetic nervous
system reflexes (including increased
heart and respiratory rates and
excitement). Levels of corticosteroids
and catechOARines are elevated.
Chronic pain, arbitrarily defined
as pain lasting six months or more,
is affected by adaptation of the
autonomic nervous system and is
characterized by more vague signs
such as sleep disturbances, appetite
disturbances, depression and decreased
libido. Corticosteroids and catechOARines
are not consistently elevated.
Any
means that renders the cerebral
cortex nonfunctional, such as hypoxia,
drug depression, electric shock,
or concussion, prevents perception
of pain. When this happens, stimuli
that would be painful to the conscious
animal may evoke motor nerve reflexes
that cause movement in the unconscious
animal without pain perception.
On the other hand, noxious stimuli
administered to unanesthetized animals
chemically paralyzed by neuromuscular
blocking agents, such as curare
or succinylcholine, will not evoke
a motor reflex because of paralysis,
but will cause pain because of the
conscious state. Hence, it is possible
that unconscious animals may feel
no pain but show movement in response
to certain stimuli, while paralyzed
animals may feel pain but cannot
respond. Thus, movement per se is
not a reliable indicator of pain.
The use of paralyzing agents is
strongly discouraged in experiments
that may cause pain, unless the
animal is adequately anesthetized
and adequate provision made for
monitoring the depth of anesthesia.
Keeping
animals calm and comfortable minimizes
the exaggerative effects that emotion
may have on perception of pain.
Animals should be maintained in
familiar and stable environments
with familiar handlers and freedom
from excessive noise and other aversive
stimuli. Visual and physical contact
with other members of the animal’s
own species may be helpful. Gentle
handling, socialization to handlers
and acclimation to laboratory procedures
are useful in some species and individuals.
Recognition
of pain - Physiological
responses to pain, particularly
acute pain, include increased blood
pressure and heart rate, pupillary
dilation, increased respiration,
elevated temperature, and an arousal
response on the electroencephalogram
(Soma 1984). These are useful in
regulating adequacy of anesthesia.
Changes in blood cell counts, cardiovascular
factors, and circulating levels
of cortisol, free fatty acids, glucagon,
and acute phase reactant proteins
may serve as indicators that an
animal is experiencing pain or distress
during an experiment (Morton and
Griffiths, 1985).
Behavioral responses to pain vary
among species, and among individuals
within species. To recognize animal
pain one must be familiar with the
normal and abnormal behavioral repertoire
of the species and individual animals.
The veterinarian and the technicians
who care for animals daily can provide
valuable information.
Behavioral
signs of acute pain include:
-
Guarding (protecting the painful
area)
-
Vocalizing (whining and crying when
moved or the painful area is palpated)
-
Licking, biting, or scratching a
particular area
-
Restlessness, pacing, and repeatedly
lying down and getting up again
-
Lack of mobility (with joint, colic
or gut pain)
-
Failure to groom, causing an unkempt
appearance
-
Abnormal resting postures, hunched
up, saw horse stances
In
chronic pain, animals become more
tolerant, making recognition of
behavioral signs more difficult.
Typically, animals become withdrawn
and inattentive and show little
other behavioral evidence of suffering
when pain becomes chronic. In the
absence of evidence to the contrary,
procedures or conditions that would
be painful for humans should be
assumed painful for animals.
Stress
and distress - Breazile
(1987) describes stress as “an
internal (physiologic or psychogenic)
or environmental stimulus that initiates
an adaptive change or a stress response
in an animal.”
Distress
is due to prolonged or intense stimuli
that evoke harmful responses that
interfere with well-being, comfort,
and/or reproduction and are capable
of inducing pathologic changes.
Distress responses may cause a variety
of disorders in animals, such as
alterations in feeding behavior,
hypertension, inefficient reproduction
or feed conversion, gastric and
intestinal ulceration, electrolyte
imbalance, urticaria, and immune
system abnormalities.
