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
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.
Lumbosacral epidural anesthesia with local anesthetics provides complete anesthesia to the caudal half of the body by blocking the intradural spinal nerve roots and the peripheral layer of the spinal cord.1 This technique is useful for pelvic and hindlimb orthopedic procedures, perineal and anal surgeries, exploratory laparotomy, and cesarean section. In addition to complete anesthesia, at least some sympathetic and motor blockade is produced with local anesthetics. Lidocaine, mepivacaine, and bupivacaine consistently cause motor blockade, while motor blockade is less intense and of shorter duration with levobupivacaine and ropivacaine.1 The duration of motor blockade is generally shorter than the duration of analgesia, and, depending on the procedure and local anesthetic chosen, motor function usually returns by the time a patient recovers from anesthesia.1-3
Doses and precautions. The type and volume of local anesthetic chosen depends on the desired result. Lidocaine (2%) administered at 1 ml/6 kg completely anesthetizes the pelvic limbs and posterior abdomen, caudal to L1, within 10 to 15 minutes and lasts 60 to 120 minutes. With a comparable volume of 0.5% to 0.75% bupivacaine, the onset is 20 to 30 minutes with a duration of four to six hours. Volumes of 1 ml/7.5 kg are adequate for pelvic, perineal, and hindlimb procedures.2,3
If the total epidural injection volume of local anesthetic and other adjunct analgesics exceeds 1 ml/5 kg, high blockade of the sympathetic nerve roots can occur and Horner's syndrome, vasodilation, and hypotension can result.2 Total drug volume should not exceed 8 ml, and doses of epidural drugs should be calculated by using the ideal lean body weight of the patient. Doses should be further reduced by 25% in pediatric, geriatric, and pregnant animals since the epidural space may be smaller than expected.2,4 Hypotension is more likely in animals with hypovolemia or cardiovascular compromise, so correct hypovolemia before administering local anesthetics epidurally. Hypotension is treated with intravenous crystalloids (20 to 30 ml/kg), colloids such as hetastarch (5 to 20 ml/kg), or vasopressors or inotropes such as ephedrine, vasopressin, or dopamine.1,2
In addition to sympathetic blockade, local anesthetic overdosage can also result in blockade of the motor nerves innervating intercostal muscles and even paralysis of the diaphragm should the local anesthetic reach the level of C5; hypoventilation, hypercapnia, and hypoxia can result.1,2 Ventilatory support, using positive pressure ventilation, may be required. Careful attention to total volume injected will reduce the incidence of these adverse effects.
Controversy exists as to whether a patient should be rotated after epidural injection of local anesthetics, which exert their action by contact with the nerve roots and spinal cord. In people, small injection volumes (< 10 ml), comparable to those used in small animals, spread under the effect of gravity to the dependent side, though other factors, including cardiovascular function and CSF pressure are also important.4 In our practice, we do not rotate the patient after epidural injection of local anesthetics other than to position the patient to prepare the surgery site. Body positioning with the head down and the hindquarters elevated could result in further cranial spread of the local anesthetic and should be avoided.
Opioids are frequently included with the local anesthetic to provide additional, often more prolonged analgesia (Table 1), but opioids do not provide anesthesia.1-4 The analgesic effects of epidural morphine, when used alone, extend to the thorax and thoracic limbs, and we occasionally use morphine in our practice for forelimb orthopedic procedures and thoracotomy to provide up to 24 hours of analgesia.2,7 The site of action of epidurally administered opioids is not completely understood, but they are thought to bind to opioid receptors in the superficial layers of the dorsal horn.4 Opioids provide visceral and somatic analgesia without motor or sympathetic blockade.1,3,4,8-10 Systemic absorption can account for some of the analgesic effects, especially for lipophilic opioids such as fentanyl. Local diffusion into the spinal cord and deposition in epidural fat and subsequent release are important as well.1,2
Morphine and precautions. Preservative-free morphine formulations (e.g. Duramorph PF—Elkins-Sinn; Astramorph PF—Astra-Zeneca Pharmaceuticals) have not been associated with neurotoxicity and are safe to use epidurally and intrathecally. Using parenteral morphine formulations containing sodium bisulfite, metabisulfite, chlorobutanol, edetate disodium, formaldehyde, or phenol as preservatives has been associated with neurotoxic effects when these formulations are placed directly on the spinal cord. Parenteral formulations can be used (0.1 mg/kg diluted in 0.26 ml/kg of sterile saline solution) as long as those preservatives are avoided.2-4
