Introduction

The use of epidural anesthesia provides excellent analgesia as well as good muscle relaxation due to its ability to produce a sensory and motor blockade, respectively.

The local anesthetics used for epidural anesthesia are shown in Table 1 and Table 2. Lidocaine, bupivacaine and ropivacaine are similar in terms of their anesthetic profile when used for epidural blockade. Lidocaine may possess a slightly shorter onset time, while bupivacaine and ropivacaine produce a longer duration of anesthesia. Both bupivacaine and ropivacaine are widely used for surgical procedures. They are of particular value for continuous epidural blockade during acute pain like trauma. When used as a 0.1% or 0.25% solution they provide satisfactory sensory analgesia with minimal motor blockade. Thus, the animal can be rendered pain free and still be able to move itself or maintain spontaneous ventilation even when the cephalic spread is high. Bupivacaine and ropivacaine usually provide 2-4 h of adequate analgesia, although this depends greatly on the dosage.

The quality of epidural blockade will be influenced primarily by the local anesthetic employed. Others factors that may influence the adequacy of epidural blockade include:

1.  Dose, volume and concentration of the local anesthetic agent

2.  Addition of a vasoconstrictor to the local anesthetic solution

3.  Patient position

4.  Patient age, epidural space volume and clinical status

The volume of the anesthetic solution administered into the epidural space may influence the cephalic spread of anesthesia. However, the relationship between spread and volume of anesthetic is neither lineal nor predictable. The essential qualities of epidural anesthesia are related to the mass of drug rather than to the variation in volume or concentration of solution (see Table 1).

Epinephrine is frequently added to local anesthetics solution (lidocaine and bupivacaine) used for epidural anesthesia. The addition of epinephrine is related primarily to its effect on the local vasculature, which causes a decrease in absorption of the local anesthetic. Bupivacaine and ropivacaine administered epidurally do not appear to benefit as much as lidocaine from the addition of a vasoconstrictor.

Although the use of epidural anesthesia alone is in theory possible, it is not advisable. In fact, the addition of light general anesthesia facilitates patient manipulation and contributes to a "stress free anesthesia". A contraindication to supplementary anesthesia would occur in only few cases where there is marked depression, such as in a cesarean with coexisting fetal depression or in patients with hemodynamic compromises. In most cases, the contribution of epidural anesthesia to the anesthetic protocol allows the practitioner to significantly decrease the total dose of central depressants, thus decreasing the risks and the negative impact of these drugs in patients with underlying diseases.

Opioids such as morphine have been used intrathecally or epidurally for the treatment of both acute pain and various chronic pain conditions. Although opioids administered by any route bind to spinal cord opioids receptors, it was speculated that epidural or intrathecal administration would provide preferential delivery and binding of opioids to spinal cord receptors, thereby allowing a lower total dose to be used. Opioids for epidural injection are diluted with an appropriate volume of sterile saline (see Table 3), and use of the basic technique.

In most cases, the entry into the epidural space is accomplished via puncture at the lumbar-sacral level (L7-S1). The epidural space is situated between the internal and external dural sheaths. These two sheaths get separated at the level of the foramen magnum and extend caudally. The external leaf forms the periosteum of the spinal channel, while the internal one comprises the true spinal dura mater.

Spinal (intrathecal) administration of analgesics is rarely used to provide clinical pain relief in animals. There are very few drugs which are marketed for epidural or spinal use. Many of the drugs used for epidural analgesia are available in preparations designed for systemic use and contain neurotoxic preservatives. Therefore, to avoid complications, special care should be taken in choosing a correct formulation.

Table 1. Local anesthetics used for epidural anesthesia in dogs.

*The dose was calculated according to the spine length, measured from the occipital bone to the first coccygeal vertebra.
CIR: constant infusion rate; L: lumbar vertebra; T: Thoracic vertebra; w/e: with epinephrine

Table 2. Local anesthetics used for epidural anesthesia in cats.

Table 3. Epidural opioids--dose and intervals.

*Drugs for epidural injection are diluted with an appropriate volume of sterile saline

References

1.  Frecknell, P.; Waterman-Pearson, A. Pain management in animals. WB Saunders Co. London, UK 2000.

2.  Matthews, K.A. Management of Pain. Vet Clin North Am Small Anim Pract 2000; 30: 703-967.

3.  Otero E. Pablo. Dolor. Evaluación y tratamiento en pequeños animales. Editorial Inter-médica. Argentina, 2004.