Control of pain and distress - Anesthetics,
analgesics, and/or tranquilizers
should be used whenever experimentation
otherwise would cause pain or distress.
The use of these three classes of
drugs must be in accordance with
currently-accepted veterinary medical
practice, and must produce in the
individual subject animal a high
level of anesthesia, analgesia or
tranquilization consistent with
the protocol or design of the experiment.
In other words, drugs must effectively
minimize pain and discomfort to
the animals while under experimentation.
If
a procedure that causes animal pain
or distress must be conducted without
the use of anesthetics, analgesics
or tranquilizers, justification
for withholding pain relieving drugs
must be thoroughly documented by
the principal investigator or course
director and approved by the ACUC.
In addition, the conduct of the
procedure must be directly supervised
by the principal investigator or
course director.
In
general, procedures which by themselves
induce no more pain, stress or discomfort
than the administration of pain-relieving
drugs, may be done without them.
This includes most injections, blood
collection and routine medical examination
and treatments.
Classification
of drugs used to control animal
pain and distress:
Analgesic - drug which alleviates pain without
causing a loss of consciousness.
(Examples include morphine, meperidine,
pentazocine, buprenorphine, aspirin,
phenylbutazone.)
Anesthetic - drug or agent that is used to
abolish the sensation of pain. Local
anesthetics such as procaine, xylocaine,
and bupivicaine produce regional
anesthesia of tissues around the
site of local infiltration or prevent
afferent nerve conduction through
an infiltrated area. General anesthetics
such as thiopental, thiamylal, and
pentobarbital when injected, or
ether, halothane, methoxyflurane,
or enflurane when inhaled, depress
the central nervous system and induce
deep sleep during which the sensation
of pain is lost.
Cataleptic - drug which produces a trance-like
state with muscle rigidity and hyporesponsiveness
to the external environment; which
is known as “dissociative
anesthesia.” Because of the
nature of its activity, ketamine
produces analgesia for low grade
pain but it does not alleviate the
type of pain which accompanies deep
abdominal surgery, manipulation
of fractured bones, or extraction
of large teeth.
Sedative - agent which allays activity and
excitement by producing a mild degree
of central nervous system depression
in which the patient is awake but
calm and free of nervousness.
Tranquilizer - drug which acts on the emotional
state to calm and quiet the animal
without loss of consciousness. These
drugs increase the threshold to
environmental stimuli and depress
many physiological functions, but
do not produce sleep or analgesia.
When used in combination with dissociative
anesthetics, a degree of general
anesthesia is produced which is
effective for certain procedures.
Promazine, acetylpromazine, xylazine
and valium are examples.
Narcotic - drugs that cause insensibility
or stupor. Examples include morphine,
meperidine, codeine, and fentanyl.
Staff
veterinarians in the OAR are available
to assist in the selection of anesthetic,
analgesic and tranquilizing agents
and the development of protocols
for the use of these drugs in experimentation.
Preanesthetic
Treatment
Anticholinergic
agents are used as preanesthetic
agents, especially before general
anesthesia, to diminish salivary
and bronchial secretions and to
protect the cardiovascular system
from vagal inhibition. Atropine
should be given to primates, pigs,
goats, sheep, guinea pigs, dogs
and cats since these species are
relatively susceptible to airway
obstruction by excessive secretions.
Atropine is ineffective in about
50% of rabbits because they have
a genetically-determined atropinesterase.
A
variety of tranquilizers provide
a significant advantage as preanesthetic
agents. Their administration facilitates
the induction of anesthesia by calming
the animal, reducing the dose of
anesthetic agent required (thereby
minimizing toxicity and risk of
overdose), and allowing for a smoother
recovery from anesthesia. Some of
these agents have analgesic properties
which continue into the post-anesthesia
period.
Local
Anesthesia
Local
anesthesia is based on the administration
of substances in or near nerve pathways
to prevent afferent nerve transmission
from a body part to the spinal cord
and/or brain. Drugs used include
lidocaine, procaine, and bupivacaine.
