ANAESTHESIOLOGY
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be rotated after epidural injection of local anaesthetics , 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 anaesthetics 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 anaesthetic and should be avoided .
Opioids Opioids are frequently included with the local anaesthetic to provide additional , often more prolonged analgesia ( Table 1 ), but opioids do not provide anaesthesia . 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 orthopaedic 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 . Mμ 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 , hyperaesthesia 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 orthopaedic 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 anaesthetic 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 anaesthesia . 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 anaesthetic , as recovery of motor blockade will occur before the patient recovers from anaesthesia . Prolonged motor blockade from bupivacaine after recovery from a short period of inhalant anaesthesia is distressing to some animals and can result in self-induced trauma .
Alpha-2-agonists Epidurally administered alpha-2-agonists provide analgesia by activating presynaptic alpha-2 adrenergic receptors located on primary afferent C fibers terminating in the dorsal horn of the spinal cord , as well as postsynaptic alpha-2 receptors located on wide-dynamic-range projection neurons in the dorsal horn . 14 Epidural administration of alpha-2-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 alpha-2-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 alpha-2-agonists is warranted .
TECHNIQUE Perform epidural injections in heavily sedated or anaesthetised 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 ster- vet360
Issue 06 | FEBRUARY 2017 | 14