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Pre-Surgical Information
Spaying or neutering your pet is a big step! There is a lot to consider
when we are planning your pet’s surgery. Here is some information to help
you make wise decisions.
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Special LibraryOur Special Library
is a collection of informative articles on a variety of health topics.
Click here . . .
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Discharge InstructionsClick here for
information on how to care for your pet at home.
Click here . . .
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Pain
Management
Once we accept the fact that animals feel pain in a manner very
similar to the way we feel pain, the question immediately arises
as to what can we do about that pain. The short answer is,
"Lots." There are a variety of drugs and techniques
available to help ease the pain our furry companions feel.
Some of this is pretty technical. Every practitioner has
certain "hobbies" within the scope of their practice.
These are areas that the practitioner finds particularly
interesting, and about which he or she develops a high degree of
skill and expertise. I personally (Dr. Nield, that is),
find pain management very interesting, and have over the years
spent a lot of time studying and researching this fascinating
field. While I can't claim to be a specialist in pain
management (it is not ethical to claim to be a specialist in
something umless you are board certified in thast field), I do
believe that we do an excellent job of managing pain at
Sunnyside Veterinary Clinic, and that we have some things to
offer that you may not find just anywhere else.
Pain is a sensation we feel when our body is damaged. That
is how nature lets us know that there is a problem. In
this respect, pain has a physiologic function, because it leads
us to avoid the thi ng
thsat caused the pain (pulling away from a hot stove), to
protect the injured part while it heals (not walking on a broken
leg), and to even begin treatment of a wound (in the case of
dogs, licking a wound).
Pain starts with receptors at the ends of nerves called nociceptors. The word itself comes from the same roots as
the words noxious and receptor. Nociceptors are activated by bad
things, like
 burns,
cuts, blunt trauma, stretching, chemical reactions etc.
They are also activated by pressure, touch, and other normal
stimuli. Some areas, like the skin, are
well-supplied with nociceptors. Other areas, like the
liver, have very few nociceptors. There are separate
nociceptors for heat, stretching, touch, and trauma. Here,
mechanical, chemical, and thermal energy causes the nociceptors
to be activated. The chemical stimuli can come from either
outside sources, such as an acid burn, or internal sources.
When tissues are injured, say by a bruise or a cut, they release
a number of substances. Histamine, prostaglandins,
bradykinins, and other compounds associated with inflammation
are released which act directly on the nociceptors.
A particularly interesting aspect of nociceptor function is
peripheral sensitization. Sensory endings in inflamed tissue
display
 enhanced
sensitivity to stimulation so that ususally non-painful stimuli
become painful (allodynia) and the perception of painful stimuli
becomes more intense (hyperalgesia). Various inflammatory
mediators like histamine, bradykinins, and prostaglandins have
been shown to cause sensitization in this way. Thus, the sensory
endings of nociceptors are modulated and, through them, the
perception of pain.
Once nociceptors are activated, they send signals up the nerves
towards the brain. There are different kinds of nerves.
There are fast "A-delta" nerves, which rapidly carry the initial
sensation sharp pain, slow "C" nerves which carry the secondary
dull, throbbing pain sensations, and very sensitive "A-beta," or
tactile nerves , which have a lower threshold of stimulation
and which are responsible for our sense of touch.
The first stop is the spinal cord. Here, a complex process
of switching, routing, and modul ation
occurs. The spinal cord is the first place the body tries
to make sense out of all the varied and assorted signals the
injured area sends it. All the different signals from all
the different types of nociceptors and all the different types
of nerve fibers converge for the first time in the spinal cord.
S ometimes
good things happen to the pain signal in the spinal cord,
sometimes bad things happen to the pain signal. In a
certain region of the spinal cord, called the dorsal horn, a lot
of bad things can happen to a pain signal. Through a
process called "dorsal horn wind-up," a form of central
sensitization, a relatively mild pain stimulus can be amplified
and modulated such that it becomes exagerated. Long after
the initial cause of the pain has subsided, dorsal horn wind-up
can result in prolonged and exaggerated pain.