Local
infiltration is accomplished by
injecting the local anesthetic agent
beneath the skin, or other tissue,
in the area where anesthesia is
desired.
A
field block is accomplished by injecting
local anesthetic agent around afferent
nerves that innervate the operative
region. Typically, the nerves are
blocked a few inches away from the
area where anesthesia is desired.
Knowledge of the regional anatomy
is essential to obtaining satisfactory
results with this technique.
A
conduction block is accomplished
by injecting local anesthetic agents
directly adjacent to the afferent
nerve(s) supplying the area where
anesthesia is desired.
Regional
or spinal anesthesia can be achieved
by injecting local anesthetic agents
into the epidural or subarachnoid
space of the spinal cord.
Several
precautions should be considered
with use of local anesthetics:
-
Systemic toxicity may result from
intravascular injection of local
anesthetic agents.
-
The time required from injection
to development of adequate anesthesia
and the duration of the anesthesia
vary with the agent used and the
method of administration.
-
Administration of local anesthetics
to animals requires experience and
skill and the use of aseptic techniques.
It is often necessary to sedate
or tranquilize the animal before
local anesthetics can be effectively
administered.
General
Anesthesia - Inhalation
Inhalant
anesthetics are administered as
gases inhaled by the subject. they
have a significant advantage over
injectable agents in that the level
of anesthesia is more easily regulated.
Even an overdose can often be corrected
by removing the source of anesthetic
and ventilating the animal’s
lungs.
Administration
Jar
method: A piece of cotton or gauze
soaked with a volatile anesthetic
agent is placed beneath a wire screen
inside a large jar. The animal to
be anesthetized is placed on the
screen and the jar is closed forcing
the animal to inhale the gas until
it is observed to be anesthetized.
Contact of the animal’s skin
with the anesthetic agent must be
avoided.
Nose
cone/open drop method: A piece of
cotton or gauze sponge soaked with
the anesthetic agent is placed in
a nose cone or beaker which is placed
over the animal’s nose and
mouth forcing the animal to inhale
the anesthetic gas. This method
is often used to maintain anesthesia
in animals induced by the jar method.
Depth of anesthesia can be regulated
by varying the amount of anesthetic
on the sponge and proximity to the
animal’s nose.
Anesthetic
machine method: Anesthetic machines,
fitted with vaporizers suitable
for the agent being used, provide
a predetermined level of anesthetic
gas in the animal’s breathing
air. These machines may be of the
open circuit variety in which exhaled
air containing anesthetic gases
and carbon dioxide are exhausted,
or of the closed circuit variety
which utilize a carbon dioxide absorption
system and the addition of oxygen
to conserve the anesthetic agent.
These machines may be used with
a face mask or with endotracheal
intubation.
For
routine anesthesia in mammals other
than rodents it is generally advisable
to appropriately pre-anesthetize
with sedatives and atropine and
then induce anesthesia with an ultra-short-acting
injectable anesthetic prior to the
administration of inhalant anesthetics.
In experiments conducted with anesthetized
animals, investigators should be
aware of the multiple metabolic
effects that may be introduced by
multiple drug anesthesia regimes.
In
some birds and snakes, gas anesthetic
agents can be administered by a
needle into the air sacs and exhausted
through the trachea in a closed
circuit system.
Inhalant
Anesthetic Agents
Ether - (diethylether) is an inexpensive,
non-toxic inhalant anesthetic agent
which has excellent anesthetic properties.
Induction and emergence are slow
with considerable struggling. Ether
is extremely flammable and should
only be used in well-ventilated
areas or in hoods which are free
of sparking potential (such as smoking,
cautery, open flames, electric motors,
and other electrical apparatus).
Long-term storage of ether may lead
to the formation of explosive peroxides.
Ether should be stored in explosion-proof
refrigerators. Animals that have
been euthanatized with ether may
retain sufficient vapors in the
hair coat and tissues to constitute
a risk of explosion. Animals larger
than rats and mice should be premedicated
with atropine to reduce the excessive
salivation and respiratory tract
secretions that result from ether
irritation of the respiratory epithelium.
