Local and regional anesthesia techniques, Part 4: Epidural anesthesia and analgesia

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

CONTRAINDICATIONS

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.

Local anesthetics

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

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.

Alpha2 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.

TECHNIQUE

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

Epidural injection is easier to achieve with the patient in sternal recumbency, with the hindlimbs flexed at the hips and stifles and hocks extended so the legs are positioned alongside the body (Figure 1). This position makes the lumbosacral space easier to identify.

Figure 1. The lumbosacral space is easier to identify if you place the patient in sternal recumbency with the hindlimbs flexed at the hips, the stifles and hocks extended, and the legs alongside the body.

Palpate the space

Locate the lumbosacral space by palpating the most anterior aspect of both iliac crests with your thumb and middle finger; the imaginary line connecting them passes through the lumbosacral intervertebral space. With your index finger, palpate the space as a depression immediately caudal to the dorsal spinous process of L7 and immediately cranial to the fused dorsal spinous processes of the sacrum (Figure 2).

Figure 2. To identify the lumbosacral space, palpate the iliac crests, dorsal spinous process of L7, and the lumbosacral space immediately caudal to L7 and cranial to the sacrum.

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

Using your nondominant hand to confirm the landmarks and the lumbosacral space, position the spinal needle over the midline with your dominant hand (Figure 3). Midline positioning is critical to avoid contact with the transverse processes of the L7 vertebra. Direct the needle bevel cranially, and advance the needle, with stylet in place, perpendicular to the skin. Position your dominant hand so that it is in contact with the animal when penetrating the skin to provide counterpressure and to avoid pushing the needle in too far. It is often helpful to hold the stylet with the index finger of your dominant hand to keep it from being pushed out of the spinal needle by the subcutaneous tissues. If the needle encounters bone, withdraw it slightly, redirect it caudally or cranially, and advance it again. You may need to adjust the needle angle to facilitate correct placement in the epidural space.

Figure 3. Once the injection site area is prepared and draped, palpate the lumbosacral space, tear a small opening in the drape at the injection site, and insert the needle until you penetrate the ligamentum flavum.

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

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.

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.

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

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.

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.

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

REFERENCES

1. Stoelting RK, Hillier SC. Local anesthetics. In: Pharmacology and physiology in anesthetic practice. 4th ed. Philadelphia, Pa: Lippincott Williams & Wilkins, 2006;179-207.

2. Scarda RT, Tranquilli WJ. Local anesthetics; Local and regional anesthetic techniques: dogs; Local and regional anesthetic techniques: cats. In: Tranquilli WJ, Thurmon JC, Grimm KA, eds. Lumb and Jones' veterinary anesthesia and analgesia. 4th ed. Ames, Iowa: Blackwell Publishing, 2007;395-418, 561-604.

3. Lemke KA. Pain management II: local and regional anaesthetic techniques. In: Seymour C, Duke-Novakovski T, eds. BSAVA manual of canine and feline anaesthesia and analgesia. 2nd ed. Gloucester, UK: British Small Animal Veterinary Association, 2007;104-114.

4. Valverde A. Epidural analgesia and anesthesia in dogs and cats. Vet Clin North Am Small Anim Pract 2008;38(6):1205-1230.

5. Hansen BD. Epidural catheter analgesia in dogs and cats: techniques and review of 182 cases (1991-1999). J Vet Emerg Crit Care 2001;11(2):95-103.

6. Swalander DB, Crowe DT Jr, Hittenmiller DH, et al. Complications associated with the use of indwelling epidural catheters in dogs: 81 cases (1996-1999). J Am Vet Med Assoc 2000;216(3):368-370.

7. Valverde A, Dyson DH, McDonnell WN. Epidural morphine reduces halothane MAC in the dog. Can J Anaesth 1989;36(6):629-632.

8. Yaksh TL. Pharmacology and mechanisms of opioid analgesic activity. Acta Anaesthesiol Scand 1997;41:94-111.

9. Stein C, Machelska H, Schäfer M. Peripheral analgesic and antiinflammatory effects of opioids. Z Rheumatol 2001;60(6):416-424.

10. Förster JG, Rosenberg PH. Clinically useful adjuvants in regional anaesthesia. Curr Opin Anaesthesiol 2003;16(5):477-486.

11. Smith LJ, Kwang-An Yu J. A comparison of epidural buprenorphine with epidural morphine for postoperative analgesia following stifle surgery in dogs. Vet Anaesth Analg 2001;28(2):87-96.

12. Pypendop BH, Siao KT, Pascoe PJ, et al. Effects of epidurally administered morphine or buprenorphine on the thermal threshold in cats. Am J Vet Res 2008;69(8):983-987.

13. Morimoto K, Nishimura R, Matsunaga S, et al. Epidural analgesia with a combination of bupivacaine and buprenorphine in rats. J Vet Med 2001;48(5):303-312.

14. Schug SA, Saunders D, Kurowski I, et al. Neuraxial drug administration: a review of treatment options for anaesthesia and analgesia. CNS Drugs 2006;20(11):917-933.

15. Vesal N, Cribb PH, Frketic M. Postoperative analgesic and cardiopulmonary effects in dogs of oxymorphone administered epidurally and intramuscularly, and medetomidine administered epidurally: a comparative clinical study. Vet Surg 1996;25(4):361-369.

16. Duke T, Cox AM, Remedios AM, et al. The analgesic effects of administering fentanyl or medetomidine in the lumbosacral epidural space of cats. Vet Surg 1994;23(2):143-148.

17. Sabbe MB, Penning JP, Ozaki GT, et al. Spinal and systemic action of the alpha 2 receptor agonist dexmedetomidine in dogs. Antinociception and carbon dioxide response. Anesthesiology 1994;80(5):1057-1072.

18. Campagnol D, Teixeira Neto FJ, Giordano T, et al. Effects of epidural administration of dexmedetomidine on the minimum alveolar concentration of isoflurane in dogs. Am J Vet Res 2007;68(12):1308-1318.

19. Pacharinsak C, Greene SA, Keegan RD, et al. Postoperative analgesia in dogs receiving epidural morphine plus medetomidine. J Vet Pharmacol Ther 2003;26(1):71-77.