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Veterinary Anesthesia & Analgesia Support Group |
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Gary Old May 2004
Epidural administration of local anesthetics and analgesics is an effective tool for veterinary anesthesia and analgesia. This technique is very useful for surgical procedures caudal to the diaphragm but is particularly well suited for procedures involving the hind limbs, pelvis and perineal region. Depending on what agent is used, desired results can be achieved with local anesthetics or analgesics. Anatomy
of the epidural space:
The
epidural space has a large amount of epidural fat that is inside the
vertebral bone column. Even with a short period of weight loss the amount
of epidural fat will not be affected. The spinal cord is surrounded by the meninges with the pia
mater adhered to the spinal cord. The arachnoid and the dura mater are a
combined layer, placing the cerebrospinal fluid between the pia mater and
the arachnoid/dural layer. There
is a venous plexus on the ventral floor of the vertebral column that will
occasionally be entered.
The
vertebral level at which the cord and the dural sac (meningeal layers) end
is variable in the dog and cat. In the dog the cord ends at L6-L7 and the
dural sac at L7-S2. In the cat, they usually end further caudally (about
one more vertebra).
Indications:
The
indication is to provide analgesia and anesthesia caudal to the diaphragm.
It is an excellent means to provide anesthesia and analgesia to the
pelvis, pelvic limbs and perineum.
Contraindications: The
main contraindications are coagulopathies, sepsis, skin infections over
the injection site, and pelvic/sacral fractures. With coagulopathies, if
the ventral venous plexus should be entered, it could cause excess
bleeding, which would put pressure on the spinal cord.
Technique:
The
animal is either heavily sedated or anesthetized. There are two ways to
administer an epidural injection. The easiest for me is to place the
patient in sternal recumbency with the hind limbs frog-legged cranially to
open the L-S space. The hair
is clipped and the skin prepped with a surgical scrub. It is recommended
that gloves be worn while performing this procedure.
I
feel for both wings of the ileum with my thumb and middle finger and find
the highest point. There is now an imaginary line between the thumb and
middle finger. With my index finger I then find the spine of L7, which is
even with, or just cranial to the line between thumb and middle finger. I insert the needle just caudal to the L7 dorsal spinal process.
Alternatively, the animal can be placed in lateral recumbency with its
legs pulled cranially. The landmarks are the same.
I
find that a 22 gauge spinal needed is my preferred needle. It is small
enough for all patients yet stiff enough not to alter course while going
through the tissue. Smaller
needles can be used but on occasion will deviate due to the small
diameter. Some veterinarians prefer 20 gauge needles for larger patients.
The increased stiffness allows for more directional control in these
patients.
It
is recommended that a spinal needle be used for two reasons. One, the
bevel is shorter than a hypodermic needle allowing you to better sense the
pop when the needle passes through the ligamentum flavum. The other is
that the stylus prevents a skin plug from clogging the needle. This would
prevent you from seeing CSF fluid and, if a skin plug were injected into
the epidural space, it could serve as a nidus for infection and
inflammation.
The
needle is advanced until a distinct pop is felt or resistance is met.
Occasionally the animal’s tail will twitch. If the needle is advanced
too far you could enter the ventral plexus or the dorsal annulus of the
disc space.
If in the epidural space, there should be little to no resistance when injecting. Initially a small amount of air or saline can be injected as a test. There should be no resistance. The plunger should always advance and not retract so as to refill the syringe. An alternative method is called the hanging drop technique. With this technique the needle hub is filled with saline after initial penetration of the skin. When the needle enters the epidural space, the saline is drawn into the epidural space due to the subatmospheric pressure within the spinal canal. I rarely use this technique and rely on the ease of injection of fluid. If resistance is met is it usually due to the needle entering the ventral tissue and it should be withdrawn slightly. Once in the epidural space it is important to check for CSF fluid. This would affect the volume of drug injected as described below. Drugs commonly used for epidural injections: Ideally
the drugs should be preservative free, although the methylparaben type of
preservative can be used without causing obvious clinical problems.
Local Anesthesia: Four
drugs have been used in epidurals:
Generally
a dose of 1ml per 4.5 kg, with a maximum dose of 6ml total regardless of
the dog’s size, is recommended. If you are trying to effect a blockade
of the cranial abdomen then a dose of 1 ml per 3-5 kg may be needed but
must be injected slowly. If CSF fluid is obtained during epidural
administration then this dose needs to be reduced by 50%. If this is not
done you may cause a block further cranially than anticipated. The drug
should be administered slowly to prevent a patchy block and to prevent the
drug from moving too far cranially. If the animal is pregnant the block
needs to be reduced 50-75% due to engorgement of the venous plexus, which
makes the epidural space smaller. Obese animals should have their dose
base on an estimated lean body weight.