The next stop for the pain signal is the brain. Once the
modified pain signal reaches the brain
it is processed further. Things like fear and anxiety are
allowed to modify the pain sensation. All of the
pre-processed signals from all of the various nerves are
processed into the perception of pain. This final result
is the sum of all the pre-processing that goes on in the
nociceptors, nerves and spinal cord combined with a lot of
post-processing that occurs in the brain itself. In some
cases, all the pain signals bouncing around can lead to a
generalized hyperexcitiablity called central sensitization,
which can lead to excessive pain.
One of the most important concepts to master
about pain management is the idea that our nerves are not
just wires that carries pain signals from point A to the
computer-like brain, which then impartially processes the pain
signal. On the contrary, our nervous system processes pain a
lot like a community processes a disaster. Several different
people may see an accident happen (the nociceptors), but each
may report it in a different way. Some people are more
excitable than others, and some may give an inflated report
(peripheral sensitization). They then pass the initial
report along to other people at the coffee shop (the spinal
cord), and there may be considerable discussion with some wild
speculation thrown in. Sometimes things get out of hand
(dorsal horn wind-up), and a exaggerated message emerges.
The coffee shop people pass their information as they perceive
it along to the newspaper (the brain), where the editorial
committe tries to process it. Like most committees,
sometimes excitable people can dominate the process (central
sensitization), leading to a response that may or may not be
appropriate for the original accident.
Because of the complexities of the nervous system, with all it's nociceptors, nerve fibers, dorsal horns, and brain synapses,
there are a lot of points at which we can control and modify the
pain signals. Lets start at the very beginning and
evaluate how various drugs can modify the pain process.
The nociceptors are where the pain response begins.
Several classes of drugs act directly on the nociceptors to
modify the pain response. Non-Steroidal Anti-Inflammatory
Drugs, or NSAID's for short, are one such class. Aspirin,
ibuprofen, meloxicam, phenylbutazone, and Rimadyl are all
NSAID's. They act to reduce the amount of prostaglandins
that are released by injury, thus reducing their direct nociceptor
stimulation and their peripheral sensitization.
A second class of drugs that act directly on the nociceptors are
the local anesthetics. These are drugs like lidocaine,
bupivicaine, and novocaine. They are injected at the site
of pain, and they totally block the nociceptors temporarily,
just like when we go to the dentist's and he injects novocaine
into our gums. Blocking the pain, even temporarily, can
have a great effect even after the local anesthetics wear off
because that prevents the spinal cord and brain from getting
over-zealous in thier modification of the pain response.
A third class of drugs that can act on the nociceptor level are
the opioids. They will be discussed in depth later, but
there are some opioid receptors present in the periphery, and
they can have a local effect there.
The next place we can influence the pain response is at the
spinal cord. Here, drugs like opioids are very useful.
Opioids are named after opium (opium-oid), which as we all know
is a very potent drug derived
 from
poppies that was commonly abused in the last few centuries.
There are many opioids, including heroin, codeine, morphine, hydromorphone, fentanyl, buprenorphine, nalbuphine, tramadol,
and a dozen others. Opioids act on opioid receptors, which
are specialize sites on nerves that bind the opioid molecule and
cause it to have an effect on the nerve. This effect is an
actual blunting of the pain response. There are lots of
opioid receptors in the spinal cord, and opioids act on them to
decrease, often dramatically, the pain response. Opioids
turn the volume down, so to speak. Way down. Anyone
who has had an opioid before will attest to their beneficial
properties.
Other drugs can have an effect on the spinal cord. NMDA
antagonists are one such class. Most people ar e
not familiar with NMDA antagonists. However, there are
NMDA receptors located in the spinal cord. These NMDA
receptors, when activated, produce a prolonged and excessive
pain response, even to injuries which should not be all that
painful. This is called dorsal horn wind-up, a form of
central sensistization. NMDA antagonists block the NMDA
receptors, preventing them from being activated. A
perplexing assortment of drugs have NMDA antagonistic
properties: ketamine (an anesthetic),
dextromethorphan (
the cough suppressant in Robitussin), and
amantidine (an
anti-viral agent, also used to treat Parkinson's Disease) are
the most commonly used.
Other drugs called Alpha-2 agonists also work to reduce pain at
the level of the spinal cord. These drugs also have strong
sedative effects
The final place we can influence the pain response is the brain.
Fortunately, there are lots of opioid receptors in the brain.
