In the January, March, and June issues of Veterinary Medicine, we discussed many simple-to-perform anesthesia techniques: infiltration anesthesia; splash blocks; digital nerve blocks; intravenous regional anesthesia; soaker-type catheters; stifle, intercostal, intrapleural, and forelimb blocks; and maxillary and mandibular nerve blocks. Another useful method is epidural anesthetic administration. Preoperative epidural injection of local anesthetics and opioids provides excellent preemptive, multimodal intraoperative analgesia; reduces the concentration of volatile anesthetic required to maintain surgical anesthesia; and provides analgesia extending into the recovery period.1-4 Continuous epidural anesthesia can be provided by placing epidural catheters. Descriptions of epidural catheterization and its possible complications are available elsewhere.2-6 In this article, we describe single-injection epidurals.
EPIDURAL INJECTION SITES
The lumbosacral intervertebral space is the most common location for epidural injection in small animals. Located between the vertebral periosteum and the dura mater, this space contains nerves of the cauda equina, fat, blood vessels, lymphatics, and, in some species, the end of the spinal cord with the remaining meninges (pia and arachnoid mater).
Access to the lumbosacral intervertebral space involves inserting a spinal needle through skin, subcutaneous fascia, and the ligamentum flavum, which forms the dorsal wall of the epidural space in dogs and cats.2-4 Below the dura mater lay the arachnoid mater and the pia mater. The subarachnoid space contains cerebrospinal fluid (CSF); injection into the subarachnoid space is referred to as intrathecal or spinal injection and typically is not purposefully performed in small animals. Thoracic epidural injections are used with some frequency in people and have been described in veterinary species but are not commonly used in dogs and cats.1-4
Epidural anesthetic injections should not be administered to patients with increased intracranial pressure, clotting disorders (because of the possibility of causing an epidural hematoma), uncorrected hypovolemia, degenerative central or peripheral axonal diseases, anatomical abnormalities that make location of landmarks difficult, or skin infection at the site of needle penetration.1-4
DRUGS FOR EPIDURAL INJECTION
Epidural administration of local anesthetics (Table 1) provides complete anesthesia, sufficient to perform surgery, to the caudal half of the body. Administering opioids epidurally provides additional analgesia often of longer duration and with fewer adverse effects than systemic administration (Table 1). If morphine is used in the epidural injection, premedication with another mu agonist opioid is recommended.
Table 1: Drugs for Epidural Use in Dogs and Cats
Other drugs, such as alpha2 agonists, nonsteroidal anti-inflammatory drugs, and ketamine, may be administered epidurally as well, though this is not common in small animals.