Methoxyflurane - (Metofane) produces excellent
general anesthesia in animals and
has a low vapor pressure which reduces
the risk of overdose in subject
animals and significant exposure
in personnel. In prolonged or very
deep anesthesia, methoxyflurane
tends to depress respiration and
cause hypotension.
Halothane - Halothane also produces excellent
general anesthesia. It has a very
high vapor pressure and if uncontrolled
can reach concentrations that are
lethal to animal subjects. Halothane
should therefore be used only with
anesthetic machines having specific
halothane vaporizers. It tends to
be more nephro- and hepatotoxic
than Metofane and can cause hypotension,
cardiac depression and respiratory
depression during prolonged or deep
anesthesia.
Nitrous
Oxide - Nitrous oxide is a gas at
atmospheric pressure and therefore
requires no vaporization; It is
non-flammable, non-toxic and relatively
inexpensive and is not sufficiently
potent to produce deep surgical
anesthesia alone. In mixtures with
oxygen at (N2O:O2) ratios of 1:1
to 4:1, it is used to supplement
more toxic anesthetics such as methoxyflurane
and halothane, thereby reducing
the amount and concentration of
these agents needed.
Carbon
dioxide - CO2 has excellent anesthetic
properties, however it is difficult
to control the level of anesthesia.
C02 may be used in the jar method
for producing brief anesthesia in
rodents or to produce unconsciousness
to facilitate exsanguination or
other terminal procedures. C02 and
02 mixtures (e.g. 70%:30% or 50%:50%)
can be used to produce brief, light
anesthesia for survival procedures.
Chloroform - The use of chloroform for any
purpose is strongly discouraged
in animal facilities housing mice.
Some inbred strains of mice are
ultra sensitive to the hepatotoxic
and other harmful effects of chloroform.
Chloroform is also toxic and carcinogenic
to humans.
All
gaseous anesthetic agents pose potential
health risks to personnel, and should
only be used when provision has
been made for scavenging of the
waste gases.
General
Anesthesia - Injection
Anesthesia
by injection has the disadvantages
of more difficult regulation of
anesthetic depth, and, in general,
more harmful side effects than inhalation
anesthesia. Administration
The
preferred route of administration
of most injectable general anesthetics
is intravenous. Advantages of the
intravenous route over others include
improved control of the depth of
anesthesia (dose) and reduced local
tissue toxicity.
Injectable
Anesthetic Agents
Sodium
pentobarbital - a short-acting barbiturate.
Aqueous solutions are irritating
to tissues, and perivascular administration
may result in tissue sloughs. For
intravenous administration, approximately
50% of the calculated dose should
be given in a rapid bolus to induce
anesthesia and avoid an anesthetic
induction excitement period. The
remainder of the dose is administered
slowly over two to five minutes
to allow monitoring and control
of the depth of anesthesia. While
intraperitoneal injections are used
for some small rodents, in larger
animals, such as rabbits and cats,
intraperitoneal injections cause
evidence of pain. Recovery is often
characterized by paddling movements
and vocalization. The excitement
phase of induction and the undesirable
features of emergence can be reduced
by pre-anesthetic administration
of tranquilizers or sedatives.
Thiobarbiturates - analogues of barbiturate anesthetics.
Thiopental (Pentothal) and thiamylal
(Surital) are ultra-short-acting
thiobarbiturates. They produce a
shorter duration of anesthesia than
sodium pentobarbital (1 5-30 minutes).
Prolongation of anesthesia with
supplemental doses may cause the
recovery period to be accompanied
by excessive muscular activity and
vocalization.
Ketamine
Hydrochloride - a dissociative anesthetic
(Cataleptic). Ketamine is often
used alone or in conjunction with
other agents in a variety of small
laboratory animals and in cats and
nonhuman primates. With Ketamine
alone, heart rate and blood pressure
are usually increased and swallowing
and cough reflexes are retained.