The adverse effects seen with epidural morphine in people, such as myoclonus, cardiovascular depression, respiratory depression, pruritus, urinary retention, ileus, and nausea, are not as common in dogs and cats, but it is still important to monitor for these signs, particularly urinary retention, after administering epidural opioids. Mu agonist opioids inhibit spinal and supraspinal detrusor motor centers to inhibit bladder motility. Bladder expression may be required in the initial postoperative period in some animals. Rarely, hyperesthesia of the pelvic area and tail have been reported after epidural morphine administration.2-4
Buprenorphine and other opioids. Epidural buprenorphine appears to be as effective as epidural morphine in relieving postoperative hindlimb orthopedic pain in dogs.11 Epidural buprenorphine also reduces the response to thermal stimulus in cats, though the effect is not as prolonged compared with morphine.12 In rats, a combination of epidural buprenorphine and bupivacaine provided comparable analgesic effects to epidural morphine and bupivacaine.13 All parenteral formulations of buprenorphine are preservative-free and safe for epidural use. Other opioids, such as oxymorphone, fentanyl, and methadone, can be used epidurally, but they provide a shorter duration of analgesia than morphine or buprenorphine do.2-4
Opioid and local anesthetic combinations
We often use preservative-free morphine, combined with bupivacaine, in our practice to take advantage of the quick onset (20 to 30 minutes) and long duration (four to six hours) of bupivacaine anesthesia. Onset of analgesia with epidural morphine can be up to 60 minutes for caudal analgesia and up to 180 minutes for forelimb analgesia, but the duration of analgesia is 12 to 24 hours.2 If the surgical procedure is expected to be less than two hours, lidocaine is a better choice of local anesthetic, as recovery of motor blockade will occur before the patient recovers from anesthesia. Prolonged motor blockade from bupivacaine after recovery from a short period of inhalant anesthesia is distressing to some animals and can result in self-induced trauma.
Alpha 2 agonists
Epidurally administered alpha2 agonists provide analgesia by activating presynaptic alpha2 adrenergic receptors located on primary afferent C fibers terminating in the dorsal horn of the spinal cord, as well as postsynaptic alpha2 receptors located on wide-dynamic-range projection neurons in the dorsal horn.14 Epidural administration of alpha2 agonists provides additional analgesia and reduces the minimum alveolar concentration of isoflurane. However, it is not uncommon to see bradycardia, hypotension, vomiting (in awake animals), and respiratory depression as a result of systemic uptake, and epidural administration of alpha2 agonists is contraindicated in patients with cardiovascular compromise.15-18
Postoperative analgesia after epidural medetomidine in dogs was comparable to epidural oxymorphone but resulted in bradycardia and second-degree atrioventricular blockade.16 Epidurally administered morphine in combination with medetomidine was associated with only minor benefits based on subjective pain scoring when compared with morphine alone in dogs undergoing repair of a ruptured cranial cruciate ligament.19 Further investigation of the benefits and potential adverse effects of epidural alpha2 agonists is warranted.
Perform epidural injections in heavily sedated or anesthetized animals. The injection site is usually at the lumbosacral space in dogs and cats and can be performed with a patient in either sternal or lateral recumbency.
Position the patient
Palpate the space
Prepare the site
Once you have identified the lumbosacral space, clip the hair, and surgically prepare the area. Place a sterile transparent drape over the area, tear a small opening in the drape at the injection site, and use a sterile disposable 2.5- to 7.5-cm, 20- to 22-ga spinal needle and sterile gloves. If the procedure is being performed in an awake but sedated patient, inject 0.5 to 1 ml 2% lidocaine (with sodium bicarbonate solution added to the local anesthetic in a 1:9 ratio of bicarbonate to lidocaine to reduce the pain of injection) subcutaneously at the site of the spinal needle insertion and wait five to 10 minutes before inserting the spinal needle at this site.
Insert the needle
The diameter of the lumbosacral epidural space is 2 to 4 mm in medium-sized dogs and < 3 mm in cats.4 As the needle is advanced, you usually feel a popping sensation as the skin is penetrated and then a second pop as the needle penetrates the ligamentum flavum and enters the epidural space. Advance the needle no farther since most of the blood vessels lie in the ventral part of the spinal canal and it is more likely that a vessel will be penetrated if advanced too far. In cats, the spinal dura mater frequently extends to S1-S2, so it is more likely that the dura and arachnoid will be punctured if the needle is advanced too deeply, entering the subarachnoid space. Occasionally, you will see the tail twitch or hindlimb movement as the epidural space is entered, indicating the needle has contacted the cauda equina. In this case, the drug may be injected without redirecting the needle.
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
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
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.
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.
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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