Local and regional anesthesia techniques, Part 4: Epidural anesthesia and analgesia - Veterinary Medicine
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Local and regional anesthesia techniques, Part 4: Epidural anesthesia and analgesia
Consider this straightforward and economical technique to relieve your patients' pain. And if you haven't already learned the technique, attend a wet lab or visit a local referral practice. You and your patients will benefit.


VETERINARY MEDICINE


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.

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.


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Source: VETERINARY MEDICINE,
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