Clinical effects: Using
local anesthetics will result in sensory, motor and autonomic blockade. If the blockade extends to the thoracic region, autonomic effects
may be significant, potentially causing bradycardia. Respiratory function
will not be impaired unless a motor block of the phrenic nerves occurs
(C3-C5). Initially, after an
epidural there may be a decrease in blood pressure and an increase in
heart rate. No significant
changes in cardiovascular function, respiratory rate or arterial blood
gases are expected with local anesthetics.
Side effects of epidural local anesthetics: The
most common side effect is prolonged paresis. This is self-limiting but
may require the animal to stay in the hospital until normal function
returns. Urinary retention is a risk: monitor urine output and bladder size. Rarely, respiratory paralysis may occur, requiring ventilation
until the effect of the block dissipates.
Opioids: Opioids are probably the most commonly used class of epidural drugs. Opioids can be used alone or they can be combined with local anesthetics.
The
most commonly used opioid for epidurals is morphine. It is the least lipid
soluble so it travels the farthest cranially and has the slowest onset of
action. Because of these qualities the duration of action is longer. In
humans, the analgesia range is from 4 to 51 hours. In canines the duration
of effect is 10 to 24 hours. Spinally administered opioids have been shown
to decrease inhalant requirements by up to 50% of the normally expected
levels.
There
is a debate about the epidural use of morphine products containing
formaldehyde as the preservative. Some anesthesiologists are concerned
about neurotoxicity from this preservative while other says it poses no
problems. I have used both morphine with formaldehyde and preservative
free morphine. While the neurotoxicity is rare, when it occurs it produces
very painful myoclonus of the affected muscles that is considered an
emergency situation requiring treatment. This may requires general
anesthesia. What I prefer to do is to administer another bupivacaine dose
epidurally until the myoclonus is eliminated. Once the latter epidural has
worn off the myoclonus is usually gone. Because of the severity of this reaction, I recommend preservative
free morphine (Astromorph 1mg/ml or Duramorph 1mg/ml).
Hydromorphone and oxymorphone are more lipid soluble and may produce more segmental effects. They will not move as far cranially and may provide more pronounced analgesia to the hindquarters. The more lipid soluble the drug, the closer the epidural dose will be to the dose required systemically. Hydromorphone and oxymorphone both contain methylparaben as a preservative. Methylparaben is considered an acceptable preservative for epidural use. Fentanyl
is very lipid soluble and is of short duration and unless given via
epidural catheter offers little advantage over morphine, hydromorphone, or
oxymorphone.
Buprenorphine
is lipid soluble and a partial mu agonist that can be used but I see
little advantage or need for using buprenorphine instead of the previous
drugs.
Side effects of epidural opioids:
With
the use of epidural opioids, particularly morphine, pruritus and urinary
retention are not uncommonly seen. Occasionally
urinary catheterization will be necessary although I have not had to do
this in any of my cases. In
humans vomiting, nausea and respiratory depression can be seen. The reasoning is that due to morphine’s low lipid solubility, it
travels rostrally and binds to supraspinal opioid receptors in the brain
causing respiratory depression. This
respiratory depression does not appear to be clinically significant.
Important points for epidural use:
1) Local anesthesia dose is 1ml /4.5kg (maximum 6ml) 2) With pregnancy decrease the dose of local anesthesia 50-75% 3) With obese dogs use lean estimated body weight 4) Old age decreases the dose and they may have stenosis of the intervetebral foramen causing increased cranial movement and drug effect 5) Increased intra-abdominal pressure will engorge venous plexus reducing the volume needed for block 6) If you obtain CSF fluid reduce the does of local anesthetics by 50%. If using a opioid/saline then no reduction is needed 7) Infuse slowly, as this can cause patchy block or to far cranial
placement of local anesthesia
Finally,
for abdominal procedures I usually use morphine in a lidocaine 0.5%
concentration. I started
using this after beginning to use morphine epidurals for spays and
castrations. I felt some of these animals awoke painful and thought this
was due to the slow onset of morphine. By adding the low concentration of
lidocaine it appears to help with pain until the opioid has achieved full
effects. If I obtain CSF, I do not reduce the dose of the epidural, as the
concentration of lidocaine does not block motor function.
Other drugs used in the epidural space:
Ketamine, alpha-2 drugs (xylazine and medetomidine) and NSAIDs have been used epidurally. 1) Medetomidine at a dose of 5ug/kg did not show evidence of analgesia but it did appear to have a supra-additive effect when combined with morphine, prolonging the duration of overall analgesia 2) Xylazine has been used at a dose of 0.02 mg/kg in combination with morphine and was not associated with any cardiovascular side effects |
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| Click on the links below for great reference material | |||||||||||||||||||||||||||||||||
| Sheilah Robertson's Clinician's Brief PDF | |||||||||||||||||||||||||||||||||
| Oklahoma State University Teaching Material | |||||||||||||||||||||||||||||||||
| Excel Epidural Dose/Volume Calculators: See Resources & Downloads Page | |||||||||||||||||||||||||||||||||
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