Confirm needle placement
Once the epidural space has been entered, remove the stylet, and inspect the hub for blood or CSF. If you see blood, a venous
sinus has been penetrated and the needle should be withdrawn and repositioned. If CSF is present, you have entered the subarachnoid
space and can withdraw or reposition the needle or, alternatively, inject one-fourth to one-half of the calculated volume
of the drug. The maximum dose of local anesthetic that can be administered spinally (intrathecally) is < 1 ml/10 kg, and only
preservative-free drugs should be injected spinally.4
Figure 4. After positioning the needle, inject 0.5 to 1 ml of air or sterile saline solution to test for resistance to injection.
If you encounter no resistance, the needle is correctly positioned in the epidural space.
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If no blood or CSF is observed in the hub, inject a test dose of 0.5 to 1 ml of air or saline solution with a sterile syringe
to check for resistance to injection (Figure 4). In our practice, we use glass syringes because they provide less resistance to injection; however, plastic syringes may
be used. If the epidural space has been correctly entered, there should be no resistance to injection of the air or saline
solution. If the plunger pushes back, the needle has not been correctly positioned.
Another method to verify correct placement in the epidural space is to use the hanging drop technique. To perform this technique,
remove the stylet before the needle penetrates the ligamentum flavum, and place a drop of saline solution or local anesthetic
into the needle hub. As the needle penetrates the ligamentum flavum, the fluid is aspirated into the needle because of the
subatmospheric pressure within the epidural space. Other ways of identifying the epidural space include the use of a nerve
stimulator or identification of appropriate pressure waveforms, though these techniques are rarely necessary.2-4
Prepare the syringe
Figure 5. Aspirate an air bubble (½ to 1 ml) into the syringe containing the drug or drug mixture. Minimal compression of
the bubble during injection indicates lack of resistance to injection and correct needle placement in the epidural space.
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Once correct needle positioning is confirmed, aspirate a small (½ to 1 ml) bubble of air into the syringe with the drug or
drug mixture, and attach the syringe to the needle. Gently aspirate to ensure the needle tip is not in a vein since placement
in a vein does not always bleed back and would provide no resistance to air injection (Figure 5). If blood is aspirated, the needle must be removed, and a new spinal needle should be obtained before placing the needle
again.
Inject the drug
When you are confident the needle tip is in the epidural space, inject the drug slowly over one to two minutes. Minimal compression
of the air bubble indicates lack of resistance to injection and correct placement in the epidural space. Once the drug has
been injected, slowly withdraw the needle from the injection site.
Observe patient response
If the epidural injection of a local anesthetic is successful, relaxation of the external anal sphincter and tail is observed.
In an awake patient, hindlimb ataxia, as well as a lack of response to the flexor pinch reflex of the pelvic limbs, develops.
Generally, once the toe reflexes have disappeared, anesthesia adequate for abdominal surgery is present. In anesthetized patients,
a lack of response to surgical stimulation, as well as a lower requirement of inhalant anesthesia to maintain surgical anesthesia,
is a good indicator of a successful local anesthetic epidural. Patients receiving epidural opioids alone will still respond
to acute pain stimuli and will not exhibit muscle relaxation. Epidural opioid administration often results in reduced requirement
for supplemental analgesics in awake patients and reduced requirement for volatile anesthetics in anesthetized patients.
CONCLUSION
This is the final article in a series on simple local and regional analgesic techniques you can use in small-animal practice.
Most of these techniques are easy to perform, do not require specialized or expensive equipment, and will improve pain management
in your patients.
ACKNOWLEDGMENT
The authors wish to thank Gregory Hirshoren, Instructional Resources, College of Veterinary Medicine, The University of Tennessee,
for the photos that accompany this article.
Christine Egger, DVM, MVSc, DACVA
Lydia Love, DVM
Department of Small Animal Clinical Sciences
College of Veterinary Medicine
The University of Tennessee
Knoxville, TN 37996
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