Here, these drugs can have a profound effect on how much things
hurt. Opioid receptors in the brain also produce some
fairly profund behavioral changes as well. This would
explain the popularity of such drugs of abuse as opium and
hashish. Many opioids produce a sense of immense
well-being, euphoria, or being high.
The concept of Pre-emptive Pain Medication is very important.
Simply put, if pain medications are given before the painful
stimulus, the over-all pain response is greatly reduced.
The nervous system will amplify and distort pain signals that
are received suddenly. The initial rush of pain signals
overwhelms the system, and nociceptors are up-regulated, dorsal
horn wind-up occurs, central sensitization happens, and we
consciously perceive the pain to be greater than it really needs
to be. Pre-emptive pain management is giving pain control
medication before the pain even starts, thus damping down the
pain response mechanisms, preparing the nervous system to handle
the up-coming painful stimulus in the most favorable way.
Multi-modal Pain Management is another important concept.
This involves using different drugs from each class. Say
we give a normal dose of drug X, but the patient is still
painful. If we give more drug X, the comfort level may
increase, but we will also start to get bad side-effects too and
the levels of drug X climb. For example, too much morphine
is not a good thing because of the severe respiratory depression
it can cause. However, if instead of giving the patient
more drug X we give a normal dose of drug Y, which acts in a
different way (or mode) than drug X, the patient will likely be
much more comfortable without experiencing any adverse side
effects. Th effects of drug X and drug Y will be additive
if not synergistic. If we add a dose of drug Z to boot,
the patient will experience great pain relief while still not
experiencing bad side-effects.
This is where the field of pain management overlaps with the
field of anesthesia. It is possible to design a pre-op and
inter-op anesthesia plan that will synergize and compliment the
post-op pain control plan. It is also possible to design
an anesthesia plan that will provide absolutely zero pain relief
(besides unconsciousness). This is where anesthesiology
becomes both a science and an art, choosing medications that
will provide maximal benefit to each individual patient.
An example of Multi-modal, Pre-emptive Pain Management
Let's consider a surgery for a ruptured ACL in a dog. This
is a major orthopedic surgery, and is known to be quite painful.
The patient is three years old, and otherwise healthy. An ideal
plan might start with a pre-anesthetic injection of
hydromorphone, a potent opioid, combined with a low
 dose
of an alpha 2 agonist such as medetomidine. This
combination provides a double-dose of pain relief plus anxiety
relief, a good thing when you are going in to surgery. (2 modes
so far)
The next step in a multi-modal approach would be to give a
little more hydromorphone as an epidural incection. This
route applies the potent opioid directly to the spinal cord.
At the same time it is often beneficial to add bupivicaine and
lidocaine, both local anesthetics, to the epidural. (Now we are
up to 3 modes)
When inducing anesthesia, it is a good idea to use a protocol
that involves ketamine, an NMDA antagonist (mode 4)
that doubles
as an anesthetic. This sets the stage to run a constant
rate infusion containing both ketamine and lidocaine and maybe
more hydromorphone. During surgery, it is helpful to apply a low
dose of bupivicaine directly inside the joint.
(mode 5)
Post-op, a constant rate infusion of hydromorphone, ketamine and
lidocaine does wonders for the immediate post-op period. (a
continuation of modes 1 and 4)
For the next week or so, oral doses of Rimadyl, a powerful NASID
makes the recovery period comfortable. (mode 6)
By using many different drugs, all at lower doses than would
be required were only one drug given, we can blunt the pain
response in many additive and synergistic ways.
Veterinarians, being both anesthesiologist and surgeon
and follow-up care provider to their patients, are in a much
better position to administer excellent pain control protocols
to their patients than many humans get when they have surgery.
The anesthesiologist never sees the patient after they roll out
of ther OR, the surgeon isn't in a position to tell the
anesthesiologist what to do, and neither are around to help the
patient cope with post-op discomfort.
Pain control is both an art and a science.
Animals feel
pain just like humans do, although they may not show their pain
in obvious ways. At Sunnyside Veterinary Clinic, we feel
an obligation to treat pain in our animal patients as if it were
our own pain. We use pre-emptive, multi-modal anesthesia
and pain control plans, sometime simple, sometimes complex, but
always tailored to each individual patient's needs.
Thank you for visiting us at our web page.
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