Ketamine produces limited anesthesia
suitable for superficial or minor
surgical procedures. There is limited
muscle relaxation and fair to poor
analgesia. Ketamine is usually administered
intramuscularly except in small
rodents where it may be administered
intraperitoneally.
A
mixture of ketamine and xylazine
provides better analgesia and muscle
relaxation than either agent alone.
Also, the duration of anesthesia
is prolonged and recovery is prolonged
and smoother. The mixture may be
administered intramuscularly or
intraperitoneally. The mixture tends
to reduce blood pressure and heart
rate in contrast to the use of ketamine
alone.
Precautions
for General Anesthesia
Regardless
of the species and anesthetic agents
involved, some principles of general
anesthesia are worth keeping in
mind. These include:
-
Maintain a patent airway. For acceptable
anesthesia, nothing must block the
ability to breathe freely and easily.
This is especially important in
allowing resuscitation in the event
of an anesthetic emergency. With
small rodents that are obligate
nose breathers (rats, mice, hamsters,
guinea pigs, rabbits), a patent
airway is easily maintained if the
nostrils are not blocked.
-
Core body temperature can fall
alarmingly, particularly in small
animals, during the course of prolonged
general anesthesia. Hypothermia,
added to other factors, can initiate
an irreversible sequence of events
leading to death. Heating pads should
always be used in animal surgery
and should be either thermostatically-controlled
or of the circulating-water type
to prevent skin injury. At the very
least, animals should not be placed
on surfaces that conduct heat away
from the body.
-
Administer to effect. Because of
wide variation within and among
species, there is no way to predetermine
the precise anesthetic dose of a
drug. General anesthesia must be
given “to effect,” as
measured by physiological parameters
and response to stimuli. In practice
with small rodents, this is not
usually possible because of size,
numbers, time, equipment, etc.;
nevertheless, most anesthetic deaths
can be attributed to failure to
obey this principle. This is especially
true of parenterally-administered
drugs such as barbiturates. Once
they are injected, there is little
the anesthetist can do to control
the outcome; therefore, great care
is necessary when administering
these drugs.
-
Barbiturates are caustic substances
when injected into living tissue
and care must be taken to completely
avoid subcutaneous or intramuscular
injections with these drugs unless
there is a specific experimental
requirement. Intravenous injections
are preferred. Intraperitoneal injections
are acceptable in rodents, when
diluted material is used.
-
Inhalant anesthetics may not be
used in animal rooms because they
may cause stress or toxicity in
animals other than the one being
anesthetized.
-
Gas scavenging systems should always
be used to remove excess anesthetic
gases.
Analgesia
Centrally-acting
analgesics include narcotic agonists
and partial agonists. The agonists
are potent analgesics that produce
a high degree of CNS disturbance
manifested primarily as respiratory
depression and sedation. Agonists
include morphine, meperidine, fentanyl,
etorphine, and codeine. Narcotic
partial agonists produce morphine-like
analgesia but also have some morphine
antagonist activities. Pentazocine,
butorphanol and buprenorphine are
partial agonists. Buprenorphine
has the advantages of long action
(6-12 hours in animals) and reversal
of respiratory depression associated
with narcotics.
Peripherally-acting
analgesics have moderate to low
analgesic potency and antiinflammatory
properties. These include aspirin,
acetaminophen, phenacetin, ibuprofen,
and phenylbutazone.
REFERENCES:
Breazile, J E. Physiologic basis
and consequences of distress in
animals. J Am Vet Med Assoc 1987;191:1212-1215.
Flecknell,
PA. The relief of pain in laboratory
animals. Lab Anim 1984;18:147-160.
Soma,
L R. Anesthetic and analgesic considerations
in the experimental animal. Ann
NY Acad Sci 1983;406:32-47.
Morton,
D B and Griffiths, P H M. Guidelines
on the recognition of pain, distress
and discomfort in experimental animals
and an hypothesis for assessment.
Vet Rec
1985;1 16:431-436.
